Development of Neutron Emission Spectroscopy ... - DiVA Portal

ACTA

UNIVERSITATIS

UPSALIENSIS

UPPSALA

2007

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 339

Development of Neutron EmissionSpectroscopy Instrumentation forDeuterium and Deuterium-TritiumFusion Plasmas at JET

LUCA GIACOMELLI

ISSN 1651-6214ISBN 978-91-554-6961-0urn:nbn:se:uu:diva-8199

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v

List of papers This thesis is based on the following papers which are referred to in the text by their Roman numerals. I L. Giacomelli, H. Henriksson, M. Tardocchi, S. Conroy, G. Ericsson, M. Gatu Johnson,

W. Glasser, G. Gorini, A. Hjalmarsson, M. Johansson, J. Källne, A. Murari, S. Popovichev, H. Sjöstrand, M. Weiszflog and JET EFDA Contributors Neutron emission spectroscopy of fuel ion rotation and fusion power components demonstrated in the trace tritium experiments at JET 31st EPS Conference on Plasma Phys. London, 28 June - 2 July 2004 ECA Vol.28G, P-5.171 (2004)

II L. Giacomelli, C. Hellesen, A. Hjalmarsson, H. Sjöstrand, J. Källne, S. Conroy,

G. Ericsson, E. Andersson Sundén, M. Gatu Johnson, W. Glasser, G. Gorini, E. Ronchi, M. Tardocchi, M. Weiszflog and JET EFDA Contributors Characterization of phoswich scintillation detectors for the focal plane hodoscope of magnetic proton recoil spectrometers for fusion neutrons UU-NF 07#11 Uppsala University, Neutron Physics Report ISSN 1401-6269 (July 2007) unpublished

III H. Sjöstrand, L. Giacomelli, E. Andersson Sundén, S. Conroy, G. Ericsson,

M. Gatu Johnson, C. Hellesen, A. Hjalmarsson, J. Källne, E. Ronchi, M. Weiszflog, G. Wikström, G. Gorini, M. Tardocchi, A. Murari, G. Kaveney, S. Popovichev, J. Sousa, R.C. Pereira, A. Combo, N. Cruz and JET EFDA Contributors New MPRu instrument for neutron emission spectroscopy at JET Review of Scientific Instruments 77 10E717 (2006)

IV L. Giacomelli, S. Conroy, G. Ericsson, G. Gorini, H. Henriksson, A. Hjalmarsson,

J. Källne and M. Tardocchi Comparison of neutron emission spectra for D and DT plasmas with auxiliary heating Eur. Phys. J. D 33 (2005) 235-241

V A. Hjalmarsson, S. Conroy, G. Ericsson, L. Giacomelli, G. Gorini, H. Henriksson,

J. Källne, M. Tardocchi and M. Weiszflog The TOFOR spectrometer for 2.5 MeV neutron measurements at JET Review of Scientific Instruments 74 3 (2003) 1750-1752

VI M. Gatu Johnson, L. Giacomelli, A. Hjalmarsson, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, C. Hellesen, J. Källne, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, A. Murari, S. Popovichev, J. Sousa, R.C. Pereira, A. Combo, N. Cruz and JET EFDA Contributors The TOFOR neutron spectrometer and its first use at JET Review of Scientific Instruments 77 10E702 (2006)

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VII M. Gatu Johnson, L. Giacomelli, A. Hjalmarsson, J. Källne, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, C. Hellesen, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, A. Combo, N. Cruz, J. Sousa, S. Popovichev and JET EFDA Contributors The 2.5-MeV neutron time-of-flight spectrometer TOFOR for experiments at JET UU-NF 07#12 Uppsala University, Neutron Physics Report ISSN 1401-6269 (August 2007) Submitted to Nucl. Instr. Meth.

VIII C. Hellesen, L. Giacomelli, M. Gatu Johnson, A. Hjalmarsson, J. Källne, M. Weiszflog,

E. Andersson Sundén, S. Conroy, G. Ericsson, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, S. Popovichev, L. Ballabio and JET EFDA Contributors Measurement and analysis of the neutron emission from ICRH and NB heated JET D plasmas using the TOFOR spectrometer UU-NF 07#13 Uppsala University, Neutron Physics Report ISSN 1401-6269 (August 2007) To be submitted to Nucl. Fusion

All papers are reprinted with the permission of the journals. Paper III is reprinted with permission from H. Sjöstrand, Review of Scientific Instruments, 77, 10E717 (2006). Copyright 2006, American Institute of Physics. Paper IV is reprinted with permission from European Physical Journal D vol 33, (2005) 235-241. © EDP Sciences, Societá Italiana di Fisica, Springer-Verlag 2005. Paper V is reprinted with permission from A. Hjalmarsson, Review of Scientific Instruments, 74, 1750 (2003). Copyright 2003, American Institute of Physics. Paper VI is reprinted with permission from M. Gatu Johnson, Review of Scientific Instruments, 77, 10E702 (2006). Copyright 2006, American Institute of Physics. Permission for republication must be obtained from the respective copyright holders.

The home pages of the journals where the papers appeared are at the following URL:

Review of Scientific Instruments: http://rsi.aip.org/ The European Physical Journal D: http://www.edpsciences.org/journal/index.cfm?edpsname=epjd

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Papers not included in the thesis 1. M. Tardocchi, G. Gorini, D. Palma, C. Sozzi, J. Källne, S. Conroy, G. Ericsson,

L. Giacomelli, W. Glasser, H. Henriksson, A. Hjalmarsson, E. Ronchi, H. Sjöstrand, M. Weiszflog, S. Popovichev and JET EFDA Contributors Control and monitoring system for fusion neutron spectroscopy on the Joint European Torus Review of Scientific Instruments 75 10 (2004) 3543-3546

2. L. Giacomelli, A. Hjalmarsson, H. Sjöstrand, W. Glasser, J. Källne, S. Conroy,

G. Ericsson, M. Gatu Johnson, G. Gorini, H. Henriksson, S. Popovichev, E. Ronchi, J. Sousa, E. Sundén Andersson, M. Tardocchi, J. Thun, M. Weiszflog and Contributors to the JET EFDA Workprogram Advanced neutron diagnostics for JET and ITER fusion experiments Nucl. Fusion 45 (2005) 1191-1201

3. H. Henriksson, S. Conroy, G. Ericsson, L. Giacomelli, G. Gorini, A. Hjalmarsson,

J. Källne, M. Tardocchi and M. Weiszflog Systematic spectral features in the neutron emission from NB heated JET DT plasmas Plasma Phys. Control. Fusion 47 (2005) 1763-1785

4. L. Giacomelli, E. Andersson Sundén, S. Conroy, G. Ericsson, M. Gatu Johnson,

C. Hellesen, A. Hjalmarsson, J. Källne, E. Ronchi, H. Sjöstrand, M. Weiszflog, G. Gorini, M. Tardocchi, A. Murari, S. Popovichev, J. Sousa, R.C. Pereira, A. Combo, N. Cruz and JET EFDA Contributors Development and characterization of the proton recoil detector for the MPRu neutron spectrometer Review of Scientific Instruments 77 10E708 (2006)

5. M. Tardocchi, G. Gorini, E. Andersson Sundén, S. Conroy, G. Ericsson,

M. Gatu Johnson, L. Giacomelli, C. Hellesen, A. Hjalmarsson, J. Källne, E. Ronchi, H. Sjöstrand, M. Weiszflog, T. Johnson, P. U. Lamalle and JET EFDA Contributors Modeling of neutron emission spectroscopy in JET discharges with fast tritons from (T)D ion cyclotron heating Review of Scientific Instruments 77 126107 (2006)

6. W. Glasser, L. Giacomelli, H. Sjöstrand, E. Andersson-Sundén, S. Conroy, G. Ericsson,

C. Hellesen, H. Hjalmarsson, J. Källne and E. Ronchi Determination of the hodoscope geometry of the MPRu neutron spectrometer UU-NF 05#11 Uppsala University, Neutron Physics Report ISSN 1401-6269 (December 2005) unpublished

7. A. Hjalmarsson, S. Conroy, G. Ericsson, L. Giacomelli and J. Källne Neutron transport simulations for the design and performance optimization of the

TOFOR neutron time-of-flight spectrometer UU-NF 05#12 Uppsala University, Neutron Physics Report ISSN 1401-6269 (December 2005) unpublished

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8. A. Hjalmarsson, S. Conroy, G. Ericsson, M. Gatu Johnson, L. Giacomelli, W. Glasser, G. Gorini, C. Hellesen, J. Källne, E. Ronchi, H. Sjöstrand, E. Sundén Andersson, M. Tardocchi, J. Thun and M. Weiszflog

Characterization of a scintillator detector with charged particles and pulse light emission

UU-NF 05#13 Uppsala University, Neutron Physics Report ISSN 1401-6269 (December 2005) unpublished

9. A. Hjalmarsson, S. Conroy, G. Ericsson, L. Giacomelli and J. Källne

Response function simulation of the TOFOR neutron time-of-flight spectrometer UU-NF 06#06 Uppsala University, Neutron Physics Report ISSN 1401-6269 (December 2005) unpublished

10. J. Ongena, A. Ekedahl, L.-G. Eriksson, J. Mailloux, M.-L. Mayoral, K. Rantamaki,

D. Van Eester, Yu. Baranov, C.D. Challis, L. Colas, F. Durodié, G. Ericsson, Ph. Jacquet, I. Jenkins, T. Johnson, M. Goniche, G. Granucci, C. Hellesen, T. Hellsten, J. Källne, K. Holmström, A. Krasilnikov, E. Lerche, I. Monakhov, M. Nave, M. Nightingale, M. Lennholm, V. Petrzilka, M. Santala, M. Vrancken, A. Walden, L. Bertalot, V. Bobkov, R. Cesario, G. Corrigan, K. Erents, L. Giacomelli, G. Gorini, A. Hjalmarsson, M. Gatu Johnson, V. Kiptily, P.U. Lamalle, M. Laxåback, M.J. Mantsinen, D. Mazon, D. Moreau, M. Mirizzi, J.M. Noterdaeme, V. Parail, G. Ravera, A. Salmi, M. Tardocchi, M. Weiszflog, JET EFDA Task Force H and JET EFDA Contributors Overview of recent results on Heating and Current Drive in JET 17th Topical Conference on Radio Frequency Power in Plasmas, Clearwater, Florida, USA, May 2007

11. A.V. Krasilnikov, D. Van Eester, E. Lerche, J. Ongena, J. Mailloux, M. Stamp,

S. Jachmich, H. Leggate, V. Vdovin, A. Walden, M.-L. Mayoral, G. Bonheure, M. Santala, V. Kiptily, S. Popovichev, T. Biewer, K. Crombe, B. Esposito, D. Marocco, M. Riva, Yu.A. Kaschuck, V.N. Amosov, G. Ericsson, L. Giacomelli, C. Hellesen, A. Hjalmarsson, J. Källne and JET EFDA Task Force Heating and LHD team, M. Isobe, M. Nishiura, M. Sasao, H. Nishimura, K. Saito, T. Seki, T. Mutoh, R. Kumazawa, Y. Takeiri, M. Osakabe, M. Goto, S. Murakami, P. Goncharov and JET EFDA Contributors ICRH of JET and LHD Majority Ions at their Fundamental Cyclotron Frequency 17th Topical Conference on Radio Frequency Power in Plasmas, Clearwater, Florida, USA, May 2007, Pre-print on Iopp website reference EFD-C(07)01/11

12. D. Van Eester, E. Lerche, P. Mantica, A. Marinoni, A. Casati, G. Ericsson, L. Giacomelli, C. Hellesen, A. Hjalmarsson, J. Källne, V. Kiptily, S. Sharapov, M. Santala, T.M. Biewerk, C. Giroud, K. Crombé, Y. Andrew, J. Ongena, E. Joffrin, P. Lomas, R. Felton, F. Imbeaux, F. Ryter and JET EFDA Contributors Recent experimental results and modeling of RF heating of (3He)-D JET plasmas: RF as a tool to study transport 17th Topical Conference on Radio Frequency Power in Plasmas, Clearwater, Florida, USA, May 2007, Pre-print on Iopp website reference EFD-C(07)01/13

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13. C. Hellesen, L. Giacomelli, M. Gatu Johnson, A. Hjalmarsson, J. Källne, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, E. Ronchi, H. Sjöstrand, M. Albergante, G. Gorini, M. Tardocchi, I. Jenkins, S. Popovichev and JET EFDA Contributors Measurement and analysis of TOFOR neutron spectra from RF and NB heated JET D plasmas 34th EPS Conference on Plasma Physics, Warsaw, Poland, July 2007

14. L. Giacomelli, A. Hjalmarsson, C. Hellesen, M. Gatu Johnson, J. Källne, M. Weiszflog,

S. Sharapov, E. Andersson Sundén, S. Conroy, G. Ericsson, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, S. Popovichev, T. Johnson and JET EFDA Contributors Neutron emission spectroscopy diagnosis of fast ions in RF D(3He) heated plasmas at JET 34th EPS Conference on Plasma Physics, Warsaw, Poland, July 2007

Contents

1. Introduction .............................................................................................................................13

2. The JET tokamak.....................................................................................................................15 2.1 The machine................................................................................................................... 16 2.2 Fusion plasmas with D and DT fuel .............................................................................. 17 2.3 Neutron emission diagnostics ........................................................................................ 19

3. Neutron emission spectroscopy of D and DT fusion plasmas at JET......................................23 3.1 The spectrometers.......................................................................................................... 24

3.1.1 MPR.................................................................................................................. 24 3.1.2 TOFOR ............................................................................................................. 26

3.2 Analysis and interpretation of NES data........................................................................ 28 3.3 NES experiments at JET................................................................................................ 30

4. Results .....................................................................................................................................31 4.1 NES experiments with the MPR for DT plasmas .......................................................... 31 4.2 Development of the MPRu focal plane detector............................................................ 34 4.3 NES diagnosis of D plasmas from the experience of DT .............................................. 41 4.4 Construction, installation and first use of TOFOR spectrometer .................................. 43 4.5 Data taking with TOFOR and first physics results ........................................................ 47

5. Conclusions and outlook .........................................................................................................57

6. Summary of the papers ............................................................................................................59

7. Sammanfattning på svenska ....................................................................................................65

8. Acknowledgements .................................................................................................................67

References......................................................................................................................................69

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1. Introduction The ultimate goal in nuclear fusion research is the production of energy on a commercial basis. The advantages of nuclear fusion relates to the type of fuel chosen. Hydrogen isotopes, deuterium (2H or D while d is used to indicate the ion) and tritium (3H or T and t for the ion), are used as reactants. T is an artificial isotope bred from lithium (Li) isotopes, abundantly available on earth, as is D (about 30 ppm in earth’s water) [1]. The products of the reactions in D or DT fuels are helium (He) isotopes (already present in the atmosphere at the 5 ppm level) and neutrons. The neutrons carry a large fraction (75 % in dd reactions or 80 % for dt) of the released nuclear fusion energy which can be converted into heat outside the chamber used for the thermonuclear combustion of the fuel. The conversion is obtained by stopping the neutron radiation in a blanket around the chamber. This results not only in heat but also in nuclear reaction processes such as the desirable T breeding and the undesirable activation of the reactor structure (as in ordinary nuclear fission reactors but with no spent radioactive fuel left over). The abundance of fuel, the lack of green house gases and fuel waste together with the inherent safety of the process make fusion an important research investment for the future. Although fusion has the principal characteristics of an ideal energy source, the drawback is that there is presently no reactor capable of burning the fusion fuel let alone making use of the released energy for practical applications such as electricity generation. The fusion reactor must operate with thermal fuel which has to be in the state of plasma, i.e., an ionized gas made up of ions and electrons in separate motion, meaning that initial energy has to be supplied to the fuel (the hydrogen ionization energy is EH=13.6 eV1 equivalent to 1.58�105 K temperature using the Boltzmann’s constant). Moreover, the Coulomb barrier between nuclei has to be overcome to make them fuse, implying kinetic energies of the order of keV corresponding to plasma temperatures of 107 K. Differently from the stars, where the large volume plasmas determine the conditions for many types of fusion reactions, the man-made plasmas have volumes in the range 10-9-102 m3. Solid pellets of DT fuel can be heated with, for instance, laser beams and the volume of the plasma produced is limited due to the so-called inertial confinement. Alternatively, plasmas of larger volumes can be confined by the use of suitable magnetic field configurations.

The obstacles meeting the fusion methods are of different kinds. They relate to the physics of plasmas in extreme fusion conditions as well as to laser technology and power deposition in the pellet (inertial fusion), to plasma-wall interactions in the reactor chamber and handling of high magnetic fields and currents (magnetic confinement fusion) besides the high power loads. With regard to plasma conditions aimed at energy production, the output power from fusion reactions has to be greater than the power used to create and maintain the fusion process itself, i.e., Pout>Pin. The power losses are Bremsstrahlung radiation and escaping particles, besides transport from the plasma. All these terms result in a relationship, the Lawson criterion that bonds together the main plasma parameters. These are the plasma temperature (T), density (n) and confinement time �E defined as the ratio between the total plasma energy and the energy loss rate. The criterion Tn�E>5�1021 keV m-3 s [2] specifies a minimum for the plasma ignition.

This can be achieved for plasmas of temperatures of the order of 107 K, but with different combinations of n and �E. In inertial confinement fusion, the density of the plasma is very high 1 The electron volt eV is defined as the kinetic energy acquired by an electron accelerated from rest through a potential difference of one volt. In SI, 1 eV=1.602�10-19 J.

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(1032 m-3) with �E�10-9 s while in magnetic confinement fusion one operates with n�1020 m-3 with �E�10 s.

A principal figure of merit for the reactor is the fusion energy gain factor Q expressed by the ratio of the power released in nuclear fusion (Pf) and the one supplied (Pin) to maintain the plasma state in thermal equilibrium, i.e., Q=Pf/Pin. For Q=1, the break-even condition is reached while the ignition happens if the fusion plasma in the reactor is self-sustained, namely, maintained in its thermal state through only the energy released by the charged reaction products [2].

The work presented in this thesis deals with magnetic confinement fusion. Modern magnetic confinement fusion research started with the advent of the tokamak2 in 1968 [3]. The concept developed over the years with a series of machines of increasing size so as to approach plasma conditions which could bring the fuel closer to ignition. An important stepping-stone was the Joint European Torus (JET) built in England, which started operation in 1983. In November 2006, after many years of research and development, the construction of a new experimental reactor ITER started and is planned to be operational in 2016. ITER is about twice as large as JET and is projected to reach Q=10 to demonstrate the reactor concept validity based on magnetic confinement fusion.

2 Acronym created from the Russian words, "TOroidalnaya KAmera ee MAgnitnaya Katushka," or "Toroidal Chamber and Magnetic Coil".

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2. The JET tokamak The largest experimental reactor for magnetic confinement fusion is JET, located at the Culham Science Center in England. The project was approved by the European Union in 1978 and came into operation in 1983 with the main aims devoted to the studies of the scaling of the plasma as fusion parameters approached the reactor range, plasma-wall interactions, plasma heating and �-particle production, confinement and subsequent plasma self-heating. A better understanding of these topics has been achieved through the experimental campaigns run over these years but new issues to address have come up along the way.

This has lately been of help to pave the way to ITER for its envisaged operating scenarios including technology aspects such as antennae for radio frequency (RF) power injection and plasma facing walls of tungsten (in the divertor) and beryllium besides diagnostics [4]. Table 1. Comparison of the main parameters of JET and ITER tokamaks. Adapted from [5]. The values for JET indicate maximum performance achieved separately.

JET ITER design values Plasma major radius [m] 3 6.2 Plasma minor radius, horizontal [m] 1.25 2 Plasma volume [m3] 100 840 Magnetic field on axis [T] 4 5.3 Plasma current in D-shaped plasma [MA] 7 15 PNB [MW] 22 33-50 PRF (ICRH) [MW] 22 20-40 Core density [m-3] 2 × 1020 1020 Pulse time [s] 60 400 Electron temperature [keV] 20 21 Ion temperature [keV] 40 18 Q value in DT plasma 0.6 10 Fusion power [MW] 16 500 Fusion energy [GJ] 0.022 in 4 s >120

ITER is designed to reach Q=10 for a fusion power level of 500 MW compared to a

maximum of 16 MW reached at JET (Table 1). It is also noteworthy that ITER will feature pulses 400-s long for a produced energy greater than 120 GJ allowing testing reactor relevant issues such as heating and radiation loads on materials.

JET has been subjected to continuous development to increase its performance during its operating period of more than 20 years. The reactor facility includes a large-scale tritium storage and processing plant allowing for DT operation besides the usual D plasma experiments. The first low (0.2 g) concentration T operation was performed in 1993 [6]. The experience gained together with the full capability of the Active Gas Handling System (AGHS) provided the conditions for the first DT experimental campaign (DTE1) in 1997 [7]. Here 20 g of T was affording operation with plasmas of large variation in the D:T mixing ratio (say from 10:1 to 1:10 and beyond). During this campaign new operation records were achieved such as 16 MW of released fusion power, 22 MJ of fusion energy, a steady fusion power of 4 MW maintained for 4 s and Q=0.6 [8]. The latest experimental campaign with tritium was held in 2003 (TTE) and made use of trace

16

amounts of T in D plasmas, i.e., a T:D mixing ratio of about 1:100. A major enhanced performance (EP1) program was undertaken during the shutdown period 2003-05 to increase the heating power and the diagnostic capabilities [9]. The restart of operations began in late 2005 followed by the resumption of the experimental campaigns in 2006 with operations in deuterium. The work presented in this thesis concerns the project, the characterization and the performance of neutron emission spectroscopy instrumentation installed for JET-EP1 and the results obtained during the last ten years of JET operations, namely DTE1, TTE and the recent D experimental campaigns. 2.1 The machine The plasma magnetic confinement at JET is the typical one for tokamaks and is achieved through the combination of poloidal and toroidal magnetic fields singularly generated, which result in a helical configuration (Fig. 1a). A central iron column is part of a transformer with eight limbs enclosing the plasma in the toroidal vacuum chamber (Fig. 1b); the primary transformer winding is the central coil and the plasma is the secondary. The poloidal magnetic field results from the toroidal plasma current induced by transformer action. Water-cooled copper coils around the vacuum chamber produce the toroidal magnetic field. In addition, around the outside of the machine there is a set of six outer poloidal field coils used to positioning, shaping and stabilizing the location of the plasma inside the vessel. The transformer action used at JET means that the machine operates in a pulsed mode, about one shot every twenty minutes lasting for about 60 s.

Fig. 1. Sketch of the JET field coils and resulting magnetic field lines, (a), and picture of the machine, (b). Courtesy of EFDA-JET; Figure: EFDA-JET [10].

Under nominal working conditions, the plasma temperature in a fusion reactor is

expected to be about 20 keV (see Table 1). Different auxiliary heating systems must be used to reach such temperatures since the basic Ohmic heating (P�) through the plasma toroidal current suffers the plasma conductivity dependence T3/2. Therefore P� on JET can only supply power to the plasma up to the limit of a few MW. Instead, the main plasma heating is supplied by auxiliary power (Paux) sources such as neutral beam (NB) and radio frequency (RF) wave injection. The

17

latter can be in the form of ion cyclotron resonance heating (ICRH) or lower hybrid current drive (LHCD).

The NB system at JET can provide up to PNB=23 MW through beams of 80 and 130 keV. The beams are obtained by accelerating ionized hydrogen isotopes (proton, d or t), neutralizing them in order to cross the tokamak magnetic field and reach the plasma core, the region of higher temperature and density during the shot. The injection is typically quasi-tangential at an angle of 60� with respect to the toroidal magnetic field axis. In the plasma, the beam atoms lose electrons due to collisions and/or charge-exchange reactions becoming ionized and as a consequence confined by the tokamak magnetic field. With ICRH, the RF is tuned to a resonant frequency of the ions in the plasma (see below) which are accelerated. JET has four antennae that can deliver 32 MW ICRH power in the frequency range 23-57 MHz. It is typically coupled to the second harmonic frequency of the main species (d or t in DTE1) or to the fundamental frequency of minority ion species (t in TTE or 3He in DT and D plasmas). Both NB and ICRH power injections give rise to fast ion populations interacting in the plasma while following different trajectories.

The trajectory of a charged particle along the tokamak magnetic field is complex and can be simplified by considering it as a combination of the orbit that a particle describes around the field line (gyro-motion) and the motion of orbit center along it. The size of the orbit is defined as

the Larmor radius Bq

mv� where m and q are the mass and the charge of the particle, �v is the

velocity component orthogonal to the magnetic field B and the corresponding cyclotron

frequency, m

qB�

�2

. Moreover, the ion motion depends on energy, position, and pitch angle

defined as v

v� �sin where v is the ion speed and v is the velocity vector. One can distinguish

between ions on passing orbits following the helical magnetic field lines so that is fixed and others that are trapped. These have variable and describe “banana” orbits which include points where the ions are reflected backwards ( � vv ) at the “banana” tips; in between these points

0|| �v . The ICRH heating is based on the resonance of the RF wave with the particles whose

gyro-motion matches the same frequency. The wave is injected orthogonally to the toroidal magnetic field and the interaction gives rise to the acceleration of the particle. The population of “heated” particles spins very fast (high �v ) with a correspondingly large Larmor radius . Depending on the magnetic field in the region of ionization/resonance, the NB/ICRH fast ions may be trapped in “banana” orbits.

The LHCD system works at 3.7 GHz with a capacity of 12 MW. The LH waves interact mainly with the electrons driving electric current in the plasma parallel to the magnetic field lines. This has effect on the plasma magnetic profile and stability.

Besides P� and Paux, there can also be contributions from the fusion ion products, i.e., the � particles in DT plasmas (P�). This heating effect was detected in the record fusion power discharges at JET with contributions up to 10 % of Paux [11]. In a fusion reactor, the main heating source must be dominated by P� whose exploitation will be tested with ITER. 2.2 Fusion plasmas with D and DT fuel The fuel in use in the JET tokamak is generally deuterium and only rarely a mixture of deuterium and tritium. These hydrogen isotopes are the preferred fuel because they have the lowest

18

Coulomb barrier and, hence, allowing for fusion reactions to take place in plasmas of a few keV temperature. The reactions of interest are [12][13]

� � � � � ��

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%100~ MeV)(14.64 p MeV)(3.71 He He d

%107~MeV16.6�He%100~MeV14.03nMeV3.54He

t d

%10~MeV23.8��%50~MeV3.02pMeV1.01t%50~2.45MeVnMeV0.82He

dd

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�

�

where the birth energy of the fusion products in MeV are given in parenthesis and the branching ratio in percent in brackets. We can note that the fusion reactions involving � production is suppressed by a factor of 10-4 or more relative to those involving nuclide production only (i.e., nuclear interaction channels in contrast to Coulomb interaction for �’s). These reactions are therefore insignificant with regard to the total fusion energy released and its division between reaction channels. The two dd nuclear reaction branches occur with equal probability for the typical ion motional states of fusion plasmas. This is illustrated by the fusion reactivity as function of temperature for d+d�3He+n, d+d�t+p and d+t��+n in Fig. 2. Here we also see that the reactivity is almost two orders of magnitude higher for dt compared to dd for fusion plasma temperatures up to say 50 keV. This is the reason for DT as the fuel of choice in the plasma of a fusion reactor. This advantage is considered to outweigh the drawback that tritium is a radioactive element that decays through 18.6-keV �’s (end point energy) emission with a half-life of 12.3 years, i.e., one has to handle the resulting contamination of certain parts of the fusion reactor. It also requires a special gas handling and processing system besides the fact that tritium needs to be produced through breeding at the same rate that it is consumed3. The use of D3He as fusion fuel is a possibility for the reason that it has a peak reactivity approaching that of DT but at a much higher temperature, i.e., about 300 keV instead of 70 keV for dt. In principle, the dt and d+3He reactions are very similar from the nuclear interaction point of view only for the difference of the much higher Coulomb repulsion between d and 3He ions which is what pushes up the temperature of the peak reactivity. The D(3He) fuel alternative is also not practical because there is no suitable breeding possibility of 3He. Moreover, as no neutrons are emitted it means that there is no volume absorption of the released fusion energy so all energy has to be absorbed on plasma facing surfaces. The non-neutronic energy release has been discussed as a benefit as it would minimize the activation of the machine, but may in total not be so. Still, all released fusion energy of d+3He reactions would be carried by charged particles whose kinetic energy is deposited in the plasma (more likely from �’s) during the slowing down (thermalization). Moreover, this reaction produces the highest energy release (18.4 MeV) compared to 17.6 MeV for dt and 3.7 MeV (average) for dd. In the latter two cases, the fraction carried by neutrons are 4/5 and 3/4 so that only 3.5 MeV and 0.8 MeV, respectively, are deposited

3 The breeding process involves 6Li and 7Li such as 6Li+n�t+� or 7Li+n�t+�+n’ nuclear reactions.

19

through slowing down and making up the plasma self-heating. There is thus only a small relative advantage to DT compared to D plasmas while D3He is the more convenient in this respect.

Fig. 2. Reactivity <�v> of the fusion reactions as function of the ion temperature Ti where those for the two branches of the dd reaction are almost identical. Adapted from [14].

The neutron emission is not only important for the fusion power deposition in reactors

but it is also a carrier of information about the plasma. In other words, the neutron emission is an essential means of diagnosing plasmas with respect, for instance, to the total fusion power and where in the plasma it is generated, besides the motional state of the fuel ion producing it. Since the plasma has to be heated to reach desired fusion conditions, auxiliary power (Paux) is injected via NB or ICRH. This power couples directly or indirectly to d and/or t ion populations whose motional state is affected and which, through the two-body kinematics, is conveyed to the reaction product emission. Neutrons therefore bring information about the quality of the auxiliary heating in terms of fusion power, plasma temperatures, and ion velocity distributions. Important characteristics of neutron measurements as a fusion plasma diagnostics are that they are non-intrusive and they can typically sustain high radiation fluxes and fluences with less instrumental radiation damage than for most other diagnostics. 2.3 Neutron emission diagnostics JET is equipped with neutron diagnostic (ND) systems to study both D and DT plasmas. These can be summarized in three categories depending on their primary diagnostic uses, i.e., to determine (a) the yield rate of the neutron emission (b) the spatial (radial) distribution of the neutron emission (c) the spectrum of the neutron emission. The main instrumentation for these is total neutron yield monitors, neutron camera and neutron spectrometers. The monitors are used to measure the uncollimated flux from the plasmas and are calibrated with activation foil measurements so that the total neutron yield rate of the plasma can

20

be determined. The JET neutron camera consists of two sets of collimator arrays (9 vertical and 10 horizontal for 19 sight lines). The camera provides data on the neutron flux and its variation with time from which the spatial emissivity distribution of the plasma is derived through reconstruction. These two types of ND systems have rather specific plasma diagnostic uses. Information on neutron spectrum is keenly desired as it can provide insight on several aspects of the physics of the plasma and determination of diagnostic parameters; i.e., neutron emission spectroscopy (NES) has multiple functions of plasma diagnostics. At JET, scintillation detectors are used in this context but it is the rather large instruments of dedicated designs that are used for routine diagnosis of D and DT plasmas. NES is the part of the ND that is most requiring with respect to instrumental development which plasma information potential is still at a rather unexplored level. This thesis is about (c), i.e., NES diagnosis of JET plasmas. The theoretical foundation for the use of NES goes back to 1967 [15] when Lehner and Pohl demonstrated that the neutron emission spectrum of a plasma in thermal equilibrium is Gaussian shaped and a simple relationship exists between the ion temperature Ti and the spectral full width at half maximum (FWHM) �, i.e., iTk� . This has been demonstrated in experiments at the TFR [16] and Alcator C [17] tokamaks. NES diagnostics were installed from the beginning at JET and the results on plasma temperature were presented in Ref. [18]. Over the last 20 years, a number of NES diagnostic uses has be demonstrated at JET. Depending also on the instrumental line of sight, the plasma toroidal collective motion can be measured from the shift of the mean position of the neutron emission spectrum [19]. Moreover, other plasma parameters can be extracted for the neutron spectra. The total number of measured neutrons in the spectrum during a discharge relates to the total neutron yield and can be used to determine the total fusion power [20]. As said before, the neutron spectrum reflects the kinematics of the nuclear fusion reactants. It can present tails due to energetic ion populations from the auxiliary heating or multi step ion interactions. For instance, in DT plasmas, the � particles produced may scatter with d or t raising their energies and resulting in energetic neutrons recognizable in the spectral tail above 15.5 MeV at 10-4 of the neutron peak value. This is referred to as �-particle knock on neutron (AKN) emission [21]. The AKN process may also be recognized in D plasmas if 3He is added as a minority for ICRH purpose [22]. Moreover, the energetic tritons generated may fuse with the deuterons producing 14-MeV neutrons. This process is called triton burn up neutron (TBN) emission and features 10-2 intensity with respect to the main dd neutron emission peaking at 2.5 MeV.

All these spectral signatures pose requirements on the NES instrumentation. As the information extracted rely on the statistical accuracy of the data, the spectrometer must feature high efficiency to the incoming neutrons and high resolution. The efficiency determines the instrumental count rate (Cn) attainable during the measurement and the time-resolved spectral capability while the resolution deals with the precision and accuracy of the measurement itself. Moreover, efficiency and resolution play opposite roles in the projected performance and a trade off has to be reached to attain detailed and reliable information, namely to recognize spectral features that differ several order of magnitude and cover a wide energy range. The wide energy coverage is a very important requirement for the spectrometer to be able to measure plasma signatures as AKN and TBN.

The performance of the spectrometer is only one aspect of the NES requirements. In order to extract the information from the neutron spectra, the detailed instrumental response and a reliable model of the plasma populations are needed. JET has been the center for the development of NES diagnosis of tokamak plasmas for over 25 years. The latest NES advancement effort was carried out as part of the enhanced performance (JET-EP1) program devoted to heating and diagnostic systems during the period 2003-05. This program included two neutron diagnostic projects, namely, the MPRu and TOFOR

21

neutron spectrometers. The first project concerned the upgrade of the magnetic proton recoil (MPRu) spectrometer and was developed as part of the present thesis. The other acronym indicates the time-of-flight (TOF) neutron spectrometer that was specifically designed to optimize the count rate (TOFOR). TOFOR is a new spectrometer that was optimized, constructed, characterized, and put into operation at JET as part of this thesis.

23

3. Neutron emission spectroscopy of D and DT fusion plasmas at JET

The instruments for neutrons emission spectroscopy (NES) can be used for the basic measurements for D and DT plasmas, i.e., neutrons from dd and dt reactions of energies around 2.5 and 14 MeV, respectively. Dedicated types of instruments must be used, not only because of the difference in energy, but also due to the fact that the neutron flux is a factor 100 greater for DT plasmas compared to D for otherwise equal condition. This means that a 2.5-MeV neutron spectrometer needs higher efficiency to balance the lower flux in order to reach equivalent diagnostic performance as a 14-MeV neutron spectrometer for DT such as in terms of count rate. On the other hand, the energy resolution can be degraded (7 % compared to 4 %) which corresponds to the Doppler broadening for D and DT plasmas of Ti=4 keV. Another factor derives from the fact that neutron detection involves a nuclear reaction such np scattering which changes the conditions for determining the neutron energy of the range considered. The main instrument for basic NES diagnosis of DT plasma is based on measuring the proton recoils with a magnetic spectrometer (the MPR technique, Fig. 3). For the 2.5-MeV emission from D plasmas, the neutron time-of-flight (TOF) technique (Fig. 3) is more effective than MPR. It was first used in the 1980’s at JET and has been under development to improve different aspects of the performance (see review in Ref. [23]) which falls into two branches. One is represented by efforts to achieve high energy resolution combined with high efficiency4. The other has been aimed at achieving high efficiency and count rate capability while relinquishing energy resolution to the minimum acceptable level (the mentioned Doppler broadening level). The latter is represented by TOFOR (TOF spectrometer optimized for rate). Below we describe the MPR and TOFOR spectrometers, the relationship between measured quantity and incoming neutron energy (response function) and the principles for analyzing and interpreting the data. The NES projects under the JET-EP1 program have been completed with the installation of the MPRu and TOFOR spectrometers in December 2004 and February 2005, respectively. The NES capability at JET is now strengthened due to the possibility of neutron energy measurements up to 18 MeV. The two instruments can be used for dedicated diagnostic measurements of 2.5-MeV dd and 14-MeV dt neutrons in D and DT plasmas, respectively. In addition new special measurements can be made such as dual sight line observations of the dd neutron emission from D plasmas.

4 TOF spectrometers were installed and operated at JET and at the large Japanese tokamak JT-60U in the 1990’s [24]. Since the design requires the optimization of the spectrometer configuration as a trade off between efficiency (�) and resolution (�E/E), they have been under development to improve different aspects of the performance. These instruments were resolution oriented, meaning that the optimization provided good �E/E such as (�, �E/E) = (5�10-2 cm2, 4.6 %) for JET and (2.8�10-2 cm2, 4.3 %) for JT-60U TOF spectrometers, respectively. The count rate was thus limited to Cn<20 kHz.

24

Fig. 3. Sketch of the neutron detection techniques based on np scattering reactions for the magnetic proton recoil (a) and the time-of-flight (b) spectrometers.

3.1 The spectrometers 3.1.1 MPR The MPR spectrometer is based on the magnetic proton recoil technique (Fig. 4). Here, the incoming neutron flux from the plasma impinges on a thin CH2 target foil. Only the head-on scattered protons ( 10-5) are collimated and enter the spectrometer electromagnet to be momentum analyzed. A beam dump exists for the neutrons that do not interact. At the exit of the vacuum chamber ( 10-4 mbar), there is a focal plane detector in the form of an array of monolithic plastic scintillation counters (hodoscope) to record the positions of the protons. The detected protons on the hodoscope produce a one dimensional position distribution (histogram) that is the convolution of the incoming neutron energy spectrum and the response function of the spectrometer. The response function of the MPR is well known due to the mapping of the dipole magnetic field and, hence, the detailed determination of proton trajectories that depend on the energy.

The MPR efficiency and resolution can be adjusted by choice of target thickness and proton collimator aperture. In general, since the relation between efficiency (�) and energy resolution (�E/E) is � � const�E/E� � � , with �!"# and depending on the type of spectrometer, it is not useful to choose a value for �E/E better than the Doppler broadening of the neutron spectrum. This can be determined in thermal DT plasmas knowing the ion temperature from the equation

iTk� where � and Ti are in keV and k�177 [25]. With Ti=4 keV as a lower limit for the ion temperature of a reactor-grade plasma, the MPR was operated with an energy resolution of 2.5 % or greater.

25

The MPR was installed at JET in 1996 as a NES diagnostic for 14-MeV neutrons from DT plasmas. The MPR was placed in front of one of the horizontal diagnostic ports of JET at 47� relative to the toroidal magnetic field axis and at 4.76� relative to the equatorial plane of the torus (Fig. 4).

Fig. 4. Schematic of the MPR detector components and dimensions (left-hand panels), and the line of sight with respect the JET tokamak to the right (Courtesy of EFDA-JET; Figure: EFDA-JET).

The spectrometer is dedicated to 14-MeV neutron measurements in DT plasmas but attempts were also made to use it when JET ran D plasma experiments. Although the magnetic setting allowed the measurement of the 2.5-MeV protons, the MPR performance for D plasmas was hampered by the background radiation, mostly due to �’s and Compton electrons. The signal-to-background (S/B) ratio was estimated to be about 10-1 for dd neutron measurements in D plasmas compared to 104 in DT plasmas [26].

The MPR was operational during the TTE experimental campaign in 2003 for 14-MeV neutron spectroscopy. However, already earlier the planning of the JET experimental activity based on D plasmas set the conditions for the proposal to upgrade the MPR spectrometer as part of the JET-EP1 program.

A principal motivation for MPRu was to be able to measure also 2.5-MeV neutrons improving the background rejection capability of the focal plane detector to achieve adequate S/B with a target value of 10 or better. An R&D project started in 2002 to develop and build a new hodoscope. The solution was to replace the array of monolithic scintillation counters with phoswich scintillators [27]. Fig. 5 shows a picture of the final MPRu hodoscope assembly. This together with other new sub-systems for data acquisition (DAQ) [28] and control and monitoring (C&M) [29] were implemented in the MPRu.

The MPRu was installed at JET in December 2004 and tested during the restart period at the end of 2005. The MPRu spectrometer is placed in the same MPR position. The survey defined a line of sight being 47� tangential and 4.89� with respect the equatorial plane of the tokamak. The reference setting for 14-MeV neutron measurements (8 mg cm-2 target foil and the 40 msr proton collimator) provides a flux efficiency of 0.5·10-4 cm2 and 2.5 % neutron energy resolution (FWHM, Gaussian shaped distribution); this corresponds to the width of the neutron emission of

26

a 4-keV thermal DT plasma. For 2.5-MeV measurements, a somewhat relaxed resolution (6.6 %) is sufficient for a target of about 1.6 mg cm-2 [30].

This thesis involves work specifically on the development of the MPRu detector through laboratory tests performed on phoswich scintillation counter prototypes for their characterization.

Fig. 5. The MPRu focal plane detector in the Torus Hall at JET ready for the installation. 3.1.2 TOFOR The time-of-flight technique is based on the measurement of the time difference between signals generated by np scatterings in two hydrogen-based detectors. The first detector (S1) is placed in the collimated neutron beam coming from the plasma while the second (S2) at some distance away from the main beam and from S1 (i.e., at a flight path L). The neutrons that scatter in S1 and emerge in the direction of S2 provide the start signal t1. The stop signal t2 comes from the neutron interactions in detector S2 (Fig. 6). With the particle masses set to mn $!mp and non-relativistic kinematics, the incoming neutron energy is obtained from the time-of-flight tTOF=t2-t1 through

� � � �

2

22'

cos2cos %%&

'(()

*

TOF

nnn t

LmEE

; here the flight path L depends also on the scattering angle ,

i.e., L=L( ). The S1 and S2 detectors are placed on a constant TOF sphere of radius R such that 2

2 %%&

'(()

*

TOFnn t

RmE . It follows that tTOF is only depending on the incoming neutron energy En.

27

Fig. 6. Sketch of the TOF technique implemented for the TOFOR spectrometer. Shown are the main geometrical parameters and the scintillators S1 and S2. Adapted from [31].

There exist constraints that have to be taken into account in the design of a TOF spectrometer. For instance, the radius of the constant TOF sphere, the area and thickness of the S1 detector and the dimensions of the S2 detector are all parameters that affect the performance of the instrument defined by its efficiency and resolution. The background radiation that determines the level of accidental coincidences in the detectors depends on R as well as on the S1 and S2 areas. The thickness of the detectors set the level of multiple neutron scatterings. The S2 area defines the solid angle for the detection of the scattered neutrons.

The TOFOR concept was presented in Ref. [32] based on the recognition of count rate as the primary design goal of the new generation of TOF spectrometers. This was attained by splitting the S1 detector into a stack of five cylindrical plastic scintillators (40-mm diameter, 5-mm thickness and 1-mm separation) read out by three PM tubes each and an array of 32 S2 detectors of trapezoidal shape (length 350 mm, width 95/134.7 mm, thickness 15 mm) read out by one PM tube each. The S2 detector ring defines a scattering angle ( =30�) that ranges between 30�-6.98� and 30�+7.87�. The detectors are located on a constant TOF sphere of radius R=704.6 mm resulting in tTOF�65 ns for incoming 2.5-MeV neutrons. The S1 and S2 detectors are Bicron BC-418 and BC-420 [33].

The S1 configuration avoids event saturation in the beam detector, providing good efficiency and minimizing neutron multiple scatterings. The use of three PM tubes allows maximum light collection in view of the low energy deposition of the scattered protons. Due to their length, the S2 detectors feature 2.4 ns light transport time and deviate from the sphere. This was compensated for by tilting the detectors whose far ends were tipped about 5� inwards.

The TOFOR spectrometer design aimed for a count rate capability of Cn�300 kHz for neutron fluxes of 2.5�106 n cm-2 s-1 from 1017 n s-1 D plasma neutron yields. The projected parameters were (�, �E/E) = (12�10-2 cm2, 5.8 %) for 2.5-MeV neutrons [34].

The DAQ system makes use of time digitizing PCI boards with 0.4-ns time resolution and event collection at rates up to 1.25-GHz peak rate or 5-MHz sustained pulse rate [35]. The C&M system implemented for the MPRu spectrometer is also used for TOFOR with different settings [29].

28

The TOFOR spectrometer was installed in the JET roof laboratory in February 2005 (Fig. 7). The vertical line of sight covers the poloidal plasma section in the range 2.7-3.1 m.

This thesis brings contributions to the different phases of the TOFOR project. Simulations were performed to assess the required performance of a TOF spectrometer for JET D plasmas and so to define its design. Detector components were tested to optimize the setup and complete the information necessary for the response function calculations. The spectrometer was built, installed, and commissioned at JET where neutron data analysis was carried out for the final validation of the response function among other tests.

Fig. 7. Picture of the TOFOR spectrometer in the JET roof laboratory and sketch of its sight line (Courtesy of EFDA-JET; Figure: EFDA-JET).

3.2 Analysis and interpretation of NES data The MPR and TOFOR spectrometers make use of np scattering reactions in a plastic target. For the MPR type of spectrometer, the target is thin CH2 foil which thickness can be varied in the range 1.6-50 mg cm-2 [36]. The NES information relates to the proton position histogram H(xp) measured at the focal plane detector. The response function contains the details of the neutron energy to proton position conversion, i.e., En to xp, and specify the relationship between the incoming neutron flux Fn(En) and the measured H(xp).

For the TOFOR spectrometer, the np recoil target is thicker (5 mm, i.e. 516 mg cm-2) in the form of a plastic scintillator (S1). The protons scattered in the detector provide the start time t1 while the neutrons emerge from it. Some of them interact in S2 producing the stop t2 signal and the response function relates the neutron TOF spectrum as a function of tTOF=t2-t1 to Fn(En). Actually, the TOF spectrum is reconstructed off line by considering individual t2 and the corresponding t1 such that t1 +!� t2-�t, t2+�t] where �t is set at 400 ns. The characterization of the spectrometer allows a detailed determination of the response function. For instance, in the case of TOFOR, Monte Carlo calculations have been performed to assess the effect of the spectrometer

29

configuration on neutrons in the energy range 1 to 5 MeV [37]. This was then folded with the contribution of the DAQ signal processing assumed to be Gaussian of 2.5 ns FWHM.

Concerning the data analysis, a suitable plasma model is essential. The model affords interpretation of the fusion reaction processes and identification of their signatures in the measured neutron spectrum. For instance, ion populations in thermal equilibrium produce neutrons with Gaussian energy distribution around the central energy, i.e., 2.5 MeV for dd or 14 MeV for dt reactions. The peak width in this case is proportional to the ion temperature though

iTk with k=82.5 in dd or 177.3 in dt cases, respectively (asymptotic values for Ti�0) [15][25]. The injection of auxiliary power heating will perturbed the equilibrium such as through

the creation of extra energetic components. For instance, the atoms of NB beams undergo charge exchange reactions with the plasma and the resulting energetic ion population slows down from the injection energy (Ebeam) till thermal energies. It can be modeled with constant distribution from Ebeam to Ebeam/2 with the energetic ions being placed in a passing orbit with pitch angles in the range of, say, 60�,10� with respect the toroidal magnetic field. The ICRH power is typically modeled to result in energetic ion population over the pitch angle range 90�,10�. In other words, the ICRH is assumed to couple directly to the resonating ions in the gyro-motion direction, giving rise to an anisotropic orthogonal ion velocity component ( �v ) which is Maxwellian within the pitch angle cone. The slowing down of the energetic ions is due to electron scattering above the

critical energy, � � 3/22

8.14 -.

/01

2 3

i ie

iimeC An

ZnATE , where Am is the atomic mass of heated atoms, Te

and ne the electron temperature (in keV) and density, Ai, Zi, ni are the atomic mass, atomic number and density of the ion (i) species. For instance, EC is about 90 keV for deuterons in D plasma.

As the energy falls below EC, the slowing down process is due to ion collisions which affect their pitch angles and make the energetic ions of preferred pitch distribution to loose their anisotropicity. These means that the energetic parts of the fuel ion population are the ones that give the clearest signatures of their presence as they are possible to recognize as causing high energy tails in the neutrons spectrum. The neutron spectral features of RF and NB injection have their specific signatures which can be connected to parameterized neutron spectral components. Other sources of specific neutron signatures that can be recognized in the spectrum are the � knock on (AKN) and the triton burn up (TBN) effects. With sufficient counts in the spectrum, AKN signature can be recognized in the spectrum at 10-2 of the peak intensity in D plasmas [22].

Specifically, the NES data recorded with the MPRu and TOFOR are analyzed with combinations of spectral components from a predefined set where to choose from depending on the plasma conditions at hand and the diagnostic use. The data analysis method is thus based on ion population components that define an assumed neutron spectrum which is folded with the response function of the spectrometers. The data are of different types. Those of the MPR (and the MPRu) are proton position histogram where there is a one-to-one correspondence between proton position (i.e., energy) and neutron energy within the instrumental resolution. In the case of TOFOR, it is the tTOF of the scattered neutron which through kinematics is related to its incoming energy. The relationship between incoming neutron energy and the measured quantity is given by the response function which is well determined for both spectrometers. The energy distribution of the incoming neutron flux from the plasma is determined by the best fit (least square fitting) to the data in terms of model component parameters.

This plasma model allows multi-component spectral analysis to extract information on the ion velocity distributions from the measured data. It is also possible to implement as input to the model the results from detailed calculations of the plasma conditions in terms of energy and pitch angle [38].

30

3.3 NES experiments at JET The MPR spectrometer was used during the DTE1 and TTE experimental campaign as a dedicated NES diagnostic for 14-MeV neutrons from dt reactions. Here, the high neutron yield of the plasmas produced high count rates up to 0.6 MHz for DTE1. This together with high spectrometer performance provided data of very high quality which opened up for new uses of NES diagnostics and so detailed information on DT plasma conditions. During the TTE experiments, on line data analysis was performed and diagnostic information was provided on a shot-by-shot basis. After the long shut down for the realization of the JET-EP1 program, the restart and commissioning of the new systems took place at the end of 2005. When plasma operation resumed in 2006, MPRu and TOFOR spectrometers started operation first with commissioning and, in the case of TOFOR, also regular data taking.

This thesis presents results on the analysis and interpretation of the neutron data collected by the MPR, MPRu and TOFOR spectrometers during the experiments at JET.

31

4. Results 4.1 NES experiments with the MPR for DT plasmas The MPR spectrometer was used for the first time in DTE1 in 1997. This gave a substantial NES data bank representing measurements of much higher quality than had previously been achieved. A principal feature was the high count rate (up to Cn0.6 MHz) that permitted time resolved studies of the plasma evolution or exploration of weak features in the neutron spectrum. Models were developed for off line analysis and interpretation to take advantage of the high quality of the data for the two-fold purpose of investigating new physics of high power plasmas created with auxiliary heating based on NB and ICRH and also with a small contribution of self-heating by �’s as well as developing the NES diagnostic methods. A number of observations were obtained with respect to the velocity distributions of the ion populations in the plasmas, their relative densities, and high order effects such as AKN, besides plasma collective motion such as toroidal rotation [19][39][12]. Data from this period was used as presented in section 4.3.

In the period 1997-2003, with JET operating with deuterium only, the MPR was used to provide information on the triton burn up neutron (TBN) emission [40]. These 14-MeV neutrons produced in D plasmas were clearly seen with the MPR while the 2.5-MeV dd neutron emission could only be barely discernable (S/B�10-1).

In October 2003, it was time for the third tritium campaign at JET, albeit only with trace amount in D plasmas (the trace tritium experiment, TTE). The MPR was used in live experiments in the sense that the whole NES analysis and interpretation machinery was in place so the results for a plasma discharge were available promptly afterwards so that the outcome could be used for setting up the next discharge. The drawback was that the neutron yield rate was down by a factor of 50 compared with DTE1 and so was the MPR count rate. However, TTE gave a number of interesting NES observational opportunities and results as exemplified by Paper I of the thesis.

For the TTE campaign, tritium was fed in D plasmas as gas puffs or by NB injection, i.e., NB(T). The use of NB(D) and NB(T) power injection gave rise to different effects on the time evolution of the plasma state as illustrated by the time traces of the total neutron yield rates Yn(t) from both dd and dt reactions in Fig. 8. Fig. 8a shows the sharp increase of Yn due to the NB(D) injection up to the 1016 n s-1 level due to 2.5-MeV neutron emission from dNB+d reactions with a further increase by a factor of 6 due to the tritium gas puff injections at tP�62.5 s and 14-MeV dNB+t reactions. The shots studied in Fig. 8b refer to experiments on the transport of T in the plasma. In the figure, Yn reaches up to the 5�1015 n s-1 level due to dd reactions with a further increase by an order of magnitude at tP=48.7 s due to NB(T) power injection supplied as a short (150 and 100 ms, respectively) beam blip. Although the NB(T) power was only one tenth of NB(D) (about 13.5 MW), its effect on Yn is ten times higher which reflects the factor of 102 higher reactivity of tNB+d reactions compared to dNB+d (cf. Fig. 2) where tNB and dNB indicates NB fast ions at energies of about 100 keV.

The yield rate Yn was measured with monitors which recorded 2.5-MeV and 14-MeV neutrons indiscriminately with approximately the same efficiency. These time evolution features in the spectrum of the neutron emission could not be observed with the MPR, or any other spectrometer, because of the low count rates.

32

Fig. 8. Observations of the effects on the neutron yield rate Yn(t) due to the NB(D) auxiliary heating when applied with 5-mg T puffing, (a), and with NB(T) on and off-axis, (b).

Sufficient counting statistics for MPR recorded spectra was afforded by time integration of the data relative to discharges which plasma conditions were steady for longer periods. This offered some interesting new NES observations. One example is provided by shot #61065 of Fig. 9 having relatively high tritium density and high dt neutron yield rate. In this spectrum, three different components can be distinguished, two of which are due to bulk (thermal, TH) dt reactions and neutral beam (NB) dNB+t reactions. The NB neutron emission is shifted to higher energy relative to the TH component as the dNB ions are preferentially injected into passing trajectories in the direction counter to the MPR viewing direction. In addition, the entire neutron emission is Doppler shifted due to collective toroidal rotation of the plasma (see below). Finally, there is an indication of a weak broad spectral component attributed to TBN. The tdd tritons come from d+d�t+p reactions with E(tdd)�1 MeV which, during their slowing down, can suffer nuclear burn up in the reactions tdd+d��+n. This higher order fusion reaction is the only 14-MeV neutron emission source in D plasmas but is dwarfed here by the main dt neutron emission due to the trace T density in the plasma.

33

Fig. 9. MPR proton position histograms (dots) of the NB(D) discharge #61065 together with best fits to data (dashed lines).

An example of a new NES observation derives from the comparisons of three similar

discharges with ICRH heating but with the antennae tuned for dipole, +904 and -904 wave injection phasing. The neutron emission was recorded along the MPR sight line angle at �!=47� with respect to the toroidal magnetic axis. The results in the form of proton recoil position histograms are shown Fig. 10 where the peak due to 14-MeV dt neutrons is shifted depending on the antenna phasing. The measured peak position can be correlated with the plasma collective

motion, namely, its toroidal velocity given by �cos5420.

�Etv � where �E is the energy shift

corresponding to the proton peak position on the hodoscope [41]. This equals +309,8!km s-1 and -279,41!km s-1 for the cases +904 and -904. The dipole phase shows no shift within the error (+57,41!km s-1) for absolute rotation measurement.

These experiments showed a clear correlation between the ,904 phasing of the ICRH power and the rotation of the fuel ion population at the level of ,300 km s-1 relative to the plasma current direction. This is certainly lower than what had been observed for the DT discharges in the DTE1 campaign (up to 700 km s-1 [42]). The dipole wave power was found to have no influence on the plasma rotation. This correlation was studied in Ref. [43] for ICRH on minority hydrogen in D plasmas but no apparent effect was observed.

34

Fig. 10. Effects of the ICRH phasing on the plasma collective motion measured by the MPR spectrometer. For dipole phasing no effects occur, (a), namely the proton histogram is peaked about the reference position, while co and counter current plasma rotation is induced for +90�5 (b), and -90�5 (c), respectively. The end of the TTE experiments was followed by a long shut down to implement the JET-EP1 program and the realization of the MPRu and TOFOR projects that this thesis research was devoted to. 4.2 Development of the MPRu focal plane detector The starting point for the development of an MPRu focal plane detector was the existing array of fast plastic scintillation counters each attached to a photomultiplier (PM) tube. The thickness of the scintillators was chosen so as stop protons of up to 18 MeV, i.e., 3.5 mm. This turned out to be unfortunate for the use of the MPR as a 2.5-MeV neutron spectrometer as the recoil protons stopping in the scintillator give rise to pulses similar to those of minimum ionizing electrons of the background. In other words, the proton signals have a characteristic (peaked) pulse height distribution that overlaps with that of all electrons of energy above, say, 1 MeV [26]. While pulse height discrimination of background radiation is effective for measurement of 14-MeV neutrons, it is not so for those of 2.5 MeV. A solution would be to use a thin scintillator to detect low energy protons and a thicker one to cover the range up to 18 MeV. This is offered by the phoswich method.

The phoswich configuration chosen consists of a laminated detector made of two different plastic materials whose light response to ionizing radiation has short and long decay times. This means that the shape of the pulse recorded in the PM tube carries information about

35

the fraction of light produced in each layer, here L1 and L2. In the present application, L1 is 0.3 mm thick, just sufficient for dd neutron measurements up to, say, 5 MeV, while L2 has a thickness of 2.2, 2.5 and 2.8 mm depending on the position in the hodoscope related to the proton energies. The protons impinge on L1 and will be stopped in the phoswich counters. By changing the MPR monolithic scintillators to those of phoswich type, range information has been added to pulse height as the means to separate protons signals from background radiation. Specifically, the new detector should stay within the physical bounds of the old one. The phoswich technique has found use in many applications, and the present focal plane detector for protons up to 18 MeV has some special requirements. Geometrically, this entails the use of an elongated scintillator of 100 mm length and widths of 10 or 20 mm with the light collected at the two short ends which presented a challenge due to the measurement and design requirements. Detailed tests were carried out in order to verify the new detector performance. This was a primary task of the present thesis research reported in Paper II.

Phoswich detector prototypes were built using the Bicron BC-404 material, the same as the original monolithic MPR scintillators, for the fast thin L1, and BC-444 for the slower L2; the decay times are 1.8 and 179.7 ns [33]. Two prototypes were tested: One with just the two scintillators (type P) and the other (P/b) with an extra layer of acrylic plastic BC-800 (backing). Fishtail, for P, or cylindrical, for P/b, light guides were attached to each end to connect to PM tubes. Monolithic scintillators of the original MPR type were also tested for comparison.

The collected data consisted of individual PM tube signals (waveforms) recorded at sampling frequencies of 1 or 2.5 GHz depending on requirements. The size of data sets collected was adjusted to the accuracy required for the tests. In order to perform quantitative analysis of data, the waveforms were integrated over the time intervals (gates) G0, G1, G2 and G3 to determine the average pulse height offset level (base line from G0) and to extract the amplitude parameters A1, A2 and A3 (Fig. 11). These were routinely base line reduced and used for the pulse shape studies in the tests [44].

Fig. 11. Average waveforms (pulse height as function of time) generated by 3 and 7-MeV protons in the phoswich detector P/b and the time gates (G0, G1-3) used to define the time-integrated information on the signal (referred to as amplitude). 3-MeV protons stop completely in L1 while 7-MeV ones reach L2.

36

The gates were chosen so that G1 covered the fast leading slope of the peaked pulse height evolution, while G2 covered the trailing slope with sensitivity to the scintillator light emitted with long decay time. In other words, the amplitude A1 reflects the integrated pulse for the main part of signals from the BC-404 layer and the early part of signals from BC-444 while A2 contains a relatively large fraction of the BC-444 pulses. Moreover, the gate G3 was used to determine the integrated amplitude value for the full waveform (A3). The pulse shape variation in recorded waveforms was expressed by the ratios A1/(A1+A2) and A2/(A1+A2), or simply A1/A2, while A1+A2 expressed the total pulse amplitude; the latter was also obtained using A3. Average waveforms for several events of data sets were also used for comparing the results for different detectors or radiation conditions.

The useful information extracted from the data concerns the pulse shape of the signals generated and their pulse height (amplitude) resolution. The former depends on the range of the interacting radiation while the latter on its energy deposition and ultimately, on the photon statistics. In fact, the accuracy of the pulse shape measurement derives from the number of photoelectrons, Ne,

produced by scintillation light at the photo cathode of the PM tube. The

amplitude values calculated from the collected waveforms reflect the underlying variation in Ne

which can be assumed to be Poisson distributed with mean value 6eN and standard deviation 6� . The resolution 7 is determined as the ratio of the measured FWHM W and the position

6 of the amplitude distribution: )/(355.2/ 6�67 W and 22 )/355.2()/( 7�6 eN . The amplitude values of the recorded waveforms thus reflect the underlying number of photoelectrons generated by the scintillator light pulse.The amplitude data from the tests consist of distributions characterized by mean value and width which were determined by the Poisson statistics with an error depending on the number of measured waveforms in the data set.

Radioactive sources are a convenient way to test scintillators and were used in the present study besides beam protons from a Tandem accelerator. The 241Am source emits �’s of 5.5 MeV. They have short range (<0.1 mm) and were used to study the scintillator light response (either L1 or L2) for known energy deposition. A limitation was that the source could not be used for measurement of L2 in P/b. Moreover, the quality of the data for L2 is poor for � signals due to the small light output.

Electrons from � sources (in this case 106Ru) are emitted with a continuous spectrum up to the end point energy (here 3.5 MeV). This feature was used in the tests for selecting minimum ionizing electrons. Their deposited energy is proportional to the detector thickness when passing the scintillator (specific energy loss of about 2 MeV g-1 cm2 [45]). In other tests, the low energy part of the � spectrum was selected by special techniques so as to study the scintillator response in terms of possible waveform shape variation with respect to the energy deposition in L1. In this case comparison was made with the results of � measurements to see also the possible dependence on the type of radiation. One can also mention, that the � source could also be used to mimic the background that the detector would be exposed to in the tokamak environment and illustrate its response.

From the above measurements performed, two examples of results are given here. The first refers to the measured waveforms for �’s irradiating L1 of phoswich P and P/b. The amplitude distributions are compared in Fig. 12 in terms of A1 and A2. The two detectors show a distinct response with P/b amplitudes significantly greater as a result of the improved light transport to the PM tubes at the ends. This corresponds to an improved A1 resolution of 7!=22 % compared to 32 % obtained for detector P.

37

Fig. 12. Examples of measured distribution of waveforms generated by 5.5-MeV � particles impinging on the fast layer L1 of the phoswich scintillators without (P) and with (P/b) backing. The waveforms are characterized by the A1-A2 amplitude distributions (see text).

The second example concerns the use of minimum ionizing electrons from the � source to determine the total amplitude response as function of the radiation interaction position along the phoswich scintillator. This is illustrated in Fig. 13 by the measured mean value 6 of amplitude A3 for the collimated electron flux impinging in the center (x=0 mm) and two positions (x=,20 and ,40 mm) towards both ends of the P/b detector with a PM tube at each. This shows that the amplitude variation from end to end is more than a factor of two for individual PM tubes which corresponds to a factor of 4 stronger longitudinal response dependence than that for the monolithic MPR scintillator [46]. However, summing the pulse output from the PM tubes attached to each end reduced the variation to 15 % besides approximately doubling the pulse height. This showed that the phoswich scintillators required the use of two PM tubes implying a significant impact on the design of the hodoscope in terms of scintillator widths and light guide geometry.

38

Fig. 13. Example of results on the pulse height response of the phoswich scintillator with backing to minimum ionizing electrons impinging at five longitudinal positions (x). The results are given as the mean value of waveforms represented by the A3 amplitude (6) of two PM tubes individually and their sum. Also shown are double exponential fits to the data. The errors shown are statistical. A Tandem accelerator provided narrow beams of protons of well-defined energy in the range Ep=1 to 7 MeV which allowed testing of the detector P/b for part of the intended range of application (1.5 to 18 MeV). This meant that one could measure how the amplitude of the phoswich signal varied with energy, and, especially, the waveform shape change when the proton range exceeded the thickness of layer L1. The results on the average waveforms for eight beam energies are shown in Fig. 14a while the amplitude distributions A1 and A2 in Fig. 14b. Here the width of the distributions for each Ep value is an indication of the amplitude resolution that can be achieved with the phoswich detector. Moreover, the amplitudes A1 and A2 increase approximately linearly with Ep up to about 5 MeV where the range exceeds the L1 thickness and the energy deposition in L2 starts. This means that the phoswich signals show localized A1-A2 distributions for protons over the intended range of use and that the amplitude resolution is sufficient for positive identification of protons, and, hence, discrimination of background. Though the gates were fixed at defined values in all the tests for comparison purposes, the dependence of the resolution on the gate duration was investigated. This resulted in the optimal G1 time interval of [-2, 30] ns for the resolution of the A1 proton distributions [47]. The results of these tests, together with the ones obtained using the � source without collimation, provided information on the background rejection capability of the new detectors and input for the simulations of the MPRu focal plane detector under the operative conditions at JET [48].

39

Fig. 14. Results on measured waveforms for proton beams of eight energies between 1 and 7 MeV presented as pulse time transient averages for each data set, (a), and A1-A2 distributions for individual events, (b). The cluster of events marked noise represent base line amplitudes proportional to the G1 and G2 gate lengths.

The test results were used for the construction design of the phoswich detectors. These were then mounted in the new hodoscope frame of the MPRu spectrometer. The mounting was an exacting task. The positions of the detectors were determined with a light-emitting diode (LED) and used as input to the MPRu response function. The accuracy of the position was estimated to be ,0.05 mm for an absolute energy determination at En=14 MeV of �En/En�10-4 [49]. This work was done in Uppsala. The assembled focal plane detector was installed in the MPRu spectrometer at JET in December 2004. New target foils were also implemented in the range 1.6 to 22.1 mg cm-2. The MPRu was then put in operation and performed the first 2.5-MeV neutron measurements at JET during the high level commissioning in April 2006 (Paper III). The performance was tested with D plasmas with the spectrometer magnetic field on to receive recoil protons from dd neutrons and off to let the detector see only background. The results for the measured A1-A2 distributions are shown in Fig. 15. Here the cluster of events around A1�160 a.u. and A2�50 a.u. can be attributed to protons of about 2.5 MeV (cf. Fig. 14b).

40

Fig. 15. Example of measured amplitude distributions in the A1-A2 representation from a central phoswich scintillation counter of the MPRu hodoscope during JET high level commissioning with the spectrometer magnetic field set to record recoil protons from 2.5-MeV dd neutrons, (a), and with the field off to record background radiation only, (b). The recoil protons events appear in the encircled region.

Out of the described A1-A2 amplitude distributions of recorded data one can selected those with A2<200 a.u. and project the selected events on the A1 axis (Fig. 16). The results for the cases with and without spectrometer fields give two different distributions representing events due to the sum (S+B) of signal protons and background, and background only (B), respectively. This information can be used to estimate the functional form B(A1) and to determine the admixture of background in S+B events selected within chosen A1 limits, and similarly the exclusion of signal event. Thus, the phoswich pulse information offers two ways for use, namely, to discriminate against background radiation in the selected data and to make correction of the remaining admixture. The aim was to obtain a S/B ratio of 10:1 for dd neutron measurements for D plasmas which has been demonstrated by the results. It is also shown that it can be exceeded with the added step of subtraction. The resolution of the A1 amplitude distribution of the 2.5-MeV protons is about 25 % (FWHM) and the efficiency of the MPRu spectrometer corresponds to 3�10-14 and 10-13 detected neutrons per JET produced neutron for D and DT plasma, respectively.

41

Fig. 16. Example of A1 amplitude distribution for the data of Fig. 15. The event selection A2<200 a.u. is applied and the background data (magnet off) are normalized for A1<150.

4.3 NES diagnosis of D plasmas from the experience of DT The results obtained by the MPR spectrometer during the DTE1 experiments were used to project the neutron spectra attainable from D plasmas (Paper IV). Two reference DTE1 shots were chosen, one with only NB and the other with only ICRH heating. Two ion populations, namely bulk and high energy, were adopted in the analysis model. The best fit to the MPR data provided their relative intensity and temperature. The study assumed identical D plasma conditions to those obtained in the analysis of the DT experiments, i.e., ion populations relative densities and temperatures with the ion densities set by the electron density ne, as nd=nt=ne/2 for DT and ne=nd for D plasmas. The ion velocity distributions were used as input to the simulation code APACHE [50] to calculate the 2.5-MeV neutron spectra from for two different lines of sight, 90� relative to the magnetic axis, as for the TOFOR spectrometer and 47� as for the MPRu.

The information obtained on the characteristics of the projected spectral shapes set the requirements on the neutron energy range corresponding to the temperature of the ion populations involved and on the count rate necessary to detect and study the effect of the auxiliary heating and the signatures of minor energetic ion reactions. The projected neutron spectra for 90� line of sight was used as reference in order to determine what the target performance characteristics of the TOFOR should be. As an aside, it was learnt about the benefit of simultaneous measurement of the neutron emission spectrum along dual sight lines such as would be offered by the MPRu and TOFOR spectrometers. In particular, this could provide better understanding of the anisotropic ion velocity distributions generated by the auxiliary heating.

The above projected performance requirements were then compared with the capability of a TOFOR type of spectrometer as derived from conceptual design [32]. Refined designs and quantitative determination of performance specifications were obtained from neutron transport calculations using the nuclear reaction and instrumental modeling code Geant4 (Paper V). These calculations examined aspects such as the best trade off between efficiency and energy resolution based on the principles mentioned in section 3.1.2. This study also explored the effect of thicknesses and detection areas of the S1 and S2 detectors (cf. Fig. 6) besides the overall geometry on the performance. TOFOR was subsequently the object of a detailed modeling once

42

the construction design was determined. With the full modeling and refinements of the neutron transport calculations, the neutron response function was determined.

An important input to the above work is the variation of light transport time for neutrons detected at different positions of the S2 detector. This was studied in laboratory tests of a prototype of 5 mm thickness instead of 15 mm for that of the construction design. For the test, minimum ionizing electrons from the 106Ru source were used for their fixed energy deposition, i.e., 1 MeV [45]. The S2 area was trapezoidal with a length of 350 mm and base (towards the PM tube) and tip sides of 100 and 65 mm. The signal time variation along the prototype was measured (Fig. 17) and found to be 2.4 ns from tip to base. This was used together with the assumption of a linear longitudinal variation as input for the calculation of the spectrometer neutron response function. The deviation from the linear position dependence at the level of a few tenths of a nanosecond is insignificant relative to the target value of 1.6 ns FWHM for the time resolution of TOFOR [32].

Fig. 17. Results on the measured time response of a prototype for the S2 TOFOR scintillator to a minimum ionizing electron source placed at different longitudinal positions (x) with a linear fit to the data.

In order to time record all neutrons scattered from S1 into S2, at a given incoming

energy, independently on the impact positions, the S2’s have been tilted in TOFOR so that the flight path is decreased/increased for neutrons hitting the tip/base of the detectors. From the measured light transport time variation, the tilt was determined to be 5.74 as the optimum value to compensate for the longitudinal time variation in S2 for incoming neutrons of 2.5 MeV. This optimization reduces the time variation to the level of 1 ns having a limited effect on the overall energy resolution of TOFOR. It is interesting to note in this context that the neutron transport calculations including the above mentioned time variation resulted in a geometry for TOFOR with a slight modification from the conceptual design. For instance, the average scattering angle was changed from 27.54 to 304, the tilt angle from 5.74 to 5.04, while the thickness of S1 and S2 remained the same (5 and 15 mm).

43

4.4 Construction, installation and first use of TOFOR spectrometer

The construction phase started in 2003 in Uppsala with testing and assembly of detector components. The characterization and system measurements made use of LED [31] and � sources besides cosmic rays. The spectrometer assembly was completed by the end of 2004 including full geometric alignment survey to within set tolerances as well as basic spectrometer calibration. The TOFOR was disassembled in January 2005 for shipment to JET where it was remounted in February 2005 at its position in the roof laboratory above the tokamak. The TOFOR symmetry axis alignment to the vertical line of sight into the plasma was specified through a photogrammetry survey. The C&M system with two pulsed sources based on a laser and a LED was connected to the TOFOR system as was the DAQ system with the five time digitizing (TD) units which record the events of all 37 TOFOR detectors stored as time trains for each discharge studied. The TOFOR data acquisition and control functions were interfaced to the JET communication network so that the instrument could be controlled remotely and data be read into the JET computer system. As soon as the parts were interfaced, the commissioning started and the testing and tuning of the spectrometer itself was done with the use of the C&M system [29] and � sources as well as with background radiation of �’s and muons. When, neutrons from the plasma became available, these were used for continued commissioning but more so for verifying the working point settings relative to their references (see below).

The first weak flux of plasma neutrons was received by TOFOR in November 2005. Despite the poor statistics, the data were sufficient to extract a time-of-flight (TOF) spectrum which demonstrated that TOFOR had recorded its first 2.5-MeV neutrons from dd reactions (Fig. 18a) [51]. The spectrum showed also two peaks with very short flight times. It was later identified that one of these peaks was due to �-rays coming together with the neutrons from the plasma. This was a gratifying observation as it implied empirical information that could potentially provide input to quantify the pulse response function of TOFOR. Moreover, it would improve the accuracy by which the translation of the data in terms of measured channel number (Fig. 18a) to spectra in terms time-of-flight units of nanoseconds (Fig. 18b).

44

Fig. 18. Results from the first 2.5-MeV dd neutron observation with TOFOR in November 2005 for JET discharge #64289: (a) recorded time-of-flight (TOF) distribution as function of channel number and (b) TOF spectrum in units of nanosecond. The peak at 65 ns is identified as due to 2.5-MeV neutrons and the small one at 4 ns is due to �’s entering the spectrometer from the plasma. The curve is a fit to the TOF spectrum using a Gaussian of variable width folded with the instrument response function.

The first description of the new 2.5-MeV neutron spectrometer at JET and the results

from its use during the JET high level commissioning were presented at a conference in June 2006 (Paper VI). For the commissioning (March 2006), the first high neutron yield pulses were produced and TOFOR measured neutron TOF spectra that showed the effect of auxiliary heating systems as can be seen in Fig. 19. The spectra are background reduced from the accidental events due to particles interacting with detectors S1 or S2 separately. The 2.5-MeV neutron peak from thermal dd reactions corresponds to about 65 ns while the shoulder on the low time (high energy) side is the signature of the energetic ion population generated by the RF resonance (Fig. 19a). In the NB case (Fig. 19b), the TOF spectrum features a wide bulk peak and a steep edge at tTOF�60 ns. Both spectra show also the contribution of the multiple scattered neutrons in the detectors S1 and S2 and/or in the surrounding TOFOR materials giving events in the regions 40<tTOF<60 ns and tTOF>70 ns [37]. Their contribution is of about 1 and 5 % of the peak intensity, respectively.

45

Fig. 19. TOF spectra for the plasma time periods 47-57 s, (a), and 65-68 s, (b), of JET pulse #65745 corresponding to the individual RF(ICRH) and NB heating phases, respectively.

Paper VII is a full report on the TOFOR instrument which includes the setting of working

points and some examples of measured TOF spectra chosen to illustrate the instrumental capabilities. Among the important characteristics achieved by TOFOR are the capability to measure neutrons over a large energy range (target being 1 to 5 MeV or beyond), the high count rate (up to about 400 kHz) for a JET plasma of maximum neutron yield rate and the control of the admixture of the accidental event ( 2

nC8 �9 In addition, it has been demonstrated that the TOFOR response function is determined to high accuracy but this is an aspect that will be the objective for fine tuning with the help of new empirical data as is exemplified below. The TOF spectra measured at JET were used to evaluate the TOFOR performance under operational conditions. As in general the spectrum features a peak at 4 ns due to the � radiation coming from the plasma and the main neutron contribution peaked at 65 ns, the former was first used to verify the energy resolution of the instrument [52].

The instrumental resolution depends on the geometry and on the DAQ signal processing (i.e., electronics). This includes the effects of the signal pulse height and resolution. The TOFOR geometry is optimized for dd neutrons, while in case of � interactions the light transport within the detector, the multiple scattering in the spectrometer structure and the electronics give contribution to the measurement resolution. The �!peak in the TOF spectra was used to estimate the contribution of the DAQ system, �electronics, which also include the effects of multiple scatterings. The � peak measured in the experiment featured a width of 3.2 ns (FWHM) from which one obtains

nslightselectronic 2.1124.2

355.22.3 22

22 %&'

()*�%

&'

()*� ��

where 2.4 ns is the light transport time across the S2 detectors (Paper V). This value is 13 % larger than the one assumed in the initial response function

calculations and makes the time resolution change from 5.0 % to 5.4 %. The temperature Ti of the Ohmic plasma can now be assessed from the width of the neutron peak (3.8 ns, FWHM) of the TOF spectrum considering the contributions of the geometry (3.2 %) and the electronics [51]. Since iT� 5.82 and

� � � � ns.....

selectronicgeometrynTi602190

355283 22

2222 ��%

&'

()*��

46

it follows that ns..�tiT 413552 � � and keV

.E

t�tTi 7.1

5822

2

-.

/01

2%&'

()* .

Fine-tuning of the constant fraction discrimination (CFD) energy thresholds and of the PM tube gains were done, and the electronics contribution to the TOFOR resolution was determined by comparing the signal processing time at different stages of the DAQ system for a few scintillation detectors. The signals from cosmic rays and � background interactions were acquired from the linear fan-in-fan-out (FIFO) modules and processed to simulate the CFD modules. This resulted in a time distribution that was compared to the actual CFD response. The study showed that the CFD modules reacted differently to the signal amplitude notwithstanding the shortest time delay (2 ns) was implemented in their internal settings [53]. Fig. 20 shows the distribution of the time difference �telectronics of the simulated and real CFD signals as a function of their pulse amplitude. This can be separated into the pulse amplitude dependence (time walk) on the top of which sit the contribution of the photon statistics. This test gave better understanding of the signal processing contribution to the resolution and verified what was estimated in [54]. From the distribution shown in Fig. 20a for background radiation, one can assess a FWHM of about 1 ns in the region of pulse heights lower than 0.5 V in absolute value.

Fig. 20. Example of electronics contribution to the instrumental resolution. The time difference �telectronics of the simulated and real CFD signals as a function of their pulse amplitude for cosmic rays and � background interactions in detector S2-20. In (b), the 3D histogram of the distribution presented in (a). Cable length set the average value of �telectronics about 4 ns.

The above findings for the neutron signals of interest were verified through the analysis of neutron TOF spectra collected for Ohmic plasmas for a number of JET shots. A Gaussian distribution was assumed to model the DAQ signal processing and was included in the response function. The best fit to the Ohmic neutron data the plasma thermal component folded with the response function resulted in a DAQ contribution to 1.5 ns FWHM implying a TOFOR time resolution of 4 %.

It can be noted that this is higher than aimed for (2.9 %) with the practical outcome of having to acquire slightly greater statistics than would have been needed with better time resolution. This applies to measurement of Ohmic plasmas while the study of discharges with

47

auxiliary heating is practically unaffected. The design objectives of TOFOR can thus be considered achieved also in this respect.

4.5 Data taking with TOFOR and first physics results The TOFOR spectrometer was in full operation at the start of the JET experiments after the shutdown, i.e., campaigns C15-C17 from April to December 2006 and C18-19 from January to April 2007. It was the main diagnostic in some experiments and was also found use in most discharges since JET restarted in 2006. Since it was a new instrument, it served as an explorative plasma observation tool.

A dedicated plasma experiment was carried out on May 19th 2006 to test the capabilities of TOFOR. For this purpose, plasmas were set up so as to produce high neutron yield rates achieved with auxiliary heating. It was also desirable to have a steady yield rate for extended plasma periods which could be used for time integration to increase the statistics. The other objective of the test was to produce neutron emission conditions giving broad spectra, and especially, to change the spectrum during a discharge with the help of the applied auxiliary power. To this end, NB heating was applied to plasmas started with Ohmic heating, and then adding ICRH heating as shown in Fig. 21.

Fig. 21. Time evolution of the auxiliary NB and ICRH power for discharge #66463 (top panel) and the neutron yield rate (bottom panel). The time integration periods for collected TOFOR data are indicated at the top representing the NB and NB+RF steady state periods and the three transient periods T1, T2 and T3.

48

Specifically, NB power (up to 15 MW) was applied at plasma time tP=47 s to prepare the plasmas for the application of the RF power at which point (tP=49.5 s) it was ramped down to about 6 MW. The RF power, in the form of ICRH tuned to the 2nd harmonic frequency of deuterons, was switched on to a low (insignificant) level of 1 MW and stepped up to 6 MW to reach steady NB+RF heating conditions in the range 51 to 55 s. This gave rise to two time periods of rather steady yield rate (tP=48-49.5 s and 51-55 s) considered as representing typical average conditions for NB and NB+RF heating, respectively; the ending 0.5 s of the RF power period was without NB. The experimental details and the results for the two steady state periods are presented in Paper VIII. In addition, the analysis of selected discharges has started of data for the main transition periods, namely the Yn(t) ramp up period due to the start of the NB injection (transition period T1, tP=47-47.5 s), the ramp down/up of NB/RF (T2, tP=49.5-50.5 s) and the only RF (T3, tP=55.5-57 s) besides the thermal plasma conditions created by Ohmic heating before and after the auxiliary power injection.

The data set for this experiment consists of seventeen shots produced with different ICRH frequencies, i.e., plasma resonance positions. Moreover, a pre-collimator was used to change the viewed radial range of the plasma. By default, data were also collected for neutral beams of different energies, namely, 80+130 keV (standard), and 80 or 130 keV only. The data collected afforded the verification of the TOFOR response function and plasma models implemented for the off line data analysis. Count rates up to 10 kHz were reached providing the capability for time resolved spectral analysis. Here the study of the measured TOF spectra during the two steady state periods of #66463 is briefly presented.

The analysis was performed with a two-component neutron model, i.e., bulk (B) (due to low energy dd reactions) and high-energy (HE) (involving reactions between high energy and bulk deuterons). The HE component is clearly seen only for the NB+RF period, specifically, due to RF. Examples of measured TOF spectra for the steady state periods with the fits and the corresponding neutron energy spectra obtained with the parameterized plasma model as input are shown in Fig. 22. The four glitches in the TOF spectrum of Fig. 22b for tTOF>70 ns have been addressed as an artifact in the response function. Their effects are marginal since they appear in the region corresponding to low energy neutrons (En<2 MeV). The figure shows that the data are well described with the chosen analysis model which justified its use to extract quantitative information on the plasma response for different heating scenarios of NB and RF power injection; the results are summarized in Table 2 and are discussed in details in Paper VIII.

49

Fig. 22. Comparison of the TOF and energy spectra for shot #66463 obtained for the steady conditions of external heating with NB in (a) and NB+RF in (b). Here the four glitches for tTOF>70 ns are due to an artifact in the response function.

Examples of measured and fitted TOF spectra for the neutron emission during the

transient periods of discharge #66463 are shown in Fig. 23. The fitting approach was the same as for the steady state periods but for the transient T1 with the ramp up of the NB power. Here it was found that a bulk neutron component was not adequate but a good description of the data was obtained with a slowing down distribution of deuterons coming from the NB injection. In other words, the measured TOF spectrum and, hence, the deduced neutron spectrum, clearly show the signatures of the anisotropicity of the fast ions coming from the NB source. It can also be noted that the HE contribution due to the RF injection in T2 is much narrower than that for the steady state period showed in Fig. 22 due to its ramping up. Transient T3 instead shows the effects of the diminishing RF power.

50

Fig. 23. Comparison of the TOF and energy spectra for shot #66463 obtained for the transient conditions T1, T2 and T3.

Quantitative spectral information was extracted in terms of amplitudes (A in arbitrary units) and temperatures (T in keV) of the model components from the best fit to the data. The amplitude A corresponds to the integral of the intensity I of the component shown in the neutron spectrum. An illustration of the result that can be obtained for steady state and transient periods are provided by discharge #66463 (Table 2). Here one can note that the neutron emission is dominated by contributions from high-energy deuterons coming from the NB injection in its ramp up phase (transient T1) before the steady state NB phase of higher amplitude ascribed to bulk ion reactions. During the next transient (T2), the effect of RF is seen both in the increase of the bulk values for A and T and the appearance of the HE component. As the NB+RF steady state conditions are reached, TB increases, but with reduced amplitude, and the RF tail temperature THE increases from 50 keV to above 200 keV. Finally, at the end of the RF period (T3), temperatures fall as do also the amplitudes, especially, that of the bulk.

51

Table 2. Results on the analysis of the TOF spectra relative to the time intervals tP considered for the different heating conditions during the shot #66463 performed in the dedicated NES experiment at JET. Temperatures (T) are in keV while amplitude values (A) in arbitrary units and normalized. The standard error is indicated.

Phase T1 NB T2 NB+RF T3 �tTOF [s] 47-47.5 48.0-49.5 49.5-50.5 51.0-55.0 55.5-57

Bulk T - 24,1 23,1 36,2 21,5 A - 30.0,0.5 26.1,0.6 12.8,1.1 1.2,0.2

NB T * - - - - A 3.5,0.3 - - - -

HE T - - 50,9 227,36 127,27 A - - 4.4,1.0 2.9,0.3 0.9,0.1

* Fixed slowing down distribution for NB(D) injection. The data for the Ohmic phases of the discharge (tP<47 s and tP>57 s) were also studied

but required summation of several discharges to obtain sufficient statistics to verify that the width of the neutron spectrum was consistent with the Doppler broadening due to deuterons of the expected ion temperature range, typically, Ti<3 keV.

The TOFOR was used in several other experiments with ICRH auxiliary heating which have been the objective for analysis and interpretations in broader perspectives of plasma study [55]. NES observations of the effects of the location of the power deposition of auxiliary NB and ICRH heating systems can be found in [38]. One interesting example is the studies of ICRH power tuned to the 3He ion resonance and the dependence of the coupling efficiency on concentration [56]. The analysis of the TOFOR data collected in these experiments was carried out to compare the effects of the different RF scenarios.

The minority heating experiments performed on January 17th 2007 made use of helium concentrations below 3 %. TOFOR measured high-energy neutrons, which featured tails in the low time region of the TOF spectra (40-45 ns) as shown in Fig. 24 [22]. The analysis was carried out with three plasma components, bulk (B), super thermal (ST) and high energy (HE), in order to model the main plasma conditions and the resonant HE deuterons that slow down to an isotropic velocity distribution. The analysis showed HE deuteron populations of Ed<2.9 MeV which unlikely have been produced by spurious deuteron 2nd harmonic ICRH heating. These data are noteworthy as they indicate the first observation of so-called knock on effects with TOFOR as has previously been seen with the MPR for DT plasmas at mentioned in section 4.1. In the present experiment, the knock on is due either to 3He ion accelerated by the RF (3HeRF) or by � particles from d+3HeRF��p. In the first case, the 3HeRF ions suffer knock on collisions with deuterons during their slowing down and the fast deuterons thus created give rise to the high-energy dd neutron emission tail. However, the situation is rather complex and multi-step scattering processes were also considered for explaining such observations. In fact, the measured deuteron energies are compatible to d+3He nuclear fusion reactions due to the favorable reactivity (see Fig. 2). Depending on the flux surface where they are generated, the 3.7-MeV �’s are more likely to remain confined and slow down in the plasma compared to the 14.6-MeV protons. The � knock on may thus accelerate the deuterons at the observed TOFOR temperatures. These results confirm that TOFOR can provide wide energy coverage needed to measure energetic and exotic plasma populations and, thus, open up the possibility of studying the � slowing down process in D plasmas at JET.

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Fig. 24. Measured TOF spectra for discharge #69426 with a three-component fit of bulk (B), super thermal (ST) and high energy (HE).

Other experiments made use of larger 3He concentrations in the range 5-18 % [57].

Particularly, the experimental session held on January 15th 2007 aimed at the study of the influence of 3He ICRH heating on internal transport barrier (ITB) formation depending on the 3He concentration. The ITB is a plasma configuration featuring steep gradients in the temperature and density profiles caused by a reduction in the transport of particles across the barrier in the poloidal direction. Preliminary analyses of TOFOR data show evidence of the formation of very energetic ion populations in two of them with temperature higher than 200 keV when the 3He concentration passed 15 % after the barrier disappeared.

In shot #69393, the auxiliary power injected reached steady conditions of 16 MW NB and 4.4 MW ICRH during tP=44.5-49.5 s. The ITB barrier developed within tP=44.5-47 s. The 3He concentration varied strongly during the period of applied auxiliary heating, i.e., from about 5 to 40 % during tP=44-51 s (Fig. 25a). Fortunately, this also provided relatively high neutron yield rate so TOFOR spectra could be collected with a time resolution of 1 s. These spectra were normalized to the peak intensity for comparison (Fig. 25b). As mentioned, 3He concentration exceeded 15 % considered to be a critical limit for the ICRH coupling. With TOFOR it was thus possible to observe a neutron tail developing in tP=47-48 s and reaching its maximum within tP=49-50 s. For tP>50 s the tail disappeared which seems to be related to the 3He concentration that change the absorption mechanism of the RF power. The time resolved spectra were analyzed assuming three plasma components of bulk (B), slowing down from NB injection (NB) and high energy (HE) types for modelling the isotropic ions and the ones affected by the heating.

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Fig. 25. Example of TOFOR observations of the neutron emission from plasma subjected to ICRH tuned to the resonance frequency of 3He ions in D plasma with varying 3He concentration, (a), and the resulting 1 s time resolved TOF spectra for different plasma time periods, (b): Data from JET discharge #69393 set up to study internal transport barriers (ITB) effects in combination with RF heating.

The result of the analysis of the TOF spectra measured in tP=49-50 s is shown in Fig. 26.

The data are well described by the fit and, in particular, the need for a HE component shows that fast deuterons have been created by the RF power. This indicates that some of the RF power has escaped absorption on 3He and been taken up by the deuterons instead, as also confirmed by other diagnostics [58].

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Fig. 26. Example of TOFOR observations attained in the ITB experiments. The TOF spectrum measured for tP=49-50 s is analyzed with a three-component model (B, NB and HE) resulting the neutron energy spectrum shown in the bottom panel.

Table 3 presents the results attained for shot #69393 in terms of temperatures and amplitudes of the model components for the 1 s time resolved TOF spectra of Fig. 25b. The first three TOF spectra present similar characteristics with respect to bulk temperature and amplitude

ratio %%&

'(()

*:: 4.21.2

NB

B

AA . The RF power has apparently no effect. In these time intervals the

barrier existed and the 3He concentration is about 17 %. The TOF spectra obtained within tP=47-

50 s feature higher bulk temperature and an average 6.1�NB

B

AA . The main characteristic is the

development of a tail due to HE deuterons with temperatures of the order of 200 keV and amplitudes comparable to the bulk populations in the last two time intervals. This effect disappears within tP=50-51 s as the RF power is switched off. Looking at the time trace in Fig. 25a, it seems that the creation and evolution of the high energy RF population is related to 3He concentration greater than 15 %.

55

Table 3. Results on the analysis of the TOF spectra from the TOFOR data of shot #69393 performed in the ITB experimental session at JET. Temperatures (T) are expressed in keV while amplitude values (A) in arbitrary units. The standard error is indicated.

�tTOF [s] 44-45 45-46 46-47 47-48 48-49 49-50 50-51 Bulk T 21,2 20,1 25,1 27,2 28,3 32,3 18,6 A 14.0,1.5 23.3,0.7 22.3,2.2 18.1,2.3 16.5,0.7 15.0,3.0 1.3,0.4 NB T * * * * * * * A 5.8,0.5 11.2,3.1 9.7,0.6 9.8,0.6 11.2,3.4 9.0,0.7 0.8,0.2 HE T - - - 197,25 245,18 233,17 - A - - - 5.5,0.6 16.4,0.9 18.8,1.0 -

* Fixed slowing down distribution for NB(D) injection.

The analysis of the TOFOR data for other JET shots featured by high 3He concentration (i.e. larger than 10 %) was performed for comparison. The experimental session held on May 18th 2006 on the mode conversion in hybrid scenario study made use of comparable 3He concentration as for the ITB experiments though the ICRH heating was tuned to a different frequency. Here the HE ion populations were observed at temperatures about 100-150 keV. These results can be taken as examples of how the new TOFOR data will help clarify the complex IRCH power absorption mechanisms which is important for finding the best RF heating scenarios for JET plasmas in view of ITER.

Besides the interesting new plasma physics results that have been achieved, the results themselves have demonstrated the performance of the TOFOR spectrometer as a NES diagnostic for D plasmas. The target specifications with respect to energy resolution, count rate capability and neutron energy coverage have been achieved. Moreover, the detection efficiency is sufficiently high so the count rate capability of about 0.4 MHz will be exploited at JET at the expected neutron yield rate limit of about Yn=1017 n s-1. It can thus be noted that TOFOR will have comparable NES diagnostic capabilities to those of the MPR spectrometer for DT plasmas which was an overall goal of the TOFOR spectrometer project. This research has thus contributed to developing a spectrometer that allows advanced NES diagnosis of D plasmas in tokamaks which has not previous been possible.

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5. Conclusions and outlook This thesis research has been devoted to the development of new instrumentation for neutron emission spectroscopy (NES) of fusion plasmas of deuterium (D) and deuterium-tritium (DT). This has involved the existing spectrometer of the magnetic proton recoil (MPR) type which has been upgraded to MPRu as part of the thesis. The MPR has been in use at JET since 1997 primarily for measurements of the 14-MeV neutrons from d+t��+n reactions in DT plasmas. The MPRu has been equipped with a new focal plane detector to permit also measurements of 2.5-MeV neutrons from d+d�3He+n reactions, i.e., for NES diagnosis of D plasmas. The other instrument is a 2.5-MeV neutron time-of-flight (TOF) spectrometer of new design optimized for high count rates; it is dubbed TOFOR. TOFOR is a spectrometer dedicated to NES diagnosis of D plasmas and was developed with the goal to achieve the same high performance as the MPR had demonstrated for DT plasmas. In other words, the TOFOR and MPRu spectrometers should provide JET with NES diagnostic capability for most plasmas produced with both D and DT that had not existed earlier. Both projects have been completed with installation at JET and have been demonstrated to perform as projected. The MPRu focal plane detector consists of phoswich scintillation counters and was developed based on tests carried out in this thesis work. The objective to improve the signal-to-background (S/B) ratio was largely achieved with a value of S/B�10 in the measurement of 2.5-MeV neutrons from dd reactions. This means that the MPRu can be used as complementary instrument for NES diagnosis of D plasmas as intended. For diagnosis of DT plasmas, the improved background rejection capability should allow unimpeded MPRu measurements also of the weakest spectral components for the 14-MeV neutron emission (S/B�2�104). The present limit of the achievable S/B ratio with the phoswich technique is the pulse height resolution which is an intrinsic one related to the photon statistics of scintillator signals produced by protons in the energy range down to 1.5 MeV. The results of this work would thus indicate that the use the MPR type of spectrometer for measurement of the 2.5-MeV neutron emission from dd reactions in DT plasmas will require another type of focal plane detector or enhanced surrounding shielding. The TOFOR spectrometer was developed from scratch using some basic information on the requirements calculated as part of this thesis from the successful use of the MPR to measure extended neutron energy distributions (,20 % around 14 MeV) for DT plasmas with auxiliary heating. It was found that the study of similar conditions in D plasmas would require a considerably greater energy range (-40 % to above +100 %) around 2.5-MeV. This together with the initial neutron transport calculations quantified the specifications of TOFOR with respect to using it as a D plasma diagnostic whose performance should be close to the standard that the MPR spectrometer had shown achievable for DT plasmas. With this design target, the TOFOR spectrometer was constructed, installed and commissioned at JET. The results presented in this thesis have demonstrated that the objectives were met but they have also indicated different paths of further development. The main one is the replacement of the electronics thresholds for pulse height discrimination with new boards which complement the present time digitizing capability but with a finer time resolution (30 ps) with information on the magnitude of the scintillator light pulse of the detected particle. If this can be realized, it would open up possibilities for further refinements in the optimization of the use of the time-of-flight technique for fusion neutron measurements that TOFOR has started. Physics results obtained as part of this thesis are exemplified by the interpretation and use of the MPR measurements during the first DT experiments (DTE1) at JET and by the on line analysis of the data collected during the trace tritium experimental (TTE) campaign of 2003. In

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this case, the correlation of the plasma toroidal rotation with the wave phase of the radio frequency (RF) power injection was observed. With the new MPRu it will be possible to make measurements also for D plasmas, especially, when JET approaches neutron yield rates close to the maximum, a factor of 10 higher than in present MPRu tests. Of special interest is the possibility of using the MPRu together with TOFOR to provide dual sight line observations that can be expected to enhance NES diagnostic capability at JET.

The TOFOR measurements performed at JET have been used to refine the response function as well as the models used to interpret the data. This has been developed to the level that the first physics results have been produced as reported in one paper of this thesis while other results are included in the summary as examples of the studies under way. The most noteworthy achievements are those of the effects of auxiliary heating on the plasma, specifically, those relating to the high-energy component of the deuteron population. Here, the direct effect of neutral beam injection of deuterium and RF heating of deuterons has been studied as well as the synergy between the two. Moreover, RF heating of minority 3He in D plasma has been found to have various effects: For low 3He concentration (<3 %), the RF heating creates fast 3He ions that can affect the deuteron population, namely, through the knock on effect. Another observation was that the RF heating tuned to 3He switches to absorption on deuterons depending on the 3He concentration. If this is high (>15 %), the deuterons are accelerated to high energies to give rise to a corresponding neutron tail that TOFOR detects.

TOFOR is now part of the diagnostic complement at JET. The experience gained and the results obtained are the basis to project that TOFOR will play a central role in the study and optimization of auxiliary heating with the aim to attain high fusion power using D plasmas. Especially interesting research could be done with TOFOR in conjunction with the new ICRH heating systems which has as an aim to provide input for ITER.

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6. Summary of the papers Paper I L. Giacomelli, H. Henriksson, M. Tardocchi, S. Conroy, G. Ericsson, M. Gatu Johnson, W. Glasser, G. Gorini, A. Hjalmarsson, M. Johansson, J. Källne, A. Murari, S. Popovichev, H. Sjöstrand, M. Weiszflog and JET EFDA Contributors Neutron emission spectroscopy of fuel ion rotation and fusion power components demonstrated in the trace tritium experiments at JET 31st EPS Conference on Plasma Phys. London, 28 June - 2 July 2004 ECA Vol.28G, P-5.171 (2004) This paper presents the investigation of the dependence of the total neutron yield rate on the neutral species injected with the NB systems and on the position of their energy deposition and the results obtained with the MPR spectrometer during the trace tritium experimental (TTE) campaign at JET in October 2003. Specifically, the MPR provided detailed information on the RF wave power effects studied with regard to different ICRH injection scenarios of the antenna phasing. This gave the first observation of generation of toroidal rotation in the fuel ion population of tokamak plasmas. Velocities of about 300 km s-1 were observed whose co/counter direction changed depending on the +90�;-90� phasing. The operational capability to perform analysis and interpretation of the MPR results on shot-by-shot basis is also mentioned in the paper. This afforded live control room participation in the JET experimental program. My contribution to Paper I is participation in analysis of data and writing of the paper. I made the presentation at the conference. Paper II L. Giacomelli, C. Hellesen, A. Hjalmarsson, H. Sjöstrand, J. Källne, S. Conroy, G. Ericsson, E. Andersson Sundén, M. Gatu Johnson, W. Glasser, G. Gorini, E. Ronchi, M. Tardocchi, M. Weiszflog and JET EFDA Contributors Characterization of phoswich scintillation detectors for the focal plane hodoscope of magnetic proton recoil spectrometers for fusion neutrons UU-NF 07#11 Uppsala University, Neutron Physics Report ISSN 1401-6269 (July 2007) unpublished The paper is about the tests performed at the INF laboratory and at the Tandem Lab accelerator at Uppsala University for the characterization of the prototype phoswich detectors. These make use of the same scintillation material and similar configuration as for the new scintillation counters implemented in the new MPRu focal plane detector (hodoscope). The paper describes the setups, the radiation sources, and the data analysis methods used to assess the performance of the detectors. The analysis method is essential for the exploitation of the information in the data and different techniques were examined and presented in the paper. Similarly, the quality of data is a limiting information factor that was examined in terms of pulse height resolution of the recorded waveforms for different time integration periods as well as the effect of noise. These results were used as figure of merit for comparing the performance of different detectors configurations. It was found that the use of an extra layer of acrylic plastic in the detector configuration improved the light collection and so the resolution of the measurement. Different radioactive sources were used

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to study various aspects of the phoswich detector response to ionizing radiation of relevance for the application in the MPRu hodoscope. To these was added the use of proton beam in the energy range 1 to 7 MeV which could also be used to mimic the actual operating conditions of the phoswich detector in the MPRu hodoscope at JET. These results provided essential information for the assessment of the background rejection performance of the new MPRu hodoscope and specifications for the construction design and the data analysis techniques to be used. The paper presents results from initial experiments with the MPRu at JET demonstrating that the detector met the objectives of making the MPRu useful as 2.5-MeV neutron spectrometer for diagnosis of D plasmas. My contribution to Paper II is participating in all aspects of the tests which includes all experimental work and data analysis and interpretation. I have carried the main responsibility for the most the tests and have finalized the results forming the material of the paper. Paper III H. Sjöstrand, L. Giacomelli, E. Andersson Sundén, S. Conroy, G. Ericsson, M. Gatu Johnson, C. Hellesen, A. Hjalmarsson, J. Källne, E. Ronchi, M. Weiszflog, G. Wikström, G. Gorini, M. Tardocchi, A. Murari, G. Kaveney, S. Popovichev, J. Sousa, R.C. Pereira, A. Combo, N. Cruz and JET EFDA Contributors New MPRu instrument for neutron emission spectroscopy at JET Review of Scientific Instruments 77 10E717 (2006) This paper presents the details of the MPRu spectrometer with the new focal plane detector installed at JET in December 2005. It describes the initial commissioning and use of the MPRu with fusion neutrons from D plasmas. Preliminary data obtained during the high level commissioning period in April 2006 are presented on the detector’s capability to handle the interference of background radiation on the measurement of 2.5-MeV neutrons from d+d�3He+n reactions. The first data analysis provides a signal-to-background ratio of S/B�10, thus, confirming the projected performance. My contribution to Paper III is participation in the experiments at JET and assessment of the MPRu performance with respect the test results from Paper II. Paper IV L. Giacomelli, S. Conroy, G. Ericsson, G. Gorini, H. Henriksson, A. Hjalmarsson, J. Källne and M. Tardocchi Comparison of neutron emission spectra for D and DT plasmas with auxiliary heating Eur. Phys. J. D 33 (2005) 235-241 The paper presents the results on the projection to D plasmas of the MPR spectra measured for 14-MeV neutrons during the DTE1 experimental campaign in 1997 at JET. The DTE1 data for NB and RF heated plasmas were analyzed to extract information on the underlying deuteron and triton velocity distributions, and the result were used to construct equivalent velocity distributions in D plasmas. The same plasma conditions in terms of ion temperatures and densities are assumed to simulate the corresponding spectrum for 2.5-MeV neutron emission from d+d�3He+n reactions. The study defined the characteristics of the dd neutron spectra from which the requirements on the TOFOR design were determined with regard to detection efficiency and neutron energy coverage to achieve the desired performance. Moreover, the comparison of the spectral features attainable along the two lines of sight planned for the MPRu (47� tangential) and

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the TOFOR (90� vertical) spectrometers is carried out to assess the potential of dual sight line NES diagnosis and the quality of the extracted information. My contribution to Paper IV includes the analysis of the existing 14-MeV DT data taken with the MPR and the determination of the parameters of interest used as input for the simulations of 2.5-MeV neutron spectra of dd reactions that I performed. Paper V A. Hjalmarsson, S. Conroy, G. Ericsson, L. Giacomelli, G. Gorini, H. Henriksson, J. Källne, M. Tardocchi and M. Weiszflog The TOFOR spectrometer for 2.5 MeV neutron measurements at JET Review of Scientific Instruments 74 3 (2003) 1750-1752 This paper presents the TOFOR concept with details on the project design. It includes the results of light transport tests performed to characterize a prototype S2 detector. The importance of light transport effects is illustrated and its measured value was used for the optimization of the TOFOR geometry. Neutron transport calculations were run to make the first quantitative explorations of the relationship between the geometrical TOFOR design and performance in terms of efficiency and resolution. Comparison was done with the results of the optimized geometry. My contribution to Paper V includes running lab test to determine the light transport in the prototype S2 detector and the definition of the optimized TOFOR geometry which were used in the calculations as empirical input to quantify the performance. Paper VI M. Gatu Johnson, L. Giacomelli, A. Hjalmarsson, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, C. Hellesen, J. Källne, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, A. Murari, S. Popovichev, J. Sousa, R.C. Pereira, A. Combo, N. Cruz and JET EFDA Contributors The TOFOR neutron spectrometer and its first use at JET Review of Scientific Instruments 77 10E702 (2006) The paper describes the concept and realization of the TOFOR spectrometer installed at JET roof laboratory in February 2005. It is a preliminary report on the TOFOR spectrometer and its subsystems for data acquisition (DAQ) and control and monitoring (C&M) and interfacing to JET. The performance of TOFOR is illustrated with results on neutron time-of-flight spectra using preliminary setting of TOFOR operating points. My contribution to Paper VI includes participation to the different phases of the TOFOR project, from the optimization of the design to the tests of components and construction, from the test of the instrument in Uppsala to the shipping and final assembly at JET. I worked on the characterization of the DAQ and C&M systems and I was involved in the data analysis and writing of the paper.

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Paper VII M. Gatu Johnson, L. Giacomelli, A. Hjalmarsson, J. Källne, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, C. Hellesen, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, A. Combo, N. Cruz, J. Sousa, S. Popovichev and JET EFDA Contributors The 2.5-MeV neutron time-of-flight spectrometer TOFOR for experiments at JET UU-NF 07#12 Uppsala University, Neutron Physics Report ISSN 1401-6269 (August 2007) Submitted to Nucl. Instr. Meth. This paper provides a detailed description of the TOFOR spectrometer which was built to high requirements in terms of instrumental specifications for neutron emission spectroscopy (NES) measurements of D fusion plasmas in tokamaks. The TOFOR installation at JET is also described. The performance is illustrated with results from the experiments of the last campaign 2006-07 with auxiliary heating in terms of NB and RF power which tested the capability of measuring neutron spectra with large energy coverage around 2.5-MeV and operating at high count rates. The results on the measured high count rate up to 40 kHz and the projection up to 400 kHz for maximum neutron yield operation at JET demonstrate that TOFOR has achieved the required performance providing a figure of merit for NES diagnosis of tokamak plasmas. My contribution to Paper VII includes what is mentioned under paper VI. In addition, I have worked on the fine-tuning of the setting of TOFOR working points with special studies of the electronics contribution to the instrumental resolution so to attain a better characterization of the response function. I have provided input material for the paper and contributed to its final version. Paper VIII C. Hellesen, L. Giacomelli, M. Gatu Johnson, A. Hjalmarsson, J. Källne, M. Weiszflog, E. Andersson Sundén, S. Conroy, G. Ericsson, E. Ronchi, H. Sjöstrand, G. Gorini, M. Tardocchi, S. Popovichev, L. Ballabio and JET EFDA Contributors Measurement and analysis of the neutron emission from ICRH and NB heated JET D plasmas using the TOFOR spectrometer UU-NF 07#13 Uppsala University, Neutron Physics Report ISSN 1401-6269 (August 2007) To be submitted to Nucl. Fusion The paper describes the first dedicated NES experiment run with TOFOR at JET on May 19th 2006. Its aim was to study the physics of NB and NB+RF heated plasmas in deuterium and in doing so also test the TOFOR performance. The focus was on subjecting the plasma to different heating scenarios and to explore how these manifested themselves in the neutron emission spectrum. The paper describes different heating scenarios to attain reasonably steady state plasma conditions for NB with injection of deuterons of different energy in combinations with RF power in terms of ICRH wave for absorption on deuterons at different radial positions in the plasmas. The measurements led to a detailed test of the TOFOR response function which was refined as part of this study. The paper also describes the first use of an improved model for the analyses and interpretation of NES data. The data as time-of-flight spectra are described in terms of a two-component model for quantitative assessment of the plasma response to the auxiliary heating scenarios and the results are presented as neutron spectra. Among the physics results of the experiment is the observation of high-energy tail in the deuteron velocity distribution due to RF power injection. This effect is greatly enhanced for in-board resonance position of the RF wave.

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My contribution to Paper VIII includes the active participation in running the experiment and providing input to improve the TOFOR response function. I was involved in the data analysis and contributed to the final version of the paper.

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7. Sammanfattning på svenska I denna avhandling presenteras utveckling av instrumentering för neutronemissionsspektroskopi (NES) vilket användas för att studera fusionsplasma bestående av endast deuterium (D) eller an blandning av deuterium och tritium (DT). NES-mätningar är centrala för att diagnostisera ett fusionsplasmas tillstånd, t.ex. för att bestämma vad som händer i plasmat på vägen mot makt att producera mer energi än vad som använts för att nå fusionstillståndet. Syftet med fusionsforskning är nämligen att lära sig skapa och kontrollera att plasma i reaktorn där fusionsbränslet har värmts upp till mycket hög temperatur (dvs. över 100 millioner grader som motsvarar 10 keV rörelseenergi) vid att tryck på 1 till 2 atmosfärer. Under dessa förhållande börjar fusionsbränslet att brinna vilket inte har uppnåtts ännu, men genom att injicera en stor mängde tilliggs effekt i bränslet har man lyckats att få bränslet att gläda. Den testreaktor som har nått längst är JET, som är av tokamaktyp, i vilken plasmat innesluts med hjälp av kraftiga magnetfält. I denna reaktor har man genom att injicera 30 MW tilläggs effekt alstrat en fusion effekt på 16 MW, där kvoten mollan dessa är ett mått på plasmats fusion kvalitén (Q=16/30=0.6 i detta fall). Nästa steg i utvecklingen har att nå Q=10 som man avser att nå den nya tokamaken ITER som har 10 gånger större volym jämför med JET. ITER planeras att kunna producera 500 MW och kan kanske nå bränslets antändningspunkt Q�<!där inga tilläggs effekt längre är nödvändig utan förbränningen upprätthålls av plasmats egenuppvärmning. Neutronspektroskopi rör mätning av bränslepartiklarnas energi som är ämnet för detta forskningsprojekt, speciellt effekten av hur tilläggseffekt som injiceras i plasmat påverkar rörelseenergi hos bränslejonerna som förekommer i plasmat.

d+d�3He+n i ett D-plasma och d+t�4He+n i ett DT-plasma är då två fusionsreaktionerna som är av intresse för neutronspektroskopi, eftersom neutronerna som emitteras kommer att få större delen av den frigjorda energin. Genom att mäta neutronernas fördelning rumsligt och tidsmässigt samt dess energi fördelningen (spektrumet). Från neutronemissionen kan man erhålla information om förbränningsprocessen och i synnerhet om det underliggande rörelsetillståndet hos bränslejonerna. Plasmat kan värmas upp på ett varsamt sätt så att bränsle jonerna hela tiden befinner sig i termiskjämviktsläge vilket ger en energi fördelning i neutronspektra som har den typiska klockformen. Från denna fördelning är bredda ett direkt mått på bränslet temperatur. När tilläggseffekt injiceras störs jonernas rörelsetillstånd och neutronspektra får en mer komplicerad form som kan tolkas får att information om bränslejonerna tillstånd, ett speciellt fall är inslagen av supertermiskkomponenter som härrör från injicerarns. Huvudsyfte med NES-mätninger är att mäta neutronspektra runt 2.5 och 14 MeV, vilket är centralenergierna för d+d och d+t fusionsreaktionerna, med tillräcklig noggrannhet så att det kan användas för att extraherar den nämnda informationen. För detta krävs spektrometrar med hög detektionseffektivitet och räknehastighetsförmåga, okänslighet för bakgrundsstrålning, bra energiupplösning och stor energibandbredd. Denna avhandling är ett bidrag till utvecklingen av två nya neutronspektrometrar, TOFOR och MPRu, som ingick som delar av programmet JET-EP1 (enhanced performance) vilket pågick under tiden 2003-05.

TOFOR är en neutronspektrometer av flygtids typ (TOF) var huvudsyfte är att diagnostisera D-plasma och är optimerad (O) för hög räknehastighet (R). I termer av räknehastighet är TOFOR en faktor 100 bättre än vad som tidigare här uppnåtts på JET med andra TOF-spektrometrar. Detta har gett mojligheter att studera extra uppvände plasman som tidigare inte kunnat studeras. TOFOR:s prestanda har testas på JET då räknehastighet på 40 kHz har uppnåtts vid en tiondel av den neutronemission som JET maximalt kan producera. De nådda resultaten pekar alltså mot att TOFOR kan klara räknehastigheter upp till 400 kHz vilket var ett

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av design målen. TOFOR har redan givit en rad nya resultat som lett till ökad kunskap om hur extrauppvärmningen påverkar plasmat av vilka några presenteras i denna avhandling. I synnerhet uppmärksammas observationer av hög energisvansar i neutronspektra som skapas av den tilläggseffekt som injiceras i form av radiofrekvensvågor (RF). Från dessa resultat har man kunna bestämma ”svanstemperaturer” på flera hundra keV medan plasmats grundtillstånd har en temperatur på 3 till 8 keV vilket också kan observeras med TOFOR under gynnsamma förhållanden.

MPRu är en uppgraderad version av den existerande MPR-spektrometern (som står för ”magnetic proton recoil”) som är byggd speciellt för att diagnostisera DT-plasmas; MPRen har använts på JET sedan 1996 med data från en period under 1997 samt en under 2003 med begränsade DT-plasma. De först test mätningarna på JET har gjorts som visar att MPRu är ca hundra gånger bättre MPR vad beträffa bakgrundsokänslighet när det gäller studiet av d+d reaktioner. Kvantitativt rapporteras i avhandlingen där kvoten signal-till-bakgrund har förbättrats till 10/1 från 1/10 vilket bedöms var tillräckligt för att kunna använda MPRu som ett komplement till TOFOR för att diagnostisera D-plasma. Vad beträffa, MPR-data för DT plasma har dessa använts på två sätt i avhandlingen, där data från 1997 är av mycket hög kvalitet. Dessa data analyserads och användes för att beräkna de neutron-emission-spektra som TOFOR skulle mäta för D-plasma. Denna information användes för att bestämma de prestanda som TOFOR skulle designas för. Den andra användningen gäller data från 2003 som analyserades och tokades med resultat som visade att toroidal rotation kunde induceras med RF-uppvärmning och att riktningen kunde styras. Avslutningsvis kan det noteras att spektrometrarna TOFOR och MPRu kommer utgöra huvudinstrumenten för NES-diagnostisering på JET för de planerade experimenten under de närmaste tre till fyra åren. JET forskningsprogram kommer att till stor del inriktas till frågor som är av vikt för ITER som kommer vara strakt beroende av neutronmätningar under dess experimentperiod om ca 25 år.

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8. Acknowledgements “I've got a theory that if you give 100 percent all of the time, somehow things will work out in the end.”

Larry Bird The work I have presented in this thesis concerns my contribution to projects the realization of which corresponds to more than one man career. Many people have contributed to the different phases of the instruments’ construction and I want here to express my gratitude to them.

First to Jan Källne whose ideas and will made the projects happen. He also gave me the chance to be part of the group, fully experience the research in this field and supervised my work. Göran Ericsson, Sean Conroy, Matthias Weiszflog and Giuseppe Gorini with their devotion to different aspects of the projects contribute to their actualization. I am grateful to Anders Hjalmarsson and Marco Tardocchi with whom I have spent very long working days in Uppsala and at JET. Working with Anders, Sean and Marco has been inspiring over these years. I want to thank the numerous colleagues and friends of the TOFOR, MPRu and MPR crews, Maria Gatu Johnson, Carl Hellesen, Philip Magnusson, Henrik Sjöstrand, Erik Andersson Sundén, Emanuele Ronchi, Mikael Höök, Peter Andersson, Wolfgang Glasser, Gustav Wikström, Niklas Kronborg, Diego Palma, Hans Henriksson and Luigi Ballabio whose commitment, efforts, and teamwork made the projects a reality.

I want also to express my gratitude to Alain Lioure and Frederic Le Guern for their collaboration during the projects, to Sergey Popovichev, Garry Kaveney, Trevor Edlington, Brian Syme and Kate Bell for their assistance during the installation of the spectrometers at JET, to Andrea Murari, Elena De La Luna, Jerzy Brzozowski, Jef Ongena, Dirk Van Eester, Ernesto Lerche, Thomas Johnson, Anatoli Krasilnikov and Vasili Kiptily for their precious collaboration and for the discussions on plasma physics during the experimental campaigns, not forgetting the JET operators and personnel. I want to thank Lars Einarsson, Torbjörn Hartman, Yngve Kjellgren, Jonas Åström for their collaboration during the setup of the tests of the phoswich detectors and Armando Foglio Para for his interest in my work over these years.

My gratitude to Susanne Söderberg who solved many practical aspects of my stay at the department and in Sweden and to Ib Koersner and Teresa Kupsc for their help with the computer system. My appreciation goes to Michael Österlund, Jan Blomgren and to the many other friends and colleagues that I have met at the department during these years for the nice working environment. I want also to thank the friends I have met at the TSL, ISV/IKP, IRFU and Astronomy departments and at Milano Bicocca University.

During these years in Uppsala I have been lucky to meet a lot of good friends from different countries in the World. So here my applaud goes to the Argentinian-Italian-Russian-Cuban-French-Spanish-Bolivian-American-Polish-Swedish community. And I am very grateful to the Ulleråker Dream Team, able to change the small gym into the Boston Garden on every Thursday. Great merit to Ned Carter for keeping track of everybody and to everybody for the competitive basketball and great amusement.

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My deep gratitude to Alessandro Retinó for his loyal friendship. I want also to thank the many friends that patiently have always been waiting for me in Italy. Massimo, Matteo and Silvia Notari. Christian Agnesi, Katia Sorice, Simone Bravi and All the Members of The Alcoholic Group. La Confraternita InterUniversitaria Pezzama has been growing during these years establishing new and strong networks around Europe. Big thanks to Enzo Trivellone, to Michele Nicoli and Laura Piatto and …, to Matteo Mazzuccato and Isabella Santagostino. My deep gratitude to Vincenzo Di Mauro, for his precious friendship and time. Grazie ancora a Voi, Miei Cari Genitori e kiitos paljon Jaana sinun rakkaudestasi.

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