17th IAEA Technical Meeting on Research Using Small Fusion DevicesLisbon, 22 to 24 October 2007

Joint Experiments on the tokamaks CASTOR and T-10

Department of Applied PhysicsGhent University

B- 9000 GentBelgium

Presented by Guido Van OostFor the CRP “Joint Research

Using Small Tokamaks” teams

G Van Oost1, M Gryaznevich2, E Del Bosco4, A Malaquias6, G Mank6 , M Berta5, 13, J Brotankova3, R Dejarnac3, E Dufkova3, I Ďuran3, M Hron3, P Peleman1, J Sentkerestiova3, J Stöckel3, B Tál5, V Weinzettl3, J Zajac3, S Zoletnik5, V Budaev11, N Kirneva11, G Kirnev11, B Kuteev11, A Melnikov11, M Sokolov11, V Vershkov11, I El Chama Neto15, J Ferreira4, R Gonzales14, C R Gutierrez Tapia18, H Hegazy8, P Khorshid12, A W Kraemer-Flecken16, L I Krupnik19 ,Y Kuznetsov7, A M Marques Fonseca17, A Ovsyannikov9, L Ruchko7, E Sukhov9, A Singh10, V Soldatov16, A Talebitaher12, , G M Vorobjev9

1Dep. of Applied Physics, Ghent Univ., Belgium; 2EURATOM/UKAEA Fusion Association, Culham SC, Abingdon, UK;

3Institute of Plasma Physics, Association EURATOM/IPP, Czech Rep.; 4INPE, São José dos Campos, Brazil; 5KFKI-

RMKI, Association EURATOM, Budapest, Hungary; 6IAEA, NAPC Physics Section, Vienna, Austria; 7Institute of

Physics, Univ. of São Paulo, Brazil; 8EAEA, Cairo, Egypt; 9Saint Petersburg State Univ., Russia; 10Plasma Physics

Lab., Univ. of Saskatchewan, Canada; 11RRC “Kurchatov Institute”, Moscow, Russia; 12Plasma Physics Research

Center, Teheran, Iran; 13Széchenyi István Univ., Association EURATOM, Győr, Hungary; 14CICATA-IPN, Mexico;

15Univ. Tuiuti Paraná, Brazil; 16Forschungszentrum, Julich, Germany; 17IST, Lisbon, Portugal; 18ININ, Mexico;

19Phys-Tech Inst., Kharkov, Ukraine

OUTLINE

• Introduction

• Transport barriers and ExB shear

• JE1 on CASTOR

• JE2 on T-10

• Conclusions

Introduction

Transport barriers: physical

mechanisms

Possibilities for CRP JE’s

TRANSPORT BARRIERS, ELECTRIC FIELDS,

TURBULENCE

The understanding and reduction of turbulent transport in magnetic confinement devices is not only an academic task, but also a matter of practical interest, since high confinement is chosen as the regime for ITER and possible future reactors since it reduces size and cost.

Generally speaking, turbulence comes in two classes: electrostatic (emphasis) and magnetic turbulence(CASTOR). Over the last decade, step by step new regimes of plasma operation have been identified, whereby turbulence can be externally controlled, which led to better and better confinement.

The electron heat conduction, however, which normally is one to two orders above the collisional lower limit, remained strongly anomalous also in the regime with suppressed electrostatic turbulence. In that case it became the dominant heat loss channel. From this, it is conjectured that magnetic turbulence drives the anomalous electron heat conduction. Experiments at the late Rijnhuizen Tokamak Project (RTP) and the T-10 tokamak which specifically addressed the electron thermal transport properties of the plasma have strongly corroborated this conjecture.

The physical picture that is generally given is that by spinning up the plasma, it is possible to create flow velocity shear large enough to tear turbulent eddies apart before they can grow, thus reducing electrostatic turbulence. This turbulence stabilization concept has the universality, needed to explain ion transport barriers at different radii seen in limiter-and divertor tokamaks, stellarators, reversed field pinches and mirror machines with a variety of discharge- and heating conditions and edge biasing schemes.

Electron ITB (T-10, TEXTOR)• From previous work [K.A.Razumova et al ,Nuclear

Fusion 44 (2004) 1067] we know that e-ITBs are formed when dq/dr is low in the vicinity of rational magnetic surface with low m and n values.

• Later, experiments with a rapid plasma current ramp up were performed. In this case, due to (p + li/2) 1/Ip

2 a rapid change of the magnetic surface densities in the central part of plasma takes place, while current penetration in this region occurs only after t>50ms. Therefore, confinement changes observed in the plasma core are the result of a magnetic surface density change only.

Self-consistent pressure profile ?

• Tokamak plasmas have a tendency to self-organization: the plasma pressure profiles obtained in different operational regimes and even in various tokamaks may be represented by a single typical curve, called the self-consistent pressure profile.

• About a decade ago local zones with enhanced confinement were discovered in tokamak plasmas. These zones are referred to as Internal Transport Barriers (ITBs) and they can act on the electron and/or ion fluid.

• Here the pressure gradients can largely exceed the gradients dictated by profile consistency. So the existence of ITBs seems to be in contradiction with the self-consistent pressure profiles (this is also often referred to as profile resilience or profile stiffness).

• Therefore, the interplay between profile consistency and ITBs has been investigated.

Conclusions e-ITB

1. It is shown that self-consistent profiles exist in tokamak plasmas for the normalized pressure, This profile relates to the lowest level of plasma instabilities, hence to the best confinement.

2. The radial distribution of plasma transport coefficients is determined by the necessity to maintain the self-consistent pressure profile under different external impacts.

3. It is shown that the well-known effect of density decrease in the local ECRH heating region (“density pump out”) is connected with p(r)/p(r0) conservation.

4. Under suitable conditions there may be regions in the tokamak plasma (the ITB regions), where p may be steeper than determined by the self-consistent profile condition.

5. The only possibility to organize more peaked profile than the self-consistent pressure profile is to form the pronounced ITB in plasmas.

6. The suggested hypothesis about gaps between turbulent cells in the vicinity of low-order rational surfaces could be the key to all presented experimental results.

Razumova et al., PPCF

Although turbulence measurements have been performed on many magnetic

confinement devices during the last decades, the additional insight gained from

these experiments is relatively limited.

This can be attributed to a number of reasons:

•Firstly, only a very coarse spatial resolution was achieved in many

measurements of electric fields and turbulence.

• Secondly, simultaneous measurements of different fluctuating quantities

(temperature, density, electric potential and magnetic field) at the same

location, needed for a quantitative estimation of the energy and particle

transport due to turbulence were only performed in a very limited number of

cases.

•Thirdly, theoretical models were often only predicting the global level of

turbulence as well as the scaling of this level with varying plasma parameters.

The investigation of the correlations between on the one hand the occurrence

of transport barriers and improved confinement in magnetically confined

plasmas, and on the other hand electric fields, modified magnetic shear and

electrostatic and magnetic turbulent fluctuations necessitates the use of various

active means to externally control plasma transport.

It also requires to characterize fluctuations of various important plasma

parameters inside and outside transport barriers and pedestal regions with high

spatial and temporal resolution using advanced diagnostics, and to elucidate

the role of turbulence driving and damping mechanisms, including the role of

the plasma edge properties.

The experimental findings have to be compared with advanced theoretical

models and numerical simulations.

BivBiviprne1

rE

diamagnetic driving term rotation driving terms

Er is connected with: • perpendicular heat- and particle transport• perpendicular momentum transport• poloidal flow

Coupling between particle-, heat- and momentum transport!!

Investigation of Er can elucidate plasma transport

Modify Er through: • particle- and energy supply

• supply of momentum (e.g. through NBI)

• modification of the plasma current profile (poloidal magnetic field)

Active Control of Transport

Radial electric fields and rotationThe radial electric field Er follows from the generalized Ohm’s law

How to modify the edge turbulence(to improve the confinement)

Turbulent structures are torn to several pieces Size as well as the amplitude is reduced Transport coefficients become smaller Transport barrier forms at the shear layer J. Stöckel Global confinement improves

tor

rad

tor

torradpoloidal B

E

B

BEv

2

Transport coefficients are proportional to the radial size of turbulent structuresRadial electric field poloidal rotation

0Shear dr

dEr

Er > 0 Er < 0

Poloidal cross sectionB is to the screen

How does ExB shear suppression work ?

• ExB shears enters quadratically into theories: sign irrelevant

• Er and Bθ contribute to the shear; Er/RBθ is toroidal angular speed

• Shearing rate not constant over flux surface (larger on LFS than on HFS)

RB

E

B

RB

dr

vd r2

BE

Plot of radial electric field Er , toroidal angular speed E /RB , and the EB shearing rate as a function of flux surface label for a high performance, deuterium VH-mode plasma in DIII-D

is proportional to the square root of the toroidal flux inside a given flux surface

Although the derivative of Er vanishes near = 0.5, the EB shearing rate is appreciable across the whole plasma

Plasma conditions are 1.2 MA plasma current, 1.6 T toroidal field, 9.8 MW injected deuterium neutral beam power, and 4.7 1019 m-3 line averaged density. Discharge is a double-null divertor.

Improvement and development of diagnostics

MAGNETIC PROBES

In the plasma interior magnetic fields can be measured with spectroscopic methods.

From magnetic field measurements performed outside the plasma important properties

like plasma current, energy content and MHD fluctuations together with their mode

structure can be inferred. Such measurements utilize different types of coils.

As the discharges became longer, the evaluation of B from its measured time derivative

has become increasingly difficult, because the integration needs a precise determination

of possible offsets in the preamplifiers.

Recently, Hall probes have been used on TEXTOR and CASTOR to measure the

absolute value of B directly together with its fluctuations in the boundary plasma of

tokamaks. The strongest objection against the use of Hall sensors in the reactor type

tokamak is their vulnerability to radiation damage (esp. to the high neutron fluxes). This

is presently being investigated.

Joint Experiments on Small Tokamaks:

edge plasma studies on CASTOR

G. Van Oost1, M.Berta2, 13, J.Brotankova3, R.Dejarnac3, E. Del Bosco4, E.Dufkova3, I.Ďuran3, M. P. Gryaznevich5, M.Hron3, J,Horacek3, A. Malaquias6, G. Mank6, P.Peleman1, J.Sentkerestiova3, J.Stöckel3, V.Weinzettl3, S.Zoletnik2, B. Tál2, J.Ferrera4, A. Fonseca7, H. Hegazy8, Y. Kuznetsov7, A. Ossyannikov9, A. Singh10, M.Sokholov11, A.Talebitaher12

1) Department of Applied Physics, Ghent University, Ghent, Belgium2) KFKI-RMKI, Association EURATOM, Budapest, Hungary3) Institute of Plasma Physics, Association EURATOM/IPP.CR4) INPE, São José dos Campos, Brazil5) EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, UK6) IAEA, NAPC Physics Section, Vienna, Austria7) Institute of Physics, University of São Paulo, Brazil8) EAEA, Cairo, Egypt9) Saint Petersburg State University, Russia10) Plasma Physics Laboratory, University of Saskatchewan, Canada11) RRC “Kurchatov Institute”, Moscow, Russia12) Plasma Physics Research Center, Teheran, Iran13) Széchenyi István University, Association EURATOM, Győr, Hungary

CASTOR tokamak

Major radius 40 cmMinor radius 8,5 cmToroidal magnetic field 0,5-1,5 TPlasma current 5 - 20 kAPulse length < 50 ms

Central electron temperature 100-300eVCentral ion temperature 50-100 eVPlasma density 0.5-3.1019 m-3

Energy confinement time <1 msEdge density/ temperature ~1018 m-3/10-40 eV

The oldest tokamak in the world Operational till end 20061958-1976 Kurchatov Institute as TM 11977- 2006 IPP Prague

Research is focused onedge plasma (turbulence, electric fields, ….)

Modern tokamak (operational in Culham Laboratory, UK until 2002)

Offered to IPP- ASCR, Prague

Tokamak COMPASS-D

Radial edge turbulence structure

Left graph: Radial profiles of floating potential Φf (red circles), and

density of ion saturation current Jsat (Jsat=Isat/A, A is 2π x radius x

length of the probe). Right graph: Phase velocities of fluctuations

obtained from Φf (red circles), and Jsat (blue triangles). Horizontal lines

in the right panel show velocities calculated from the gradient of Φfl.

VSL

VSL

Jsatf

Flow measurements with a high temporal resolution - Gundestrup Probe

B,I

1

2

34 5

6

78

Bt, Ip

Top ViewB

Polar diagram of Ion saturation current

Several (eight) segments with a different orientation w.r.t. magnetic field lines

The Gundestrup cauldron (Denmark)

v, v,M ,M ,E , , ,T, n θ//rplflee

Mach probe head

“Mach probe”: TEXTORo main feature:

• 16 plates to measure toroidal and poloidal rotation simultaneously at 2 radial positions

o front pins:• measure n, Vf, E, Er and their fluctuations

o min. radius: r=39 cm (R0=214 cm)

B

r

Radial profiles of (a) the floating potential , (b) the radial electric field , (c) the shear, and (d) the ion saturation current averaged over 4 ms before (open symbols) and during (filled symbols) biasing. The vertical dashed-

dotted line marks the position of the LCFS.

Radial profiles of the toroidal velocity and poloidal velocity averaged

over 4 ms before (open circles) and during (filled circles) biasing

Fast bolometry• Two arrays of the fast AXUV-based bolometers with 16 and 19 channels

were installed in the same poloidal cross-section of CASTOR.The set with unique temporal resolution of 1 µs and spatial resolution of about 1 cm and a very low signal to noise ratio enables the visualization of fine structures in the radiated power profile. This allows to estimate their position and poloidal rotation velocity by using the normalized RMS and cross-correlation techniques, and finally to identify them with turbulent structures in the edge

plasma region measured by Langmuir probes.

• Fast bolometry on CASTOR: Cross-correlation between the fluctuating part of the signal from one bolometer array and all channels of the second array. Left: before biasing (6 – 9.9ms), periodic structures located from 60mm on LFS to -10mm on HFS and from -15mm to -50mm on HFS, frequency 30kHz, velocity 2.3 km/s.

Right: during biasing (10.5 – 14.4ms), biasing voltage +150V, some structures on r = 60mm on LFS and r = -45 on HFS, no periodicity, velocity about 1km/s (on r = 60mm) or 2km/s (on r = 20mm).

Absolute plasma potential, radial electric field and turbulence rotation velocity measurements

in low-density discharges on the T-10 tokamak S.V. Perfilov1, A.V. Melnikov1, L.G. Eliseev1, S.E. Lysenko1, V.A. Mavrin1, R.V. Shurygin1, D.A. Shelukhin1, V.A. Vershkov1, G.N. Tilinin1, S.A. Grashin1, L.I. Krupnik2, A.D. Komarov2, A.S. Kozachek2, A. Kraemer-Flecken3, S.V. Soldatov3, G. Ramos4, C.R. Gutierrez-Tapia5, H. Hegazy6, A. Singh7, J. Zajac8, G. Van Oost9 and M. Gryaznevich10

 1 Nuclear Fusion Institute, RRC "Kurchatov Institute", 123182, Moscow, Russia,2 IPP, NSC “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine3 Institut für Plasmaphysik, Forschungszentrum Jülich, EURATOM Association, Jülich, Germany4 CICATA, Instituto Politécnico Nacional, Mexico5 Instituto Nacional de Investigaciones Nucleares, Mexico6 Egyptor project, Egypt7 University of Saskatchewan, Canada8 Institute of Plasma Physics, Prague, Czech Republic9 Ghent University, Belgium10 EURATOM-UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, UK

• Rational surfaces play a key role in the establishment of ITBs, as has been observed in stellarators, too[Brakel R et al 2002 Nucl.Fusion 42 903].

• However, this does not exclude a possible supporting role of ExB shear in ITB formation near rational surfaces (interaction between neighbouring cells). Recent work on DIII-D and gyrokinetic simulations [Waltz R.E et al 2006 Phys. Plasmas 13 052301 ] hints at possible synergy between ExB shear and effects of rational surfaces.

• Large profile corrugations in electron temperature gradients at lowest-order singular surfaces lead to the buildup of a huge zonal flow ExB shear layer which provides a trigger for the low power ITB observed in DIII-D.

• The direct experimental study of plasma radial electric fields is the key issue to clarify ExB shear stabilization mechanisms.

• The plasma turbulence rotation velocity measurements, compared with ErxBtor drift rotation velocity may explain whether turbulence moves together with the plasma or independently.

HIBP plasma potential and Er profiles in T-10

Trilateral Euregio Cluster