Localised Neutron Emissionat the edge of high density JET Trace Tritium -

ELMy H-mode plasmas

A.Murari6 on the behalf of G. Bonheure1, S. Popovichev2, L.Bertalot3,S. Conroy4, J. Mlynar5 and JET-EFDA

Contributors1 TEC Trilateral Euregio Cluster – ERM, Brussels, Belgium

2 UKAEA, Culham UK3 ITER, EFDA-Garching, Germany

4 VR, Sweden5 IPP.CR, Prague, Czech Republic

6. Consorzio RFX, ENEA, Padova, Italy

Tenth Meeting of the ITPA Topical Group on Diagnostics Moscow, 10 – 14 April 2006

In close collaboration with TF DT

Overview

• Introduction • Tomography with MFR (Minimum Fisher

Information Regularization) method• 2D Time resolved reconstructions from

trace tritium gas puff experiments• Edge Localised Neutron Sources

(ELNS): strong emission at the edge• Summary and discussion

Introduction: Neutron cameras• Two multi-collimator arrays

with 19 channels available in total , 10 horizontal and 9 vertical

• Channel spacing ~ 15-20 cm near plasma centre

• Time resolution is ~ 10 ms• Detection of 14 MeV neutrons

with NE213 and Bicron detectors

• Detection of 2.5 MeV neutrons with NE213 detectors

Each channel measures the neutron emissivity integrated over its viewing cone

The local neutron emissivity tells us about:

• spatial dependence of the neutron emission;

• alpha particles (and other charged fusion products) spatial distribution;

• spatial distribution of fuel ions e.g tritium diffusionand particle transport in TTE tritium gas puff experiments (G.Bonheure, submitted to NF) ;

Importance of knowing the local neutron emissivity

During TTE “spatial asymmetries” in the neutron emission were observed and reported in:

- 2004: in only RF heated plasma scenarios (Tritium minority scheme)

- 2004: Strong reverse shear plasmas (current holes)

- 2006: report on ELMy - H mode plasmas: localisedEmission has been observed at the edge for all high density ELMy H mode scenarios

KN3 Neutron camera - TTE Profiles Raw data Shot 61372 S1 -8.1.7 T transport in impurity seeded ELMy H mode plasmas

Emissivity centre moves from ch 18 to ch 15

0

1E+14

2E+14

3E+14

4E+14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

KN3 Channels

n m

-2

Horz Vert

Time evolution

T + Ar

Time: from T puff start at 60.10 s up to 60.45.s in 50 ms step

2D inverse methods: Tomography analysis with MFR method

• This method is new for JET and is under development with

expertise from J.Mlynar

• Implementation of neutron camera setup (with and without

finite cone widths)

• Two implementations (Matlab) and C++ (L.Zabeo)

• Fast version generates several hundreds of frames per shot

• No additional assumptions on neutron emission

• Initial validation phase with ‘Phantom tests’ : With a free

boundary, side wings are visible at boundary (10% level) –

disappear when boundary is zero constrained

Tomography analysis – high ne

• DT - 14 MeV neutrons 2-D images (2fps)

• Bicron detectors• A High density ELMy-H

mode with T puff

• At time shortly after T puffing, the neutron cameras detect a high emissivity in upper and low field side vessel region which last > 50 ms !

Tomography analysis (same pulse with NE213 detectors)

• DT - 14 MeV neutrons 2-D images (2fps)

• NE213 detectors• A High density ELMy-H

mode with T puff

• At time shortly after T puffing, the neutron cameras detect a high emissivity in upper and low field side vessel region which last > 50 ms !

DT neutrons emissivity in #61372 @ 20.22s

Results coherent with other tomographic methods (thanks to V. Kiptily).

In particular the validated approach (C.Ingesson) used at JET since many years gives similar results.

Since in this method there is a poloidal smoothing the emission is less concentrated but qualitatively the phenomenon is very similar.

Tomography analysis: low ne

• DT - 14 MeV neutrons 2-D images (15fps)

• Bicron detectors• A Tritium puff case

(ELMy-H mode with low density)

Summary of experimental Evidence • 2D and Time resolved neutron emissivity was reconstructed with the

MFR method

• Sequence of 2D images of neutron emissivity from tritium gas puff experiments were obtained

• In high density ELMy H-mode plasmas (ITER-like, ne > 5 1019), an ‘anomalous shape“ of neutron emissivity is systematically observed in the upper low field side edge region in images of neutron emissivity

• This effect is transient (50-150ms) and associated with tritium puff

• It is detected by both sets of detectors (NE213 and Bicrons)

• Raw data shows good statistics (> 1000 counts)

• Similar images are obtained with raw data. This excludes problems associated with various raw data corrections and tomographic reconstructions (which in any case agree).

What is then causing the anomalous shape ?

Edge Localized Neutron Source (ELNS)Let’s suppose that the source ha the banana shape of the figure

Source shown is only schematic

Up-down symmetry (not uniform intensity but more intense near banana tips)

How will be the reconstruction of neutron camera signal like this ?

Reconstructed image from phantom

Resolution of ELNS is very poor

Conclusions• In high density ELMy H-mode plasmas from tritium

gas puff experiments , ‘transient anomalous shape’ is found in 2D images of neutron emissivity

• It is likely that an ELNS is produced transiently (50-150ms) by puffing T in high density ELMy H-mode plasma and in presence of high power D beam

• The present set-up of neutron camera is not optimized to resolve neutron source in the edge region

• This ELNS seem to be due to a) higher neutral beam power deposition at the edge at high density b) slower T diffusion at high density

Discussion: issues related to extended neutron source near the edge

Further tritium experiments on JET- Adding lines of sight to KN3 is not an option- Use alpha particles losses diagnostics

Vertical neutron camera in ITER:

- Significant neutron emission can occur even at the edge of the plasma

- It would be important to ensure that the design guarantees good coverage of the edge region (requirement of 10% accuracy in neutron profile)

- Is there a risk involved in full T fuelling in ITER in similar operational regimes?

(localised losses, temperature pedestal 1-5 keV)

Edge Localized Neutron Source (ELNS)• Tritium gas puff: Build-up of tritium density for 50-150 ms due to ‘slow’ tritium diffusion

• High density: high number of D beam ions deposited (see figure on the right) in edge region (r > 0.75)

• Significant beam target reactivity ( ~ half central value at r = 0.75), source extends near up to H-mode edge pedestal

•source shape due to trapped beam ions (> 70% in this region)

ELNS is a high density effect

Forward calculation

How will the reconstruction be for neutron camera signal like this ?

Tomography analysis (same pulse with NE213 detectors)

• DT - 14 MeV neutrons 2-D images (15fps)

• NE213 detectors• A Tritium puff case

(ELMy-H mode with low density)

Tomography analysis

• DT - 14 MeV neutrons 2-D images (15fps)

• Bicron detectors• A Tritium puff case

(ELM-H mode with moderate density)

Tomography analysis (same pulse with NE213 detectors)

• DT - 14 MeV neutrons 2-D images (15fps)

• NE213 detectors• A Tritium puff case

(ELMy-H mode with moderate density)