Improved Confinement in JET High Plasmas with an ITER-like Wall - - PowerPoint PPT Presentation

1 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Improved Confinement in JET High  Plasmas with an ITER-like Wall

Clive Challis

Culham Centre for Fusion Energy Paper EX/9-3 IAEA FEC, St Petersburg 13-18 October 2014

2 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Acknowledgements

Co-authors:

• E. Belonohy1,2, M. Beurskens3, P. Buratti4, P. Drewelow2, L. Frassinetti5, J. Garcia6,
• C. Giroud3, N. Hawkes3, J. Hobirk2, E. Joffrin6, D. Keeling3, D.B. King3, C. Maggi2,
• J. Mailloux3, C. Marchetto7, D. McDonald8, I. Nunes9, G. Pucella4, S. Saarelma3,
• S. Sharapov3, J. Simpson3 and JET Contributors*

1EFDA CSU, Culham Science Centre, Abingdon, OX14 3DB, UK 2Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany 3CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK 4Unità Tecnica Fusione, C.R. ENEA Frascati, CP65, 00044 Frascati, Italy 5VR, Fusion Plasma Physics, KTH, SE-10044 Stockholm, Sweden 6CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 7Instituto di Fisica del Plasma, CNR, 20125 Milano, Italy 8EFDA CSU Garching, D-85748 Garching, Germany 9Instituto de Plasmas e Fusão Nuclear, IST, Universidade de Lisboa, Portugal

*See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia

3 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Outline of talk

• Comparison of JET power scans

with different wall materials:

– Carbon wall (C-wall) – ITER-like wall with Be main chamber & W divertor (ILW)

…and different plasma shapes:

– Low triangularity (low ) – High triangularity (high )

• Identification of key factors that

contribute to power scaling of confinement

• Implications for prediction of plasma

performance Power scan with JET ITER-like wall at constant: IP, B, Meff, ne, a, R, 

IPB98(y,2) scaling experiment

4 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Overview of power scan experiments

plasma shapes

high  (U~0.36) low  (U~0.15)

Scan IP(MA) B(T) ILW high  1.4 1.7 ILW low  1.4 1.7 C-wall high  1.4 1.7 C-wall low  1.7 2.0

plasma current & field

• H-mode (type I ELMs)
• q95~3.9
• current ‘overshoot’ for access

to high 

• No performance degrading

MHD at analysis time

• ‘engineering’ parameters

constant in each scan

analysis time

plasma conditions time evolution q-profiles

5 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Weak power degradation of confinement

E~P-0.25 E~P-0.65 E~P-0.30 E~P-0.35

• All scans show weak power degradation of confinement except C-wall high 

IPB98(y,2) (E~P-0.69)

1.4MA/1.7T 1.4MA/1.7T 1.7MA/2.0T

C-wall high  ILW high  ILW low  C-wall low 

6 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Wall recycling affected C-wall scaling

• Increasing neutral deuterium

population in vessel (e.g. by gas injection) can degrade confinement

• Close wall proximity of high  plasmas

led to collinear increase in neutrals with power for C-wall (Joffrin EPS 2014)

• Wall recycling appears to affect confinement scaling in C-wall high  power scan

ILW data may be overestimated due to reflections

7 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Pedestal & core play role in scaling

• Strong increase in plasma thermal energy with power in

ILW scans due to increase in:

– Pedestal pressure (strongest for high ) – Core pressure peaking (strongest for low )

Wped/P~P-0.61 Wped/P~P-0.48 Wcore/P~P-0.12 Wcore/P~P-0.11

radius pressure Wpedestal Wcore

8 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Pedestal consistent with stability analysis

• Increase in pedestal pressure with power consistent with edge stability modelling of

peeling-ballooning stability boundary (Maggi this conference EX/3-3)

• Pedestal parameters

before ELM

• Pedestal temperature

varied at fixed width to find peeling-ballooning stability boundary using ELITE

9 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Density peaking correlated with collisionality

• Density peaking behaviour varies between power scans:

– ILW high  : weak increase with power – ILW low  : strong increase with power (compensates weak pedestal increase) – Correlated with collisionality for C-wall and ILW power scans (as previously seen on AUG & JET – Angioni Nucl Fusion 2007)

10 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Core Ti/Te increases with power

• Electron temperature peaking constant in ILW power scans
• Increase in core Ti/T

e with power contributes to increase in plasma stored energy

11 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014 w/o electromagnetic, ExB & fast ion effects full simulation

Core Ti/Te modelled

Pabs~6MW Pabs~12MW

measurements

• Quasi-linear TGLF simulations capture increase in Ti/T

e with power and shows

importance of:

– Ion heating (Pi/Pe larger at high power) – Electromagnetic effects & fast ion pressure (reduces ITG transport at high power)

• Non-linear simulations may be needed to fully reproduce T peaking (Citrin PRL 2013)

ILW low 

12 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Positive feedback on  leads to weak power degradation

temperature ‘stiffness’ classical fast-ion collisions increase fast-ion slowing-down time ITG physics increase temperature peaking peeling- ballooning paradigm increase pedestal temperature increase fast ion pressure increase NBI power increase beta increase core temperature increase fast-ion heating to ions classical fast-ion collisions particle pinch increase density peaking reduce collisionality classical collisions decouple ions & electrons See mechanism described by Garcia this conference TH/5-2

13 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Neutrals affect confinement scaling

ILW low  low gas

(~0.3x1022/s)

medium gas

(~0.9x1022/s)

high gas

(~1.8x1022/s)

• Increasing neutrals by D gas injection

helps control W contamination

• Strong power degradation of

confinement at N,th<1.5 & high D gas

• Weak increase in pedestal pressure

with power at high gas not consistent with peeling-ballooning analysis (Maggi this conference EX/3-3) ILW low  - high gas

See also Cesario Plasma Phys Control Fusion 2013

14 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Consistency with wider dataset

• ILW database shows:

– Strong power degradation of confinement at high D gas & low  with H98~0.7-1.0 (‘baseline’ plasmas) – Weak power degradation of confinement at low D gas & high  with H98>1 (‘hybrid’ plasmas)

• Behaviour reproduced in power

scans, suggesting that positive feedback loops on  may be key factor in confinement improvement of JET ‘hybrid’ plasmas H98=1 fit to N<2 data

JET-ILW dataset from Beurskens Nucl Fusion 2014

15 Challis EX/9-3 IAEA FEC St Petersburg 13-18 Oct 2014

Conclusions

• Weak power degradation of confinement observed in JET high  plasmas

at low & high , associated with: – Low neutral particle density in vacuum vessel (more easily achieved with ILW than C-wall at high ) – Multiple positive feedback loops affecting 

• Strong power degradation seen with high neutral deuterium density in vacuum vessel

(through gas injection or wall recycling) – Pedestal fails to reach modelled pedestal limit at high power

• Prediction of plasma performance requires:

– Self-consistent modelling of  feedback loops – Understanding of effect of neutrals on pedestal