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Improvement of long distance ICRF coupling to ELMy H-mode plasmas by gas puffing at JET

Topical Group on Heating and Current Drive Coordinating Committee on ICRH (CCIC) April 10, 2008, Culham. Improvement of long distance ICRF coupling to ELMy H-mode plasmas by gas puffing at JET. I . Monakhov, M.-L. Mayoral and P. Jacquet. Relevance of JET ICRF coupling conditions to ITER

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Improvement of long distance ICRF coupling to ELMy H-mode plasmas by gas puffing at JET

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  1. Topical Group on Heating and Current Drive Coordinating Committee on ICRH (CCIC) April 10, 2008, Culham Improvement of long distance ICRF coupling to ELMy H-mode plasmas by gas puffing at JET I. Monakhov, M.-L. Mayoral and P. Jacquet

  2. Relevance of JET ICRF coupling conditions to ITER Experimental set-up, discharge scenarios, measurement technique etc SOL parameters during gas injection in different scenarios Coupling improvement vs gas injection rate Coupling improvement vs position of gas injection inlets ‘Between-ELM’ and ‘during-ELM’ coupling vs antenna-plasma distance High-power gas-assisted long-distance operations in ELMy H-mode Gas-assisted coupling improvement vs core plasma confinement Issues for discussion Lots of interesting stuff; data not fully analysed yet; some extra time and effort required to finalise the conclusions

  3. Relevance of JET ICRF coupling conditions to ITER JETITER >5 cm (behind limiter)  Antenna strap recess 4.5 cm (behind the first wall) 14 cm (min)  Separatrix to wall/limiter distance 12 cm (min); ~14 cm (average) 2-31019 m-3  Typical density at separatrix  3-41019 m-3 2 cm  Near SOL typical density decay length  2-4 cm weak  Far SOL density profile flattening  noticeable (?)  Cut-off density and spectrum 

  4. Experimental set-up and plasma configurations • ICRF: • f=47 MHz; - phasing; antennae A,B,(C),D • 0.5MW (coupling studies);5-8 MW (high-power shots) • Plasma: • Deuterium (+ few % Hydrogen) • ELMy (type-I) H-mode • 10-14cm ROG (midplane separatrix-limiter gap) • Two ‘high-triangularity’ configurations: • ‘ITER-AT’ • BT=3.1 T, Ip=1.9 MA, dl~0.50, du~0.38, PNBI=15 MW, • =CH • ‘high recycling’, high SOL density, good coupling • ‘HT3-mod’ • BT=1.55 T, Ip=1.5 MA, dl~0.35, du~0.45 PNBI=8 MW, • =2CH • ‘low recycling’, low SOL density, poor coupling

  5. Typical discharge scenarios ‘ITER-AT’, high recycling ‘HT3’, low recycling Injected power Outer gap Gas injection rate Line-averaged density, Oct7 D Oct1, vertical chord B4 coupling resistance

  6. GIM10 GIM12 GIM9 GIM11 Gas inlets: position and magnetic connection to antennas ‘ITER-AT’ configuration • GIM 6 magnetic connection • antenna A – strong • antenna B – strongest • antenna C – marginal • antenna D – no Divertor ring GIMs should affect all antennas in the same way

  7. Gas inlets: position and magnetic connection to antennas • GIM 8: • A – marginal • B – strong • C – marginal • D – no ‘HT-3’ configuration, pulse #70680 • GIM 2: • A – strong • B – marginal • C – no • D – no • GIM 6: • A – marginal • B – strong • C – marginal • D – no GIM 9: A,B,C,D – same

  8. SOL density vs gas injection rate ‘ITER-AT’ (high recycling) ‘HT3’ (low recycling) Line-averaged density at R=3.73 m (near SOL) vertical chord Octant 8 vs gas injection rate • Linear dependence of SOL average density on gas injection rate • Different efficiency of gas injection modules (GIMs) • Higher gas injection efficiency in ITER-AT (high recycling) configuration particularly for divertor ring inlets (GIM9 and GIM10)

  9. SOL density profiles Reliable data available for ‘ITER-AT’ (high recycling) configuration only ‘ITER-AT’ 1.4 1022 el/sec GIM9+GIM10+GIM6 no gas puffing

  10. Coupling analysis in ELMy plasmas Not straightforward: what is representative Rc value – average, max or min? • ‘Average’ – not much sense due to short and strong ELM perturbations; sensitive to ELM frequency • ‘Maximum’ – relevant mainly to ELM tolerance studies • ‘Between-ELM’ – most adequate for conservative estimates of RF system power injection capabilities Our approach: use smoothed running maximum and minimum in ~20ms sliding window of fast (10kHz) Rc measurement to get temporal behaviour of ‘ELM-maximum’ and ‘between-ELM’ coupling

  11. ‘Between-ELM’ coupling improvement during gas injection Up to 5-fold Rc increase in both scenarios (depending on GIMs and antennas)

  12. Coupling improvement vs GIM location ‘ITER-AT’ configuration Introduction of gas injection from GIM6 clearly have stronger effect on antennas A and B (magnetically connected to GIM6), as compared with toroidally symmetrical injection from divertor ring (GIM9+GIM10) However, antenna D also benefits from GIM6 (no magnetic connection!) in line with ‘global’ SOL density increase during gas injection. Conclusion: there are two contributing factors - ’global’ and ‘local’ - to ICRF antenna coupling improvement

  13. Coupling improvement during gas injection ‘HT-3’ configuration • Coupling improves for all antennas including those which are not magnetically connected to GIMs • Biggest improvement for antennas magnetically connected to GIMs • ~Linear dependence on injection rate for all antennas

  14. Coupling improvement during gas injection ‘HT-3’ configuration

  15. Dependence on antenna-plasma distance ‘HT-3’ low recycling , pulse #70683 • In low recycling configuration both ‘between-ELM’ and ‘ELM-maximum’ coupling ‘linearly’ decrease with ROG • ‘ELM-maximum’ (i.e. coupling perturbation during ELMs) seems to be decreasing faster Antenna B GIM6 ~2 1022 l/sec ‘ELM-maximum’ ‘Between-ELMs’

  16. Dependence on antenna-plasma distance ‘ITER-AT’ high recycling, four pulses with different gas injection rates Gas ‘ELM-maximum’ ‘Between-ELMs’ • ‘ELM-maximum’ coupling linearly decrease with ROG • ‘between-ELM’ coupling looses dependence on ROG during strong gas puff ?

  17. High-power long-distance operations in ELMy H-mode ‘ITER-AT’: up to 8 MW ICRF power injected (trip-free) over 14cm ROG ~1.8 1022 el/s from GIMs 6, 9 and 10

  18. High-power long-distance operations in ELMy H-mode ‘HT3’: up to 5 MW ICRF power injected (trip-free) over 14cm ROG 2 1022 el/s from GIM 8

  19. ICRF coupling to ELMy H-mode plasma over ITER-relevant antenna-plasma distances studied in two scenarios with distinctly different SOL plasma behaviour – ‘high-’ and ‘low- recycling regimes’ Linear decrease of ‘ELM-maximum’ and ‘between-ELM’ coupling with increasing antenna-plasma gap was recorded, the absolute values of coupling resistance being strongly dependant on the plasma scenario Gas injection was demonstrated as a powerful tool for ‘between-ELM’ antenna coupling improvement with up to 5-fold coupling resistance increase in both plasma scenarios The impact of gas injection on coupling of individual antennas around the torus has both ‘global’ and ‘local’ component, the latter being related to magnetic connection of the gas inlets to antennas Gas injection allowed to inject up to 8MW ICRF power (trip-free) into ELMy plasma over 14cm plasma-limiter gap (~19cm separatrix-strap distance) Conclusions

  20. Supplementary slides

  21. Confinement vs gas injection ‘ITER-AT’ high recycling, four pulses with different gas injection rates

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