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Current rampdown at JET experimental results and modelling tasks

Current rampdown at JET experimental results and modelling tasks. I Nunes G Saibene G Sips. Preparing ramp-down for ITER. Rampdown phase in ITER must: Terminate the burn including H-L transition Avoid flux consumption (mainly from the CS coils) Avoid vertical instabilities

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Current rampdown at JET experimental results and modelling tasks

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  1. Current rampdown at JETexperimental results and modelling tasks I Nunes G Saibene G Sips

  2. Preparing ramp-down for ITER • Rampdown phase in ITER must: • Terminate the burn including H-L transition • Avoid flux consumption (mainly from the CS coils) • Avoid vertical instabilities • Remain in X-point as long as possible until limiter plasmas are ok (few MA) • Maintain particle and power handling • Avoid disruptions (large heat loads, hence damage to PFCs) • Experiments at JET • Current ramp-down rate • Heating • Plasma shape I Nunes - ISM: ITER ramp-down simulation at JET

  3. Varying Ip ramp-rate Vary current ramp rate from 0.07MA/s to 0.5MA/s (~300s to ~80s in ITER) I Nunes - ISM: ITER ramp-down simulation at JET

  4. Ramp-down summary • Slowest ramps (0.07 and 0.14MA/s) • Transition type I ELMy H-mode/L-mode • bp and li approx. constant before transition • Small variation of bpand li after transition • Faster ramp (0.28 and 0.5MA/s) • transition from type I/III ELMy H-mode before the end of additional heating • Increase of li and bp before the end of additional heating due to transition to a lower confinement regime • Larger variation of li and bp 0.1s after heating switch-off *Times relative to heating switch-off **Times relative to 0.1s I Nunes - ISM: ITER ramp-down simulation at JET

  5. li evolution during the ramp-down phase • Evolution of li during the ramp-down phase (with heating) for all 4 cases as function of dIp/dt • Evolution is within the projected ITER VS capability • Typically ITER should be in H-mode up to ~0.5IpFT and after in L-mode/ohmic I Nunes - ISM: ITER ramp-down simulation at JET

  6. What if H-mode is lost during flat top? • H-L transition at the end of the current flat top… • dIp/dt=0.14MA/s • WDIA = 4.6MJ • DW(0.1s) = 3.655MJ • DW(1.8s) = 1.386MJ • Note: Elongation decreases and minor radius increases during ramp-down #72207 NBI+ICRH Ip bp H98y Ha WDIA li q95 a k I Nunes - ISM: ITER ramp-down simulation at JET

  7. Marginal H-mode during ramp-down #72244 • dIp/dt=0.14MA/s • Note: Very close to L-H threshold  large single ELMS  could cause large disruptions NBI+ICRH Ip bp H98y Ha WDIA li q95 a k I Nunes - ISM: ITER ramp-down simulation at JET

  8. Can we avoid sudden W drop? #74405 • H-L transition during ramp-down with slow turn-off of additional heating (simulation of soft stop at JET or decay of Pa) • dIp/dt=0.5MA/s • Large variation of plasma size before end of heating • Note: TOG increased by 10cm NBI+ICRH Ip bp H98y Ha WDIA li q95 a k I Nunes - ISM: ITER ramp-down simulation at JET

  9. Decreasing k • Possible to strongly decrease k (small k more stable plasma) • Slowest increase of plasma inductance for faster and larger reduction of k although not lower enough in ohmic plasmas ohmic I Nunes - ISM: ITER ramp-down simulation at JET

  10. Ohmic ramp-down using k reduction • Same reduction of elongation with slower Ip rate • Good control of li and almost constant flux In JET it is possible to control li and flux if H-mode is lost during ramp down (in X-point) without using flux I Nunes - ISM: ITER ramp-down simulation at JET

  11. H-mode ramp-down using k reduction #72463 • Type I/type III/dithering ELMs • dIp/dt=0.5MA/s • Plasma shrinking TOG=60cm • Avoid fast transition of WDIA and li • TOG increased by 70 cm NBI+ICRH Ip bp H98y Ha WDIA li q95 a k I Nunes - ISM: ITER ramp-down simulation at JET

  12. Summary • Safe landing: H-mode + moderate current ramp (JET examples 0.07MA/s and 0.14MA/s) • H-L transition during ramp-down: moderate dIp/dt shows better control of li and flux consumption doesn’t seem to be an issue in these examples Future work: • How to extrapolate to ITER requires more work and simulations  accurate contribution to total flux from resistive, external and inductive flux • Transport modelling (ITER profiles, H-L transition, etc.) (V Parail and I Voitsekhovitch)… I Nunes - ISM: ITER ramp-down simulation at JET

  13. Modelling work • Select which ramp-down cases we would like to have: • H-mode • Ohmic • Different ramp-down rates • Different heating levels • What is important to model? To match the entire ramp-down (1 or 2 cases)? Or concentrate on the typical behaviour such as • li excursion • Stored energy excursion • Effect of elongation/minor radius (ohmic) • Combination of the above • How to model the H-L transitions? • Which JET pulses? The underlined cases? I Nunes - ISM: ITER ramp-down simulation at JET

  14. What if H-mode is lost during flat top? • H-L transition at the end of the current flat top… • dIp/dt=0.14MA/s • WDIA = 4.6MJ • DW(0.1s) = 3.655MJ • DW(1.8s) = 1.386MJ • Note: Elongation decreases and minor radius increases during ramp-down #72207 NBI+ICRH Ip bp H98y Ha WDIA li q95 a k I Nunes - ISM: ITER ramp-down simulation at JET

  15. Ohmic ramp-down using k reduction • Same reduction of elongation with slower Ip rate • Good control of li and almost constant flux The red case!! I Nunes - ISM: ITER ramp-down simulation at JET

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