Exploiting 3-D Fields to Improve the Efficacy Exploiting 3-D Fields to Improve the Efficacy of ELM Pace Making with Vertical Jogsof ELM Pace Making with Vertical Jogs

J. M. Canik, S.P. Gerhardt, J-W. Ahn

Advanced Scenario and Control TSGITER Support

2010 NSTX Research ForumPPPL

Dec 1-3, 2009

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Background and XP Goals

• Background– ELM pacing via vertical jogs was demonstrated in 2009.

• Results submitted to Nuclear Fusion as part of H-mode workshop.

– In a sense, it didn’t work as well as 3-D field triggering.• Lithium conditioned discharge didn’t ELM despite large jogs.• 3-D field pulses (~2 kA, 4 msec, ~50 Hz) showed clear triggering.

– Synergism has been observed between 3-D fields and “kicks” in JET.• ELMs are triggered at kick-amplitude and EFCC amplitude which are insufficient to trigger

ELMs alone.

– RMP coils and internal radial field coils mean that either (both) technique(s) may be available for ITER

• XP Goal: Improve the efficacy of vertical jogs in NSTX through the use of small 3-D fields.– Test if the “triggering efficiency” of jogs can be improved with 3-D fields.

• Test the frequency range of applicability

– Collect good MPTS data for pre-/post- ELM profile analysis. • Understand profile effects (ELITE?)

– Collect IR camera data.• Do peak heat fluxed decrease with increasing ELM energy?

NSTXNSTX NSTX RF 10– RMP + VJ Pacing (Canik) Month day, 20**

ELMs can be triggered with vertical jogs or 3D fields in NSTX

3

900 A EFC Current

950 A EFC Current

1300 A EFC Current

1000 A EFC Current

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Results from JET … and for ITERSee APS invited talk by E. de la Luna

• Physics of the triggering is not clear.• Density reduction is larger with kicks.• Rotation damping is much smaller with kicks alone.• “Simultaneous application of EFCC perturbation and kicks, both

below threshold, result in fELM control & reduced divertor peak power.”

• Average ELM energy is reduced by ~3 compared to natural ELMs.

• Peak heat flux is not reduced like 1/fELM.

– Wetted area is reduced for small ELMs

• Very difficult to fuel these discharges and maintain good confinement.• Could be useful for ITER

– During current ramp when q95 is changing (RMP may not work).

– Expected that 25 Hz kicks available with z=5cm and 50 Hz kicks available with z=2.5 cm (within limits of VS power supply).

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Options Exist for Timing Between Jogs and n=3 Field Application in NSTX

Jogs

n=3

Jogs

n=3

Jogs

n=3

1: Jogs with no n=3 fields

2: Jogs with Constant n=3 fields

3: Jogs with Synchronous Pulsed n=3 fields

• Use n=3 fields that alone are insufficient to generate ELMs.

• Minimize the rotation damping from the n=3.• Minimize vertical position oscillations.• Complicated waveforms.

• Was done in 2009.

• Pacing observed, though required large jogs.

• Similar to JET case.

• May still provide too much n=3 field.

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Tentative Run Plan (Always assuming success on previous step)

• Establish a discharge with ELM pacing through jogs. (5 shots)– 134318 is maybe a template (change IP?)

• Reduce jog amplitude until triggering no longer observed. (2 shots)• Add small n=3 fields to recover pacing. (10 shots)

– Scan amplitude of n=3 field, but don’t go so high that the braking is overwhelming.

• Test upward jogs (maybe) (2 shots)• Split n=3 fields into pulses to reduce average n=3 field. (10 shots)

– Scan amplitude or width of the pulses.

• Pick configuration single configuration and scan MPTS timing to resolve the dynamics. (5 shots)

34 shots= 1 Good Day

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Critical Issues & Diagnostics Addressed in the Proposed Plan

• Is it possible to reduce the jog amplitude when 3-D fields are applied?– Scan n=3 strength and jog amplitude.

• How small can the 3-D fields be and still help with the triggering?– Try to minimize both perturbations.

• Can pulse 3-D fields do the job?– For a configuration that works with steady n=3, try pulses.

• What is the frequency range over which ELMs can be paced this way?– If successful above, increase the perturbation frequency (and maybe

amplitude) to find limits.

• How do the ELM energy, divertor load, peak power change as a function of ELM frequency?– Collect IRTV data throughout.– Don’t forget to take the reference shots.

• How do profiles change through this cycle?– Stagger the MPTS timing to catch different parts of the ELM cycle.

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Techniques Have Been Developed For Generating Vertical Jogs

• Synchronous jogs in drsep and zmaxis used to move plasma up and down.

• ELM pacing most successful for jogs near large-amplitude limit.

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ELM Pacing With Vertical Jogs is Possible in NSTX

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ELMs Can Be Triggered By 3-D Fields in NSTX

• Threshold coil current for ELM-triggering is ~950 A ->ΔB/B = 610-3

– No triggering at 900 A (natural ELMs start at ~0.5s in control discharge)

– Intermittent ELMs at 950 and 1000 A

• ELM frequency appears to increase with n=3 field magnitude– ELMs become more regular– Trend clouded by tendency of

plasma to lock high currents-too much braking.

– Can significantly smaller n=3 fields be used to increase the efficacy of jogs.

900 A EFC Current

950 A EFC Current

1300 A EFC Current

1000 A EFC Current

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Other questions identified from 2009 XP-945

• Upward jogs?– Need equilibrium drsep sufficiently negative to avoid transiently biasing

upwards?

• drsep effects of triggering efficacy at fixed jog size?– Is jogging more efficient at making ELMs when drsep becomes more negative?– Some information from last year indicates as much.

• Try in a lower-triangularity equilibria. – closer to JET, AUG cases.

• Improved profiles before/after an ELM.– Stagger jogs w/ respect to the MPTS timing.– Data for ELITE?

• Explicit voltage perturbations? (PCS modification).– Seems unnecessary?

NSTXNSTX NSTX RF 10– RMP + VJ Pacing (Canik) Month day, 20** 12

Other questions identified from 2009 XP-945

• Upward jogs?– Need equilibrium drsep sufficiently negative to avoid transiently biasing

upwards?

• drsep effects of triggering efficacy at fixed jog size?– Is jogging more efficient at making ELMs when drsep becomes more negative?– Some information from last year indicates as much.

• Try in a lower-triangularity equilibria. – closer to JET, AUG cases.

• Improved profiles before/after an ELM.– Stagger jogs w/ respect to the MPTS timing.– Data for ELITE?

• Explicit voltage perturbations? (PCS modification).– Seems unnecessary?

Keep only these 2.