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Can RMP-ELM effect be a Resonance with Zero Shear Rational q in Pedestal?

Can RMP-ELM effect be a Resonance with Zero Shear Rational q in Pedestal?. Both ideal and vacuum response have broad resonance Narrow q resonance reminiscent of the snake on JET: q =2 filament model matched B & Te perturbations

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Can RMP-ELM effect be a Resonance with Zero Shear Rational q in Pedestal?

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  1. Can RMP-ELM effect be a Resonance withZero Shear Rational q in Pedestal? • Both ideal and vacuum response have broad resonance • Narrow q resonance reminiscent of the snake on JET: • q=2 filament model matched B & Te perturbations • Zero shear point readily modified by tiny current, ~1kA Magnetics 1kA model B cfexp Snake when qmin hits 2on JET: q Core Edge Edge Core 4 Model flux • Can this arise with the ELM? • High bootstrap leads to zero or weak shear point, • Flux readily perturbed when this has rational q value • Narrow q window, collisionality, b dependence • May not be snake – point is topology is readily altered, could be ideal-like distortion, or just changed turbulence ECEJET ECEJET SXRJET core edge [Hender et al. 2004]

  2. How to test this on NSTX • Key issue is to have edge bootstrap to reach weak/zero shear • High lithium, high shape, double (???) null, lowest possible density, fiddle with X point location (advice from Rajesh) • Can you still make large ELMs then? • (Harder to reverse shear in ST – but weaken it…?) • Configure RMP coils for optimal edge resonance • N=3 fields I guess! • q95 ramp as usual • Distortion requires least free energy at integer q(then get 3 separate 12/3 filaments, but only 1 needs to grow) • Look for: • Complete ELM suppression • Narrow q resonances in any effect • eg ELM frequency, density pump, turbulence measurements • Finger like structures (what diagnostic?) • Ladders on magnetic spectrogram or SXR • Add a bit of Argon or Neon to light up filaments/fingers on SXR?

  3. Reserve notes on this model… • (note other ideas being kicked around – shielding currents change mod B  particle transport)

  4. Questions / Issues • Addresses: • Collisionality, and possibly shape dependence • q95 resonance • Resolved? • Expect n=3 modes every 1/3rd in q – but non unity q resonances have triple the filament length – need more energy to drive. • Questions: • Do we really get zero shear in RMP suppressed discharges? Is this at rational q in case that work? • How does this effect impact transport • Changes turbulence? • Leads to more localized and stronger shielding response? (momentum balance and particle transport through NTV) • Why density not heat pump out? • Model RMP effect to confirm surface easily perturbed by RMP (as opposed to filament current) • Should it stop rotation or lead to palpable islands?

  5. Thoughts on testing • This mode should work when • There is very good bootstrap: • Good shape, low collisionality  test in weak shape • The effect is more challenging when: • Magnetic shear is naturally more positive  double null • Look for experimental signatures: • Above shape and collisionality dependencies • Suppression when measured qmin hits rational • Measure Jedge better • Look for filament signatures: • Localized impurity emission, SXR camera, 2D TS • Should see resonances in a partial response (ie ELM frequency) at higher order rationals – eg every 1/3rd or every 1/6th step in q95. • If this is a static snake – then need to see if rotation & other profile measurements consistent with that – are they? • N=2 field should have different resonant point in q95

  6. Possible modeling needed • Vacuum and IPEC response to I coil in plasma with zero shear rational q point in pedestal • Do we see flux surface easily perturbed by RMP, in the way that the filaments did on JET? • Consider also shielding effect • is this more likely to be overcome with zero shear point? • Will shielding lead to enhanced rotation or density response at zero shear point? • Transport response • Stellarator transport code needed to compute turbulent response to this (snake-like) and other field perturbations • Does such a code exist?

  7. Summary • This may be a viable model… • Captures many parameter effects • But does not yet explain: • Transport in particles rather than heat • Whether filaments actually form & how we should/should not see them • More to do to ‘model the model’ and get data to test further • Comparing expected response to experimental signatures (on dependencies or structures expected in plasma) may test it. • Model may need to be augmented / bolted into other models to explain some aspects, like transport response.

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