Author: mcohen
Last saved by: ksilber
Company: PPPL

Title: Global MHD and ELM stability dependence on edge current, n*q-ped, and edge collisionality
Group: Macroscopic Stability
Submitted By: S.A. Sabbagh, T.E. Evans, L.E. Zakharov
Affiliation: Columbia U, GA, PPPL
Email: sabbagh@pppl.gov
Milestone: R10-1, R10-3, MDC-2, PEP-23
ITPA: R10-1, R10-3, MDC-2, PEP-23

Brief Description:
Global kink-ballooning, RWM, and ELM instabilities have been studied using MHD models. Common physics elements of MHD stability theory - edge current and profile in connection with primary MHD resonance, and the combined effect of edge collisionality on thermoelectric currents outside the pedestal region - will be varied to test theoretically expected favorable and unfavorable stability conditions for these modes in experiment. This understanding is critical for key ST-CTF and ITER goals including sustained high normalized beta operation with minimal fluctuation, and ELM mitigation.

Background:
Improved stability of key MHD modes including kink-ballooning, RWM, and ELMs appears to have common elements based on past experimental and theoretical findings. Edge current and profile strongly affects low-n kinks and RWMs. Reduced, or negative edge current was shown to be strongly stabilizing in experiments (Sabbagh, PoP 1991). Proximity to the MHD resonance m = n*qa is also expected to strongly affect kink-ballooning modes and RWMs (Bogomolov, NF 1987), and this resonance plays a key role in ELM mitigation (Evans, NF, 2008). Zakharov has postulated that field lines become stochastic outside of the H-mode pedestal, which suggests that the correct q value to use for the resonance condition might be q just outside the pedestal. Currents in the edge region, including thermoelectric currents set up on edge field lines (Staebler, NF 1989; Evans, JNM 2009) affect ELM stability. These currents are sensitive to the plasma collisionality outside of the H-mode pedestal, applied resonant magnetic fields, and edge field line geometry. Thermal gradients that can form along the edge field lines at high collisionality give rise to the current and are thought to destabilize ELMs. At low collisionality, these currents weaken and ELMs are expected to stabilize. In this regime of low density and high temperature, ELMs may remain completely stabilized as pressure increases, if the potential onset of ballooning modes yields a soft, enhanced-transport limit to the pressure gradient. The sensitivity of these effects to MHD stability have yet to be examined in combination in ST plasmas. Favorable combinations of these effects may lead to significant improvements in MHD mode stability, including ELM mitigation.

Experimental Approach/Plan:
Target plasmas near marginal stability to kink-ballooning, RWM, and ELMs will be exposed to variations of the key parameters mentioned. n*q-ped will be scanned through integer values by both increasing and decreasing it (which theoretically should lead to different kink, RWM stability). Slow Ip and Bt ramps (both positive and negative) will be used to change edge current and profile with fixed, or nearly fixed n*q-ped to best maintain a resonance condition. Current in the edge region, including thermoelectric currents, will be changed by (i) changing edge collisionality by running on/off the LLD, (ii) changing edge collisionality by applying external RMPs (favorable applied field spectra from XP818 can be used), (iii) changing the geometry of the edge field lines by scanning LSN, DND, and USN configurations - both the field line geometry and plasma collisionality can be changed, as the plasma will move off the LLD.

Machine Time Requested (run days):
1.5

Minimum Useful Machine Time (run days):
1

Special Requirements

Operations/Development:
Ability to run on/off the LLD; use of n = 3 applied field, and the possible use of the favorable n = â€œ2 + 3â€� configuration used in XP818.

Diagnostics:
RWM sensors, divertor IR camera, halo current detectors, USXR, etc.

Analysis:
EFIT with MSE, DCON, VALEN, MISK, TRIP3D, IPEC, etc.