Resistive wall mode stabilization of high-β plasmas in the National Spherical Torus Experiment

The resistive wall mode (RWM) poses a limit to the maximum β that can be sustained in magnetic fusion experiments. RWM stabilization physics at low aspect ratio is studied in high-β National Spherical Torus Experiment (NSTX) [M. Ono, S. M. Kaye, Y.-K. M. Peng et al., Nucl. Fusion 40, 557 (2000)] plasmas (βt up to 39%; βN up to 6.8) to understand and alleviate this constraint. Plasmas with increased q in NSTX have been maintained with β above the computed ideal no-wall β limit for more than 20 wall times with no signs of RWM growth in cases where toroidal rotation ωϕ>ωA∕4q2 across the entire plasma cross section. Plasmas that violate this stability criterion can suffer a RWM induced collapse within a few wall times. This critical rotation profile for stabilization is in agreement with drift-kinetic theory applied to low frequency magnetohydrodynamics modes [A. Bondeson and M. S. Chu, Phys. Plasmas 3, 3013 (1996)]. A toroidally symmetric array of internal sensors has been used to observe n=1–3 RWMs in NSTX. This array consists of Bp and Br sensors both above and below the midplane at 12 toroidal locations instrumented to detect toroidal mode numbers of n=1–3. RWM perturbations exceeding 30G have been measured with mode growth rates on the order of 5ms. Small modes (δB<10G) which cause minor drops in β, with growth rates ∼1500s−1 have been observed when βN exceeds 6. Resonant field amplification of an externally applied error field by the stable RWM has been observed.