Resistive Wall Mode Stabilization and Plasma Rotation Damping Considerations for Maintaining High Beta Plasma Discharges in NSTX

Maintaining steady fusion power output at high plasma beta is an important goal for future burning plasmas such as in ITER advanced scenario operation and a fusion nuclear science facility. Research on the National Spherical Torus Experiment (NSTX) is investigating stability and control physics to maintain steady high plasma normalized beta with minimal fluctuation. Resistive wall mode (RWM) instability is observed at relatively high rotation levels. Analysis including kinetic effects using the MISK code shows a region of reduced stability for marginally stable experimental plasmas caused by the rotation profile falling between stabilizing ion precession drift and bounce/transit resonances. Energetic particle (EP) effects are stabilizing but weaker than in tokamaks due to a reduced EP population in the outer plasma. Calculations for ITER show that alpha particles are required to stabilize the RWM at anticipated rotation levels for normalized beta of 3. Combined RWM and new beta feedback control capability were used to generate high pulse-averaged normalized beta with low fluctuation. Non-resonant braking by applied 3-D fields was used to alter plasma rotation compatibly with beta feedback. A newly implemented RWM state space controller produced long pulse, high normalized beta plasmas at low internal inductance. Neoclassical toroidal viscosity (NTV) torque by applied 3-D fields could be used to actuate rotation control and avoid rotation profiles unfavorable for RWM stability. As the ExB frequency is reduced, the NTV torque is expected to increase as collisionality decreases, and maximize when it falls below the ∇B drift frequency (superbanana plateau regime). Increased non-resonant braking was observed at constant applied field and normalized beta in experiments when rotation and ExB frequency were reduced to low values. The RWM multi-mode spectrum is computed in high beta plasmas using the multi-mode VALEN code. The computed RWM growth rate for instabilities and natural mode rotation for stabilized modes agrees with experiment. The computed multi-mode RWM spectrum shows significant amplitude in low-order ideal eigenfunctions other than the least-stable eigenfunction of single-mode analysis.