Analysis of toroidal momentum dissipation by non-axisymmetric fields in high beta, low aspect ratio tokamak experiments

Sustained passive stabilization of ideal MHD modes in tokamaks and the spherical torus (ST) can be obtained by maintaining high plasma rotation. However, the rotation has been theoretically predicted and experimentally found to decay, eliminating passive stabilization and impeding sustainment of high beta. Understanding the physical mechanisms leading to plasma momentum dissipation is extremely important to determine how the favorable plasma rotation can be sustained and maximized and how the plasma rotation profile can be controlled in the future tokamaks. The present work first quantitatively examines the agreement between electromagnetic torque theory and localized resonant plasma rotation damping by resistive MHD instabilities. The drag caused by the interaction of the tearing mode with the wall eddy currents can quantitatively explain localized resonant plasma toroidal rotation damping induced by the tearing mode. The remainder of the study focuses on quantitative comparison of theory to the observed global plasma rotation damping by applied non-axisymmetric fields and ideal MHD instabilities. Plasmas with beta below, approaching, and above the calculated no-wall beta limit are created to study the non-resonant plasma toroidal rotation damping physics. At low beta, external applied field perturbations are used to study the braking effects of n = 1 and n = 3 field configurations. At beta close to the no-wall limit, resonant field amplification (RFA)/stabilized RWM effects are added to the model in computing the braking magnetic field. At beta well above the no-wall limit, the unstable RWM damps the plasma rotation strongly, and the theoretically computed mode eigenfunction is used to determine the field. An NBI source term, resonant EM torque, fluid viscous force and neoclassical toroidal viscosity (NTV) torque in both plateau and collisionless 1/nu are included in the model. Inclusion of a broad toroidal and poloidal field spectrum is required for quantitative agreement. Computation in NSTX plasmas shows that the plateau regime NTV formulation is applicable at the plasma edge, while the 1/nu regime NTV torque is dominant at the radial position of peak torque as the ion collision frequency drops below the ion transit frequency. When the effect of toroidally trapped particles is included, NTV theory is a viable model to be used to predict torque balance in tokamak plasmas. The theory and experiment agree to order one when comparing the change in the plasma angular momentum profile to the computed NTV torque profile caused by an applied non-axisymmetric field perturbation, RFA, or RWM destabilization.