The resistive wall mode and feedback control physics design in NSTX

The National Spherical Torus Experiment has been designed to investigate the physics of ST global mode stabilization. NSTX has R=0.86 m, a<0.7 m, B0<0.45 T, and Ip<1.5 MA. Maximum toroidal and normalized beta have exceeded 31% and 5.5, respectively, with normalized beta reaching 7.4 li. Sudden beta collapses have been correlated to violation of the n=1 ideal MHD beta limit using time-evolving EFIT reconstructions of experimental discharges. The resistive wall mode (RWM) was observed in experiments maximizing plasma coupling to the stabilizing conducting plates. A large rotation damping rate of -300 kHz/s was observed in RWM discharges in contrast to -75 kHz/s for plasmas exhibiting n=2 and 3 rotating modes. The computed RWM perturbed field structure from experimental equilibria was input to the VALEN code and the computed n=1 mode growth time of 4.6 ms agrees well with the experimental value of 5 ms. Increased beta improves wall coupling, and passive stabilization of an equilibrium with normalized beta of 5.2 and pressure peaking factor of 2.2 yields a growth time of 23.5 ms. This plasma would be completely stabilized by a proposed global instability feedback system. (author)