Maximum Controllable Displacement: Experimental Results from NSTX, and Comparison with Theoretical Results and Other Tokamaks

Optimization of the ITER PF design requires information on the limits to axisymmetric stability control performance, as well as the safe margins in operation experienced in present-day devices with configurations relevant to ITER. Maximum controllable displacement, $\Delta Z_{max}$, the largest displacement for which the plasma is still controllable, was recently proposed as a performance metric for the power supplies of the vertical stabilization loop on ITER. $\Delta Z_{max}$ is obtained. Experiments, which turn off the vertical position control and then turn it back on after the plasma has moved a given distance, are performed in order to measure $\Delta Z_{max}$. The experimental measurements of $\Delta Z_{max}$ for the NSTX under various conditions relevant to ITER are presented. Employing a linear magnetohydrodynamics model of the plasma, the theoretical $\Delta Z_{max}$ is obtained. The results are compared to similar measurements from other Tokamaks (DIII-D, C-Mod) to establish model accuracies and validate design predictions for ITER. This work was supported by DoE contract No. DE-AC02-76CH03073.