Vertical position feedback system for PBX-M

The PBX-M (Princeton Beta Experiment-Modified) tokamak produces highly shaped discharges that can be vertically unstable. The PBX-M vertical position control-system hardware is described, results from plasma operation are shown, and results of analyses of the PBX-M device with respect to vertical displacements of the plasma are presented. A rigid displacement model indicates that terms proportional to plasma position and velocity are required in the control function to effectively damp oscillatory motion of the discharge. Simulations from the Princeton Tokamak Simulation Code (TSC) have confirmed the severity of these instabilities. Sawtooth analysis of soft X-ray data is used to independently assess the effectiveness of the system in operation