A digital, Nyquist solving, feedback control system for ZTP

Active equilibrium control is complicated on air-core Reversed Field Pinches (RFP) compared to tokamaks by the comparatively high resistance of the RFP plasma. The high resistive impedance of the RFP plasma cannot be predicted precisely, and changes during the course of the discharge, so the precise poloidal field system network response cannot be calculated accurately. Without a precise knowledge of the poloidal circuit response, a feedback transfer function cannot be calculated which has high bandwidth, high gain, and satisfies the Nyquist stability criterion. This problem illustrates the general problem of designing a stable feedback system when the forward transfer function is not well known. Recent technology has made a new approach possible. The introduction of high speed, floating point, Digital Signal Processors (DSP) allows implementing high speed digital filters or transfer function in real time. The authors propose to build the ZTP equilibrium feedback controller around a personal computer and this board. The input to the DSP board will be the signals from the plasma position diagnostic loops. The output will control the equilibrium power supplies. (ZTP2 is a small RFP operated at Los Alamos.)