Initial implementation of a multivariable plasma shape and position controller on the DIII-D tokamak

Abstract

A model-based multivariable controller for plasma shape control has been successfully implemented on the DIII-D tokamak. Good steady-state control of the plasma boundary shape and X-point position was demonstrated in lower single-null ohmic plasmas over several seconds of several discharges. Dynamic control for programmed rapid plasma shape variation showed significant lags in response (resulting from design choices and model error, and expected from simulation), but was robustly stable for all degrees of freedom explored. The control design was based on a linear plasma response model derived from fundamental physics assumptions, which was extensively validated against DIII-D experimental data. This physics-based model is readily extendable to next-generation device designs and to new operating regimes of existing devices. Controllers produced with robust control design methods were tested and improved using results of closed loop simulations. A comprehensive simulation of the tokamak plant including plasma response, power supplies, and coil circuit configuration allowed verification of the controller implementation in the plasma control system. This comprehensive simulation can be regulated by the plasma control system computer in exactly the same way the plasma control system controls the actual tokamak.