Real Time Control System for the National Spherical Torus Experiment (NSTX)

The NSTX is a national facility for the study of plasma confinement, heating, and current drive in a low aspect ratio, spherical torus (ST) configuration. The ST configuration is an alternate magnetic confinement concept which is characterized by high {beta} (ratio plasma pressure to magnetic field pressure) and low toroidal field compared to conventional tokamaks, and could provide a pathway to the realization of a practical fusion power source. The NSTX depends on a real time, high speed, synchronous, and deterministic control system acting on a system of thyristor rectifier power supplies to (1) establish the initial magnetic field configuration; (2) initiate plasma within the vacuum vessel; (3) inductively drive plasma current; and (4) control plasma position and shape. For the initial ''day 0'' 1st plasma operations (Feb. 1999), the system was limited to closed loop proportional-integral current control of the power supplies based on preprogrammed reference waveforms. For the next ''day 1'' phase of operations beginning mid-summer 1999 the loop will be closed on plasma current and position. The ultimate ''day 2'' system is envisioned to include real time reconstruction of the plasma internal current distribution so that control can be exercised over internal plasma parameters such as current and pressure profile. This paper addresses the day 1 system, with emphasis on the magnet power supply control. Companion papers address plasma control