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The National Spherical Toms Experiment (NSTX) has as one of its primary goals the demonstration of the attractiveness of the spherical toms concept as a fusion power plant. Central to this goal is the achievement of high plasma [Beta](=2[[Mu].sub.0] [is less than] p [is greater than]/[B/sup.2] a measure of the efficiency of a magnetic plasma confinement system). It has been demonstrated both theoretically and experimentally that the maximum achievable [Beta] is a strong function of both local and global plasma parameters. It is therefore important to optimize control of the plasma. To this end a phased development plan for digital plasma control on NSTX is presented. The relative level of sophistication of the control system software and hardware will be increased according to the demands of the experimental program in a three phase plan. During Day 0 (first plasma), a simple coil current control algorithm will initiate plasma operations. During the second phase (Day 1) of plasma operations the control system will continue to use the preprogrammed algorithm to initiate plasma breakdown but will then change over to a rudimentary plasma control scheme based on linear combinations of measured plasma fields and fluxes. The third phase of NSTX plasma control system development will utilize the rtEFIT code, first used on DIII-D, to determine, in real-time, the full plasma equilibrium by inverting the Grad-Shafranov equation. The details of the development plan, including a description of the proposed hardware will be presented.

The National Spherical Toms Experiment (NSTX) is a proof of scientific principle experiment as a magnetic fusion containment device. A primary goal of NSTX operations is control of the plasma current, position and shape in real time for a wide range of plasma pressure and current density profiles. In order to employ the best calculation of the plasma current, position and shape, it is planned to implement the equilibrium analysis code, EFIT[1], in real-time, RTEFIT[2]. EFIT inverts the Grad-Shafranov equation and performs a least squares fit to the magnetics data. EFIT and RTEFIT are also capable of providing the plasma current profile and the plasma pressure profile from analysis of diagnostic data. The calculation time for RTEFIT using the present NSTX control computer system is comparable to the expected energy confinement time on NSTX and is thus slower than desired. A computer upgrade based upon the new G4 (MPC7400) processors will permit the RTEFIT calculation loop to complete in about 2 ms. The presence of the passive plates further complicates the control algorithm to be used in conjunction with RTEFIT. The planned approach is to measure the eddy currents in the passive plates and to use the transient response of the coils to minimize the total shell current effect.