Digital Coil Protection System (DCPS) algorithms for the NSTX centerstack upgrade

A significant upgrade is planned for the National Spherical Torus eXperiment (NSTX) in which plasma current and confining magnetic field intensities will nearly double while plasma duration will more than double. Changes will include replacing the existing centerstack with a new one containing a thicker TF inner Leg, a new OH solenoid coil, and an expansion to six of the complement of PF1 coils controlling plasma divertor shape. Other coils will remain in service with hardware modifications as needed for their approximately three-fold increases in magnetic forces. The new Digital Coil Protection System (DCPS) will avoid the costly need to further upgrade coils and their mechanical supports to withstand control misoperations that could otherwise take them well beyond the operating envelope of critical variables (i.e., forces, stresses and temperatures) actually needed for the new regime of plasma experiments. This improvement would be impossible using the present protective scheme of independent overcurrent trips for each coil circuit which ignores effects of current combinations. In the upgraded NSTX, the DCPS will operate on a fixed cyclic repetition rate fast enough to avoid latency issues, e.g., 1 millisecond, monitoring currents in the plasma, in the OH solenoid coil, in 12 PF coil circuits, and in the TF coil. Its algorithms will calculate in real time the present and projected maximum future values of each critical variable that would be reached if protective action were commanded now, with and without a full plasma disruption event. The DCPS will command the protective action to begin if any such calculated critical value exceeds its operating limit; power supplies are then bypassed by applying zero volts to coil circuit terminals.