Vessel current compensation for improved equilibrium reconstruction and control just after plasma formation in DIII-D*

High fidelity equilibrium reconstructions are essential for accurate plasma control and equilibrium-based analysis. Equilibrium reconstruction and plasma control usually suffer from reduced equilibrium accuracy at the starting phase of plasma operation for tokamaks, because of massive eddy currents induced in the vessel wall from large variation of ohmic heating coil flux and control coil field. A magnetic response function model [Y. Jiang, RSI 2017] provides a set of time-dependent Green’s functions, which contain a group of 3 parameters calibrated by fitting magnetic measurement with individual coils energized at varying frequencies. The present result shows that high \chi^2, indicating poorer quality of a fit to data in equilibrium reconstruction, corresponds well with the eddy current response level in the magnetic signals used in equilibrium calculation. Instead of the DC Green’s function that EFIT [L.L. Lao, NF 1985] now uses, the AC Green’s functions provide the vessel current compensation for the magnetic sensors to optimize the equilibrium reconstructions in the presence of large induced currents. This new technique will improve the accuracy of equilibrium reconstructions during plasma startup and other transient events.