Performance of the C-Mod shape control system

The poloidal field system for Alcator C-Mod (a high-field compact tokamak) can generate elongated, single or double null diverted plasmas. The shape control system consists of hardware (hybrid multiplier, real-time control computer, and associated electronics), the user interface (a set of X-Windows applications), and the underlying algorithms used by the interface to calculate the control matrices. The shape control computer generates demands to the power supplies, based on programmed waveforms and feedback from the diagnostics. Beginning in April 1993 we have used the shape control computer to program and/or feed-back control coil currents, power supply voltages, magnetic field quantities and gas pressure during plasma breakdown, and to tailor the fields during the current ramp. Representative data from the 1993 run and an evaluation of the overall performance of the shape control computer will be presented.