Digital real-time plasma control system for Alcator C-Mod

A new digital plasma control system (DPCS) has been implemented for Alcator C-Mod. The new system was put into service at the start of the 2005 run campaign and has been effectively controlling the machine since then. This system not only provides a drop-in replacement for the original C-Mod control system, which was becoming increasingly difficult to maintain, it also provides a flexible platform to explore new non-linear and adaptive control schemes. It consists of two 64 input, 16 output CompactPCI digitizers attached to a rack mounted Linux server, which performs both the I/O and the computation. During initial operation, the system is set up to directly emulate the original C-Mod control system: this comprises a matrix of linearized observers, which estimate the 16 physical quantities under control, 16 PID channels for the conditioning of the errors between the physical quantities and their pre-programmed target waveforms and a matrix of controllers, for feeding back to the power supplies and other actuators. The matrices, the target and feed-forward waveforms and the other control parameters are time dependant and switch according to the particular phase of the plasma discharge. The compatibility with the previous control system allows the existing user interface software and data structures to be used with the new hardware. The controlling program is written in IDL, from Research Systems Incorporated (RSI), and runs under standard Linux. Interrupts are disabled during the plasma pulses to achieve real-time operation. Emulation of the original control algorithms is accomplished using 50 μs per iteration, with the time evenly split between I/O and computation, so rates of about 20 kHz are achievable. Reliable vertical control can be maintained with cycle rates as low as 5 kHz so there is considerable headroom for extra computation. The next steps are to explore the operation of the system on multiple time-scales, the use of additional computers in the real-time loop and the implementation of user provided signal processing and control schemes. These would include real-time analysis of power supply demands, real-time MHD modelling, and adaptive response to non-standard (unplanned) discharge conditions.