Feedback control of helicon current drive power on the DIII-D tokamak

A feedback control system has been implemented to regulate the power output of the helicon current drive system on the DIII-D tokamak. A 1.2 MW klystron delivers 476 MHz RF injected to the plasma by a travelling wave antenna (TWA). A PID controller implemented with a National Instruments C-RIO FPGA allows user-defined control of the output power. The FPGA controls the klystron's output power by actuating a digitally-controlled variable attenuator to change the input power to the klystron. Modeling employing system identification indicated a discrete controller running faster than 20 kHz could provide 20-200 Hz modulation while rejecting 660 Hz disturbances. Software was developed to leverage the C-RIO as both a discrete PID controller running at 40 kHz and a data acquisition system digitizing up to 100 kS/s to monitor performance during tuning. The controller is tuned to match set point waveforms with 0.5 ms response time. This poster describes the controller's technical development, tuning, and potential applications.