Low level RF control system development for the PEFP proton accelerator

For the Proton Engineering Frontier Project (PEFP) proton accelerator, the RF amplitude and the phase stability requirements of the accelerating field are within 1 % and I degree, respectively. As a prototype of the low level radio frequency (LLRF) system, a simple digital proportional integral (PI) control system based on a commercial field programmable gate array (FPGA) board is designed and tested. The main features are a sampling rate of 40 MHz, which is four times higher than the down-converted cavity signal frequency, a digital in-phase and quadrature detection, and a simple proportional-integral feedback algorithm. A simulation study to determine the optimum gain is performed by using MATLAB and a state-space model of the RF cavity system. In the test with a dummy cavity, the field amplitude and phase are maintained within I % and I degree with an intentionally applied perturbation of 10 % in the field amplitude and 12 degrees in the field phase. In addition, the feedback control system improves the shot-to-shot stability by an order of magnitude compared with an open loop control.