We utilize Low-level Radio Frequency (LLRF) to control accelerator beamlines and are pioneers of implementing fully digital LLRF/RF controls. The high power sources that drive electromagnetic fields inside accelerating cavities must be precisely controlled against noise perturbation for optimal beam quality. To do this, the LLRF controller measures the magnitude and phase of a sample of the cavity field, and feeds back to the high power RF amplifier in hard real time. Typical feedback loop response times are 0.5 to 2.0 microseconds.
Achieving <0.02° of stability at 2856 MHz, we have applied this technology to the timing distribution system in the latest FEL (free electron laser) facilities, synchronizing remote lasers and RF references with combined jitter and long-term drift as low as 20 femtoseconds rms. Using Innovative techniques to reduce crosstalk and close-in phase noise, we have achieved short-term SRF cavity stability better than 0.01°. With well-designed digital (FPGA) programming, we have also demonstrated automated cavity turn-on and robust cavity operation in the presence of microphonics.