A digital Low Level RF Control System for the S-DALINAC

The superconducting cavities of the S-DALINAC have a high loaded quality factor and are very susceptible to microphonics. To stabilize the amplitude and phase of the cavities’ fields an analog control system has been used for 20 years [1]. To improve the stability and the availability of the low level RF control system it is currently replaced with a digital one. The 3 GHz signals coming from the cavities are converted down to the base band using hardware I/Q demodulators. The base band signals are digitized by ADCs and fed into an FPGA which executes the code implementing the control algorithm. The computed control signal is I/Q modulated before it is send to the cavity again. The superconducting cavities are operated with a self-excited loop algorithm whereas a generator driven algorithm is used for the low Q room temperature bunching cavities. Additionally, a 6 GHz RF board allows the operation of a new 2f buncher. Parameters can be adjusted via an EPICS IOC running on a standard PC. All signals from the FPGA can be monitored in real-time by the operator.