Prototype design of a digital Low-Level RF system for S3FEL S-band Transverse Deflecting Cavities

Transverse Deflecting Cavities (TDCs) are generally adopted for electron beam diagnosis. Three sets of S-band and two sets of X-band TDCs are planned at Shenzhen Superconducting Soft X-ray Free Electron Laser (S3FEL) to accurately measure the temporal distribution of ultra-short electron bunches. The microwave system of one TDC consisting of a Low-Level Radio-Frequency system (LLRF), a solid-state amplifier, a klystron, and several waveguide couplers is operated in pulse mode with a maximum repetition rate of 50 Hz. Its microwave stabilities for amplitude and phase are required to be better than 0.05%/0.05{\deg} (RMS). This article will introduce the prototype design of the hardware, firmware, and software of the digital LLRF system for S-band TDCs. We use a homemade local oscillator and commercial cards based on the MicroTCA standard in hardware design. The firmware design will use an IQ demodulation and a reference-tracking algorithm to eliminate the measurement noise and drift. The software design is based on the Experimental Physics and Industrial Control System (EPICS), achieving data acquisition, slow control, and interface display functions. This technical report will also show some preliminary test results.