A VSWR-based fast protection system for high-power RF amplifiers

In this paper, a fast protection system based on Voltage Standing Wave Ratio (VSWR) measurement was designed for the infrared free electron laser (FEL) facility at Anhui University. The facility produces a tunable laser covering 2.5–200 μm by adjusting the electron-beam energy, and it injects 6–30 MW of microwave power into the accelerating tube via klystrons. Our system focuses on the matching characteristics of the microwave path, is inherently independent of output power, and provides klystron protection across the full operating range. The system performs in-situ sampling of the klystron output through a waveguide directional coupler. The sampled microwave signal undergoes diode-based envelope detection and high-speed analog-to-digital conversion, generating a digital signal stream. An FPGA-based platform then computes the output VSWR in real time and triggers a rapid protection response. The coupler was designed using a reflected-power model, with its directivity optimized to 37 dB, significantly improving VSWR measurement accuracy—achieving a theoretical accuracy of 94.6% when VSWR <1.4. By optimizing the conversion function based on the detector model and experimental data, the power measurement accuracy reaches 98.8%. Simultaneously, real-time power calculation leveraging transfer functions is hardware-accelerated through the FPGA, providing critical fast-response protection capabilities. The system has been developed and applied in klystron conditioning experiments. It consistently achieved ∼288.8 ns (VSWR-based, exclude path delay) and ∼59 ns (ultrafast, exclude path delay) response times during wide-range power sweeps, demonstrating a deployable approach to rapid, pulse-level protection that generalizes to other high-power RF sources.