High-power test of normal conducting cavities with real-time resonant frequency tracking

Normal-conducting radio frequency cavities are essential in particle accelerators, but their operational stability is often compromised by thermal-induced resonant frequency detuning during high-power operation. This paper introduces a digital low-level radio frequency system that performs autonomous frequency tracking and compensation in a standalone configuration. Specifically, during powering up and rf conditioning, the tracking operates without dependence on external instrumentation. The system employs a vector modulator driven by two orthogonal analog sinusoids of equal amplitude to modulate the reference signal, achieving an adjustable output bandwidth of ± 1.5 6 MHz with a precision of 95.3 Hz entirely within the LLRF framework. A phase-locked loop controller embedded in the system dynamically synchronizes the rf frequency to the cavity resonance, while a real-time spectrum analyzer implemented in the FPGA monitors frequency deviations in a closed-loop manner. Experimental results verify that the system can resolve spectral components within a 1.56-MHz bandwidth and generate arbitrary-frequency rf signals over a 3-MHz span using only its integrated LLRF resources. In high-power tests, the embedded PLL-based tracking maintained the reflected power at 8.5% of the incident power under thermal detuning exceeding 200 kHz, thereby ensuring operational stability through a unified LLRF-based approach.