Speaker
Description
In the J-PARC LINAC, the beam monitor and LLRF systems operate with independent RF references. The LLRF system at 312 MHz and 960 MHz incorporates active environmental compensation to enhance stability. However, the 324-MHz beam-monitor RF reference lacks such functionality, resulting in humidity-dependent phase drift. To improve environmental long-term stability, we have developed a feedback-based reference compensation system implemented on a MicroTCA.4 platform. The system takes the 324-MHz beam-monitor reference from the SSA output and down-converts it to 12 MHz on the MicroRTM, enabling IF-phase measurement in the ADCs and fast feedback computation in the FPGA. The feedback-corrected I/Q signals drive the IQ modulator on the MicroRTM to regenerate a stabilized 324-MHz reference, which drives SSA and is redistributed to monitor stations, enabling real-time compensation of environmental phase drift with minimal additional hardware. The system is integrated into the existing LLRF infrastructure at MEBT1, achieving full synchronization, compact installation and cost-effective operation. Long-term studies demonstrated suppression of humidity-induced phase fluctuations and improved reference stability, contributing to enhanced beam-monitor reliability and supporting future high-power beam upgrades at J-PARC. This paper presents the system design, FPGA implementation, and long-term measurements characterizing humidity-induced drift and the stabilized performance with feedback.
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