First annular-ring coupled structure cavity for the Japan Proton Accelerator Research Complex linac

The first annular-ring coupled structure (ACS) cavity for the Japan Proton Accelerator Research Complex (J-PARC) linac has been developed in order to confirm and establish its fabrication processes. This cavity includes 34 accelerating cells, which is 3.4 times as many as the prototype cavity (buncher) of the J-PARC ACS. The cell frequencies before brazing were tuned within the required accuracies. After brazing, the accelerating- and coupling-mode frequencies were 972.19 and 972.63 MHz on average, respectively, which were higher than the operating frequency of 972 MHz. Although the accelerating-mode frequency can be corrected to 972 MHz using adjustable plungers, these frequency errors adversely affected the on-axis electric field. This cavity was successfully conditioned up to 1.6 MW. This power corresponds to an accelerating field of 4.7 MV/m, which is 10% higher than the design value of 4.2 MV/m. In order to find the issue caused by the excited coupling cells, this cavity was also conditioned at the higher and lower frequencies that were detuned from the operating frequency of 972 MHz. Here we present the frequency tuning processes, the low-level rf measurements, and the high-power test results. We also discuss the most reasonable scenario for frequency tuning in future work