Discoloration of RF antenna coil surface after long-term operation of J-PARC ion source

Abstract

In J-PARC (Japan Proton Accelerator Research Complex) center, the continuous operation period of the Radio Frequency (RF) negative hydrogen ion (H−) source equipped with the internal RF antenna coil is extended step by step in these 6–7 years for the goal to supply stable beam during the entire period of J-PARC user operation each year. In 2022, the continuous ion source operation for 4,001 hours (5.5 months) was achieved with 60 mA H− beam current at the ion source exit, 25 Hz repetition, mainly 500 μs beam pulse width, and 25 kW RF power injection. As the lifetime of the RF ion source is mainly limited by the deterioration of the enamel coating on the RF antenna coil, a detailed evaluation of the antenna surface is required to ensure the feasibility of the further extension of the operation period. In the present study, surface discoloration on the RF antenna coil observed after 5 months of operation is investigated by the application of a digital microscope and SEM/EDS analyses. The EDS analyses show that depositions of the sputtered stainless steel elements and the injected cesium are the most possible candidates for impurities on the enamel coating. The dimension measurements of the RF antenna thickness before and after the long-term operation also support the assumption that the discoloration is due to the deposited elements and hence insulation of the RF antenna coil by enamel coating is maintained. Possible mechanisms leading to antenna deterioration starting from element depositions on the antenna coating are also discussed in the summary part of this manuscript.