Design and operation of a helium gas stripper for high-intensity uranium beams at the RIKEN radioactive isotope beam factory

Since its commissioning, the RIKEN Radioactive Isotope Beam Factory (RIBF) has achieved a sustained increase in uranium beam intensity by more than two orders of magnitude. A key enabling technology has been a helium (He) gas stripper, which overcomes the severe lifetime limitations of conventional carbon foil strippers under high-intensity irradiation. This paper presents a comprehensive account of the design, long-term operation, and performance of the He gas stripper at the RIBF. A large-scale windowless He gas target was realized using a five-stage differential pumping system and high-flow gas recirculation, enabling stable operation with areal densities up to 0.7 mg/cm2. A gas diffuser, optimally positioned with respect to the Mach disk of the He jet, significantly improved stage-to-stage gas separation. At high uranium beam intensities, beam-induced gas heating gives rise to a pronounced gas-thinning effect in the He gas stripper. This effect was characterized using time-of-flight measurements and simplified thermal modeling, showing that purely thermal descriptions overestimate the gas temperature rise. The results suggest that nonthermal energy dissipation processes, most plausibly vacuum-ultraviolet radiation from excited He atoms, limit the temperature increase. To suppress He leakage into the cyclotron and maintain stable vacuum conditions under high-intensity operation, a nitrogen (N2) gas-jet curtain technique was developed. This method provides effective sealing of the He flow, replacement of residual He with N2, and a moderate pre-stripping effect, enabling stable long-term operation at the highest uranium beam intensities at the RIBF.