Measurement of H0 particles generated by residual gas stripping in the Japan Proton Accelerator Research Complex linac

The Japan Proton Accelerator Research Complex (J-PARC) linac is a high-intensity accelerator in which beam loss is a critical issue. In the J-PARC linac, H− beams are accelerated to 191 MeV by a separated drift tube linac (SDTL) and subsequently to 400 MeV by an annular-ring coupled structure (ACS). Because there are more beam loss mechanisms in H− linacs than in proton linacs, it is imperative to investigate the beam loss circumstances for beam loss mitigation. Electron-stripping phenomena, which generate uncontrollable H0 particles, are characteristic beam loss factors of H− linacs. To clarify the beam loss causes in the J-PARC linac, a new diagnostic line was installed in the beam transport between the SDTL and ACS. In this diagnostic line, H0 particles were separated from the H− beam, and the intensity profiles of the H0 particles were successfully measured by horizontally scanning a graphite plate in the range where H0 particles were distributed. By examining the intensity variation of the H0 particles with different residual pressure levels, we proved that half of the H0 particles in the SDTL section are generated by the residual gas stripping in the nominal beam operation of the J-PARC linac.