Suppression of Beam Loss at the First Arc Section in the J-PARC Linac

J-PARC (Japan Proton Accelerator Research Complex) linac has recently started its continuous operation to provide a 181-MeV H^- beam to the downstream 3-GeV synchrotron and a neutron target. Immediately after the commencement of the continuous operation, significant residual radiation and beam loss were observed at the first bending magnet in the first arc section. The measured radiation and beam loss were localized in the side opposite to the deflection of the H^- beam. A possible beam loss source is the H⁺ produced by double electron-stripping of H^- in a residual gas scattering at the front end part of the linac. To confirm this supposition, an H⁺ signal was measured with a wire scanner monitor at the beam energy of 3 MeV by separating H⁺ from the H^- beam with horizontal dipole magnets. Then the separated H⁺ is successfully removed by a beam scraper originally for beam chopping. With this H⁺ suppression, the beam loss at the first bend was suppressed by factor of 24-31 without sacrificing the beam chopping performance. Consequently, the residual radiations were reduced to a comfortable level for the continuous operation.