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

Abstract

J-PARC (Japan Proton Accelerator Research Complex) linac has recently started its continuous operation to provide a 181-MeV H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> beam. A possible beam loss source is the H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> produced by double electron-stripping of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> in a residual gas scattering at the front end part of the linac. To confirm this supposition, an H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> signal was measured with a wire scanner monitor at the beam energy of 3 MeV by separating H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> from the H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> beam with horizontal dipole magnets. Then the separated H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> is successfully removed by a beam scraper originally for beam chopping. With this H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> 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.