Foil hits reduction by minimizing injection beam size at the foil in j-parc rcs

The uncontrolled beam loss caused by the foil scattering of the circulating beam during multi-turn charge-exchange injection is one of the main sources for high residual radiation at the injection area of J-PARC (Japan Proton Accelerator Research Complex) 3-GeV RCS (Rapid Cycling Synchrotron). It is thus one concerning issue for beam intensity ramp up and operation at the designed 1 MW beam power. We studied to reduce circulating beam hits on the foil by minimizing the vertical injection beam size at the foil and using a smaller size of vertical foil. The vertical injection beam size was reduced to 1.2 mm (σ) from its original 1.8 mm, so as to reduce the vertical foil size from 20 mm to 14 mm. As a result, the number of circulating beam passing through foil was significantly reduced due to smaller foil size. A 30% foil hit reduction, as expected in the numerical simulation was achieved in the measurement.