Numerical Simulation of a Multi-Cusp Ion Source for High Current H- Cyclotron at RISP

The rare isotope science project (RISP) has been launched in 2011 to support a wide range science program in nuclear, material, and bio-medical sciences as well as interdisciplinary programs. The production of rare isotope beams at RISP is currently configured to include facilities for both an In-flight Fragmentation (IF) system and an Isotope Separator On-Line (ISOL) system, which will utilize a 70 MeV H− cyclotron. The cyclotron will deliver 70 kW proton beam power to ISOL targets, where rare isotopes are generated and re-accelerated by a linear accelerator.A multi-cusp ion source used widely in H− cyclotrons is designed to have cusp geometries of magnetic field inside the ion source chamber, where ions are confined and enhanced plasma densities. Therefore the magnetic confinement fields produced by a number of permanent magnetic poles help to increase H− beam currents. In this work a numerical simulation is performed to understand the effect of multi-cusp magnetic fields when the number of magnetic poles is varied from 6 to 14. It is found that the larger number of magnetic poles provides a stronger ion confinement yielding higher extracted H− ion currents while the extracted electron current becomes lower.