Design and Experiment Study of 21 mA H- Multi-cusp Ion Source

The ion source is one of the key devices for the high-intensity cyclotron, which exerts influence on the beam intensity and applications of the machine. As part of the cyclotron research and development effort, China Institute of Atomic Energy (CIAE) has been developing arc discharge H- ion sources based on volume production and multi-cusp magnetic configuration since 1997. In 2000, 5.2 mA H- beam was reached and 15 mA H- beam was achieved in 2010. After a long break, H- ion source development started again in 2022 in order to improve performance of the main H- ion source of the CIAE 18 MeV cyclotron and to fulfill the requirement of future other high intensity proton beam cyclotrons for the high intensity H- ion. A new H- ion source and test stand was built at CIAE in 2023. The design of this new ion source was based on the experience on our previous 15-20 mA H- ion source and the source at TRIUMF. Major efforts included the study of the virtual filter magnetic field for filtering out fast electrons, multi-cusp confining magnetic field, four-half-circle ring filaments length and location, improvement of extraction structure and upgrading of vacuum system and power supplies. Up to now, more than 21 mA of H- beam were obtained at a high voltage of 35 kV from a plasma electrode hole of 13 mm in diameter when the hydrogen flux was 25 sccm. At this output the maximum arc power available is only 4.5 kW and the e/H- ratio is about 5. When the hydrogen flux is higher or extraction high voltage is higher, H- beam currents can reach 22 mA. However, the H- beam is very unstable with terrible sparks at extraction and acceleration gaps in this case, about five times a minute. This appears to be due to the lack of a differential pumping structure, which also results in an unexpectedly high hydrogen flux. In the experiment, the relationship between arc power and H- beam intensity was determined, also the optimal location of the four-filament structure was determined, which is obviously related to the virtual filter magnetic field distribution. At present, the emittance scanner is being processed and the measurement program is also updated and improved, the beam emittance has not been measured and analyzed at this stage. Subsequently, the vacuum chamber structure will be improved, the extraction structure also will be optimized to obtain brighter H- beam, and a long-lasting filament will be developed to reduce maintenance time.