Beam dynamics design and irradiation experiment of beam loss for a CW 100-mA proton RFQ

For a high-current Radio Frequency Quadrupole (RFQ) accelerator, space charge will strongly affect the stability of beams, which brings challenges and difficulties to the beam dynamics design. For an in-depth study of this space charge effect, a project has been proposed by Peking University and the Institute of Modern Physics of the Chinese Academy of Sciences. This project requires a 162.5 MHz RFQ to accelerate a 100-mA continuous-wave (CW) proton beam to 3 MeV. The beam dynamics design of this RFQ is based on the four-section procedure with improved radial focusing strength along the RFQ. We have investigated the relationship between limiting current and beam transmission to analyze the reasons for emittance growth and beam loss. We have also carried out an irradiation experiment based on this beam dynamics design to study the irradiation damage of the electrode surface, as this can be one main reason for reduced transmission or unstable operation that most existing high-current RFQs encounter after long-time operation. The results show that low-energy beam losses need to be kept as low as possible to prolong the life of the RFQ electrodes. The final simulated transmission efficiency reached 99.8%, as confirmed by the code toutatis.