Practical 2.45-GHz microwave-driven Cs-free H− ion source developed at Peking University

A practical 2.45-GHz microwave-driven Cs-free H− source was improved based on the experimental H− source at Peking University (PKU). Several structural improvements were implemented to meet the practical requirements of Xi'an Proton Application Facility (XiPaf). Firstly, the plasma chamber size was optimized to enhance the plasma intensity and stability. Secondly, the filter magnetic field and electron deflecting magnetic field were enhanced to reduce co-extracted electrons. Thirdly, a new two-electrode extraction system with farther electrode gap and enhanced water cooling ability to diminish spark and sputter during beam extraction was applied. At last, the direct H− current measuring method was adopted by the arrangement of a new pair of bending magnets before Faraday cup (FC) to remove residual electrons. With these improvements, electron cyclotron resonance (ECR) magnetic field optimization experiments and operation parameter variation experiments were carried out on the H− ion source and a maximum 8.5-mA pure H− beam was extracted at 50 kV with the time structure of 100 Hz/0.3 ms. The root-mean-square (RMS) emittance of the beam is 0.25π⋅mm⋅mrad. This improved H− source and extraction system were maintenance-free for more than 200 hours in operation.