Development of an electrostatic beam focus lens equipped with electron suppression grids for a microwave ion source to inject high-current ion beams into a linac

An electrostatic beam focus lens equipped with electron suppression grids (SG) was developed to focus and inject high-current ion beams into a linac from a microwave ion source developed for improving the availability of linac application systems. The SG suppresses electron acceleration by the lens electric field. This contributes to the confinement of electrons in beam plasma in the drift space between the ion source and lens. The confined electrons neutralize ion space charge and prevent the divergence of ion beams by space charge force, which enables an efficient transportation of high-current ion beams through the lens from the microwave ion source with small beam loss. High-current hydrogen beams of around 40 mA with 30 keV beam energy, 400 μs pulse width and 90% proton ratio were focused successfully using the developed lens with high transport efficiency of about 70%; without the SG this efficiency was less than 30%. The 90% beam half-width of the focused beam was around 1 mm, which would be acceptable for injection into a radio-frequency quadrupole (RFQ) linac. The microwave ion source system including a low-energy beam transport can be compact to less than 1 m in length with the developed electrostatic lens in comparison with a solenoid coil lens system which is more than 2 m length. Long-time operation stability (16 h×3 days with 0.8% duty: a condition for a typical application; 110 h with 50% duty: 62.5 times of the condition corresponding to 6875 h operation) was confirmed with a focused beam current of 35 mA and change in the current was less than 2%.