Simulation of beamlet deflection and its compensation by aperture displacement for a NBI negative ion source prototype

Abstract

A one-RF-driver prototype of the negative ion source for the NBI of CFETR is being tested and optimized at Southwest Institute of Physics, aiming for H− beam of 200 keV in 200 A/m2. For the NBI negative ion source, it's essential to have a good beam optics with small divergence and aiming trajectory. However, beamlet deflection is unavoidably caused by the magnets inserted in the extraction grid, and it results in the zig-zag beam profile. Measures must be taken to compensate the deflection. In this work, the beamlet deflection and its compensation using the aperture displacement method are studied for the one-RF-driver prototype with numerical simulation. The deflection angle of a single beamlet is calculated and the compensation scheme of displacing the aperture on the steering grid (SG) is designed. A 200 A/m2 uniform beamlet has a deflection angle of 8 mrad at the extraction/acceleration voltage 8 kV/192 kV, EG magnets’ flux peak ±45 mT. The angle can be fully compensated with 0.7 mm displacement of SG aperture. The compensation effect is analyzed for different conditions. The scheme's dependence on the voltage ratio, also the adaptiveness to the magnetic field and the beam property are learnt: For maintaining the compensation effect, a modification within 0.02 mm is needed when the acceleration voltage varies between 152 kV and 232 kV while keeping the extraction voltage unchanged. An increase of 0.1 mm of the displacement is needed when the magnetic field peak is enhanced 5 mT and the necessary increase is 0.15 mm when the non-uniformity is enhanced 10%. The compensation scheme is verified with multi-beamlet simulation in the last, taking the beamlet-beamlet interaction and the long distance transport into account.