H- ion source for CERN’s Linac4 accelerator: simulation, experimental validation and optimization of the hydrogen plasma

Linac4 is the new negative hydrogen ion (H^-) linear accelerator of the EuropeanOrganization for Nuclear Research (CERN). Its ion source operates on the principle of Radio-Frequency Inductively Coupled Plasma (RF-ICP) and it is required to provide 50 mA of H^- beam in pulses of 600 μs with a repetition rate up to 2 Hz with ratio of e^-/H^-<8. We have developed a Particle-In-Cell Monte Carlo Collision (PIC-MCC) code to simulate the RF-ICP heating mechanism and performed optical emission spectroscopy measurements to validate the simulations. The code self-consistently solves the interaction between the RF fields and the plasma response. We present an analysis of the capacitive to inductive transition characteristic of the initial phase of the discharge as well as a parametric study of the RF current, hydrogen pressure and magnetic configuration (cusp and filter fields) in the high density regime (10¹⁹ m^-3), representative of the nominal operation of the Linac4 ion source during beam extraction. The simulation results, confirmed by experimental data, provide guidelines for the optimization of the ion source operational and design parameters.