Fluid-model analysis on discharge structuring in the RF-driven prototype ion-source for ITER NBI

A 2D self-consistent fluid plasma model is employed for studying the plasma in the radio-frequency (RF)-driven negative hydrogen ion source prototype developed to equip the neutral beam injector systems for ITER. The source is considered in its usual configuration with a cylindrical driver and magnetic filter field (MF) produced by permanent magnets arranged in a magnet frame movable along the expansion chamber. The model accounts for the RF injection, the bias potential applied at the plasma grid (PG) and the losses of particles and electron energy in the third dimension, i.e. along the magnetic field lines, are evaluated. The study is focused on the role of the processes which govern the spatial structuring of the plasma parameters caused by the topology of the MF, the PG bias and the losses along the MF. The obtained numerical results are analyzed based on the contribution of the local and nonlocal processes in the electron- and negative hydrogen ion-balance and electron energy balance equations. It is shown how the fluxes associated with the diamagnetic drift caused by the gradient of the electron temperature, and the E × B-drift as well as the electron heating and the negative ion drift in the dc electric field are involved in the formation of the pattern of the plasma parameters. Effects due to the partial penetration of the MF in the driver are also investigated. A comparison with the experimental data shows that, accounting for the losses along the MF, the present model is a reliable tool for study and optimization the ITER relevant sources.