The neutral beam injection heating system of the future ITER fusion experiment will rely on the acceleration and neutralization of negative hydrogen ions. Cesium seeding into negative-ion sources is a prerequisite to obtain the required ion current at a maximum pressure of 0.3 Pa and a tolerable level of co-extracted electrons. Both the stability and the delivered current density of the ion source depend highly on the cesium conditions of the production surface during plasma and vacuum phases. The Monte Carlo code CsFlow3D was developed and applied to simulate the distribution and time dynamics of neutral and ionic cesium during long pulses of the IPP prototype radio frequency (RF)-driven ion source. These transient simulations show that the release of cesium accumulated within the ion source during the operation phase is the dominant mechanism to obtain cesium flux onto the production surface for negative ions (plasma grid). Studies of the spatial distribution of the deposits and the corresponding time dynamics and cesium flux profile are presented during long pulses. These show that the cesium flux onto the plasma grid has a high time variation which is a consequence of the transient release of reservoirs. While a high ionic flux is generated within the first seconds of the discharge, a broad neutral-dominated flux peak evolves during the total pulse interval.
Physics of the Advanced Plasma Source: a review of recent experimental and modeling approaches