RF power transfer efficiency of H- ion sources: fluid modeling of accelerator source geometries

The plasma in RF driven negative hydrogen ion sources is sustained via inductive coupling with large power levels of up to 100 kW and low frequencies around 1 MHz. This leads to currents of around 100 A flowing over the RF coil and corresponding voltages in the kV range. The associated risk of arcing limits the reliability of the whole ion source. The required power level can be reduced via optimizing the RF power transfer efficiency eta, which is typically only in the range of 50 to 60% for H- sources used for neutral beam injection systems. In order to study the optimization of eta systematically, a self-consistent fluid model has been set up and successfully validated with experimental measurements at the BATMAN Upgrade test bed. For H- sources applied at particle accelerators, no experimental measurements of eta are available so far. In order to gain a first insight into the RF power transfer efficiency of these sources, exemplary simulations were carried out with the fluid code. The simulated plasma parameters are in good agreement with results from OES measurements. eta shows an increasing trend with larger source radius and a virtually constant value with increasing RF power. For benchmarking these first results, dedicated measurements at an accelerator source setup are inevitable.