A Langmuir probe system for high power RF-driven negative ion sources on high potential

A fully automated Langmuir probe system capable of operating simultaneously with beam extraction has been developed and commissioned for the negative hydrogen ion source testbeds at IPP Garching. It allows the measurement of temporal and spatial distributions of the plasma parameters within a single plasma pulse (<5 s). This system can operate even in the presence of multi-harmonic RF interference due to a novel transformer-based RF compensation system. Analysis methods of the probe data are described in the paper along with a discussion of errors. Measurements of the plasma parameters for RF powers (30-80 kW) and source pressures (0.3-0.8 Pa) both in plasma generation region and near the plasma grid have been carried out. The plasma generation region has both a high density (>1018 m-3) and hot (Te > 10 eV) plasma with bi-Maxwellian electron energy distribution at low pressures. The plasma found near the plasma grid is very different being of low density (≤1017 m-3) and very cold (Te < 2 eV). This plasma is also strongly influenced by the presence of caesium, the potential of the plasma grid, and if an ion beam is extracted from the source. Caesium strongly reduces the plasma potential of the source and enhances the negative ion density near the plasma grid. Extracting an ion beam is observed to reduce the electron density and increase the potential near the plasma grid. Applying a potential greater than the plasma potential to the plasma grid is found to significantly decrease the electron density near the plasma grid.