Negative hydrogen ion transport in RF-driven ion sources for ITER NBI

The injection of energetic neutral atoms is a major component of plasma heating in fusion experiments. In order to fulfill the requirements of the ITER neutral beam injection (NBI), a RF-driven ion source for negative ions has been developed at the MPI fur Plasma Physik (IPP Garching). Negative hydrogen ions are generated on a converter surface by impinging neutral particles and positive ions under the influence of magnetic fields and the plasma sheath potential. A 3D negative ion trajectory calculation including a Monte Carlo description of reactions and collisions with plasma particles was used to calculate the total and spatially resolved extraction probabilities for realistic field topologies and geometries of the large scale extraction system LAG. The experimentally observed increase in extracted ion current by the use of chamfered aperture collars agrees with the results of the ion transport simulation. Profiles of the extraction probability on the converter show that most of the extracted negative ions are created in the vicinity of the plasma grid apertures. These areas of intensified extraction probability are influenced by the magnetic field configuration. The ion extraction probability is affected by the long ranging magnetic filter field. The short ranging electron deflection field, however, which is generated by magnets near the converter surface, does not significantly influence the extraction probability.