Ion flux measurements using a Mach-Langmuir probe in the ITER prototype neutral beam injection ion source

Abstract

Neutral beam injection systems as foreseen for ITER use radio-frequency (RF) ion sources at low pressure, where negative hydrogen ions are mainly produced via surface conversion of neutral atoms and positive ions at a plasma facing grid (PG). Up to now there is only limited knowledge about how fluxes and directed velocities of the positive ions are affected by external parameters such as power, pressure and the horizontal magnetic filter field which causes plasma drifts and vertical asymmetries in the vicinity of the PG. For this reason a combined Mach-Langmuir-probe diagnostic is used at multiple positions in the expansion and close to the extraction system in the prototype RF ion source (1/8 of the full ITER ion source size) to measure the positive ions directed velocity and flux as well as the plasma parameters simultaneously. With increasing RF power the flux towards the PG is found to increase linearly, its magnitude being controlled by the plasma density. Towards ITER-relevant pressures the ion flux decreases, in contrast to the directed velocity, which increases non-linearly, reaching around 5 km s−1 at a pressure of 0.3 Pa. The magnetic filter field is discovered to strongly bent down the ion flow in front of the PG. As a result, the ions at the lower half of the PG flow almost exclusively parallel to it, wherefore the flux which impinges onto the lower PG half is reduced by around one order of magnitude.