Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI

Experimental measurements with Langmuir probe and laser photodetachment in beam extraction region of a cesium-seeded negative ion source for NBI has been conducted in order to investigate the response of charged particles to applied external field. The profiles of probe saturation current and H− ion density in the direction normal to the plasma grid (PG) surface were measured by scanning the tip position. By comparing the results before and during beam extraction, the charged particle responses due to the beam extraction have been analyzed. During beam extraction, probe saturation current increases since H− ions are extracted and electrons flow into the extracting region for charge neutrality. The maximum increment of the probe saturation current due to electron flow appears in the range of 15 - 25 mm apart from the PG surface. Meanwhile, the maximum decrement of the H− ion density is at around 18 mm from the PG. In the region far from the PG, probe saturation current increment is low as well as the decrement of the H− ion density. The extrapolations of the profiles suggest that the depth of the influence on the plasma by beam extraction is 42 mm from the plasma grid surface.