Spatial Distributions of Charged Particles and Plasma Potential before and during Beam Extraction in a Negative Hydrogen Ion Source for NBI*)

In order to understand the physics of the production and extraction mechanism of negative ions, experiments with a single Langmuir probe are carried out to investigate the local spatial distribution of the plasma in the vicinity of the plasma grid in a negative hydrogen ion source. The results show that the electron density is lower in the aperture region than that in its peripheral region. It is attributed to the cusp magnetic field introduced with the electron deflection magnets near the plasma grid. In addition, the linkage of the filter field with the electron deflection magnetic field contributes to the increment of electron density. The profile of plasma potential indicates that an electric field exists between the aperture and its peripheral region, and then a local negative ion flow can be introduced near the plasma grid. © 2015 The Japan Society of Plasma Science and Nuclear Fusion Research