Evaluation of negative ion distribution changes by image processing diagnostic

Distributions of hydrogen Balmer-alpha (H alpha) intensity and its reduction behavior close to a plasma grid (PG) surface have been observed by a spectrally selective imaging system in an arc discharge type negative hydrogen ion source in National Institute for Fusion Science. H alpha reduction indicates a reduction of negative hydrogen ions because the mutual neutralization process between H+ and H- ions causes the dominant excitation process for H alpha emission in the rich H-condition such as in ionic plasma. We observed a significant change in H alpha reduction distribution due to change in the bias voltage, which is used to suppress the electron influx. Small H alpha reduction in higher bias is likely because the production of negative ions is suppressed by the potential difference between the plasma and PG surface.