Evaluation of negative ion distribution changes by image processing diagnostic

Abstract

Distributions of hydrogen Balmer-α (Hα) 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. Hα reduction indicates a reduction of negative hydrogen ions because the mutual neutralization process between H+ and H− ions causes the dominant excitation process for Hα emission in the rich H− condition such as in ionic plasma. We observed a significant change in Hα reduction distribution due to change in the bias voltage, which is used to suppress the electron influx. Small Hα reduction in higher bias is likely because the production of negative ions is suppressed by the potential difference between the plasma and PG surface.