Studies of electronegative Ar/O2 discharge in a constricted hollow anode plasma source using dual probe technique

Uniform high density electronegative plasmas find important application for plasma heating by neutral beams in fusion devices and in material processing. In this work we present a constricted hollow anode plasma source (CHAPS) for the investigation of electronegative Ar/O2 discharge. The source consists of a series of equidistant stainless steel parallel plates acting as cathode with a small stainless steel tube as the anode. The plasma comprise of highly uniform density outside the cathode plates and an intense glow near the anode1. The negative oxygen ion fraction is determined by measuring the electron density by hairpin probe and ion density using a planer Langmuir probe. The negative ion density in the bulk plasma is typically 1016 m−3 and increases monotonically as a function of pressure and power. The electronegativity of the discharge is typically close to 0.5 and remains constant with the applied power. Whereas the negative ion density in the anode region varies from 1016 m−3 to 1017 m−3 with electronegativity of 3.0 near the edge of the anodic glow and decreases to 1.0 with the distance from the anode glow.