Experimental and theoretical study on the formation of hybrid discharge structure in a compact rf-driven negative hydrogen ion source

Abstract

This study reports on an experimental and theoretical investigation of the formation of a hybrid discharge structure in a compact rf-driven negative hydrogen ion source. This structure suggests deposition of dc power in the expanding plasma region of the source, leading to a continuous axial increase of the plasma potential which is needed to drive the volume-produced negative hydrogen ions towards the extraction system. The dc power deposition is established by means of applying a bias that is higher than the plasma bulk potential at the first electrode of the extraction system. The 2D fluid-plasma model employed reveals the spatial distribution of the main plasma parameters of electron density, electron temperature and plasma potential, and it describes the mechanism of dc power deposition when the plasma electrode is biased. Optical emission spectroscopy is applied for diagnostics of the three regions of the source: rf power deposition, plasma expansion/dc power deposition, and extraction. The experimental results are not only for the electron ensemble (electron density and temperature), but also for that of the positive ions (ion densities) and neutrals (atomic and molecular temperatures and densities). The theory and experiment show good agreement, confirming the presence of the hybrid discharge structure.