High proton ratio plasma production in a small negative ion source

Abstract

A proton ratio was measured in the small (1.4 l) arc discharge multicusp source at a pressure of 1.4 Pa. A measured proton ratio in an extracted positive ion beam was 50%. By applying the transverse magnetic field as the magnetic filter in the source, the proton ratio drastically increased to 90% even in such a small source. To clarify the proton production processes, coupled rate equations were numerically solved to estimate particle (H+, H2+, H3+, and H0) densities in the various size of ion source. It has been clarified that a main proton production process is different with the source size. In the large size source, which has better confinement of the source plasma, the ionization of H atom becomes effective to increase the proton yield. In the small ion source, the dissociation of H2+ (H2++e→H++H+e) is the dominant process for the proton production. By adopting the magnetic filter in the small source, H2+ production by primary electron impact is suppressed in the extraction region. Molecular ions are dissociated or neutralized by thermal electrons in the extraction region. The main process of the proton production H2++e→H++H+e has the large cross section with the low temperature electrons, thus the high proton yield beam can be extracted.