A constricted plasma source for negative ions

A constricted plasma source is described which is characterized by a dense plasma close to the anode while the plasma expands outwards in a low pressure region (0.6 Pa) because of differential pressure between the cylindrical hollow cathode through which the gas is injected. Detail characterization of plasma parameters namely electron density (n$_{e})$, positive ion density (n$_{+})$ and plasma potential (V$_{p})$ by hairpin, ion flux and floating emissive probes, respectively, reveals the presence of negative ions adjacent to the anode fall region of the expanding plasma when a mixture of Ar/O$_{2}$ was used as compared with the pure electropositive Ar gas. A linear increase of negative ion density (10$^{16}$ m$^{-3})$ with modest operating powers (400 W) as achieved at low working pressure makes this source highly promising for cesium free negative ion sources.