Plasma Electrode Bias Effect on the $\hbox {H}^{-}$ Negative-Ion Density in an Electron Cyclotron Resonance Volume Source

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative-ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the H extracted current reach a maximum (~1-2 times 10⁹ cm^-3 and ~0.5 mA/cm², respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed.