Research progress on ionic plasmas generated in an intense hydrogen negative ion source

Abstract

Characteristics of ionic plasmas, observed in a high-density hydrogen negative ion source, are investigated with amulti-diagnostics system. The ionic plasma, which consists of hydrogen positive-and negative-ions with a significantly low-density of electrons, is generated in the ion extraction region, from which the negative ions are extracted through the plasma grid. The negative ion density, i.e., the ionic plasma density, as high as the order of 1×10 17 m -3 , ismeasured with cavity ring-down spectroscopy, while the electron density is lower than 1×10 16 m -3 , which is confirmed withmillimeter-wave interferometer. Reduction of the negative ion density is observed at the negative ion extraction, and at that time the electron flow into the ionic plasma region is observed to conserve the charge neutrality. Distribution of the plasma potential ismeasured in the extraction region in the direction normal to the plasma grid surface with a Langmuir probe, and the results suggest that the sheath is formed at the plasma boundary to the plasma grid to which the bias voltage is applied. The beam extraction should drive the negative ion transport in the ionic plasma across the sheath formed on the extraction surface. Larger reduction of the negative ions at the beam extraction is observed in a region above the extraction aperture on the plasma grid, which is confirmed with 2D imagemeasurement of the Hα emission and cavity ring-down spectroscopy. The electron distribution is alsomeasured near the plasma grid surface. These various properties observed in the ionic plasma are discussed.