Production of negative hydrogen ions in a large multicusp ion source with double-magnetic filter configuration

The production of the negative hydrogen ions in a large multicusp ion source has been investigated in a double-magnetic filter (DMF) configuration. In the DMF configuration, the energetic electrons are trapped by the mirror of a magnetic multicusp field, and only the thermal electrons are present in the center of the arc chamber. A large amount of H- ions of more than 1 x 1012 cm-3 are produced in the center region. The electron current extracted with the H- ions can be suppressed by a magnetic filter field in front of a plasma grid without affecting the H- ion current. The experimental data obtained in the DMF configuration are compared with those obtained in a single-magnetic filter (SMF) configuration. In the SMF configuration, the energetic electrons are present in the center region, and the magnetic filter in front of a plasma grid reduces the electron density and temperature, which results in the reduction of H- ion current as well as the electron current. The extracted electron current and the optimum gas pressure for the H- ion current are lower in the DMF configuration that those in the SMF configuration. (author)