Study of the materials on plasma electrode surface for negative ion extraction in hydrogen and deuterium operation

Tantalum was compared with tungsten as filament material for negative ion extraction in the discharge of hydrogen and deuterium. For hydrogen discharge, tantalum filament operation showed about 40 % increase of negative hydrogen ion current and decrease of co-extracted electron current by 10 % against tungsten filament operation. Tantalum filament operation with deuterium indicated lower intensity of co-extracted electron current compared with tungsten while negative deuterium ion current showed slightly higher intensity. Dependence of extracted currents upon the plasma electrode bias voltage was linked to the local plasma parameters measured by a Langmuir probe. The effects due to the filament materials in hydrogen and deuterium plasmas are discussed.