Effect due to plasma electrode adsorbates upon the negative ion current and electron current extracted from a negative ion source

The intensity of negative hydrogen (H−) ion current and that of electron current extracted from a negative ion source show different characteristics against the change in plasma electrode bias depending upon the material covering the plasma electrode surface. The knowledge of these characteristics is of importance for a proper design of an efficient H− ion source. This paper discusses this subject based upon two kind of experiments i) experiments with plasma electrodes covered with tungsten or tantalum evaporated from filaments made of these metals ii) experiments with caesium covered plasma electrode. The tantalum and caesium covered plasma electrodes lead to an enhancement of the extracted H− ion current by a factor of 2 compared to the tungsten coverage on the plasma electrode. The electron current is also affected by the material covering the plasma electrode. The reasons for observing these characteristics are also elucidated.