Importance of electric field for H− extraction in a volume-type hydrogen negative ion source

The effect upon extraction of negative hydrogen ions (H(-)) due to electric field near the extractor hole of a H(-) source is studied experimentally and theoretically. Probe measurements show that the extraction electric field penetrates into the plasma in the region near the extractor hole. Based on this observation a three-dimensional H(-) trajectory calculation that takes into account the local electric field distribution near the plasma electrode has been carried out. The validity of the trajectory calculation was examined by comparing the results with experimentally measured changes in H(-) current detected by a Faraday cup due to irradiation of a pulse laser beam in the region close to the extractor hole. The calculation results qualitatively explain the changes in H(-) current observed in the experiment. The calculation results also predict that the amount of H(-) current passing through the extractor hole changes with the electric field: the penetration of the electric field substantially enhances the H(-) extraction current, because it produces an electric field to attract H(-) toward the extraction hole.