Characteristics of co-extracted electron beam current in sheet plasma-type cesium-free negative-ion source

•This study designs a TPDsheet-U to produce and control negative hydrogen ions in plasma without Cs.•The reduction of the co-extraction electron current is a common and essential issue for the negative ion source.•Co-extracted electrons are successfully suppressed by setting a soft magnetic plate on plasma grid. In this study, we designed a magnetized-sheet plasma produced by TPDsheet-U (Test plasma Produced by Direct current for sheet plasma upgrade) to produce and control negative hydrogen ions in plasma. A negative hydrogen ion (H−) is produced by volume production and extracted from the peripheral of the magnetized-sheet plasma by a two-grid system. This study represents the basic design of a single-aperture grid and the characteristics of the developed extraction system on TPDsheet-U. The reduction of the co-extraction electron current is a common and essential issue for the negative-ion source. Co-extracted electrons are successfully suppressed by setting a soft magnetic plate on plasma grid. We investigate herein the effects of the developed extraction system on the extracted H− and co-extracted electron beam current.