Characteristics of the extracted negative-ion beam in a cesium-free negative-ion source using TPDsheet-U

The progress toward realizing a high-performance cesium (Cs)-free negative-ion source based on volume production in a magnetized sheet plasma device (TPDsheet-U) is reported. In the experiments, H− ions were successfully extracted from sheet plasma using single-/multi-aperture grids. The experimental results of the single-aperture grid show that the negative hydrogen ion current density (JC(H-)) performance of Cs-free H- ion beams in TPDsheet-U is about one-fourth that of Cs-containing H− ion sources in the ITER negative-ion neutral-beam injector (NNBI). (i) The JC(H-) was ~7.5 mA/cm2 at an extraction voltage of 10 kV, a discharge current Id of 90 A, and a gas pressure of 0.3 Pa. (ii) The electron current JEG(e) with this soft magnetic filter (SMF) was only ~10% of the electron current without the SMF; The JEG(e)/ JC(H-) ratio was reduced below 0.4 at y < 18 mm for a single-aperture grid with the SMF at Id = 50 A and external magnetic field strength of 38 mT.