We report the characteristics of extracted beam current in the test plasma produced by direct current for sheet plasma upgrade producing negative ions by volume production without cesium (Cs) seeding. The negative hydrogen ion beam is extracted by a two-grid extraction system, which is located at the periphery of the sheet plasma. Experimental observations show that (i) negative hydrogen ions are successfully extracted from the sheet plasma by single/multi-aperture grids and (ii) the ratio of the extracted electron current IEG(e) and the hydrogen negative ion current IEG(H−), IEG(e)/Ic(H−), decreases from 8.0 to 2.0 with an increase in the height of the electron fence (HEF), which is a filter that prevents electron diffusion from the extraction region.
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 (J
C(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 J
C(H−) was ∼7.5 mA cm−2 at an extraction voltage of 10 kV, a discharge current I
d of 90 A, and a gas pressure of 0.3 Pa. (ii) The electron current J
EG(e) with this soft magnetic filter (SMF) was only ∼10% of the electron current without the SMF; The J
EG(e)/J
C(H−) ratio was reduced below 0.4 at y < 18 mm for a single-aperture grid with the SMF at I
d = 50 A and external magnetic field strength of 38 mT.
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