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.
Divertors are required to remove the impurities such as helium ashes in a magnetic conferment-type fusion reactor. Detached plasma is one of the major systems to suppress the heat load from a core plasma. The electron temperature and density have been measured with the linear divertor simulation device TPDsheet IV. To model the ion density, a zero-dimensional model has been developed for solving the system of rate balance equations. We have confirmed the characteristic of ion density on gas pressure with rate equation. The molecular ion density decreases with the gas pressure, therefore molecular recombination reaction may contribute to detached plasma generation.
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