Abstract
The achievements of the deuterium beam operation of a negative-ion-based neutral beam injector (N-NBI) in the large helical device (LHD) are reported. In beam operation in LHD-NBIs, both hydrogen (H) and deuterium (D) neutral beams were generated by changing the operation gas using the same accelerator. The maximum accelerated deuterium negative-ion current () reaches 46.2 A from two beam sources with the averaged current density being 190 A m−2 for 2 s, and the extracted electron to accelerated ion current ratio () increases to 0.39 using 5.6 V high bias voltage in the first deuterium operation in 2017. An increase of electron density in the vicinity of the plasma grid (PG) surface, which is considered the main reason for the increase of co-extracted electrons in a beam, is confirmed by the half-size research negative-ion source in the neutral beam test stand at the National Institute for Fusion Science (NIFS). The deuterium negative-ion density is also larger than the hydrogen negative-ion density in the vicinity of the PG surface using the same discharge conditions. In the latest experimental campaign in 2018, increases to 55.4 A with the averaged current density being 233 A m−2 for 1.5 s using the shot extraction gap length. The low of 0.31 can be maintained by using high discharge power. The various parameters mentioned above are defined in detail below.
Highlights
Production of high intensity negative ion beams with low co-extracted electrons is a common issue for a negative-ion-based neutral beam injector (N-NBI) for heating and current drive in magnetically confined fusion device
Increase of co-extracted electrons for deuterium negative ion beam operation has been observed by the ITER half size source [13], which is an important issue for the realization of ITER-NBI
After the Large Helical Device (LHD) experiment campaign, we investigated the characteristics of the negative ion density and the electron density by a research negative ion source (RNIS) in the neutral beam test stand (NBTS)
Summary
Production of high intensity negative ion beams with low co-extracted electrons is a common issue for a negative-ion-based neutral beam injector (N-NBI) for heating and current drive in magnetically confined fusion device. We generated both hydrogen and deuterium negative-ion beams in N-NBIs by changing the operation gas using the same accelerator. Comparison of the beam properties such as the extracted negative ion current and the co-extracted electron current and source properties with H2 and D2 gases will clarify the production and extraction mechanism of the negative ions This should contribute to the ITER and future NBIs. This paper describes the results of hydrogen and deuterium beam operations in LHD-NBI. Hydrogen negative ion current(Iacc(H) ):This is the electrical current to the grounded grid of the accelerator and the components downstream of the grounded grid when the ion source is operated with H2 gas. The gas flow is kept constant before and during source operation and ion extraction, but the gas pressure in the source will change significantly during source operation because the gas is heated by the discharge, there will be significant dissociation of the gas, and because ions are extracted from the source
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