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Description
Negative-ion based neutral beam injectors (N-NBI) are utilized for plasma heating and current drive in the Large Helical Device (LHD). In the previous operation, we achieved 2.9 MW deuterium beam injection using the negative ion source optimized hydrogen operation. The deuterium negative ion current was reduced to 55.4 A and the average current density was 223 A/m$^3$ which is 0.65 times lower than the nominal hydrogen negative ion current [1]. On the other hand, it is necessary to consider how the stripping loss due to the neutralization reaction in the accelerator affects the deuterium operation. The difference of the stripping loss between hydrogen and deuterium is examined using two different approaches. The first is the measurement of the optical beam emission. An optical line-of-sight (LOS) is set to the angle of 33$^\circ$ to the negative ion beam emitted from the ion source. The wavelength of beam emission spectrum reflects the energy distribution of beam particles by the Doppler effect. The low-energy peaks are distributed in the energy band corresponding to the extraction voltage, and also the flat energy tail is observed in the lower energy region. Secondly, the pressure distribution inside the accelerator is estimated from the accelerator structure and vacuum pressures in the beam line. Since the neutralization cross section is large at the low energy, the loss of negative ions at the extraction section is larger than that in the acceleration section. Particularly inside of the extraction grid, a lot of negative ions are lost and neutral particles with the same energy are produced. These neutrals are the cause of the low-energy peaks in the energy distribution. The beam emission spectrum is estimated from the energy distribution and the cross section of the Balmer alpha emission. It is confirmed that the peak spectrum is asymmetric with a flat tail on the low energy side. The fraction of particles lost inside the accelerator increases from 0.16 for hydrogen to 0.24 for deuterium. Assuming that the negative ion extraction efficiency is followed by the Child-Langmuir law, the value of the negative ion extraction efficiency is 0.84 for hydrogen and 0.53 for deuterium. Therefore, the current ratio of $I_{D^-}/I_{H^-}$ should be 0.63, which is considered to be the cause of the degradation of the injected beam power in deuterium operation.
[1] K. Ikeda, et. al., Nucl. Fusion 59 (2019) 076009.
