Isotope effect on H− / D − volume production is studied by measuring both VUV emission and negative ion density in the source. In a double plasma type source, under some discharge conditions, extracted D − currents are nearly the same as H − currents, although VUV emission intensity (corresponding to production of vibrationally excited molecules) in D2 plasmas is slightly lower than that in H2 plasmas. Considering the factor √ 2 due to mass difference, D − ion density in the extraction region of the source is higher than H − ion density. In another experiment with a rectangular arc chamber, axial distributions of H − / D − ion densities in the source are measured directly using a laser photodetachment method. Relationship between H − / D − production and plasma parameter control with using a magnetic filter (MF) is discussed. Furthermore, relative intensities of extracted negative ion currents are discussed compared with the negative ion densities in the source. Production and control of D2 plasmas are well realized with the MF including good combination between the filament position and field intensity of the MF. Extracted H − and D − currents depend directly on negative ion densities in the source.

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A beam steering technique using aperture displacement was examined to correct the negative ion beam deflection due to the magnetic field for electron suppression in a large-area multibeamlet H− source. The total deflection angle was estimated, including the effect of the deflection by the electron suppression magnetic field and the beam steering by the aperture displacement, both by linear optics theory and by three-dimensional beam trajectory simulation. Two methods were compared; one used the displacement both of the grounded grid (GG) apertures and of the exit part of the extraction grid (EG exit) apertures, and the other used the displacement only of the EG exit. The beam steering experiments were performed using a large-area multibeamlet H− source with both displacement methods, and the results were compared with the theoretical estimations. As a result, both methods were effective to correct the beam deflection. In particular, the displacement of only the EG exit with a simplified displacement structure achieved a large steering angle by a small displacement. The steering angle in the experiment was a few mrad smaller than the estimations. Based on these results, the aperture displacement of the EG exit was applied to the 1/5 segment of a H− source in the Large-Helical-Device neutral beam injector, where the GG apertures are displaced only focusing of the large-area multibeamlet. In this case, 1.0 mm of the displacement is concluded to be proper to sufficiently compensate the beam deflection at 180 keV, from extrapolation of the beam energy characteristics at 100–140 kev.

Using the photodetachment technique we investigated the hydrogen negative ion density in the extraction region of a magnetically filtered multicusp ion source when argon is added to hydrogen. We found that the negative-ion density goes up at most by a factor of 1.5 when argon is added to low base hydrogen pressure (0.05 or 0.1 Pa), but goes down when argon is added to higher initial hydrogen pressure (0.5 or 1 Pa). Adding argon did not enhance the Werner and Lyman bands in the vacuum ultraviolet, which indicates that argon addition does not increase the production rate of vibrationally excited H2 molecules. The increase of the negative-ion density by adding argon into a low-pressure hydrogen discharge is probably due to the increase of the low-energy electron density.

H- ion density in the extraction region of a volume-production-type ion source is measured, using the photodetachment method, and directly compared to H− ion beam extracted from the ion source in order to understand the effective production of H− ions. It is observed that there is qualitative similarity between H− density and extracted H− beam current, as expected. Characteristics of H− ion production under two values of magnetic strength are compared to study the effect of the filter field. It was found that the weaker magnetic field of 750 G cm is not sufficient for the filter field since high energy electrons can come into the extraction region across the magnetic field.

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