28pC02
Hα イメージング分光計測による大型負イオン源内の水素負イオン挙動の解明
Evaluation of H- Behavior in Large Negative Ion Sourceby Hα Imaging Spectroscopy
池田勝則、中野治久、津守克嘉、木﨑雅志、永岡賢一、長壁正樹、竹入康彦、金子修
Katsunori IKEDA, Haruhisa NAKANO, Katsuyoshi TSUMORI, Masashi KISAKI,
Kenichi NAGAOKA, Masaki OSAKABE, Yasuhiko TAKEIRI and Osamu KANEKO
核融合科学研究所
National Institute for Fusion ScienceHydrogen negative ions have been effectively produced on a cesium-covered
metal plasma grid (PG) surface faced to arc discharge in an ion source for neutralbeam injectors on LHD [1] and JT-60U [2]. For the international thermonuclear
experimental reactor (ITER), high current D- source using RF discharge is developing
now [3]. It will be required to involve operating stable 1-hour beam generation withhigh energy (1 MeV) and uniform beam extraction from a wide grids with a lot ofapertures. In contrast, behavior of negative ions in the extraction region near thePG surface is a key issue for such severe conditions, but it is not well understood.
We developed Hα imaging spectroscopy to investigate H- dynamics during beam
extraction, and was installed on the 1/3 scaled development negative ion source inNIFS. A combination with three optical filters and an aspherical lens has been set
on the bias insulator sandwiched between the arc chamber and the PG flange asshown in Figure 1. Spectral images were transferred to a CCD detector by a glass
fiber image conduit for high voltage insulation. The center of the sight line set on
the right above extraction apertures at z = 11 mm from the PG, and the viewing
angle covered from the magnetic filter flange to the PG. We took a 16-bit
monochrome image data with the 80 ms exposure time during arc discharge.

Fig. 1 Schematic drawing of a hydrogen negative
ion source. Hα imaging system has been installedon the bias insulator.

Fig. 2 Waveform of the hydrogen arc discharge
power and the H- density (a), and the signal
intensity of hydrogen emissions (b).

Fig. 3 Two dimensional image of the reduction
Hα signal between before and after extraction
voltage applied.

Fig. 4 Signal reduction profile of Hα along
the z-axis perpendicular to the plasma gridsurface on the apertures (a) and on the PG
surface (b).

Figure 2(a) shows the waveform of the arc discharge power and the H- density
measured by a cavity ring-down spectroscopy [4] under the condition of Cs seedingwith 0.2 Pa hydrogen gas pressure and 0.2 V low bias voltages. Discharge condition
was constant before and after beam extraction applied Vext = -8 kV high extraction
voltage between the PG and the extraction grid (EG) which was short-circuited by
the grounded grid. The H- density at the position of z = 9 mm was reduced during
beam extraction. We observed same signal drop on the Hα line intensity measured
by a visible spectrometer at z = 11 mm as shown in Fig 2(b). The reduction of Hαsignal (ΔHα) was mainly caused by the reduction of excited hydrogen populationfrom the mutual neutralization processes expressed as ; Hm+ + H- → H(n=3) + Hm
(m=1,2,3), due to the decreasing of H- density. We found an amazing 2D structure ofthe reduction ΔHα between before and after applied extraction voltage in theextraction region as shown in Figure 3. In the region closed to the PG (z < 10 mm),the reduction of the Hα signal near the apertures is much larger than that near thePG surface. Reduction of the Hα intensity is observed toward the plasma inside
farther than 20 mm from the PG surface as shown in Figure 4. This result indicatesthat H- ions produced at the PG surface is penetrated to the depth of the extraction
region, which is consistent with the H- distribution measured by CRDS [4]. It isconsidered that H- ions accumulated at a position away from the PG are flowing
toward the PG apertures.
[1] Y. Takeiri, O. Kaneko, K. Tsumori, et al., Nucl. Fusion 46, S199 (2006).
[2] A. Kojima, M. Hanada, Y. Tanaka, et al., Nucl. Fusion 51, 083049 (2011).
[3] R. Hemsworth, H. Decamps, J. Graceffa, et al., Nucl. Fusion 49, 045006 (2009).
[4] H. Nakano, K. Tsumori, et. al., "H- Density Response to Applied Bias and Extraction
Voltage in Negative Hydrogen Ion Source" AIP Conference Proceeding of 3rd International
Symposium on Negative Ions, Beams and Sources, Jyväskylä (2012), O-215.