H. Nakano

K. "Sasaki

K. Tsumori

M. Goto

M." Kisaki

S. Nishiyama

National Institute for Fusion Science (NIFS-Repository)

The investigations on the production processes andextraction mechanisms of negative hydrogen ions arestill insuﬃcient in cesium-assisted negative hydrogen ionsource for NBI heating. We started this research activityfrom this academic year with the intention of investigat-ing the behaviors of atomic hydrogen and cesium va-por in a negative hydrogen ion source. In this academicyear, we tried to detect the laser absorption signal inthe Balmer-α line of atomic hydrogen in the large-scale,arc-discharge negative hydrogen ion source in NIFS.The system for laser absorption spectroscopy isshown in Fig. 1. The light source was a diode laser (NewFocus, Vortex II), which emitted tunable single-modelaser radiation in the wavelength range of the Balmer-αline of atomic hydrogen. The laser beam passed throughthe plasma in the region close to the grid electrode (theplasma grid) for extracting negative ions. The transmit-ted laser beam was detected using a photodiode. Thewavelength and the scanning frequency of the laser beamwere monitored using a wavemeter and a Fabry-Perotinterferometer, respectively. The output power of thediode laser was approximately 15 mW, but the laserpower injected into the plasma was attenuated to avoidthe optical saturation.We observed no absorption signals in the geome-try of simple absorption spectroscopy even in a high arc-discharge power of ∼ 80 kW. This indicates a low densityof atomic hydrogen at the n = 2 state, which may be dueto a low electron temperature in the extraction region ofthe negative ion source. Then, we employed frequency-modulated absorption spectroscopy. In this method, thefrequency of the laser beam was modulated in two man-ners. One was the wide (∼ 100 GHz), slow (1 s) scan witha triangle waveform to measure the absorption spectrumin the entire wavelength range of the Doppler-broadenedBalmer-α line. The other frequency modulation had a si-nusoidal waveform at a frequency of 600 Hz. The secondfrequency modulation was superposed on the ﬁrst mod-ulation. The signal from the photodiode was connectedto a lock-in ampliﬁer which ampliﬁed the signal compo-nent at the second harmonic of the sinusoidal frequencymodulation (1.2 kHz). The amplitude of the second fre-quency modulation was optimized experimentally. Thismethod (hereafter “the 2f method”) can yield a signalwhich is similar to the second-order derivative of the ab-sorption line proﬁle with a much higher sensitivity thanthe simple absorption spectroscopy. The 2f componentin the amplitude modulation of the laser beam, whichoccurred simultaneously with the frequency modulation,was subtracted to obtain the correct 2f spectrum.???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????Fig. 1: System for laser absorption spectroscopy in thenegative hydrogen ion source in NIFS.Fig. 2: Examples of 2f spectra at the Balmer-α line ofatomic hydrogen.Figure 2 shows typical spectra obtained by the 2fmethod. It is understood from the principle of the 2fmethod that the positive peak is observed at the peak po-sition of the absorption line proﬁle. The negative peaksare observed at the inﬂection points of the absorptionline proﬁle. Therefore, we can estimate the absorptionline proﬁle of the Balmer-α line from the 2f spectrum.The Doppler-broadening of the absorption line proﬁleestimated from the 2f spectra gave us an estimation of∼ 3000 K as the translational temperature of atomic hy-drogen in the negative ion source. A greater absorptionwas observed at a higher arc-discharge power, but thetemperature of atomic hydrogen was insensitive to thearc-discharge power as shown in Fig. 2.218§9. Laser-aided Spectroscopic Diagnostics in a Negative Hydrogen Ion SourceSasaki, K., Nishiyama, S. (Hokkaido Univ.),  Nakano, H., Goto, M., Tsumori, K., Kisaki, M.

Laser-aided Spectroscopic Diagnostics in a Negative Hydrogen Ion Source

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