22P085-R
Spectral characteristics of saturation spectroscopy at the Balmer-α line ofatomic hydrogen in magnetized hydrogen plasma
有磁場水素プラズマにおける水素原子バルマーアルファ線の飽和吸収分光スペクトル特性
Koichi Sasaki1 , Renge Asakawa2 , Motoshi Goto3 , and Nader Sadeghi4
佐々木 浩一1 ，浅川 れんげ 2 ，後藤 基志 3 ，サデギ ナダー 4
1

Division of Quantum Science and Engineering, Hokkaido University,Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan1
北海道大学大学院工学研究院量子理工学部門，〒 060-8628 札幌市北区北 13 条西 8 丁目
2
Department of Electrical Engineering and Computer Science, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan2
名古屋大学大学院工学研究科電子情報システム専攻，〒 464-8603 名古屋市千種区不老町3National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
3核融合科学研究所，〒 509-5292 岐阜県土岐市下石町 322-6
4
LIPhy, Universite Joseph Fourier & CNRS, Grenoble, France
4
ヨゼフフーリエ大学グルノーブル校およびフランス国立科学研究センター，フランス

We developed a system of saturation spectroscopy at the Balmer-α line of atomic hydrogen with
the intention of applying it to the diagnostics of LHD plasmas. The spectrum was composed of abroadband oﬀset and many peaks which were assigned as fine-structure components of the Balmerα line with Zeeman splitting. The saturation parameter or the amplitude of the saturation spectrum
was discussed by referring to the theory of saturation spectroscopy. In addition, the physical originof the broadband oﬀset component was speculated on the basis of the pressure dependence of thesaturation spectrum.
1. Introduction
Laser absorption spectroscopy has a much finer
resolution than optical emission spectroscopy, and is
widely used for investigating the velocity distributionfunctions of various species in plasmas by measuring Doppler-broadened spectra of their transition lines.
On the other hand, studies of the particle balance inLarge Helical Device (LHD) at the National Institutefor Fusion Science (NIFS) need a spectral resolution
finer than the Doppler broadening. A required information for understanding the particle balance is thelocation of ionization (H + e → H+ + e + e). The location of ionization is expected to be almost the sameas the location of excitation (H + e → H∗ + e) in theLHD plasma. In principle, the location of excitation
is known by measuring the spectrum of the Balmer-αline of atomic hydrogen with Zeeman splitting, since
the spatial distribution of the magnetic field strength isknown in LHD and the spectrum with Zeeman splitting is a fingerprint of the magnetic field strength.
However, because of a high temperature of atomic hydrogen in LHD, the details of Zeeman splitting aremasked by the Doppler broadening. Under the aforementioned background, in this work, we developed asystem of saturation spectroscopy at the Balmer-α lineof atomic hydrogen.
2. Experiment
We examined the performance of a saturation
spectroscopy system in a linear machine with a uniform magnetic field of 350 G along the axis. The
experimental apparatus is schematically shown inFig. 1. The light source for saturation spectroscopy
was an oscillator-amplifier system of diode lasers,
which yielded tunable, single-mode, cw radiation witha power of approximately 200 mW. The wavelengthof the master oscillator (New Focus, Vortex II) wasscanned for a range of 50 GHz within a period of 5 ms.
The power of the master oscillator was approximately
15 mW, and a part of the master oscillator beam (< 1mW) was picked up using a beam splitter and was usedas the probe beam. A collimator was used for avoiding
the divergence of the probe beam. In addition, a partof the master oscillator beam was picked up to measure the rough wavelength and the frequency scan using a wavemeter and a Fabry-Pérot spectrum analyzer,
respectively. The other part of the master oscillator
beam was injected into a diode laser amplifier (TOP-Digital
oscilloscope

PROBE
BEAM
Collimator
λ/2

1

Pin hole
APD

Antenna

PUMPBEAM

Diode laser
amplifier

Isolator
Isolator

Wavemeter

0.8leb 0.6orpα 0.4

Diode laseroscillator

Pump: OFF (a)0.2

Pump: ON

0-10 -5 0 5 10 15 20
Relative frequency (GHz)
0.4

Spectrum
analyzer

Figure 1: Experimental apparatus.

TICA, BoosTA) to obtain the intense pump beam.
The probe and pump beams were launched into theeb 0.3
roplasma source from the counter axial directions, and0 pα
they were overlapped carefully. Therefore, the present
eb 0.2experimental configuration was sensitive to the σ tranor
psitions between energy levels with Zeeman splitting.
α∆ 0.1
A beam splitter was used for detecting the transmitted probe beam using an avalanche photo diode. A pin(b)0hole transmitted the probe beam, while it eliminated-2 0 2 4 6 8 10 12
the optical emission from the plasma eﬃciently. The
Relative frequency (GHz)electrical signal from the avalanche photo diode was
Figure 2: Spectra of (a) simple laser absorption specrecorded using a digital oscilloscope, together with thetroscopy and (b) saturation spectroscopy.
signal from the spectrum analyzer.
σ transitions of the Balmer-α line at 350 G and theircross-over signals.
3. Results
Spectra of the probe beam transmitted through aplasma are shown in Fig. 2(a), which was observed 4. Discussion
According to a theory of saturation spectroscopy,
with and without the pump laser beam. The origin
theamplitudeof the peak in the saturation spectrum isof the relative frequency axis corresponds to a wavelength of 656.285 nm. The vertical axis shows the directly related to the saturation parameter. From theabsorbance, which is the product between the absorp- amplitude of the peaks shown in Fig. 2(b), the saturation coeﬃcient for the probe beam (αprobe ) and the ab- tion parameter was estimated to be less than 0.2 ∼ 0.3.
sorption length (ℓ), and is obtained from the intensi- On the other hand, the theoretical saturation parameter,
ties of the incident (I0 ) and transmitted (It ) probe laser which is given by the laser intensity, the relaxation frebeams using the Lambert-Beer law αℓ = − ln(It /I0 ). quencies of relevant energy levels, and the transition
The pump laser power in front of the optical window probability, was evaluated to be ∼ 5. Hence, the amfor launching the pump beam into the plasma was 146 plitude of the saturation spectrum shown in Fig. 2(b)mW. When the pump laser beam was switched oﬀ, we is smaller than that expected by the theoretical satuobserved a smooth spectrum with two broad peaks, as ration parameter. Another experimental result that isshown in Fig. 2(a), which was understood by the over- diﬃcult to be understood by the theory of saturation
laps of many Doppler-broadened fine-structure com- spectroscopy is the existence of the broadband oﬀsetponents of the Balmer-α line [1]. On the other hand, component in the saturation spectrum. We will diswhen the pump beam was switched on, we observed cuss the possibility of collisional transport in the vemany dips in the spectrum of the probe beam, as shown locity space at the conference, by referring the pressure dependences of the amplitudes of the peak andin Fig. 2(a).
Figure 2(b) shows a saturation spectrum, which oﬀset components.
was obtained by dividing the diﬀerence between thetwo spectra (∆αprobe ) shown in Fig. 2(a) by the spec- References
trum without the pump laser beam (α0probe ). We ob- [1] M. Aramaki, Y. Okumura, M. Goto, S. Muto, S.
served many peaks with high spectral resolutions. The
Morita, and K. Sasaki: Jpn. J. Appl. Phys. 44
peaks shown in Fig. 2(b) are found to be understood as(2005) 6759.
/