22P041-P
Multi-emission Line High-resolution Plasma Spectroscopy on
a Micro-hollow Cathode Atmospheric-pressure Helium Discharge
マイクロホローカソード大気圧ヘリウム放電の複数発光線同時高分解プラズマ計測
Masahiro Hasuo1, Shinichi Namba2, Keisuke Fujii1, Taiichi Shikama1 and Hiromichi Azuma1
蓮尾昌裕1, 難波愼一2，藤井恵介1，四竈泰一1，東 裕通1
1, Graduate School of Enginerring, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan
2, Graduate School of Enginerring, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Japan
1, 京都大学大学院工学研究科 〒606-8501 京都市左京区吉田本町
2, 広島大学大学院工学研究科 〒739-8527 東広島市鏡山1-4-1
We have simultaneously observed line profiles of neutral helium 23P-33D (587.6 nm), 21P-31D (667.8 nm)
and 23P-33S (706.5 nm) emissions from a 1 mm-diameter hollow cathode plasma at gas pressures from0.03 to 1 atm. We estimate the Lorentz and Gauss widths from the observed profiles, and then deduce theelectron temperature and density and the gas temperature from the widths. The electron density increases as
an increase in the gas pressure with the power of about 0.8, and reaches 9.4 x 1021 m-3 at 1 atm. The electronand gas temperatures are estimated to be about 15000 K and 600 K, respectively, and their pressure
dependences are not clearly observed.

1. Introduction
Atmospheric-pressure plasmas gather muchnotice for many kinds of applications because of
their freedom from vacuum enclosure and highparticle density [1]. However, since the size of suchplasmas is usually small, the plasma diagnostics
with an electric probe cannot be applied.
Recently, Namba et al. have demonstrated
passive emission spectroscopy for the hydrogen
Balmer-α and helium 21P-31D line profiles of a
micro-hollow cathode atmospheric-pressure plasma
to estimate the electron density, ne, and temperature,
Te, and the gas temperature, Tg [2]. Here, we
propose a method to estimate ne, Te and Tg in such aplasma with observing line profiles of helium
21P-31D (667.8 nm), 23P-33S (706.5 nm) and23P-33D (587.6 nm) emissions simultaneously. For
the purpose, we use a multi-wavelength-range
fine-resolution (MF) spectrometer developed byourselves [3].
2. Experiment
Figure 1 shows a schematic illustration of theexperimental set-up. A hollow cathode DC plasma
is generated between brass cathode and anodeseparated by a 1 mm-thick ceramic insulator plate,
all of which have an 1 mm-diameter discharge holein the center [2]. The discharge chamber equips an
observation window from the cathode side.
Emission from the plasma is collected by anachromatic lens and introduced to the entrance slitof the MF spectrometer [2]. The introduced
emission is collimated to be a parallel beam by aconcave mirror (Mc; focal length: f = 54 mm). The
parallel light beam is incident on a diffraction
grating (2400 grooves/mm). The diffracted lightbeams are focused on a CCD detector (Andor,DV435-BV) by three concave mirrors (M587.6,M667.8, M706.5; f = 54 mm) set at the locations whichcorrespond to the wavelengths of the observed
emission lines.

Fig. 1. A schematic illustration of the experimental
set-up and MF spectrometer.

3. Results and discussion
Figure 2 shows examples of the observed spectra
at gas pressures of 0.03, 0.14, 0.52 and 1 atm
together with the spectra of a low-pressure helium
glow discharge. The pressure dependentbroadenings are clearly seen for all the lines, andthe magnitudes of the broadening are different fromeach other. The broadening is caused by electron,
ion and atom collisions, which contribute theLorentz width, WL, and the gas temperature, which
contributes the Gauss width, WG. The natural
broadening caused by spontaneous emission is
negligible in comparison with the observed widths.

WL at the pressures higher than 0.2 atm in thefollowing analysis.
For the observed emission lines, the Stark
broadening coefficients caused by electron and ion
collisions have been calculated as a function of Te[4] and the broadening coefficients due to atom
collisions have been measured at several Tg [5-8].
Since these broadening coefficients are different toeach other for the emission lines, we numericallysolved a simultaneous equation for the measured
three WL to estimate ne, Te and Tg.
Figure 4 shows the results. It is found that neincreases as an increase in the gas pressure with thepower of ~ 0.8, while the dependence of Te and Tg
are not clearly observed. From ne and Tg, the degreeof ionization at 1 atm is estimated to be 7 x 10-4.

Fig. 2. Observed line profiles of the 21P-31D (667.8 nm),23P-33D (587.6 nm) and 23P-33S (706.5 nm) emissions.
+ : data points. The lines are the fitted results with a
Voigt function convoluted to the instrumental function.

In order to extract WL and WG from fitting to theobserved spectra, we have to determine theinstrumental function of the MF spectrometer. For
this purpose, we measured and reproduced
respective spectra of a low-pressure helium glow
discharge, which have negligible Lorentz widths
and a well-determined Gauss widths from the gastemperature. Fig. 3 shows the extracted WL and WG.
The fitted results with convolution of the
instrumental function are shown by lines in Fig. 2.

Fig. 3. (a) WL and (b) WG as a function of gas pressure.
The horizontal lines are the instrumental widths.

At lower pressures, WL became smaller than theinstrumental Lorentz width, WLI. On the other hand,
WG is smaller than the instrumental Gauss width,WGI, in almost all the cases. The scatter of WG
increases with an increase in the gas pressure
because WL becomes one order of magnitude larger
than WG at higher pressures, and then the accuracyof the WG estimation decreases. Therefore, we use

Fig. 4. (a) ne and (b) Te and (c) Tg as a function of gas
pressure.

Acknowledgments
This work was partly supported by Grant-in-Aidfor Scientific Research (B) (No.21340170).
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