7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Absorption length dependence of He I resonance line in He arcjet plasma

K. Okuda1, H. Kawazome2 A. Saito1, J. Kono1, R. Hamada1, Y. Kanbara1, R. Suzuki1, N. Ohno3,
N. Tamura4, Y. Hayashi4, Y. Hamaji4, S. Masuzaki4, H. Okuno5, K.Yamasaki1 and S.Namba1
1
Hiroshima University,
2
National Institute of Technology, Kagawa College, 3Nagoya University,
4
National Institute for Fusion Science,5Nishina Center for Accelerator-Based Science, RIKEN
e-mail: m224257@hiroshima-u.ac.jp
Emission spectrum analysis of plasma and neutral
particles is widely used to estimate plasma parameterssuch as electron temperature and density [1]. In estimatingplasma parameters using emission spectra, it is assumed
that the observed emission from the plasma is notabsorbed or scattered until it reaches the detector.
However, if the optical thickness cannot be neglected, it isnecessary to take into account the absorption of emission
by atoms or ions over the optical path, which is termed
self-absorption. Therefore, it is essential to quantitatively
evaluate the effect of self-absorption to determine plasma
parameters using emission spectra accurately. The He I
resonance lines, which lie in the vacuum ultraviolet region,
are the example where the self-absorption effect cannot beneglected. Since the emission of He I resonance lines isvery intense, various information can be expected to beobtained by analyzing the intensity ratio of emission
spectra and spectral shape. In this study, the dependenceof the He I resonance line intensity on the absorption
length was experimentally examined to estimate the selfabsorption effect quantitatively. In order to vary theabsorption length, we used a vacuum tube installed analuminum filter, where the VUV light can transmit it, but
neutral atom cannot, thus controlling the absorption length.
A schematic diagram of the experiment is shown in Fig.
1. In this study, we measured He I resonance lines (1s 1S– np 1P) from the helium cascade arc plasma, whosediameter was 8 mm. A vacuum ultraviolet spectrometerand an X-ray CCD camera were used to measure theemission spectra. A photograph of the vacuum tube used
to vary the optical length is shown in Fig. 2. The key
feature of the observation system is that the viewing angle
remained constant while the length of the absorption
region was changed. The internal collimator with an innerdiameter of 5 mm determined the solid angle to observe
the He I resonance line. In addition, the aluminum filter atthe observation window and vacuum bellows tube wereplaced between the internal collimator and the plasma, so
the optical path from the filter to the CCD camera waskept in high vacuum, so that no self-absorption occurred
between the filter and CCD camera. The inner diameter ofthe observation window and filter was 12 mm. The
observation window was large enough so that theobservation angle did not change when the observation
window was moved along the optical axis. The
transmittance for 58.4-nm wavelength (1s 1S - 2p 1P) isapproximately 0.17. The VUV spectroscopic systemobserved the anode of the He arc plasma within therange of 5.5 × 8.8 mm. The observation window wasmoved 160 mm by an actuator. By moving theobservation window and measuring the He I resonancelines, the dependence of the line intensity on the lengthof the absorption region was obtained, which represents
the effect of the self-absorption length along the opticalpath.
We will present the details of this measurement
system and the obtained data.
This work was supported by JSPS Grant-in-Aid for
Scientific Research JP23K033610B.

Fig. 1. A schematic of the experimental setup.

Fig. 2. Photo of the vacuum tube. The actuator wasused to vary the optical length.
Reference[1] R.W.P. McWhirter, in Plasma Diagnostic Techniques,edited by R. H. Huddlestone and S. L. Leonard (Academic,New York, 1965).