Investigation and Application of the Line Intensity Ratio Method for the Measurement

S. Kajita

, T. Hiraishi

, N. Ohno†, M. Okamoto

, T. Nakano

and S. Takamura
 
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
†EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan 
Japan Atomic Energy Agency, Ibaraki 311-0193, Japan
Abstract. The electron density and temperature obtained from the line intensity ratio method of He I (l = 667.8, 706.5 and
728.1 nm) are compared to the probe method in a divertor simulator NAGDIS-II. When a collisional radiative (CR) model
that does not include the effect of the radiation transport was used for the analysis, ne obtained from the spectroscopic method
was significantly higher than that from the electrostatic probe method. In the case that the effect of the radiation trapping is
taken into consideration using optical escape factor, the discrepancy becomes moderate. In hydrogen and deuterium mixture
plasmas, however, it was shown that the line intensity ratio method deduced the value that was significantly deviated from the
expected value from the CR model. It was shown that the increase of the density in 31S state or decrease of the density in 33S
was enhanced due to the injection of the molecules (hydrogen or deuterium).
Keywords: spectroscopy, collisional radiative model, line intensity ratio method
PACS: 52.25.Os, 52.70.Kz
INTRODUCTION
Electron temperature, Te, and density, ne, are important parameters for understanding the divertor physics of experimental
fusion devices. Electrostatic probe method and laser Thomson scattering (LTS) method [1] have been used for
the measurement of these parameters. However, former method could suffer damages due to high heat and particle
fluxes, and latter method requires good accessibility to the divertor region. Passive spectroscopies in combination with
an appropriate model for the excited population distribution could be the alternative methods.
With regards to He I, a collisional radiative (CR) model has been provided [2, 3], and some line intensity ratios
have been applied for the measurements. One candidate for the combinations of the line intensities is l = 728.1 nm
(21P 31S), l =667.8 nm (21P 31D), and l =706.5 nm (23P 33S). Although the spectroscopic method deduced
consistent value with other measurement method in some literatures, it has been recently reported from the results
in linear divertor simulators that the effects of radiation trapping on the population distribution of helium atoms are
considerable [4, 5]. In the present paper, the electron density and temperature evaluated from the line intensities 728.1,
706.5, and 667.8 nm are compared with those from the electrostatic probe in a divertor simulator NAGDIS (NAGoya).

DIvertor Simulator) – II. The effects of the radiation trapping of the resonance lines (11S n1P) on the evaluation of
ne and Te are discussed. Further, problems for the measurement method in mixture plasmas (H2 and D2) are shown.
EXPERIMENTAL SETUP
The experiments were performed in the divertor simulator NAGDIS-II [6]. Figure 1 (a) shows the schematic view of
the cross section of the NAGDIS-II. The plasmas, of which typical electron density ne and temperature Te are 1013
cm

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