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A Simple Method of H/He Influx Ratio Measurement as a Monitor for Machine Operation in LHD

Shigeru MORITA, Motoshi GOTO, Suguru MASUZAKI, Naoko ASHIKAWA and LHD experimental group

National Institute for Fusion Science, Toki 509-5292, Gifu, Japan

(Received 20 November 2007 / Accepted 5 March 2008 / Published 6 August 2008)

Abstract

In order to observe edge neutral recycling for fusion machine operation, the absolute values of H_α and HeI emissions have been usually analyzed with measurement of edge density and temperature. On the other hand, it is known that the temperature and density dependences of each line can be negated by taking the ratio between the two lines. The intensity ratio of H_α (6563 Å) to HeI (5876 Å or 6678 Å) visible spectral lines is adopted, and the results are presented here instead of the method usually used for monitoring the edge particle recycling and the effect of wall conditioning in LHD. The ionization events per photon were calculated for both emissions using a collisional-radiative model, and the ratio of H⁺ flux (≡ H⁰ influx) to He⁺ flux (≡ He⁰ influx) was obtained. The H_α and HeI (5876 Å) emissions in LHD have been measured using a monitor assembly with an interference filter and optical fiber array. The H⁺ /He⁺ flux ratio has been thus evaluated by integrating the H_α and HeI emissions from a 10-channel toroidal array. As an example of the measurement, all discharges performed during the past 9 years in LHD have been analyzed. The present method shows that the aftereffects of the H₂ and He glow discharge cleaning for vacuum wall conditioning on the LHD discharges become clearly visible. The replacement of deposited atoms on the carbon divertor plates was also analyzed. It is found that the replacement is completed by 40-50 shot repetition of discharges. These results indicate the effectiveness of the present method as a good monitor for fusion machine operation.

Copyright (c) 2008 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

H_α, HeI, H/He ratio, wall conditioning, plasma facing material, visible spectroscopy, LHD

DOI: 10.1585/pfr.3.S1037

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This paper may be cited as follows:

Shigeru MORITA, Motoshi GOTO, Suguru MASUZAKI, Naoko ASHIKAWA and LHD experimental group, Plasma Fusion Res. 3, S1037 (2008).