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Self-Reversal in Hydrogen Lyman-α Line Profile

Motoshi GOTO and Shigeru MORITA

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

(Received 8 December 2009 / Accepted 20 March 2010 / Published 10 December 2010)

Abstract

A detailed spectral profile of the Lyman-α line of neutral hydrogen, i.e., the transition from n = 2 to the ground state, where n is the principal quantum number, was measured with a vacuum ultraviolet spectrometer for the plasma in the Large Helical Device. Self-reversal was observed in the spectral profile when the plasma density was increased with repetitive injection of hydrogen pellets. A one-dimensional radiation transport model was used for creating the Lyman-α spectral profile, for which an emission and absorption medium with a slab geometry and constant plasma parameters were assumed. The population density of the n = 2 level generally has a peaked spatial profile even with constant ground state density because of the reabsorption effect which is essential for the emergence of self-reversal in the spectral profile. We used the n = 2 level population distribution in the medium by Molisch et al. [Radiation Trapping in Atomic Vapours, Oxford University, Oxford, 1998] and evaluated the Lyman-α spectral profile as a function of the optical thickness. The observed line profile was found to be well fitted, for example, with a ground state density of 5.2 × 10¹⁸ m⁻³ and a medium thickness of 10 cm when a Lorentzian profile having a full width at half maximum of 0.0018 nm is adopted for the emission and absorption coefficients.

Copyright (c) 2010 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

Lyman-α, self-reversal, reabsorption effect, LHD, internal diffusion barrier

DOI: 10.1585/pfr.5.S2089

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References

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This paper may be cited as follows:

Motoshi GOTO and Shigeru MORITA, Plasma Fusion Res. 5, S2089 (2010).