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Author(s):
V.S. Voitsenya, D.I. Naidenkova, Y. Kubota, S. Masuzaki, A. Sagara, K. Yamazaki
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Title:
On the Possibility to Increase Efficiency of Conditioning of Vacuum Surfaces by Using a Discharge in a Hydrogen-noble Gas Mixture
Date of publication:
Apr. 2004
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Key words:
Cleaning of in-vessel components, glow discharge, hydrocarbon film removal, mixture of hydrogen with rare gases, hydride ions
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Abstract:
Because of using the carbon-based wall protection in fusion devices, many surfaces of inner components, remote from plasma, become to be coated with carbon film. Between components subjected to C film deposition are elements of plasma diagnostics, e.g., in-vessel mirrors and windows of laser and optical methods. From time to time these elements have to be cleaned from the contaminating films. One possible method is to use a cleaning discharge with low temperature plasma (Te leq 5 eV) however, at that the efficiency of removal of hydrocarbon film molecules is quite small, ~2-3 %. The rate of C film cleaning could be significantly increased by providing discharge in oxygen (with efficiency up to 30%), but the use of this gas is impossible in fusion devices. The alternative method to enhance the efficiency of C film cleaning is to use the plasma produced by discharge in mixture of hydrogen with one of heavy rare gases, as was suggested in [4]. This approach is not in contradiction with the general conception of the fusion reactor project, where the addition of a rare gas (heavier than helium) is considered as a real method to control the periphery plasma parameters. In the present paper we analyze the published data on all important reactions in a low-temperature plasma produced in mixture of hydrogen with rare gases: Ne, Ar, Xe, Kr. The main peculiarity of such plasma is the appearance of a group of hydride ions, namely NeH^+, ArH^+, XeH^+, KrH^+, which probably is the main factor leading to increase of the rate of removing C film from contaminated surfaces compare to pure hydrogen plasma but practically do not exist as neutrals. The comparison data on cross sections of different reactions demonstrates clearly that between all rare gases, the highest concentrations of hydride ions would be in the Ar-H_2 mixture plasma.
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