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The effective charge Z(eff), taken as a line-averaged signal in the W7-AS stellarator, is measured for ECH and NBI plasmas by using Bremsstrahlung in the visible region at a wavelength of 5202 A. Any contribution of line radiation from metallic impurities is negligible in the wavelength interval chosen. As a consequence, the Z(eff) value is successfully obtained even in the low density region of ECH plasmas. Typical values measured for Z(eff) range from 1 to 2 for NBI plasmas and from 1 to 3 for ECH plasmas in the case of a freshly boronised vessel wall. In some cases of counter-injected NBI, however, large Z(eff) values of 3 to 6 are measured. This behaviour is similar to that observed in CHS. The results of Z(eff) measurements on ATF and CHS heliotron/torsatron and the W7-AS advanced stellarator are compared. This analysis is repeated for the impurity behaviour of reheat-mode discharges in W7-AS. A correlation between the temporal behaviour of tau(p) and tau(E) can be observed in both W7-AS and CHS.

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Authors: MORITA S, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE);BALDZUHN J, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: IPP 111/199 EN (1994) 38 pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
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