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Abstract

Reliable experimental determination of the mean effective charge (Z(eff)) of the plasma is of enormous importance for the impurity control in high temperature plasmas, allowing the performance of various discharge scenarios to be assessed and the benchmarking of model calculations, ideally accompanied by measurements of the Z(eff) profile. The Z(eff) is usually calculated using data from line-integrated visible bremsstrahlung measurements, electron density and temperature profiles, and the plasma geometry. On Tore Supra (TS) it had been noticed in the past that hot spots on the inner wall, MARFEs or supra thermal electrons can falsify the signal of the visible bremsstrahlung. More significant, however, was a systematic overestimation of the edge channels, giving unreasonably high edge Z(eff) values of 30 or even higher. The analysis of dedicated discharges in the present configuration of TS leads us to conclude that this is due to the influence of vessel wall reflections. A newly developed inversion method which takes reflections into account shows that the presence of reflections leads to an overestimation of the outer channels in the reconstruction of the Z(eff) profile. This is due to the larger relative contribution of the reflections on the edge channels due to the shorter path length through the plasma and the lower bremsstrahlung emission at the edge. We present a model which takes into account this effect and allows a correction procedure to be applied when calculating the Z(eff) profile. The TS data can be modelled assuming Lambertian (diffusive) reflections with a reflectivity coefficient of 20%-25%, which is in agreement with measurements of the reflectivity of the inner wall of the TS vacuum chamber.

Additional information

Authors: SCHUNKE B, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);HUYSMANS G T A, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR);THOMAS P R, Département de Recherches sur la Fusion Contrôlée, Association Euratom-CEA sur la Fusion, CEA Cadarache, Saint-Paul-lez-Durance (FR)
Bibliographic Reference: An article published in: Review of Scientific Instruments, Vol. 76 p.083501 1-5
Availability: This article can be accessed online by subscribers, and can be ordered online by non-subscribers, at: http://rsi.aip.org/
Record Number: 200518351 / Last updated on: 2005-10-04
Category: PUBLICATION
Original language: en
Available languages: en