Predicted effects of parallel temperature gradients on the overestimation of TCV divertor target Langmuir probe T(e) measurements
The tendency, in most tokamaks, for Langmuir probes embedded in divertor target plates under high recycling and detached conditions to yield electron temperatures higher than those observed by alternative methods (eg spectroscopy or divertor Thomson scattering) has long been known. In particular, the very low temperatures (~1 eV) associated with strongly recombining detached or partially detached divertor plasmas are often not reproduced by probes. Under such conditions, strong parallel field temperature and density gradients arise which may perturb the form of the electron velocity distribution, particularly at high velocities, since faster electrons originating further upstream of the divertor may travel collisionlessly to the targets. In previous work a numerical approach had been developed, allowing rapid computation of the electron velocity distribution, f(v), at the probe which is currently being cross-checked against a full kinetic PIC code. This simple numerical approach has been used in this contribution to generate velocity distributions and theoretical target probe I-V characteristics employing B2-Eirene simulations of parallel field Te and ne gradients. The results are conclusive: in TCV, at the low temperatures and high densities predicted by the code for high recycling conditions, faster electrons originating further upstream where Te is higher are too collisional to reach the probe without thermalising and the probe should read the local target temperature. We are thus obliged to conclude on the basis of this modelling that under these conditions the anomalously high measured target probe temperatures are due to alternative effects other than fast electrons.
Bibliographic Reference: An oral paper presented at: 15th International Conference on Plasma Surface Interactions in Controlled Fusion Devices (PSI-15) Organised by: National Institute for Fusion Science, Japan Held at: Gifu, Japan, May 27-31, 2002
Availability: The conference proceedings will be published in the Journal of Nuclear Materials This paper is also in report LRP 734/02, available free from: École Polytechnique Fédérale de Lausanne, Ecublens, CH-1015 Lausanne Fax +41-21-693474
ISBN: ISSN: 0458-5895
Record Number: 200215185 / Last updated on: 2002-09-06
Original language: en
Available languages: en