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Retarding field analyzers (RFA) provide an integral of the ion velocity distribution in tokamak edge plasmas, leading, in principle, to an estimate of the ion temperature. However, the presence of the RFA itself perturbs the ambient plasma, such that the measured distribution is distorted with respect to the unperturbed one far from the probe. Here, collision-less kinetic modeling is employed to investigate the modification of the plasma characteristics (temperature, particle flux, density, and electric potential) in the pre-sheath of the RFA. The kinetic equations are solved independently by means of two different numerical methods, which provide a reliable check of their results. Moreover, they are interpreted in light of a simplified kinetic analytical model. Systematic numerical studies are performed for a large range of values of the ion-to-electron temperature ratio and the parallel drift speed. In the same way that a Mach probe measures upstream-downstream asymmetries of ion saturation current in flowing plasmas, RFAs are expected to measure important asymmetries of sheath potential and ion temperature. These asymmetries can be used to estimate accurately the ion temperature in the absence of the probe perturbation.

Additional information

Authors: VALSAQUE F, Université Henri Poincaré, Vandoeuvre-lès-Nancy (FR);MANFREDI G, Université Henri Poincaré, Vandoeuvre-lès-Nancy (FR);GUNN J P, Association Euratom-CEA pour la fusion, CEA Cadarache, St.-Paul-lez-Durance (FR);GAUTHIER E, Association Euratom-CEA pour la fusion, CEA Cadarache, St.-Paul-lez-Durance (FR)
Bibliographic Reference: An article published in: Physics of Plasmas - May 2002 - Volume 9, Issue 5, pp. 1806-1814
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