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Modeling of Local Edge Plasma Perturbations Induced by a Biased Probe

The Langmuir probe is a standard diagnostic tool for laboratory plasmas, routinely used for edge plasmas in magnetic fusion devices. However, recent studies on the Alcator C‐Mod tokamak indicate the possibility of a substantial local plasma perturbation by a negatively biased divertor plate target probe under high recycling conditions when the electron temperature is under ∼10 eV. The effects of the probe perturbation of the plasma are studied using the tokamak edge plasma code UEDGE for parameters relevant to Alcator C‐Mod edge. It is found that at sufficiently negative bias voltage, the probe substantially modifies the local plasma characteristics. In particular, this is relevant to the experimentally known “death‐ray” phenomenon, where the plasma pressure near the probe strongly exceeds the upstream value. Certain features of the “death‐ray” phenomenon are reproduced in the modeling, pointing to interplay of plasma and neutrals transport, atomic physics, and sheath heat‐flux boundary conditions (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)