Investigation of the wake due to a large probe using a spatially scanning Langmuir probe

Abstract

This paper describes direct experimental investigation of the region of plasma perturbed by a probe. Two adjacent probes have been mounted on the DITE tokamak. The large “disturber” probe is a rectangular graphite plate that creates the main particle flow pattern and can be biased with respect to the torus. The smaller “search” Langmuir probe is located 0.5 m away on the same magnetic field line. It can be swept back and forth mechanically through the shadow of the main probe many times during each discharge. Through measurements of ion saturation current at many distinct radii, a complete map of the perturbed region has been built up and transformed into radial and poloidal coordinates. A numerical solution of the two-dimensional diffusion equations is compared with experimental data in the limiter shadow. Two zones of substantially different interconnection length are included in the model. The assumption of equal radial and poloidal cross-field diffusion coefficients ( D ⊥ = 0.18 m 2 s −1) is found to give a good fit with the data.