Reduced Plasma Diffusion in Positive Gradient Magnetic Field Configurations

Abstract

The effective electron and ion diffusion coefficients have been measured in magnetic field configurations with positive, zero, and negative gradients of the magnetic induction Bz across the field. The radial density profiles are approximately the same for a mirror configuration ∂Bz/∂r < 0 and a uniform field ∂Bz/∂r = 0. The measured electron diffusion coefficient is by orders of magnitude larger than the collisional one and shows the 1/B dependence of the Bohm‐type diffusion process, D⊥−=C kTe/B. The factor C has been determined to C ∼ 10−2. The noise level diminishes and the density profiles become steeper with increasing positive gradients, ∂Bz/∂r > 0, thus showing reduced plasma diffusion. A sharp drop of the electron saturation current to the probe indicates that the anomalous diffusion of the electrons is strongly reduced in certain regions where the radial electric field is weak. The stabilizing effect of an average minimum‐B field has been confirmed experimentally.