Observation of Neoclassical, Intermediate, and Pfirsch-Schülter Diffusion in the dc Octopole

Abstract

We have studied the plasma transport processes in the dc octopole with a toroidal magnetic field. The plasma diffusion is characterized by three principal collision-frequency regimes. The magnitude of the diffusion coefficient shows an inverse-square dependence on the magnetic field in all regimes. In the high-collision-frequency regime, the measured local diffusion coefficient agrees with Pfirsch-Schlüter diffusion in both rotational transform dependence and magnitude. Both for intermediate and low values of collision frequency, the enhancement of the decay rate agrees with neoclassical theory.