Diffusion coefficient scaling in the Wisconsin levitated octupole

Abstract

Measurements of cross‐field diffusion were made in the levitated octupole using two independent methods. First, the magnitude and scaling of D_⊥ with density, n, and field strength, B, were determined by direct measurements of the particle flux onto an internal ring surface. Second, the shape of the density profile and the time decay of the density at the separatrix were compared with solutions of a diffusion equation model. Both methods indicated that D_⊥ was proportional to n^−1/2 and independent of B. The plasma was created by gun injection into the purely poloidal magnetic field and the initial parameters were n_e∼5 ×10⁹ cm⁻³, T_i∼20 eV, T_e∼5 eV. The observed diffusion scaling can be understood within the framework of vortex diffusion in a high‐dielectric constant plasma if the spectrum of waves is enhanced in energy over thermal levels with an effective temperature, T*, of 10⁴ eV.