Two sources of asymmetry-induced transport

Abstract

A single-particle computer code with collisional effects is used to study asymmetry-induced radial transport of a non-neutral plasma in a coaxial Malmberg-Penning trap. Following the time variation of the mean change and mean square change in radial position allows for the calculation of the radial drift velocity vD and the diffusion coefficient D as defined by the radial flux equation Γ=-Ddn0dr+n0vD. For asymmetries of the form φ1(r)cos(kz+ωt-lθ) and periodic boundary conditions, the transport coefficients obtained match those predicted by resonant particle transport theory where the transport is produced by particles with velocities near ±(lωR-ω)/k, with ωR being the azimuthal rotation frequency. For asymmetries of the form φ1(r)cos(kz)cos(ωt-lθ) and low collision frequency, there is a second contribution to the transport produced by low velocity particles axially trapped in the asymmetry potential. These produce a stronger variation of D with ω with a peak at ω=ωR. The width of the peak Δω increases with center conductor bias and decreases with radius, while the height shows the opposite behavior. The transport due to axially trapped particles is typically comparable to or larger than that from resonant particles. This second contribution to the transport may explain the discrepancies between experiments and resonant particle theory.