Frequency dependence of asymmetry-induced transport in a non-neutral plasma trap

Abstract

A key prediction of the theory of asymmetry-induced transport is that the particle flux will be dominated by particles that move in resonance with the asymmetry. For the case of a time-varying asymmetry, the resonance condition is ω−lωR−nπv/L=0, where v is the axial velocity, L is the plasma length, ωR is the E×B rotation frequency, and ω, l, and n are the asymmetry frequency, azimuthal wavenumber, and axial wavenumber, respectively. Data are presented from experiments on a low density trap in which ω, ωR, and n are varied and the resulting radial particle flux is measured. The experiments show a resonance in the flux similar to that predicted by theory. The peak frequency of this resonance increases with ωR and varies with n, in qualitative agreement with theory, but quantitative comparisons between experiment and theory show serious discrepancies.

Two sources of asymmetry-induced transport

“…The remaining details of the experiment are given elsewhere. 7 The code follows the dynamics of single particles in prescribed fields; inter-particle fields are not included. The prescribed fields are set by the center wire potential and the asymmetric potential.…”

“…Once the base pressure is low enough to minimize electron-neutral transport, the confinement is limited by electric and magnetic fields that break the cylindrical symmetry of the trap and produce radial drifts. This asymmetryinduced transport has been studied experimentally by a number of people, [1][2][3][4][5][6][7][8][9] but detailed comparisons of the predictions of resonant particle transport theory 10 with experiment 7 show serious discrepancies. It seems clear that some important physics is missing from the theory.…”

Reduction of asymmetry transport in the annular Penning trap

“…Collisions of trapped identical particles do not result in radial transport; thus, in the limit of zero background gas pressure, the confinement time is determined by processes not included in collisional transport theory, such as field asymmetries or image current dissipation. Detailed experimental [1][2][3][4][5][6][7][8][9] and theoretical 10-12 studies of electron confinement in the Malmberg-Penning trap have concluded that the transport in the limit of low gas pressures is due to asymmetric stray electric fields. A similar device is the annular Penning trap, in which plasma is confined between concentric cylinders.…”

Trapped-Particle-Mediated Damping and Transport

“…Some of the most precise measurements of transport have been reported by Eggleston [9], using applied asymmetries which vary accurately as sin(θ ) sin(nπz/L) sin(2π f t). Here, resonant particles are thought to be important, but the data does not match the theory of simple collisional scattering out of resonance.…”

“…Thus, it now appears likely that most of the (L/B) −2 lifetime scalings from "background asymmetries" [6] can be given interpretation in terms of the (partially) known scalings for TPM transport. The measurements of transport from applied electric and magnetic asymmetries [7,8,9, 10] also should be compared to TPM predictions. "Anomalous" damping of diocotron modes [7,11,12] is almost certainly related to TPM effects, since TPM damping scales as B −3 .…”