Electron Plasma Orbits from Competing Diocotron Drifts

The perpendicular dynamics of a pure electron plasma column are investigated when the plasma spans two Penning-Malmberg traps with noncoinciding axes. The plasma executes noncircular orbits described by competing image-charge electric-field (diocotron) drifts from the two traps. A simple model is presented that predicts a set of nested orbits in agreement with observed plasma trajectories.

Figure 1

(a) Schematic of the experimental apparatus with a master cell and three off-axis and one on-axis storage cells. (b) Electron plasma spanning the master cell and cell $S$, and (c) outward spiraling trajectory of the plasma in the master cell with a cartoon of the competing diocotron drifts $v_m$ and $v_s$.

Figure 2

Bounce-average orbit of a plasma spanning the master and off-axis cell $S$, where $r_w=6.0\mathrm{mm}$ is the radius of $S$, and the master-cell center is at $x=-5.0r_w$. The orbit period is $850\mu \mathrm{s}$. (a) Trajectory of plasma density centroid, (b) centroid displacement $D$ from cell axis vs time, and (c) density contours for one orbit period, with the lowest contour being 5% of the peak density. Also shown in (a) and (c) are expected orbits predicted by a simple model using $\alpha =0.75$.

Figure 3

Signals from sectored electrodes in the master and cell $S$ during a bounce-averaged orbit. The master cell diocotron mode is observed initially ($t<0$), then the asymmetric orbit is detected when the gate is grounded ($0<t<5\mathrm{ms}$), and, finally, the storage cell diocotron mode is observed after the plasma is cut ($5<t<10\mathrm{ms}$).

Figure 4

(a) A centroid trajectory, corresponding to $\alpha =0.4$, that remains near the fixed point for $\sim 50\mathrm{ms}$; and (b) and (c) density contours at 15 and 45 ms, respectively, showing that the plasma is stable, albeit expanding.

Figure 5

Asymmetric orbit trajectories in a single cell for $V_1=$(square) 5, (circle) 8, and (triangle) 10 V. The orbital periods are $\sim 400–450\mu \mathrm{s}$. Also shown are the predictions of Eq. (3) with $\alpha =0.13$, 0.18, 0.24, respectively.