Using variable-frequency asymmetries to probe the magnetic field dependence of radial transport in a Malmberg-Penning trap
- Occidental College, Los Angeles, California (United States)
A new experimental technique is used to study the dependence of asymmetry-induced radial particle flux {gamma} on axial magnetic field B in a modified Malmberg-Penning trap. This dependence is complicated by the fact that B enters the physics in at least two places: in the asymmetry-induced first order radial drift velocity v{sub r} = E{sub {theta}}/B and in the zeroth order azimuthal drift velocity v{sub {theta}} = E{sub r}/B. To separate these, we employ the hypothesis that the latter always enters the physics in the combination {omega}-l{omega}{sub R}, where {omega}{sub R} = v{sub {theta}}/r is the column rotation frequency and {omega} and l are the asymmetry frequency and azimuthal mode number, respectively. Points where {omega}-l{omega}{sub R} = 0 are then selected from a {gamma} vs r vs {omega} data set, thus insuring that any function of this combination is constant. When the selected flux {gamma}{sub sel} is plotted versus the density gradient, a roughly linear dependence is observed, showing that this selected flux is diffusive. This linear dependence is roughly independent of the bias of the center wire in our trap {phi}{sub cw}. Since in our experiment {omega}{sub R} is proportional to {phi}{sub cw}, this latter point shows that our technique has successfully removed any dependence on {omega}{sub R} and its derivatives, thus confirming our hypothesis. The slope of a least-squares fitted line through the {gamma}{sub sel} vs density gradient data then gives the diffusion coefficient D{sub 0} under the condition {omega}-l{omega}{sub R} = 0. Varying the magnetic field, we find D{sub 0} is proportional to B{sup -1.33{+-}}{sup 0.05}, a scaling that does not match any theory we know. These findings are then used to constrain the form of the empirical flux equation. It may be possible to extend this technique to give the functional dependence of the flux on {omega}-l{omega}{sub R}.
- OSTI ID:
- 21300428
- Journal Information:
- AIP Conference Proceedings, Journal Name: AIP Conference Proceedings Journal Issue: 1 Vol. 1114; ISSN 0094-243X; ISSN APCPCS
- Country of Publication:
- United States
- Language:
- English
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