Using Variable Frequency Asymmetries to Understand Radial Transport in a Malmberg-Penning Trap

摘要

It has long been known that asymmetric electric and magnetic fields produce radial transport in Malmberg-Penning traps, and much work has been done to understand this transport. Our approach is to apply a variable frequency electric asymmetry to a low density population of electrons and to measure the resulting radial particle flux Γ as a function of radius r. The low particle density eliminates many plasma modes (which have their own frequency dependence) and allows us to focus on the transport physics. The usual azimuthal E x B drift is maintained by a biased central wire, and this arrangement also allows us to independently vary the drift frequency ω_R by adjusting either the axial magnetic field B_z or the bias of the central wire φ_(cω). Up to forty wall sectors are used in order to apply an asymmetry consisting of a single fourier mode (n, l,ω), where n is the axial wavenumber, l is the azimuthal wavenumber, and ωis the asymmetry frequency. In the current experiments, we vary ω, n, φ_(cω), and B_z. As ω is varied, the particle flux shows a resonance similar to that predicted by resonant particle theory. The peak frequency of this resonance f_(peak) k increases with ω_R and varies with n, in qualitative agreement with theory, but when quantitative comparisons are made the experimental values for fpetlk do not match those predicted by theory. Instead, the dependence of f_(peak) on φ_(cω), B_z, and r follows simple empirical scaling laws: for inward directed flux, f_(peak)(MHz) ≈ [-Rφ_(cω)(V)/rB_z(G)], where R is the wall radius, and for outward directed flux, f_(peak)(MHz) ≈0.8[-φ_(cω)(V)/B_z(G)]. These results may provide guidance for the construction of the correct theory of asymmetry-induced transport.