Asymmetry-Induced Transport with Azimuthal Perturbations at the Trapping Separatrix

Our experiments show that weak multipolar perturbations added to a trapping separatrix have large effects on asymmetry-induced transport and plasma wave damping, as suggested by recent theoretical models.\footnote{D.H.E. Dubin and Yu.A. Tsidulko, adjacent poster} Here, the pure electron plasma columns have a controlled trapping separatrix created by an applied $\theta$-symmetric wall ``squeeze'' voltage, and a controlled overall asymmetry such as magnetic tilt. Breaking the $\theta$-symmetry of the separatrix by adding multipolar potential perturbations $\phi_m$ causes large and easily characterized effects for a variety of asymmetry-induced dissipative processes. For example, the measured bulk expansion rate $\nu_P$ is a function of the angle $\Delta \theta$ between the magnetic tilt and the multipolar separatrix perturbation. This function is the sum of phase-constant (c) and phase-variable $( \theta )$ parts, i.e., $\nu_P = \nu_c + \nu_\theta \cos (2 \Delta \theta )$. For dipole or quadrupole $(m\!\! = \!\!1, 2 )$ perturbations $\nu_c \approx \nu_\theta$, so $\nu_P \approx 2 \nu_\theta \cos^2 ( \Delta \theta )$; and for higher $(m \!\! = \!\! 3,4... )$ perturbations one finds $\nu_\theta \equiv 0$, so the $\nu_P $ enhancement is phase-independent. Moreover, the two parts scale differently with magnetic field $B$, possibly explaining the puzzling $B^{-1.4}$ scalings observed experimentally.