Enhanced Superbanana Transport Caused by Chaotic Scattering across an Asymmetric Separatrix

This talk discusses a novel ``chaotic'' form of superbanana transport, and compares theory to experiments on nonneutral plasmas.\footnote{D. Dubin and Y. Tsidulko, Phys. Plas. {\bf 18}, 062114 (2011); A.A. Kabantsev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 205001 (2010).} Magnetically-confined plasmas often have one or more locally-trapped particle populations, partitioned by separatrices from one another and from passing particles. Strong superbanana transport is caused by particles that cross these separatrices in the presence of field ``errors'' (such as toroidal magnetic curvature), since trapped and passing particles respond to the field error differently.\footnote{H. Mynick, Phys. Plasmas {\bf 13}, 058102 (2006); H. Mynick, Phys. Fluids {\bf 26}, 2609 (1983).} Collisional scattering (at rate $\nu$) is one mechanism driving the separatrix crossings; theory predicts a collisional boundary layer at the separatrix energy, and collisional transport that scales as $\nu^{1/2} B^{-1/2}$. The chaotic transport of interest here occurs when the separatrix is ``ruffled'' in the direction of plasma drift; then, collisionless particle orbits (tp orbits) cross the separatrix, giving essentially random trapping and de-trapping, with transport scaling as $\nu^0 B^{-1}$. Prior theory assumed a symmetry such that these tp orbits become trapped and detrapped on the same flux surface, thereby giving zero chaotic transport and reduced collisional transport.$^3$ Here, we characterize chaotic transport without the assumed symmetry, and find quantitative agreement with pure electron plasma experiments and simulations in cylindrical geometry. A global field error consisting of a small tilt of the trap magnetic field is applied, to play the role of large-scale curvature in tokamaks or stellarators. Also, a separatrix with two trapped particle populations is produced by applying a ``squeeze potential'' to the middle section of the plasma column. When the separatrix is $\theta$-symmetric, radial transport is observed to scale as $1/ \sqrt{B}$ in agreement with standard $\sqrt{\nu}$ superbanana theory. When the separatrix is not $\theta$ symmetric, some particles transit chaotically from trapped to passing and back as they ExB drift in $\theta$ (the tp orbits). Typical field errors then cause tp orbits to trap and detrap on different flux surfaces, and enhanced transport scaling as $1/B$ is observed in the experiments, in quantitative agreement with our theory and simulations.