Enhanced Neoclassical Transport Caused by Chaos Near an Asymmetric Separatrix

Plasma loss due to apparatus asymmetries is a ubiquitous phenomenon in magnetic plasma confinement. Recent experiments have investigated the loss rate when a central squeeze potential is applied to a magnetized plasma column, creating two trapped particle populations separated by a separatrix. These populations react differently to the asymmetries, leading to a collisional boundary layer at the separatrix. A loss rate scaling as $\sqrt{\nu / B}$ due to the boundary layer is expected theoretically,\footnote{D.H.E. Dubin, Phys. Plasmas {\bf 15}, 072112 (2008).} provided that the separatrix itself is axisymmetric. However, when the separatrix is {\it asymmetric}, particles become trapped and detrapped as they follow collisionless orbits. This can lead to single-particle resonances and/or a chaotic region around the separatrix, giving enhanced transport. This effect may help explain a long-standing discrepancy between experiment and neoclassical theory, and could play an important role in tokamak and stellerator confinement. Theory and simulations of this collisionless chaotic transport will be presented.