Enhanced Neoclassical Transport and Mode Damping 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.\footnote{D.H.E. Dubin, Phys. Plasmas {\bf 15}, 072112 (2008); D.H.E. Dubin {\it et al.}, Neoclassical transport and plasma mode damping caused by collisionless scattering across an asymmetric separatrix, in preparation.} 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,$^1$ 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 a chaotic region around the separatrix, giving enhanced transport scaling as $\nu^0 B^{-1}$. This effect also damps certain plasma modes. Predictions for damping of trapped particle diocotron modes will be compared to experiments.\footnote{A.A. Kabantsev, adjacent poster.}