Enhanced superbanana transport caused by chaotic scattering across an asymmetric separatrix

Abstract This paper discusses a novel “chaotic” form of superbanana transport, and compares theory to experiments on nonneutral plasmas. Superbanana transport is caused by particles that cross local trapping separatrices (magnetic or electric ripples) in the presence of field “errors” such as toroidal magnetic curvature. Traditionally, collisions (at rate ν) cause separatrix crossings, with resulting transport that scales as ν1/2B−1/2. The “chaotic” transport of interest here occurs when the separatrix is “ruffled” in the direction of plasma drift; then, collisionless particle orbits give random trapping and detrapping. Prior theory assumed a “stellarator symmetry” and suggested that these orbits give reduced transport scaling as νp with p ∼ 1. Here, we fully characterize chaotic transport, and show that the transport is enhanced rather than reduced, scaling as ν0B−1. Experiments on pure electron plasmas provide quantitative transport measurements, with precise control of the overall field error, and of the trapping separatrix with and without ruffles. The experiments show close agreement with theory over a decade in B, for both collisional neoclassical transport, and for the distinctive chaotic transport. At low magnetic fields, transport scaling as B−p with p & 2 becomes dominant, showing preliminary agreement with bounce-resonant theory.