Recent advances in understanding neoclassical transport processes in nonneutral plasmas

Slow radial expansion of a long nonneutral plasma column of length L, caused by θ-asymmetric E or B fields, has for decades posed challenges to theory. This poster will review recent experimental and theoretical work that has begun to unravel the neoclassical processes involved. For large magnetic fields the radial transport is generally dominated by bounce-averaged dynamics, enhanced by phase-space boundary layers at separatrices caused by weak potential or magnetic ripples (“super-banana” transport). At lower magnetic fields, the transport is dominated by bounce-rotation resonances. A plateau-regime theory of this resonance transport mechanism reproduces  “Driscoll curve” (L/B)² scaling¹, assuming random localized asymmetries associated with patch potentials on the conducting walls. The plateau theory also reproduces the stronger 1/B^2.7 scaling observed for the asymmetry associated with an overall magnetic tilt². Finally, the plateau theory also predicts a reduction in transport when the external axial potential well is near - harmonic, as observed in some experiments³.¹C.F. Driscoll and J.H. Malmberg, Phys. Rev. Lett. 50, 167 (1983); ²D.H.E. Dubin, A.A. Kabantsev and C.F. Driscoll,  Phys. Plasmas 19, 056102 (2012); ³A. Mohri et al., Jpn. J. Appl. Phys. 37, 664 (1998).