Dependence of enhanced asymmetry-induced transport on collision frequency

Our previous studies\footnote{D.~L. Eggleston, Phys. Plasmas {\bf 19}, 042307 (2012).} of asymmetry-induced radial transport using a single-particle code with collisional effects have identified, for asymmetries of the form $\phi_1(r)\cos{(kz)}\cos{(\omega t - l\theta)}$, two sources for the transport: resonant particles (RPs) and axially trapped particles (ATPs). We observe that this latter type, which occurs near the radius where $\omega$ matches the azimuthal rotation frequency $\omega_R$, is often dominant at low collision frequency $\nu$ but becomes negligible at higher $\nu$. This can be understood by noting that ATPs have a lower trapping frequency $\omega_T^2= (l^2\phi_1/rB)|d\omega_R/dr|$ than RPs. In the low $\nu$ (banana) regime, the radial oscillations have amplitude $\Delta r= v_r/\omega_T$, so ATPs dominate, and the transport may even exceed the RP plateau regime level. As $\nu$ increases, collisions start to interrupt the slower ATP oscillations while the RPs are still in the banana regime, so the ATP contribution to the transport decreases. At the largest $\nu$ values, ATP transport is negligible and the observed diffusion coefficient matches that given by plateau regime RP theory.\footnote{D.~L. Eggleston and T.~M. O'Neil, Phys. Plasmas {\bf 6}, 2699 (1999).}