Gyroaverage effects on chaotic transport by drift waves in zonal flows

Finite Larmor radius (FLR) effects on E × B test particle chaotic transport in the presence of zonal flows is studied. The FLR effects are introduced by the gyro-average of a simplified E × B guiding center model consisting of the linear superposition of a non-monotonic zonal flow and drift waves. Non-monotonic zonal flows play a critical role on transport because they exhibit robust barriers to chaotic transport in the region(s) where the shear vanishes. In addition, the non-monotonicity gives rise to nontrivial changes in the topology of the orbits of the E × B Hamiltonian due to separatrix reconnection. The present study focuses on the role of FLR effects on these two signatures of non-monotonic zonal flows: shearless transport barriers and separatrix reconnection. It is shown that, as the Larmor radius increases, the effective zonal flow profile bifurcates and multiple shearless regions are created. As a result, the topology of the gyro-averaged Hamiltonian exhibits very complex separatrix reconnection bifurcations. It is also shown that FLR effects tend to reduce chaotic transport. In particular, the restoration of destroyed transport barriers is observed as the Larmor radius increases. A detailed numerical study is presented on the onset of global chaotic transport as function of the amplitude of the drift waves and the Larmor radius. For a given amplitude, the threshold for the destruction of the shearless transport barrier, as function of the Larmor radius, exhibits a fractal-like structure. The FLR effects on a thermal distribution of test particles are also studied. In particular, the fraction of confined particles with a Maxwellian distribution of gyroradii is computed, and an effective transport suppression is found for high enough temperatures.