Neoclassical polarization drift of collisionless single ions in a sheared radial electric field in a tokamak magnetic geometry

Neoclassical polarization drift is known to play critical role in the dynamical behavior of a sheared radial electric field Er in a toroidal confinement device. However, basic studies on the effect of radial electric shear on neoclassical polarization drift have not yet appeared in the literature. In the present report, the neoclassical polarization drift speed VNP of collisionless single ions is studied using a guiding-center code in a time-varying, spatially sheared Er in a realistic tokamak geometry. It is found numerically that the VNP for single ions is not only a function of the time derivative ∂ Er ∂t, but also a strong function of the radial shear Δr∂ Er ∂r if the shear length is on the same order as the ion banana width Δr. Comparison with an analytic investigation reveals that this effect is simply due to the finite banana modification to the orbital average Er. An approximate analytic formula has been presented for collisionless single banana ions in a conventional tokamak magnetic geometry. The trapped-passing boundary layer physics is not treated.