Guiding center and gyrokinetic orbit theory for large electric field gradients and strong shear flows

Abstract

The guiding center and gyrokinetic theory of magnetized particle motion is extended to the regime of large electric field gradients perpendicular to the magnetic field. A gradient in the electric field directly modifies the oscillation frequency and causes the Larmor orbits to deform from circular to elliptical trajectories. In order to retain a good adiabatic invariant, there can only be strong dependence on a single coordinate at lowest order, so that resonances do not generate chaotic motion that destroys the invariant. When the gradient across magnetic flux surfaces is dominant, the guiding center drift velocity becomes anisotropic in response to external forces and additional curvature drifts must be included. The electric polarization density remains gyrotropic, but both the polarization and magnetization are modified by the change in gyrofrequency. The theory can be applied to strong shear flows, such as are commonly observed in the edge transport barrier of a high-performance tokamak (H-mode) pedestal, even if the toroidal/guide field is small. Yet, the theory retains a mathematical form that is similar to the standard case and can readily be implemented within existing simulation tools.