Boundary layer corrections to neoclassical ripple transport in tokamaks

Abstract

The discrete nature of the toroidal magnetic field coils spoils the symmetry of the tokamak and creates small modulations of the toroidal field called ripples. Particles trapped in the ripple well drift off the flux surface and enhance both particle and heat fluxes. In the usual ripple transport calculations, the derivative of the perturbed particle distribution function in velocity space is discontinuous at the boundary between the ripple trapped and untrapped regions. To smooth out the particle distribution across the boundary, the particle distribution in the boundary layer is calculated by using the Wiener–Hopf technique. The correction to the ripple-trapped particle distribution function away from the boundary layer and the ripple transport associated with it are obtained. It is found that the particle diffusion and heat conduction coefficients are increased by about a factor of 2 and scale like ν−1/2 as (νeff/ωbδ)1/2→1. Even for very collisionless plasmas, νeff/ωbδ = 10−2, the corrections are still non-negligible and are about 15%.