Collisional transport in a low aspect ratio tokamak

Abstract

Calculations of collisional diffusivities in toroidal magnetic plasma confinement devices order the toroidal gyroradius to be small relative to the poloidal gyroradius, i.e., ρiφρiθ, where ρiφ≡mivthi/qBφ and ρiθ≡mivthi/qBθ. This ordering is central to what is usually referred to as neoclassical transport theory. This ordering is incorrect at low aspect ratio (with aspect ratio A≡R/a, where R is the major radius of the torus and a is the minor radius), where it can be the case that ρiφ>ρiθ. The correction to the test particle diffusivities is numerically calculated by comparing the diffusivities as determined by a full orbit code (which we refer to as omniclassical diffusion) with those from a gyroaveraged orbit code (neoclassical diffusion), and then corroborated by an analytic calculation. The omniclassical diffusion can be up to 2.5 times the calculated neoclassical value. The implications of this work for the analysis of collisional transport in low aspect ratio devices are discussed.