Radial guiding-center drifts and omnigenity in bumpy-torus confinement systems

Collisional transport of a high-temperature plasma across the confining field of a bumpy torus magnetic confinement system depends sensitively upon the functional form of the radial guiding center drift, and thus upon details of the confinement geometry. A general and relatively explicit formula for the radial drift is derived, using the large aspect-ratio results of a previous equilibrium study. Allowance is made for (i) arbitrary toroidal variation of the confining field; (ii) field distortion due to plasma currents; (iii) non-circular deformation of the toroidal field coils. However, the analysis pertains only to the plasma core, and not to the high-beta annuli (electron rings) which are usually present in experiments. The question of bumpy torus omnigenity - whether any bumpy torus field configuration is consistent with a vanishing, or nearly vanishing, radial drift - is also investigated. It is found that omnigenity cannot occur, at least in the vicinity of the magnetic axis