Test-particle transport due to fluctuating magnetic fields in tokamaks

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Abstract
The transport of charged test-particles in the presence of given magnetic fluctuations and collisions is investigated from two approaches, namely, the Langevin and corresponding Fokker-Planck equations. Analytic discussion of the Fokker-Planck equation shows the radial particle and energy fluxes to be purely diffusive. The Langevin approach confirms the validity of the Fokker-Planck perturbative calculation for sufficiently small fluctuation amplitudes. From the Fokker-Planck theory the authors derive an effective parallel thermal diffusivity valid for all collisionalities, and show that the high collisionality results of Braginskii and the low collisionality form previously conjectured by the authors, are recovered. Application of the theory to TFR lead to qualitative agreement with published observations.