PPPL-3935 is available in <a href="PPPL-3935.pdf">pdf</a> format (736 KB).

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<b>Omniclassical Diffusion in Low Aspect Ratio Tokamaks</b>
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<b>Authors:</b> H.E. Mynick, R.B. White, and D.A. Gates
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<b>Date of PPPL Report:</b> March 2004
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Recently reported numerical results for axisymmetric devices with low aspect ratio A found radial transport enhanced over the expected neoclassical value by a factor of 2 to 3. In this paper, we provide an explanation for this enhancement. Transport theory in toroidal devices usually assumes large A, and that the ratio B<sub>p</sub>/B<sub>t</sub> of the poloidal to the toroidal magnetic field is small. These assumptions result in transport which, in the low collision limit, is dominated by banana orbits, giving the largest collisionless excursion of a particle from an initial flux surface. However in a small aspect ratio device one may have  B<sub>p</sub>/B<sub>t</sub> ~ 1, and the gyroradius may be larger than the banana excursion. Here, we develop an approximate analytic transport theory valid for devices with arbitrary A. For low A, we find that the enhanced transport, referred to as omniclassical, is a combination of neoclassical and properly generalized classical effects, which become dominant in the low-A, B<sub>p</sub>/B<sub>t</sub> ~ 1 regime. Good agreement of the analytic theory with numerical simulations is obtained.




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