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Title: Omniclassical Diffusion in Low Aspect Ratio Tokamaks

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}/B{sub t} 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}/B{sub t} {approx} 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}/B{sub t} {approx} 1 regime. Good agreement of the analytic theory with numerical simulations is obtained.
Authors:
; ;
Publication Date:
OSTI Identifier:
827764
Report Number(s):
PPPL-3935
TRN: US0403605
DOE Contract Number:
AC02-76CH03073
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 19 Mar 2004
Research Org:
Princeton Plasma Physics Lab., Princeton, NJ (US)
Sponsoring Org:
USDOE Office of Science (SC) (US)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASPECT RATIO; DIFFUSION; MAGNETIC FIELDS; MAGNETIC SURFACES; TRANSPORT; TRANSPORT THEORY DIFFUSION; TOKAMAKS; TRANSPORT THEORY