On the minimum electron transport coefficients in tokamaks in the range of low collision frequencies
- Russian Research Centre Kurchatov Institute, Institute of Nuclear Fusion (Russian Federation)
There are two close empirical scalings, namely, the T-11 and neo-Alcator ones, that provide correct estimates for the energy confinement time in tokamaks in ohmic heating regimes in the linear part of the dependence {tau}{sub E}(n-bar{sub e}) in the range of low values of n-bar{sub e} and <{nu}{sub e}{sup *}> {<=} 1. The similar character of electron energy confinement in this range, which expands with increasing magnetic field B{sub 0}, has stimulated the search for dimensionless parameters and simple physical models that would explain the experimentally observed dependences {chi}{sub e} {approx} 1/n{sub e} and {tau}{sub Ee} {approx} n-bar{sub e}. In 1987, T. Okhawa showed that the experimental data were satisfactorily described by the formula {chi}{sub eperpendicular} = (c{sup 2}/{omega}{sub pe}{sup 2}){nu}{sub e}/qR, in deriving of which the random spatial leap along the radius r on the electron trajectory was assumed to be the same as that in the coefficient of the poloidal field diffusion, while the repetition rate of these leaps was assumed to be {nu}{sub e}/qR. In 2004, J. Callen took into account the decrease in the fraction of transient electrons with increasing toroidal ratio {epsilon} = r/R and corrected the coefficient c{sup 2}/{omega}{sub pe}{sup 2} in Okhawa equation by the factor {sigma}{sub ||} {sup Sp}/{sigma}{sub ||}{sup neo}. If one takes into account this correction and assumes that the frequency of the stochastic process is equal to the reciprocal of the half-period of rotation of a trapped electron along its banana trajectory, then the resulting expression for {chi}{sub eperpendicular} will coincide with the T-11 scaling: {chi}{sub e}{sup an} {infinity} {epsilon}{sup 1.75}(T{sub e}/A{sub i}){sup 0.5}/(n{sub e}qR) at Ai = 1. If the same stochastic process also involves ions, it may result in the opening of the orbit of a trapped ion at the distance {approx}(c/{omega}{sub pe})(m{sub i}/m{sub e}){sup 1/4}. In this case, the calculated coefficient of electron and ion diffusion D is close to D{sup an} {approx} {chi}{sub e}{sup an}/2.
- OSTI ID:
- 21405891
- Journal Information:
- Plasma Physics Reports, Vol. 35, Issue 6; Other Information: DOI: 10.1134/S1063780X09060014; Copyright (c) 2009 Pleiades Publishing, Ltd.; ISSN 1063-780X
- Country of Publication:
- United States
- Language:
- English
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