Theory of stochastic magnetic fluctuations and anomalous electron thermal conductivity in tokamaks

A primary aim in much of the theoretical tokamak research is the development of a self-consistent turbulence theory for the short-wavelength fluctuations thought to be responsible for anomalous transport. Although this is certainly a formidable task, there are now indications that it may be possible in principle and, further, that the needed transport coefficients could actually be computed in practice. This paper considers three related developments on this problem. In Section 1 a theory is developed for the universal drift mode, culminating in an explicit formula for the anomalous electron thermal conductivity. Particular emphasis is given to the dependence on the temperature ratio Tsub(e)/Tsub(i) and its relevance for interpreting experimental data. Section 2 considers the properties of specific stochastic orbits and gives numerical and analytic calculations of the Kolmogorov entropy and stochastic diffusion coefficient. Micro-tearing instabilities are considered in Section 3. These can be excited linearly by the electron temperature gradient in the presence of collisions and do not depend crucially on magnetic shear. Nonlinear effects are evaluated and shown to imply a cascade from short to long wavelengths. This may have a bearing on observed (broad) spectra. (author)