Survey of theories of anomalous transport

Energy and particle confinement in tokamaks is usually anomalous, greatly exceeding neoclassical predictions. It is desirable to develop an understanding of the underlying processes to increase our confidence in extrapolation of tokamak behaviour towards reactor regimes. The literature abounds with theoretical expressions for anomalous transport coefficients based on turbulent diffusion due to various micro-instabilities. These often purport to provide explanations of tokamak confinement at the level of global scaling laws. However, comparison with experimental data from local transport analyses offers a far more stringent test of these theories. This review presents the available theories for turbulent transport coefficients, particularly ion and electron thermal diffusivities, in a way that will facilitate a programme of testing models against data. It provides a brief description of the basis for each theory to place it in context and then presents the resulting turbulent diffusivity. Particular emphasis is placed on the validity conditions under which the expressions may be used; this is important when subjecting them to meaningful tests against data. The present review emphasizes the more recent developments, building on earlier ones by Liewer and Ross et al. The results of this work have already been of value in carrying out a programme of testing theories against high quality JET data (Connor et al and Tibone et al).