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 Y the level of global scaling laws.However, comparison with experimental data from local transport analyses offers a f3i more stringent test of these theories.This review presents the available theories for turbulent transport coefficients, paticticularly ion and electron thermal diffusivities. in a way that will hcilimte B programme of testing models against dam.It provides n 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 imponat when subjecting them to meaningful tesfs against data.The present review emphasizes the more recent developments.building on earlier ones by Liewer and Ross er d .The results of this work have already been of value in canying out a programme of testing theories against high quality JET data (Connor er a1 and Tibone er al).