Transport studies in fusion plasmas: Perturbative experiments

By inducing in a small temperature perturbation in a plasma in a steady state one can determine the conductive and convective components of the heat flux, and the associated thermal diffusivity and convection velocity. The same can be done for the density, and in principle also other plasma parameters. Experiments show that the response of the temperature in most cases is determined by diffusion. It is in principle possible to determine elements of the matrix of transport coefficients. Importantly, off-diagonal elements in the transport matrix appear to be important. Experimental techniques, analysis techniques, basic formulas etc are briefly reviewed. Experimental results are summarized. The fundamental question whether the fluxes are linear functions of the gradients or not is discussed. On a fast time scale, plasma responses have sometimes been observed that cannot be explained by a local transport model. This so called 'non-local' transport is briefly discussed.