Transport studies using perturbative experiments

By inducing a small electron temperature perturbation in a plasma in steady state one can in principle determine the conductive and convective compone nts of the electron heat flux, and the associated thermal diffusivity and c onvection velocity. The same can be done for other plasma parameters, like density or ion temperature. Experiments show that the response of the temperature in most cases is dete rmined by diffusion. It is in principle possible to determine elements of t he matrix of transport coefficients. Interestingly, off-diagonal elements i n the transport matrix appear to be important. In this paper experimental techniques, analysis techniques, basic formulas etc. are briefly reviewed. Experimental results are summarized. The fundame ntal question whether the fluxes are linear functions of the gradients or n ot is discussed. When inducing edge perturbations, often plasma responses are observed which cannot be explained by a local transport model. These so-called 'non-local ' phenomena have drawn strong attention in the last couple of years, and we will review this class of experiments as well.