Transport studies in fusion plasmas: Perturbative experiments

By subjecting a plasma in steady state to small perturbations and measuring the response, it is possible to determine elements of the matrix of transport coefficients. Experimentally this is difficult, and results are mainly limited to tranpsport driven by the pressure and temperature gradients. Importantly, off-diagonal elements in the transport matrix appear to be important. This has also implications for the interpretation of the so called 'power balance' diffusivity, determined from the steady state fluxes and gradients. 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.