The question of anomalous transport of the energy of the electrons has been puzzling tokamak physicists for nearly as long as tokamaks exist (and internal disruptions is the other riddle which has reached its twentieth anniversary...). Magnetic turbulence has been suggested as a possible answer for a long time, but linear theory gave negative results, and experimental evidence could not be obtained with existing diagnostics until recently. In the past few years, much progress has been made, and although no final conclusion can be reached yet, it is useful to regroup and compare the latest theoretical and experimental results. As recent and exhaustive reviews already exist, we will not try to cover all the existing literature, but rather adopt a more personal and maybe biased point of view. In the first part, we discuss the possible theoretical approaches, and point out the directions along which progress should be made, especially the need to obtain a dynamical description of the magnetic structure, taking into account the feedback which the induced transport has on the instabilities. In the second part, we review and discuss the experimental evidence in favour of magnetic turbulence, and discuss the relevance of the observations to the explanation of the measured transport coefficients. Although we try to cover most of the available data, many illustrations will be taken from the Tore Supra tokamak, which is well endowed with diagnostics of the fine scale magnetic structure.
A comparative study of transport in stellarators and tokamaks