Magnetic turbulence and the transport of energy and particles in tokamaks

The transport losses observed in tokamak experiments far exceed the predictions of collisional transport theory. This `anomalous' transport is attributed to the turbulent fluctuations known to exist in the plasma interior. Generally, two extreme cases are treated in the theory: electrostatic turbulence in which the confining magnetic field is unchanged and the transport is due to fluctuating electric fields, or alternatively, magnetic turbulence in which the confining magnetic field is significantly perturbed by fluctuating current flows in the plasma. This review deals only with this second case and outlines both the existing theories and the experimental evidence. There are serious difficulties in developing a fully self-consistent theory and with the measurement of the relevant fluctuating quantities deep in the hot plasma. Consequently, the present body of theoretical and experimental evidence does not lead to any definitive conclusion regarding the role of magnetic turbulence. However, the global scaling laws which describe fairly successfully the results from a wide range of tokamaks do have parametric dependences close to those predicted for magnetic turbulence. Further progress depends largely on the use of the most advanced fluctuation measurement techniques to a similarly wide range of different tokamaks and operating modes.