Perturbative studies of turbulent transport in fusion plasmas

One powerful way to investigate transport is to study the dynamic plasma response to externally applied small perturbations. This article describes the experimental techniques and basic theory underlying the perturbative approach. It then reviews the most recent results on electron heat transport and discusses their interpretation based on key concepts from electrostatic turbulence theory. Power modulation studies have allowed a thorough assessment of the concept that a threshold exists in the inverse temperature gradient length, above which transport driven by electrostatic turbulence modes sets up. The heat waves clearly show a change in propagation properties when crossing the threshold, with a discontinuous jump of the incremental heat diffusivity that becomes significantly larger than the power balance one. With the help of simple semi-empirical models, power modulation provides a unique tool to determine threshold and stiffness levels and investigate their parametric dependence or compare amongst different machines