  The understanding of the outward radial transport of heat in magnetic
confinement fusion devices is a priority for the development of economically
viable fusion reactors. Here, we analyze the radial propagation of
spontaneously generated electron temperature (T_e) fluctuations measured using
the Electron Cyclotron Emission (ECE) diagnostic in Wendelstein 7-X, which
disposes of 32 channels, covering a large part of the plasma minor radius. By
applying a relatively new statistical analysis technique (the Transfer
Entropy), the present work provides, for the first time, a view of the detailed
mechanism of electron heat transport in fusion plasmas. It is shown that
rational surfaces have a significant impact on radial heat transport and may in
fact be essential in setting the global energy confinement. The reported
observations provide support for various explanatory concepts suggested in
literature: namely, critical gradients, non-locality, and self-organization.
