Study of radial heat transport in W7-X using the transfer entropy

Abstract

In this work, we analyze data obtained using the electron cyclotron emission radiometer at the Wendelstein 7-X stellarator using a relatively new technique: the transfer entropy. Thus, we detect the propagation of information and find that it occurs in a stepwise fashion: we observe both 'trapping zones' and radial 'jumps', when the information is apparently skipping over intermediate positions. Using scans of the rotational transform, we observe that the 'trapping zones' appear to be associated with rational surfaces. Power scan experiments show that these 'jumps' increase in importance when power is increased, thus enhancing the effective diffusivity. The observations are interpreted in terms of a resistive magneto-hydrodynamic model, which displays behavior similar to the experimental results. The 'trapping zones' are explained in terms of zonal flows associated with rational surfaces, while the 'jumps' are ascribed to mode coupling effects, i.e. the transmission of turbulent energy via the magnetic field.