Signatures of turbulence spreading during the H-L back-transition in TJ-II plasmas

The spatio-temporal evolution of turbulence and plasma flows has been studied experimentally by means of Doppler reflectometry during the back-transition from the high to low confinement mode in the TJ-II stellarator. Signatures of spatial spreading of the turbulence are found inside the radial position of the sheared E x B flow as the H-L back-transition is approached. The results point to the possible role of radial spreading of turbulence in determining the width of transport barriers. (letter)