Electromagnetic turbulence measurements during the L-H transition in the TJ-II stellarator

It is well known that sheared flows can suppress turbulence and help to improve plasma confinement. Fluctuating, turbulence-generated zonal flows are a benign non-linear turbulence saturation mechanism, consistently observed in simulations and reported in several experiments. Because of their transport modulation capability zonal flows have been considered natural candidates for triggering the L-H transition. Theoretically, both electrostatic fluctuation-induced Reynolds stress and magnetic fluctuation-induced Maxwell stress can play a role in generating zonal flows [1]. Whereas measurements of Reynolds stress with multi-pin Langmuir probes have been conducted in several devices [2], only a few measurements of Maxwell stress have been reported [3]. In this work we present local measurements of magnetic properties and Maxwell stresses in the proximity of the L-H transition in the TJ-II stellarator.