TJ-II:L-H transition studies: characterization of plasma turbulence using Gas Puff Imaging, Probes, Doppler reflectometry and HIBP diagnostics: Difference between revisions

At TJ-II, spontaneous L-H transitions are achieved in neutral beam injection (NBI) heated plasma. Abrupt as well as gradual transitions are achieved depending, among other plasma parameters, on the heating power and magnetic configuration topology. Doppler reflectometry (DR) measurements show an increase in the negative radial electric field (Er) together with a reduction in plasma turbulence at the transition [1]. These measurements together with measurements obtained using a dual Langmuir probe system [2] show that the trigger of the L-H transition is more correlated with the development of fluctuating radial electric fields than steady-state Er effects. This conclusion is further stressed when operating close to the L-H transition threshold conditions, where pronounced oscillations in both, Er and density turbulence measured by DR show a characteristic predator-prey relation [3]. These experimental observations are consistent with L-H transition models based on turbulence-induced sheared/zonal flows.  In addition, HIBP measurements show a reduction in the plasma turbulence and associated flux not only in the plasma edge region but also in the plasma core [4].