Characterization of impurities in L- and H-modes created by divertorbiasing and plasma triangularity in TdeV

Abstract

A characterization of the H-mode in Tokamak de Varennes (TdeV) is presented,combining results obtained from spectroscopic measurements with observationsof global plasma parameters. Controlled L- and H-modes are obtained byapplying an external bias on the divertor plate and/or by varying the plasmatriangularity using similar steady-state discharges with electron cyclotronresonance heating (ECRH). A qualitative study of the impurity emissionasymmetries is presented and discussed by proposing a simple model whichconsiders that the emissivity has radial and poloidal components. The poloidalvelocities are small with no significant variations with respect to the platebiasing, EC power and plasma triangularity. Toroidal rotation velocities showmore consistent trends and have larger values in the H-modes with respect tothe L-modes. No significant shear in the radial electric field is observedbetween the H- and L-modes. Differences in the electric field and in theE×B shear are observed for several discharges, but they are notconsistent overall. A study on the impurity transport shows low diffusivityvalues in the plasma centre and high values in the edge during the H-mode. Theenergy confinement time decreases with the severity of the edge localizedmodes and, consequently, with the neutral fluxes. The presence of a toroidaltorque which affects the particle transport inside the plasma plays animportant role in the TdeV H-modes.