Infoscience

Journal article

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

A characterization of the H-mode in Tokamak de Varennes (TdeV) is presented, combining results obtained from spectroscopic measurements with observations of global plasma parameters. Controlled L- and H-modes are obtained by applying an external bias on the divertor plate and/or by varying the plasma triangularity using similar steady-state discharges with electron cyclotron resonance heating (ECRH). A qualitative study of the impurity emission asymmetries is presented and discussed by proposing a simple model which considers that the emissivity has radial and poloidal components. The poloidal velocities are small with no significant variations with respect to the plate biasing, EC power and plasma triangularity. Toroidal. rotation velocities show more consistent trends and have larger values in the H-modes with respect to the L-modes. No significant shear in the radial electric field is observed between the H- and L-modes. Differences in the electric field and in the E x B shear are observed for several discharges, but they are not consistent overall. A study on the impurity transport shows low diffusivity values in the plasma centre and high values in the edge during the H-mode. The energy confinement time decreases with the severity of the edge localized modes and, consequently, with the neutral fluxes. The presence of a toroidal torque which affects the particle transport inside the plasma plays an important role in the TdeV H-modes.

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