Shear modulation experiments with ECCD on TCV

Anomalous electron transport is determined by turbulence, which in turn is affected by magnetic shear. A novel application of Electron Cyclotron Current Drive (ECCD), aiming at localized shear modulation, has been applied on TCV tokamak for experiments on shear-dependent electron transport. Pairs of EC beams, absorbed at the same radius but oriented for co and counter injection, are modulated out of phase in order to force a local modulation of current density at constant input power. Off-axis deposition (ρdep=0.24) is performed for sawteeth control. A significant impact on local shear is achieved with IECCD∼0.1IOH even if the modulation period is much shorter than current diffusion time. Although source (heat and particle) terms are constant, both electron density and temperature are modulated during alternated ECCD. Thomson Scattering is the diagnostic for local Te and ne measurement, in order to overcome suprathermal problems on ECE from high field side. Once equilibrium effects are taken into account for appropriate mapping of TS measurements onto flux coordinates, Te and electron pressure modulation, peaked on-axis, is confirmed at all radii internal to EC deposition. Best confinement (Δne,0Te,0=+12%) is for co-injection, when shear drops from ∼0.5 to less than 0.2. (author)