(EX/P3-3) Modulated ECCD experiments on TCV

Abstract.  Experiments were previously performed on TCV using Switched Electron Cyclotron Current Drive (SECCD) in which the total heating power and plasma current were kept nearly constant in time while the current density profile was modulated. It was observed that SECCD leads to a relative electron diffusivity modulation amplitude of ±65%. A direct measurement of the current density profile is not available on TCV; however, electrodynamic calculations show that ±55% shear modulation (from s=0.20 to 0.70) is achieved during SECCD experiments, and that the shear modulation is localized at the CD layer position. These results provide strong experimental evidence that electron heat transport is shear dependent: heat transport is reduced when shear is low, confirming a general observation. New experiments have been performed to address some open issues raised after the previous campaign. In particular, the scaling of transport modulation with shear, the localization of transport modulation with respect to shear, and the impact of shear modulation on particle diffusion are studied. The work is also aimed at confirmation of the electrodynamic model of the SECCD technique. To these ends, the experiments have been performed at different deposition radii, different SECCD amplitudes and different toroidal injection angles. In addition, more effort has been dedicated to modelling using the ASTRA transport/diffusion code. This provides a more accurate prediction of current density profile evolution, together with the associated global (internal inductance, energy content, confinement time) and local (magnetic shear, electron thermal diffusivity, particle diffusivity) parameters, relevant to the issue of the shear/confinement relationship. Results from these new experiments as well as modelling will be presented.

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