Radial electrical field in non axi-symmetrical tokamak plasmas - study through doppler reflectometry

Nuclear fusion research aims at producing plasmas mainly heated by fusion reactions between Tritium and Deuterium ions. This work deals with the problem of turbulent transport, which is one of the main limiting factors in the performance of tokamak operation. It is focused on the radial electric field (Er, pointing outwards/inwards from the plasma), which can generate transport barriers when its shearing rate is sufficient to cause a turbulence de-correlation. We have investigated the mechanisms causing the spontaneous generation of the radial electric field inside the last closed magnetic surface. In the Tore Supra tokamak, a Doppler reflectometer allows a quasi-direct measurement of the electric drift velocity due to Er. The effect of ripple (a periodic variation of the magnetic field between two coils, in the toroidal direction) is shown by comparing the measurements with predictions from various models, corresponding to different diffusion regimes (ripple-plateau, local trapping). In some special experimental conditions, a locally positive radial electric field has been measured inside the last closed flux surface in Tore Supra, which contrasts with the usual negative Er in this region. This suggests the presence of other non-ambipolar mechanisms. A discussion on the possible role of MHD activity and islands based on the Doppler reflectometry measurements is made. (author)