Decay of enhanced density and damping of plasma flows after the electrode biasing termination on the CASTOR tokamak

Electrode biasing is a standard tool for modification of both edge and global plasma parameters on the CASTOR tokamak (R=0.4 m,a=85 mm,B _T=1.3 T,I _p1≈9 kA,q _a≈10). During a steady state phase of a discharge, a polarization voltage is applied on an electrode immersed into the edge plasma. This voltage causes radial currents that create radial electric field and, due to theE×B drift, they cause an enhanced rotation. Then, as a consequence, the interaction with walls decreases and particle confinement and density increase. Recently, the decay of plasma density and plasma flows after the termination of the biasing period was investigated on the CASTOR tokamak. These observations are linked to processes and mechanisms that control generation of radial electric fields in plasmas and damping ofE×B sheared flows and that therefore represent a key issue for understanding the transition to improved confinements modes. In the contribution, measured time scales of the transition to the ohmic regime after the biasing termination will be shown. Further, possible consequences of these measured scales for the valuation and explanation of important processes in the plasma will be discussed.