Electrode biasing on TEXTOR: A tool for fundamental physics studies

The application in TEXTOR of an externally controlled radial electric field E_r, imposed by means of an electrode, has allowed to ascertain many aspects of the physics of the creation of E_r and of its effect on radial transport. Radial conductivity was shown to depend on parallel viscosity with the latter's nonlinear response to E_r providing the basic ingredient for E_r bifurcation, typical for L- to H-mode transitions. Simultaneous time and space resolved measurements of E_r and of the plasma flows in the edge by means of a newly developed inclined Mach probe have allowed to further substantiate the role of parallel viscosity and of neutral collisions in the damping of rotation. The causal role of grad E_r in bringing about the transport changes has been proven by showing that the field shear is spatially correlated with and temporally leads the density gradient, as well as by comparison with theoretical modeling.