Experimental studies of the physical mechanism determining the radial electric field and its radial structure in a toroidal plasma

Experimental studies on radial structures of plasma rotation and radial electric field in toroidal plasmas are reviewed. In this context, the perpendicular and parallel viscosities that determine the toroidal/poloidal rotation velocity and radial electric field profiles are discussed. Experimental studies of parallel viscosity and the comparison with the neoclassical values in heliotron/torsatron and stellarator devices, are described. The anomalous perpendicular viscosity, which is dominant in dictating the toroidal rotation in tokamaks, is also discussed. Even without external momentum input, plasma rotation and radial electric field are sustained by non-ambipolar flux of off-diagonal terms of the transport matrix. The effects of radial electric field shear and the bulk rotation velocity shear on the improvement of particle, momentum and heat transport in bulk and edge plasma regimes are also discussed.