Diagnostics for current density and radial electric field measurements:: overview and recent trends

The measurements of the current density and the radial electric field profiles in tokamaks have considerably gained importance since advanced operational scenarios have come into play. A detailed knowledge of these profiles is desirable for operational scenarios involving reversed magnetic shear and/or the active control of internal and edge transport barriers. Unfortunately, the current density as well as the radial electric field are among the plasma parameters that are most difficult to diagnose. Routine methods to measure the current density profile are based on the Faraday effect (i.e. polarimetry) and the motional Stark effect (MSE). The most advanced diagnostics for the measurement of the radial electric field in the plasma core are the heavy ion beam probe and again the MSE. This paper will briefly explain the need for detailed measurements of the current density and radial electric field profile. Subsequently, the various diagnostics to measure these parameters will be reviewed. The emphasis will be especially put on recent trends, rather than on an exhaustive overview.