Electron cyclotron radiative transfer in fusion plasmas

The approaches developed to describe radiative transfer of electron cyclotron (EC) radiation in a plasma in local thermodynamic equilibrium, taking due account of the nonlocal nature of the process owing to reabsorption and wall reflection, are reviewed. As for a cylindrical plasma with specularly reflecting walls an analytical solution of the equation of radiative transfer can be derived, an exact formula for the net power density radiated locally can be given in this case. Making use of an improved expression for the EC absorption coefficient, a detailed numerical analysis can be carried out varying both the wall reflection coefficient and the radial profile of the plasma temperature, which reveals that a reversal of the net power density profile can occur on the plasma outboard for sufficiently high wall reflectivity. A comparison with Beamstrahlung radiation profiles as well as approximate EC net power density profiles as calculated with the CYTRAN code developed by Tamor is performed. The total EC radiated power is also evaluated; its scaling with the reflection coefficient is found to be very well described by a scaling following from a recently established global model for the EC radiation, which improves the well-known Trubnikov scaling. The results obtained are discussed in view of their possible relevance to affecting the plasma temperature profile. (authors)