Effects of strong density fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Abstract

The Electron Cyclotron Emission Imaging (ECEI) diagnostic, with multiple lines of sight (LOS), where each LOS behaves like a conventional 1D radiometer, measures the electron temperature and its fluctuations on ms time scales and is specially suitable for 2D or quasi-3D visualisation of MHD phenomena. Its excellent poloidal resolution enables measurements of the poloidal velocity of MHD events. In the case of optically thick plasmas, where optical thickness is proportional to the product of electron density and the temperature, the intensity of the electron cyclotron radiation equals the level of the black body emission. However, when measuring at the plasma edge, where the density and temperatures are relatively low, the electron cyclotron emission no longer equals the black body radiation and the density contribution cannot be neglected. The ECEI diagnostic has an oblique observation angle [1] enhancing the Doppler shift of the resonant positions and the deflection of the beams with the largest incident toroidal and poloidal angles. The measurements of the ECEI are focused on hot H-mode plasmas where the hot electrons of the Maxwellian tail obey relativistic mass increase, and so the resonances are shifted towards the higher magnetic field. Taking into account all these effects the radiation transport equation is solved in order to obtain the radiation temperatures and correct resonance positions called the warm resonances [2, 3, 4]. For the discharge studied in this work, the forward model has shown that the presence of the density fluctuations can influence the radiation temperatures. Therefore the knowledge on both, density and temperature, is very important for the interpretation of the ECEI signal.