Mode analysis limitations of ECE-I & ECE measurements at the plasma edge

¹ DIFFER -Dutch Institute for Fundamental Energy Research De Zaale 20 5612 AJ Eindhoven the Netherlands
² Max-Planck-Institut für Plasmaphysik D-85748 Garching Germany
³ Department of Applied Science, the University of California at Davis, Davis CA 95616 USA

^* e-mail: branka.vanovac@ipp.mpg.de

Published online: 25 March 2019

Abstract

Interpretation of the Electron Cyclotron Emission (ECE) and ECE Imaging (ECE-I) measurements concerning the measurement position and the radiation temperature (T_rad) at the plasma edge is associated with significant uncertainty. Various limitations such as low and high-density limits, relativistic and Doppler shift-broadening, mode overlap were identified in the past. Here, we analyse the influence of density profile variation onto the radiation temperature in H-mode plasmas at ASDEX Upgrade tokamak. We show that, in the region of steep gradients, the variation of the density profile leads to an outward-shift of the measurement position, towards lower T_rad. The analysis is extended towards the study of the contribution of the ordinary mode (O-mode) emission to the measured T_rad at the plasma edge. With this analysis, we show the qualitative agreement between experimental observations and modeling when taking into account the O-mode contribution to the ECE measurements. All of these effects are assessed via the Electron Cyclotron Radiation transport forward model that is now routinely used to model the response of the ECE-I and ECE systems in different plasma scenarios.