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The electron cyclotron emission (ECE) in fusion devices is non-trivial tomodel in detail at frequencies well below the fundamental resonance where theplasma is optically thin. However, doing so is important for evaluating thebackground for microwave diagnostics operating in this frequency range. Here wepresent a general framework for estimating the ECE levels of fusion plasmas atsuch frequencies using ensemble-averaging of rays traced through manyrandomized wall reflections. This enables us to account for the overall vacuumvessel geometry, self-consistently include cross-polarization, and quantify thestatistical uncertainty on the resulting ECE spectra. Applying this to ITERconditions, we find simulated ECE levels that increase strongly with frequencyand plasma temperature in the considered range of 55-75 GHz. At frequenciessmaller than 70 GHz, we predict an X-mode ECE level below 100 eV in the ITERbaseline plasma scenario, but with corresponding intensities reaching keVlevels in the hotter hybrid plasma scenario. Benchmarking against the SPECEraytracing code reveals good agreement under relevant conditions, and thepredicted strength of X-mode to O-mode conversion induced by wall reflectionsis consistent with estimates from existing fusion devices. We discuss possibleimplications of our findings for ITER microwave diagnostics such as ECE,reflectometry, and collective Thomson scattering.

Modelling the electron cyclotron emission below the fundamental resonance in ITER