Effects of relativistic broadening on the spatial resolution of electron cyclotron emission measurements for ITER

High temperatures in ITER plasmas result in relativistic broadening of electron cyclotron emission (ECE). Successful use of the ECE diagnostic requires assessment of spatial resolution and determination of accessible regions in the plasma. Using a synthetic diagnostic that has been previously developed and specialized for ITER, the predicted resolution of ECE frequency measurements can be determined. In addition, the fundamental physics describing the ECE behavior can be quantitatively shown to demonstrate the effects of high-temperature scenarios that are uncommon in contemporary devices. This includes the overlap of ECE harmonics that limits accessible regions in the plasma. Such a discussion is useful for the design and engineering of the ECE systems for devices such as ITER. These studies have shown that, for a baseline full-field ITER plasma, ECE X-mode second harmonic has a spatial resolution of <5 cm in the accessible region, meeting ITER requirements. The studies have been expanded to include scenarios spanning the ITER operational range, which provides a general assessment of performance. Low-temperature scenarios show minimal relativistic broadening, which agrees with results from contemporary devices. Higher temperature scenarios (∼40 keV) still allow ECE measurements with spatial resolution (∼15 cm) within ITER requirements for the higher Te range.