Optimization of ECE diagnostic for W7-X stellarator

The interpretation of diagnostic data at W7-X stellarator (under construction in Greifswald) will be supported by the concept of integrated data analysis. This concept is demonstrated to be feasible for the Thomson scattering diagnostic on W7-AS. For the ECE diagnostic, the basic idea is to fit the spectral intensity Iω±Δω simulated by modelling the data (the antenna, the receiver, etc) for a Te profile. Simulations are performed with a newly developed ray-tracing code, which includes an option to calculate the emission by electrons with an arbitrary distribution function. The antenna for (standard) ECE measurements will be installed on low-field-side (lfs) near the bean-shaped plane, where ∇B is largest, without spectral overlapping of cyclotron harmonics. In order to estimate the limit of spatial resolution, which comes from relativistic broadening of the emission line, the calculated ECE spectrum, i.e. radiation for each frequency channel Iω±Δω, is 'mapped' onto the radial coordinate. Vice versa, using the obtained radial uncertainties for disturbance of the Te parametrized profiles, the sensitivity of the ECE spectrum with respect to the implemented radial Te-profiles is checked. In the initial stage, W7-X will operate in the 1019 m-3 range of densities, with strong ECR heating by X2-mode at 140 GHz. For a highly localized deposition profile, one has to expect the appearance of a suprathermal population of electrons. The question how to distinguish the thermal and non-thermal contributions in the ECE spectrum is also discussed. The attractive idea of using vertical chords with B ≅ const as sightlines for the reconstruction of the electron distribution function will fail due to the diamagnetic effects (no way to find the chord with B ≅ const for an appropriate range of β-values). However, a high-field-side (hfs) observation (especially along the same sightline as lfs) appears to be much more promissing: despite the not so high spatial resolution of a hfs ECE diagnostic, the existence of suprathermal electrons can be identified by comparison of both lfs- and hfs-ECE spectra. Moreover, the energy range of the non-thermal fraction can be identified. The capability of (possible) hfs multichord imaging is also checked. (author)