Implementation of a Mid-Infrared Spectroscopic System for Tokamak à Configuration Variable

Infrared thermography on TCV uses mid-infrared (MIR) camera imaging in the 3.7–4.8 μm range to infer heat deposition on the walls from the wall-temperature evolution. However, this measurement can be affected by MIR plasma emission collected along the line of sight. This work presents the implementation of a MIR spectroscopic system on TCV to spectrally resolve emission collected from the divertor region in the spectral range of the infrared thermography diagnostic. The system collects MIR emission through an optical fiber and uses a Czerny–Turner spectrometer configuration for spectral dispersion. The spectrometer optics are placed inside a cooled vacuum enclosure to reduce noise from background emission of the spectrometer components. In this project, the spectrometer hardware was assembled, the optics were aligned, and the cooling system was characterized. A workflow was also developed to convert raw spectroscopic images into absolutely calibrated spectra. The wavelength calibration was mainly performed using the absorption band of CO2 naturally present in air, while a blackbody-ratio method was also investigated as a more flexible alternative. However, this method was limited by its strong sensitivity to the accuracy of the blackbody-source temperature. As part of the commissioning of the spectroscopic system on TCV, a dedicated plasma scenario was developed to investigate the MIR emission properties of different regions of the divertor. Several distinct spectral features were observed. When the line of sight did not terminate near the strike point, the continuum spectrum was consistent with a wall blackbody profile, with a narrow peak near 4.05 μm identified as the deuterium 5 → 4 transition. When the line of sight terminated on the wall near the strike point, the spectral distribution became significantly more complex and was no longer described by a clear blackbody profile. These observations suggest that emission from the plasma-surface interaction region may contribute significantly to the overall spectral intensity. Finally, the continuum emission was used to estimate the wall temperature. The inferred temperatures were of the same order of magnitude as VIR infrared thermography estimates, although discrepancies remain due to emissivity assumptions and differences in the viewed wall region. Overall, this preliminary work demonstrates the potential of spectrally resolved MIR measurements on TCV as a complement to filtered infrared thermography, both for identifying plasma-emission contributions and for providing continuum-based temperature estimates. It also shows that further spectroscopic investigation of MIR emission near the strike point is important for the reliable application of infrared imaging diagnostics.