Investigating emissivity evolution of NSTX-U graphite and Li-coated stainless steel for IR thermography

Infrared (IR) thermography is essential to measuring the spatial and temporal heat flux evolution in fusion tokamaks, as well as performing power balance studies. As a result, the emissivity of the plasma-facing components (PFCs) in any tokamak must be correctly characterized to guarantee accurate temperature measurements. The emissivity of W PFCs have been shown to evolve significantly [J. Gaspar et al., FED 149 (2019)] due to changes in surface roughness, plasma conditioning, and surface temperature. We present recently conducted experiments where we observe the emissivity evolution in Li-coated stainless steel, post-mortem NSTX-U ATJ graphite, and unexposed NSTX-U R6510 graphite. Both graphite and stainless steel samples underwent Li deposition in LTX-β. Samples were transferred in-vacuo utilizing the Sample Exposure Probe from LTX-β to the Sample Exposure Station for plasma conditioning and surface analysis. A Telops IR camera with a wavelength spectrum of 7.7-9.3 µm was used to analyze the emissivity of the heated samples in an ultra-high vacuum environment as a function of temperature. XPS and TPD were used to characterize the samples.