The case for multiband infrared imaging in high-Z armored fusion devices

A multiband interpretation of infrared emission allows for accounting of spatial and temporal variations in emissivity for metallic armor materials and thus accurate determination of surface temperature and heat flux in a fusion device. Without adequately accounting for true emissivity of material in the imaged scene, the situation where true surface emissivity is less than expected can lead to inference of temperatures less than actual. In machines which push the temperature of plasma-facing materials near the limits of recrystallization and melting of high-Z materials, such a situation could be catastrophic for first-wall integrity and machine operation, particularly when there is little flexibility in strike point positioning for high performance plasmas.

The multiband interpretation of infrared radiation relies on calibration of ratios between bands rather than their integrated intensity as used in traditional infrared cameras. First demonstrated in a dual-band approach on NSTX which is appropriate when emissivity is constant with wavelength, expanding to multiband allows compensation of variable emissivity with wavelength and other conditions. The approach proposed for multiband detection is by splitting the full image frame into multiple sub-frames, each integrating a separate portion of the observed wavelengths of light. The motivation for such an approach, hardware and interpretive aspects, as well as limitations, are presented.