Emissivity of silver and stainless steel from 80 K to 300 K: Application to ITER thermal shields

Abstract

The emissivities of thermal shield mock-up samples for ITER have been measured at sample temperatures between 80K and 300K using an optical method employing a primary standard broadband detector. These thermal shields, made from SS304L stainless steel coated with silver, are designed to operate at 80K, protecting the superconducting magnet system of the ITER reactor from higher temperature regions. Our results show that the silver coating of the thermal shields can have an emissivity as low as 0.0035 at 80K, approximately ten times lower than the emissivity of the bare polished stainless steel plate. We demonstrate that the emissivity of different regions of a thermal shield assembly can be determined in a single measurement cycle, providing further emissivity data on the insulating spacer used to separate shield plates as well as emissivity data on a silver coating repair method. The temperature dependence of the emissivity for the silver coating agrees well with a theoretical estimate based on the Drude model including phonon and surface-assisted scattering.