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Description
The cooling line for the mechanical cryocooler for the Mid Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) consists of several meters of small-diameter stainless steel tubing at a normal operating temperature of about 18 Kelvin. Over much of its length this line is surrounded by surfaces at significantly higher temperatures, so it is gold-plated to minimize absorption of thermal radiation. However, the length located inside the blanketed thermal volume containing the JWST science instruments is not gold-plated. Periodic warming of the entire cooling line via cryocooler recycling is required to evaporate water frozen onto the outer surface of the gold-plated tubing. During this process, the line’s temperature will be held for an extended period of time at a temperature as high as 200 Kelvin, and the amount of thermal energy radiated onto surfaces near the instruments is a concern. Thus, it was deemed important to measure the total hemispheric emissivity of the un-plated tubing used in the cooler line as a function of temperature. We describe the measurement technique and present the results.