“Over the last couple weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins. Soon, the cryocooler is about to experience the most challenging days of its mission. By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins. But first, the cryocooler must make it through the ‘pinch point’ – the transition through a range of temperatures near 15 kelvins, when the cryocooler’s ability to remove heat is at its lowest. Several time-critical valve and compressor operations will be performed in rapid succession, adjusted as indicated by MIRI cryocooler temperature and flow rate measurements. What is particularly challenging is that after the flow redirection, the cooling ability gets better as the temperature gets lower. On the flip side, if the cooling is not immediately achieved due to, for example, larger than modeled heat loads, MIRI will start warming.
“Once the cryocooler overcomes the remaining heat loads, it will settle into its lower-power steady science operation state for the rest of the mission. This pinch point event has been extensively practiced in the cryocooler testbed at NASA’s Jet Propulsion Laboratory (JPL), which manages the MIRI cryocooler, as well as during Webb testing at the agency’s Goddard Space Flight Center and Johnson Space Center.
