Test of a Sub-4K Mechanical Cooler for IXO and Other Space Based Sensors

X-ray Microcalorimeter Spectrometer Sensors on missions such as the International X-ray Observatory (IXO) require cooling to temperatures around 50mK to achieve the required sensitivity in the 0.6-10 keV band. Cooling an X-ray Sensor such as a Transition Edge Sensor (TES) to 50mK without the limitations on lifetime, mass, volume and reliability penalties of stored cryogen systems can be achieved with a multiple stage mechanical cryocooler. While no single cryocooler technology is appropriate for all of the stages, a hybrid cryocooler can be used. Fortunately, three cooler technologies that are each optimized for efficiency over the appropriate parts of the temperature range are rapidly maturing. For the lowest temperature stage, an Adiabatic Demagnetization Refrigerators (ADR) efficiently cools between 50mK and 2K to 4K. Next, a helium Joule Thomson cooler can efficiently pump the heat to 14K. Finally, a multistage pulse tube cooler efficiently pumps the heat from 14K to 300K. An existing ADR cooler, such as that demonstrated by NASA Goddard to TRL 5, can be cooled by a hybrid JT and pulse tube cooler similar to the cooler that NGST is building for the JWST/MIRI instrument, if its temperature is lowered from 6K to below 4K. The MIRI cooler leverages extensive NGST cooler heritage with >60 years of on-orbit performance with 11 pulse tube coolers currently operating continuously in orbit without failure.

In this poster we present test results of a laboratory demonstration JT cooler stage with the sub-4K temperatures needed by the ADR cooler. By basing the test on the 6 K cooler technologies developed for the JWST MIRI program, the current development program provides the next step to reach the goal of TRL6 in time to support the IXO mission. This successful test provides demonstration of TRL 4 for the missing components required for an IXO cooler.