Sorption Cooler

The focal assembly can be cooled to 22 K by a single-stage sorption refrigerator with 315 mW of net refrigeration. This includes a 10% margin on active HEMT loads measured by UCSB and a 40% margin on parasitic loads. The gas is precooled by passive radiation to about 60 K. Spaceflight heritage is provided by the two-stage Brilliant-Eyes Ten Kelvin Sorption Cooler Experiment (BETSCE), flown on the shuttle in 1995.

The size and performance of the cooler are based on the measured performance of a cooler of nearly identical configuration now in final system tests. This cooler will fly on the UCSB long duration balloon (LDB) cosmic microwave background experiment and cool HEMTs like those proposed for the COBRAS/SAMBA mission to 25 K. A photograph of the 25 K compressor assembly is shown below.

The early validation of this system provides a solid hardware basis for size, power, mass, cost, and schedule estimates. In addition, breadboards for the flight electronics and preliminary versions of the ground support electronics now exist. The LDB cooler is built to flight standards, and incorporates high pressure tanks and a low temperature sorption bed to stabilize the cold-end temperature to less than 1 mK/sec. It has also demonstrated enormous tolerance to contamination through the use of large diameter porous plugs for the Joule-Thomson flow restrictor.

System robustness is enhanced through the use of a passive gas-gap thermal switch connecting the focal assembly to the radiative cooler. The gas-gap thermal switch is non-conductive when the sorption cooler is operating. If the sorption cooler fails and the focal assembly warms above 50 K, the solid nitrogen sublimates and provides a thermal path out of the focal assembly, keeping its temperature to 62 K. Below is a photograph of a commandable version of this switch that significantly exceeds the on-conductance and switch ratio requirements.

Return to the Planck Information page for a more complete description of Planck (COBRAS/SAMBA).

Return to the Smoot Group page for a complete description of Dr. Smoot's group's research activities.

Revised 16 August 1996; smoot@cosmos.lbl.gov