Charlie Danaher, Danaher Cryogenics, and Simon Chase, Chase Research Cryogenics
Originally published in Cold Facts magazine, February 2023, Volume 39, Number 1
Making advances in science requires the proper tools. Often those tools must be developed by scientists themselves. That’s the story of most science missions studying Cosmic Microwave Background radiation (CMB). Sub-Kelvin coolers developed by Chase Research Cryogenics (CSA CSM) have played a strategic role in many of these missions. Let’s look at two exciting examples.
One such mission is the POLARBEAR experiment. The instrument was developed by an international collaboration – University of California, Berkeley, Lawrence Berkeley National Lab, University of Colorado, Boulder, UCSD, Imperial College, University of Paris, KEK, McGill University and Cardiff University – and installed in the Huan Tran Telescope (HTT) at the James Ax Observatory on the Chajnantor Science Reserve in the Atacama Desert in Chile, at an altitude of 17,000 feet.
A Chase cooler was used to cool the focal plane to ~0.25 K, and to keep it there for about 24 hours. Cooling the focal plane to 0.25 Kelvin results in the thermal carrier noise being inconsequential compared to the thermal background noise of the atmosphere. The 0.25 K focal plane is the coldest stage of the receiver and is surrounded by a series of nested thermal envelopes. Upstream of the focal plane is the 4 K shield containing cooled optics, and outside of that is the 50 K envelope, which includes a series of IR thermal shields.
POLARBEAR was designed to make measurements of the CMB polarization on small angular scales to detect the B-mode polarization and put limits on inflation. A second mission objective is to reconstruct the lensing potential of the CMB with hopes of providing limits on the neutrino mass. POLARBEAR was the first instrument to utilize a lenslet-coupled planar array detector architecture for CMB observations. The receiver saw first light on January 10, 2012, and began its first observing season in April 2012.
The Chase cooler used in the POLARBEAR receiver was a GL10. This cooler has a sorption-pumped, He3 pot, backed by another He3 pot, which is, in turn, backed by a He4 pot. This architecture yields a cold stage temperature of 250 mK or better. The beauty of the Chase cooler is that it is a self-contained refrigerator that can be mounted to an instrument’s 3 K stage plate. This architecture allows for the scientists to design the system schematically, treating the sub-K cooler as a building block, as they’d treat the cryocooler. Integration of the GL10 is simple in that, in addition to the mechanical mounting to the 3 K stage plate, it only needs electrical connection made to external power supplies and thermometer readers.
Looking forward, let’s discuss an exciting astronomical mission under development called Taurus. Taurus is a NASA, ARPA-funded, balloon-borne Polarimeter for Cosmic Reionization and Galactic Dust. Taurus is a joint program involving Washington U-St. Louis, Princeton University, University of Illinois Urbana-Champaign, University of Iceland and NIST.
Taurus aims to measure the probability that a light ray traveling from the CMB will be scattered on its way to an instrument, whether that instrument is on Earth or in space. Knowing that probability allows scientists to understand how “blurry” that light is. Current measurements tell us that tau is around 5%, but a much more precise measurement of this probability will provide new information about how quickly the first stars formed.
A more precise measurement of tau will also help scientists attempting to measure the mass of neutrinos through their effect on the CMB. Massive neutrinos leave their signature through a different kind of blurring of the CMB: gravitational lensing. Light bends as it passes by massive objects in the universe on its way to Earth. This bending, or lensing, distorts the light we see from the CMB. A more precise measurement of tau will yield a better interpretation of this lensing signal, and thus to measuring the neutrino mass.
Additionally, because Taurus will be largely outside of Earth’s atmosphere, it will be able to measure with high sensitivity the polarized glow of dust in our galaxy, at frequencies that would be blocked by our atmosphere. The newly released Chase continuous mini dilutor (CMD) refrigerator is being evaluated for the Taurus project. This cooler can provide continuous cooling at 100 mK, which offers the possibility of continuous observation. However, what’s most attractive about the CMD is its low power draw, its low mass and the simple yet robust operation.
Cryostats destined for telescope receivers will almost surely continue to be one-off, custom systems. However, scientists who are looking for a lab cryostat to host a Chase cooler, now have a commercial choice. Danaher Cryogenics (www.danahercryo.com, CSA CSM) has partnered with Chase (www.ChaseCryogenics.com) to offer a full line of integrated cryostats.