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NIST optimises pulse tube refrigerator

USA: Scientists from the USA’s National Institute of Standards and Technology (NIST) claim to have dramatically reduced the time and energy required to chill materials to temperatures near absolute zero.

They claim that their prototype refrigerator could prove a boon for the burgeoning quantum industry which widely uses ultracold materials. NIST says it is now working with an unnamed industrial partner to commercialise the refrigerator.

It’s estimated that the prototype – a modification of the commonly used pulse tube refrigerator – could save an estimated 27,000,000W of power per year. 

Ultracold refrigeration is essential to the operation of many devices and sensors, from stabilising qubits in a quantum computer to maintaining the superconducting properties of materials to cooling NASA’s James Webb space telescope.

For decades, the pulse tube refrigerator (PTR) has been the workhorse device for achieving temperatures as cold as the vacuum of outer space. These refrigerators cyclically compress and expand high pressure helium gas. The PTR has proven its reliability, but it is also power-hungry, consuming more electricity than any other component of an ultralow temperature experiment.

When NIST researcher Ryan Snodgrass and his colleagues took a closer look at the refrigerator, they found that manufacturers had built the device to be energy efficient only at its final operating temperature of 4K. At higher temperatures the PTR is extremely inefficient — a big issue during the cooldown process from room temperature.

During a series of experiments, Snodgrass, along with NIST scientists Joel Ullom, Vincent Kotsubo and Scott Backhaus, discovered that at room temperature, the helium gas was under such high pressure that some of it was shunted through a relief valve instead of being used for cooling. By changing the mechanical connections between the compressor and the refrigerator, the team ensured that none of the helium would be wasted, greatly improving the efficiency of the refrigerator.

In particular, the researchers continually adjusted a series of valves that control the amount of helium gas flowing from the compressor to the refrigerator. The scientists found that if they allowed the valves to have a larger opening at room temperature and then gradually closed them as cooling proceeded, they could reduce the cooldown time to between one half and one quarter of what it is now. 

Currently, scientists must wait a day or more for new quantum circuits to be cold enough to test. Since the progress of scientific research can be limited by the time it takes to reach cryogenic temperatures, it is thought that the faster cooldown provided by this technology could broadly impact many fields. 

The technology developed by the NIST team could also allow scientists to replace large pulse tube refrigerators with much smaller ones.

The researchers, who include scientists from the University of Colorado Boulder, describe their method in an article posted online April 23 in Nature Communications.

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