Operation of gas bearings at cryogenic temperatures

Future space missions require efficient, low-temperature cryocoolers to cool advanced instruments or serve as the upper-stage cooler for sub-Kelvin refrigerators. Cooling loads for these missions are up to 300 mW at temperatures of 4-10 K, with additional loads at higher temperatures for other subsystems. Many of these missions have low jitter requirements, making turbo-Brayton cryocoolers an ideal candidate since the only vibration source that can impact jitter is from the low density gas flowing in tubes, which is extremely low. In addition, these cryocoolers are inherently efficient at low temperatures. The primary limitation in implementing Brayton cryocoolers at temperatures below 10 K has been the development of high-efficiency, low-temperature turbines. In large cryogenic expansion turbines used in liquefaction plants, the bearings along with either a brake compressor or an alternator operate at a warm temperature such that mechanical and electrical losses are not transferred to the cold turbine. The only parasitic loss in these machines is heat conduction down the shaft from the warm zone to the cold turbine. Up to this point, turbine developments for space cryocoolers at 10 K and colder have used this configuration. The primary advantage to this approach is being able to operate the gas bearings at warmer temperatures with fewer concerns of low viscosity and thermal contraction adversely impacting the performance of the bearings. Unfortunately, in very small machines heat conduction from the warm end to the cold end can become a large fraction of the turbine power. The use of an isothermal turbine, in which the gas bearings and alternator operate at the primary load temperature, is routinely used at temperatures above 20 K, but to date has not been applied to lower temperatures. This paper reviews the challenges of operating gas bearings at low temperatures and reviews the experimental and analytical work performed to address these challenges.