The Planck sorption cooler: Using metal hydrides to produce 20 K

The Jet Propulsion Laboratory has built and delivered two continuous, closed cycle, hydrogen Joule–Thomson (J–T) cryocoolers for the ESA Planck mission, which will measure the anisotropy in the cosmic microwave background. The metal hydride compressor consists of six sorbent beds containing LaNi4.78Sn0.22 alloy and a low-pressure storage bed of the same material. Each sorbent bed contains a separate gas-gap heat switch that couples or isolates the bed with radiators during the compressor operating cycle. ZrNiHx hydride is used in this heat switch. The Planck compressor produces hydrogen gas at a pressure of 48bar by heating the hydride to ∼450K. This gas passes through a cryogenic cold-end consisting of a tube-in-tube heat exchanger, three pre-cooling stages to bring the gas to nominally 52K, a J–T value to expand the gas into the two-phase regime at ∼20K, and two liquid–vapor heat exchangers that must remove 190 and 646mW of heat, respectively. Gas evaporated from the liquid phase is recovered by three hydride beds at ∼0.3bar and 270K. Each cooler was designed to provide 1W cooling at ∼20K for a total input power of 470W, excluding electronics. The performance of these coolers is mainly a function of the compressor interface and final pre-cooling stage temperatures. We present results from the testing of these two coolers for the input power, cooling power, temperature, and temperature fluctuations over the flight allowable ranges for these interfaces.