Metal Hydride Compressors with Gas-Gap Heat Switches: Concept, Development, Testing, and Space Flight Operation for the Planck Sorption Cryocoolers

Two closed-cycle cryogenic refrigerators were used to generate temperatures of ~18 K via evaporation of liquid hydrogen at the interfaces with radiofrequency and infrared sensors on an Earth-orbiting spacecraft that measured the anisotropy of the cosmic microwave background (CMB) during the European Space Agency (ESA) Planck Mission from June 2009 until October 2013. The liquid hydrogen phase was continuously generated in each Planck Sorption Cryocooler (PSC) by coupling a Joule−Thomson (J−T) expander to hydrogen gas initially pressurized to nominally 3000 kPa (i.e., ~30 bar) and subsequently discharged at pressure of 30 kPa (i.e., ~0.3 bar) by desorption and absorption using LaNi4.78Sn0.22Hx contained in six individual sorbent beds. The pressures were varied by alternately heating and cooling this hydride that included temperature modulation with an integrated Gas-Gap Heat Switch (GGHS). The novel GGHS used the low-pressure hydride ZrNiHx to vary thermal conductance between the bed containing the LaNi4.78Sn0.22Hx sorbent and the rest of the compressor system. The design features and development of these hydride components are described along with details of fabrication and assembly. The results obtained during extended laboratory testing are also summarized. The predictions from this preflight testing are compared to the performance observed while operating in orbit during the Planck Mission. This review ends with a summary of lessons learned and recommendations for improved systems.