Superfluid liquid helium control for the primordial inflation polarization explorer balloon payload.

The Primordial Inflation Polarization Explorer (PIPER) is a stratospheric balloon payload to measure the polarization of the cosmic microwave background. Twin telescopes mounted within an open-aperture bucket dewar couple the sky to bolometric detector arrays. We reduce detector loading and photon noise by cooling the entire optical chain to 1.7 K or colder. A set of fountain-effect pumps sprays superfluid liquid helium onto each optical surface, producing helium flows of 50-100 cm3 s-1 at heights up to 200 cm above the liquid level. We describe the fountain-effect pumps and the cryogenic performance of the PIPER payload during two flights in 2017 and 2019.