  The Primordial Inflation Polarization Explorer (PIPER) is a stratospheric
balloon payload to measure 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 cm^3 / s 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.
