Sub-Kelvin cooling for two kilopixel bolometer arrays in the PIPER receiver

TL;DR

The paper details the thermal design and implementation of a sub-Kelvin cooling system using a continuous adiabatic demagnetization refrigerator for the PIPER balloon-borne telescope, enabling sensitive measurements of primordial gravitational waves.

Contribution

It introduces a robust CADR-based cooling system for two kilopixel bolometer arrays operating at 100 mK in a balloon environment, with detailed control and performance analysis.

Findings

CADR provides ~10 μW cooling at 100 mK

System maintains stable sub-Kelvin temperatures during flight conditions

Successful integration of cooling system in balloon environment

Abstract

The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne telescope mission to search for inflationary gravitational waves from the early universe. PIPER employs two 32x40 arrays of superconducting transition-edge sensors, which operate at 100 mK. An open bucket dewar of liquid helium maintains the receiver and telescope optics at 1.7 K. We describe the thermal design of the receiver and sub-kelvin cooling with a continuous adiabatic demagnetization refrigerator (CADR). The CADR operates between 70-130 mK and provides ~10 uW cooling power at 100 mK, nearly five times the loading of the two detector assemblies. We describe electronics and software to robustly control the CADR, overall CADR performance in flight-like integrated receiver testing, and practical considerations for implementation in the balloon float environment.