Thermal architecture for the SPIDER flight cryostat

TL;DR

This paper details the design and thermal analysis of a cryogenic system for the SPIDER balloon-borne microwave polarimeter, enabling long-duration flights with extremely low background noise.

Contribution

It presents a novel cryogenic architecture combining large liquid helium tanks, superfluid helium cooling, and thermal shielding for balloon-borne microwave observations.

Findings

Achieved a hold time exceeding 25 days.

Maintained base temperatures of 4 K and 1.5 K.

Ensured low instrumental background for sensitive measurements.

Abstract

We describe the cryogenic system for SPIDER, a balloon-borne microwave polarimeter that will map 8% of the sky with degree-scale angular resolution. The system consists of a 1284 L liquid helium cryostat and a 16 L capillary-filled superfluid helium tank, which provide base operating temperatures of 4 K and 1.5 K, respectively. Closed-cycle helium-3 adsorption refrigerators supply sub-Kelvin cooling power to multiple focal planes, which are housed in monochromatic telescope inserts. The main helium tank is suspended inside the vacuum vessel with thermally insulating fiberglass flexures, and shielded from thermal radiation by a combination of two vapor cooled shields and multi-layer insulation. This system allows for an extremely low instrumental background and a hold time in excess of 25 days. The total mass of the cryogenic system, including cryogens, is approximately 1000 kg. This enables conventional long duration balloon flights. We will discuss the design, thermal analysis, and qualification of the cryogenic system.