An ASME-Compliant Helium-4 Evaporation Refrigerator for the SpinQuest Experiment

TL;DR

This work addresses the need for a documented, standards-compliant cryogenic solution for high-radiation, fixed-target spin experiments by delivering a $1\,\text{K}$ liquid $^4$He evaporation refrigerator for SpinQuest (E1039) integrated with a $5\,\text{T}$ superconducting magnet. The authors develop a methodology to apply ASME BPVC VIII and ASME B31.3 alongside Fermilab FESHM requirements, including an overpressure protection strategy and remote, radiation-hardened instrumentation. Commissioning in July 2024 demonstrated stable cooling powers in the $1$–$4\,\text{W}$ range, robust operation through magnet quenches, and a total heat load not exceeding about $1.5\,\text{W}$ under beam and microwave loading. The study provides a practical, traceable design-path for future high-intensity polarized-target cryogenics and discusses potential extensions to helium-3 systems.

Abstract

This paper presents the design, safety basis, and commissioning results of a 1 K liquid helium-4 (4He) evaporation refrigerator developed for the Fermilab SpinQuest Experiment (E1039). The system represents the first high power helium evaporation refrigerator operated in a fixed target scattering experiment at Fermilab and was engineered to comply with the Fermilab ES\&H Manual (FESHM) requirements governing pressure vessels, piping, cryogenic systems, and vacuum vessels. The design is mapped to ASME B31.3 (Process Piping) and the ASME Boiler and Pressure Vessel Code (BPVC) for pressure boundary integrity and overpressure protection, with documented compliance to FESHM Chapters 5031 (Pressure Vessels), 5031.1 (Piping Systems), and 5033 (Vacuum Vessels). This work documents the methodology used to reach compliance and approval for the 4He evaporation refrigerator at Fermilab which the field lacks. Design considerations specific to the high radiation target-cave environment including remotely located instrumentation approximately 20 m from the cryostat are summarized, together with the relief-system sizing methodology used to accommodate transient heat loads from dynamic nuclear polarization microwaves and the high-intensity proton beam. Commissioning data from July 2024 confirms that the system satisfies all thermal performance and safety objectives.

Figures (14)

• Figure 1: Simplified schematic of the refrigerator system showing the major components and connections to the pumping stack through the gate valve and bypass line. Arrows indicate key features within the shell–nose assembly.
• Figure 2: Shown is the target within the high radiation-shielded enclosure, left, and a zoom in on the refrigerator placement within the polarized target, right.
• Figure 3: A cross-sectional view of the phase separator showing the baffles above and the liquid helium path.
• Figure 4: The above model shows the valve location of the run and bypass helium routes. The run and bypass needle valves are remotely controlled. The Bypass valve reroutes liquid from the separator to the nose for fast filling. The run valve allows for throttling of the separator pre-cooled liquid through the lower heat exchanger and to the nose.
• Figure 5: A model zoomed in the target housing of the insert along the central axis of the refrigerator. A microwave horn is label delievering the 140 GHz to the target cells. Three target cells are shown with NMR loops inside. The beamline, shown as a red dashed line, passes through the beam window to interact with the target. All three target cells are submerged within liquid helium.