Technical Summary: The Cryogenic System of DMRadio-50L

Problem Statement

The DMRadio-50L experiment aims to search for axion dark matter in the 5 kHz – 5 MHz frequency range using a lumped-element LC resonator coupled with a toroidal magnet. To achieve the necessary sensitivity to detect the minute electromagnetic power excesses predicted for axions, the experiment requires operation at millikelvin (mK) temperatures. This extreme cooling is essential to minimize thermal occupation of the resonator mode and reduce the added noise of cryogenic amplifiers.

However, the scale of the DMRadio-50L detector presents significant engineering challenges. The detector assembly has a mass approaching 200 kg (dominated by the superconducting magnet, shielding, and support hardware) and occupies a volume of at least 0.5 m³. The cryogenic system must simultaneously:

1. Provide sufficient cooling power at multiple temperature stages (40 K, 4 K, Still, and Base) to support this large thermal mass.
2. Maintain a base temperature below 50 mK for the sensitive resonator and readout electronics.
3. Support a 200 kg mechanical load, with specific distribution requirements (65 kg on the still stage, 25 kg on the base stage).
4. Operate within a standard laboratory environment with a 14 ft ceiling height.
5. Mitigate mechanical vibrations from pulse tube cryocoolers, which can couple into the readout chain as noise.
6. Ensure seismic survivability in a region prone to earthquakes.

Methodology

To address these competing thermal, mechanical, and spatial constraints, the authors developed a hybrid cryogenic architecture. This system combines a horizontally mounted dilution refrigerator (DR) with a vertically oriented payload cryostat. This design, previously employed in the SuperCDMS experiment, allows for the decoupling of mechanical support and thermal transport functions.

Key Design Features:

• Hybrid Architecture: The payload cryostat houses the experiment and provides cooling at ~40 K and ~4 K via a PT425-RM pulse tube cryocooler. The horizontal Bluefors LH400 DR provides cooling for the Still and Base (mK) stages. The two subsystems share a common vacuum space connected by soft vacuum bellows.
• Thermal Pathways:
  • 40 K and 4 K Stages: Cooling is provided by two independent pulse tube cryocoolers (PT425-RM in the payload, PT420-RM in the DR). Thermal links utilize C101 oxygen-free high-conductivity copper (C101 Cu) cold fingers and flexible thermal straps.
  • Still and Base Stages: Cooling is delivered from the DR's Still and Mixing Chamber (MXC) plates to the payload via long, horizontal cold fingers.
  • Flexible Connections: To accommodate differential thermal contraction and minimize vibration transmission, the system employs flexible thermal links. These include concentric cylinders with beryllium copper (BeCu) fingerstrips for radiation shielding and mechanical compliance at the 40 K stage, and overlapping high-purity aluminum strips for the 4 K stage.
• Mechanical Support and Vibration Isolation:
  • The 200 kg load is supported by G10 tubes and stainless steel (SS) structures.
  • The base payload plate is suspended using a compression-based Vespel SP-1 suspension, which offers high mechanical stiffness with ultralow thermal conductivity.
  • The base cold finger is suspended concentrically inside the still cold finger using Vespel screws and a Kevlar string "puck" suspension to provide lateral stability without significant conductive heat load.
• Seismic Design: The system is seismically bolted to the ground. Internal structures, such as the S2 DR plate and still plate, are reinforced with additional steel supports. The Vespel supports were load-tested with liquid nitrogen to verify integrity.
• Instrumentation: A comprehensive control system monitors temperatures, pressures, and heater powers using Lakeshore controllers and Pfeiffer gauges, with data logged to a central database and visualized via Grafana.

Key Contributions

The paper details the conceptual design, technical implementation, and measured performance of this custom cryogenic system. Specific contributions include:

• Integration of Large Mass: Successfully designing a system to cool a 200 kg detector assembly to mK temperatures within a standard laboratory footprint.
• Hybrid Thermal Management: Demonstrating a modular interface between a horizontal DR and a vertical payload cryostat that allows for independent testing, maintenance, and operation while maintaining high thermal conductance.
• Specialized Thermal Links: The development of specific flexible thermal connections (e.g., the BeCu fingerstrip radiation shield and the Al strip thermal link) that balance high thermal conductance with mechanical compliance and vibration isolation.
• Seismic Validation: Providing a design that survived a magnitude 4.6 earthquake with no structural damage, validating the robustness of the seismic anchoring and internal reinforcement strategies.

Results

The system was commissioned and tested, yielding the following performance metrics:

• Cooling Times: The S2 payload plate reached steady-state temperatures in approximately 60 hours when the DR was thermally coupled, compared to 75 hours in independent operation.
• Steady-State Temperatures:
  • 40 K Stage (S1): Stabilized at 31.2 K (DR) and 52.0 K (Payload).
  • 4 K Stage (S2): Stabilized at 2.7 K (DR) and 3.4 K (Payload/Ring).
  • Still Stage: The DR still plate reached 571 mK, while the payload still plate stabilized at 696 mK.
  • Base Stage: The DR MXC plate reached 21 mK. The base payload plate stabilized at 36 mK ± 5 mK, successfully meeting the requirement of <50 mK.
• Thermal Conductance: Measurements of temperature drops across the thermal paths confirmed that the long cold fingers are the dominant thermal resistance, consistent with the design model.
• Earthquake Survivability: During a magnitude 4.6 earthquake, the system sustained no structural damage. While transient temperature fluctuations were observed (attributed to the distortion of BeCu fingerstrips), the system returned to normal operation after readjustment in the next cooldown cycle.

Significance

The authors state that the cryogenic system successfully meets or exceeds the thermal design requirements for the DMRadio-50L experiment. The system provides the necessary cooling power and thermal stability to support the sensitive resonator and readout chain, enabling the search for axion dark matter in the targeted frequency range.

Furthermore, the paper highlights the system's role as a versatile testbed for quantum sensors, such as the radio-frequency quantum upconverter (RQU). By providing a stable, low-bath temperature environment with ample volume for test circuitry and access to a high-Q superconducting resonator, the platform facilitates the development of next-generation sensors capable of readout below the standard quantum limit. The modular and compliant design offers a generalizable solution for other large-scale experiments requiring mK cooling, demonstrating that large detector masses can be effectively cooled within standard laboratory constraints.