Design, construction, and testing of the PandaX-xT cryogenics system

This paper presents the design, construction, and performance testing of the PandaX-xT cryogenics system, which utilizes two Gifford-McMahon cooling towers and an emergency liquid-nitrogen coil to efficiently manage approximately 43 tons of liquid xenon, achieving a cooling power of about 1900 W at 178 K and maintaining stable pressure fluctuations for a 1-tonne prototype.

The Gist

Imagine you are trying to keep a giant, super-sensitive camera cold enough to take a picture of a ghost. That's essentially what the PandaX-xT experiment is trying to do. It's a massive machine designed to hunt for Dark Matter (the invisible stuff that holds the universe together) and other rare cosmic events. To do this, it needs to hold about 43 tons of liquid xenon—a heavy, noble gas that has been frozen into a liquid state.

But here's the problem: keeping 43 tons of liquid xenon cold is like trying to keep a giant block of ice from melting in the middle of a hot summer desert. If it gets even a tiny bit too warm, the liquid boils, the pressure spikes, and the experiment fails.

This paper describes the design and testing of the refrigeration system (the "cryogenics") built to solve this problem. Think of it as building the ultimate air conditioner for a very special, very heavy freezer.

Here is how the system works, broken down into simple parts:

1. The Main Cooling Team: The "Heavy Lifters"

The primary job of keeping the xenon cold is handled by two massive industrial refrigerators called AL600 Gifford-McMahon cryocoolers.

• The Analogy: Imagine two giant, high-powered fans that don't blow air, but instead suck heat out of the system. They are so powerful that together they can remove about 1,900 Watts of heat.
• How it works: These machines have a "cold finger" (a metal rod) that gets extremely cold. The xenon gas touches this rod, turns into liquid, and drips down into the main tank.
• The Trick: To keep the temperature perfectly steady (so the liquid doesn't boil or freeze too hard), the system uses electric heaters. It's a bit like a thermostat that constantly adds a tiny bit of heat to keep the temperature from dropping too low, ensuring it stays exactly where it needs to be.

2. The Emergency Backup: The "Fire Extinguisher"

What happens if the power goes out or the main refrigerators break? The liquid xenon would start boiling immediately, creating dangerous pressure.

• The Solution: The team built an Emergency Liquid Nitrogen (LN2) Coil.
• The Analogy: Think of this like a fire extinguisher for heat. It's a long, coiled tube sitting inside the system, filled with super-cold liquid nitrogen.
• How it works: If the pressure gets too high, a valve opens, and liquid nitrogen rushes into the coil. It acts like a giant ice pack, instantly absorbing the heat and stopping the xenon from boiling.
• The Test: They tested this by turning off the main fridges and heating the system up. The nitrogen coil successfully handled a heat load of 1,500 Watts, proving it could save the day in an emergency.

3. The "Test Kitchen"

Before building the full 43-ton monster, the scientists built a prototype (a smaller model) to make sure everything worked.

• They built a 1-tonne tank (about the size of a large swimming pool) and a smaller test tower.
• They filled these with liquid xenon and ran them for weeks.
• The Result: The pressure inside the tank stayed incredibly stable. Even with the system running for a month, the pressure only wiggled by a tiny amount (less than 1 kPa). This is crucial because if the pressure wiggles too much, the "ghost-hunting" signals get scrambled.

Why Does This Matter?

The PandaX-xT experiment is the next big step in the search for Dark Matter. To be sensitive enough to find these elusive particles, the detector needs to be massive (43 tons of xenon) and perfectly stable.

This paper proves that the team has successfully engineered a cooling system that is:

1. Strong enough to handle the massive heat load of a 43-ton tank.
2. Stable enough to keep the pressure from fluctuating, ensuring clear data.
3. Safe enough with a backup system that can save the experiment if the main power fails.

In short: The scientists have built a super-stable, super-cold "freezer" with a built-in emergency backup, ready to house a giant tank of liquid xenon and help us solve one of the universe's biggest mysteries.

Technical Summary

1. Problem Statement

The PandaX-xT experiment is a next-generation dark matter search project located at the China Jinping Underground Laboratory (CJPL). It aims to utilize a massive 43-ton dual-phase liquid xenon (LXe) time projection chamber to search for Weakly Interacting Massive Particles (WIMPs), neutrinoless double beta decay, and astrophysical neutrinos.

The primary engineering challenge addressed in this paper is the development of a high-performance cryogenics system capable of:

• Managing the thermal load of a massive 43-ton xenon target (estimated at ~1500 W).
• Maintaining stable internal pressure fluctuations (standard deviation < 1 kPa) to ensure stable electroluminescence signals, which are critical for detector performance.
• Providing redundancy and emergency cooling capabilities in case of primary system failure or power loss.
• Improving upon the cooling power and stability of previous generations (PandaX-4T, XENONnT, LZ).

2. Methodology and System Design

The authors designed, constructed, and tested a prototype cryogenics system featuring a hybrid approach combining long-term mechanical cooling with emergency liquid nitrogen (LN2) cooling.

A. System Architecture

The prototype consists of three main components:

1. Primary Cooling Towers (2 units): Each tower houses an AL600 Gifford-McMahon (GM) cryocooler (Cryomech, USA).
   • Specs: Rated at 600 W at 80 K and 1000 W at 178 K.
   • Heating Control: Equipped with a custom 1300 W cartridge heater and Pt100 sensors to precisely regulate the cold finger temperature.
   • Design: The cold finger is isolated from the xenon gas by a copper adapter and sealed with indium wire to allow maintenance of the coldhead without breaking the main detector vacuum.
2. Emergency LN2 Cooler: A dedicated backup system using a 9.6 m stainless steel helical coil submerged in the xenon gas space.
   • Mechanism: Controlled by a PLC and electronic valves, it injects LN2 when detector pressure exceeds a preset threshold (230 kPa) and stops when it drops below a lower threshold (200 kPa).
   • Capacity: Designed to provide >1500 W of cooling power during emergencies.
3. Prototype Test Vessels:
   • A 1-tonne liquid xenon detector vessel (1355 mm diameter) to simulate the full-scale detector environment.
   • A test tower with a 2.2 kW heater to simulate high thermal loads (~15 kg of xenon) for stress testing.

B. Control and Safety

• Control: Siemens PLCs (SIMATIC S7-1500) manage temperature regulation, pressure monitoring, and valve actuation.
• Safety: Includes rupture discs (3 barg), safety valves (2.5 barg), and an uninterruptible power supply (UPS) for critical control systems.
• Vacuum: Multi-layer insulation (MLI) and a dual-stage pumping system (turbomolecular + dry screw/roots pumps) maintain vacuum levels < $1 \times 10^{-3}$ Pa.

3. Key Contributions

• Hybrid Cooling Strategy: Successfully integrated high-power GM cryocoolers for continuous operation with a high-capacity LN2 coil for emergency scenarios, ensuring robustness for a 43-ton detector.
• Modular Maintenance Design: The cooling towers are designed with a DN80 pneumatic valve isolation mechanism, allowing the coldheads to be serviced or replaced without venting the main detector's vacuum chamber.
• High-Power Heater Integration: Developed a specific 1300 W heater assembly for the cold finger to enable precise temperature setpoint control and stability testing.
• Prototype Validation: Constructed and tested a full-scale prototype (1-tonne vessel) to validate thermal load calculations and pressure stability before the final 43-ton deployment.

4. Experimental Results

The system underwent rigorous vacuum and liquid xenon testing:

A. Vacuum and Coldhead Performance

• Base Temperature: The AL600 coldheads achieved minimum temperatures of 28.6 K and 30.1 K without heater load.
• Cooling Power: At 178 K, the two coldheads combined provided approximately 1900 W of cooling power (exceeding the ~1500 W requirement).
• Stability: Temperature fluctuations at the cold finger were kept below 0.02 K at steady state.

B. Liquid Xenon Stability (1-tonne Prototype)

• Pressure Stability:
  • Single Unit Operation: With 800 kg of xenon, the system maintained pressure with a standard deviation of 0.31 kPa (AL600-1) and 0.35 kPa (AL600-2) over periods of 12 to 30 days.
  • Dual Unit Operation: Running both coldheads simultaneously with 1300 W of simulated heat load, the pressure fluctuation was 0.27 kPa (mean 192.76 kPa).
• Thermal Load Verification: Measured heat loads (218 W at 80 slpm flow; 282 W at 140 slpm) closely matched theoretical predictions, confirming the accuracy of the thermal model.

C. Emergency LN2 Cooler Performance

• Cooling Capacity: During tests where the primary cryocooler was turned off, the LN2 coil successfully maintained stable pressure under 1500 W of heating power.
• Response: The system effectively cycled the LN2 valve based on pressure thresholds, demonstrating a cooling capacity exceeding the design goal of 1500 W.

5. Significance

• Scalability: The results confirm that the designed cryogenics system can handle the thermal load of the final 43-ton PandaX-xT detector (estimated ~1500 W heat load) with a safety margin.
• Signal Stability: Achieving pressure fluctuations below 1 kPa (specifically ~0.3 kPa in tests) meets the stringent requirements for stable electroluminescence signals, which is crucial for the experiment's sensitivity to dark matter.
• Reliability: The dual-coldhead redundancy and the high-capacity LN2 emergency backup ensure the experiment can operate continuously even during power failures or mechanical faults, a critical factor for long-duration physics runs.
• Benchmarking: The system's pressure stability (0.27–0.35 kPa) is superior to XENONnT (2 kPa) and comparable to or better than LZ (0.0558 kPa) and PandaX-4T (0.25 kPa), validating the design improvements over previous generations.

In conclusion, the PandaX-xT cryogenics system prototype has been successfully validated, demonstrating the capability to support the next generation of large-scale liquid xenon dark matter experiments with high efficiency, stability, and safety.