Superconducting Magnet Cooling

What is a Superconducting Magnet?

A superconducting magnet is a magnet made from superconducting wire coils that, when cooled below a critical temperature, exhibit zero electrical resistance and produce very strong magnetic fields. Because of their extreme performance, superconducting magnets are widely used in MRI systems, particle accelerators, research laboratories, power grid infrastructure, and other advanced scientific and industrial equipment. However, superconducting magnets require cryogenic cooling to reach and maintain superconductivity, typically involving liquid helium (LHe) or cryocooler-based systems.

Challenges in Cooling Superconducting Magnets

When a new superconducting magnet is installed, it must be cooled down from ambient temperature (~300 K) to cryogenic temperatures. A conventional approach is:

1. Pre-cool using liquid nitrogen (LN₂) down to 77 K,
2. Then transition to liquid helium (LHe) cooling toward ~4 K.

But this method has drawbacks:

• Risk of quenching: Residual LN₂ must be fully removed before using LHe, or trapped nitrogen can cause instability in temperature and electrical resistance.
• Inefficient use of LHe: Using expensive liquid helium to cool from 77 K to 4 K is thermodynamically wasteful because 4 K liquid is used to cool material already at a comparatively high 77 K temperature.
  Thermal stress and shock: Rapid temperature changes can damage delicate magnet structures. The large temperature difference between the cryogen and magnet poses risk to the magnet structures during cooling and requires constant monitoring and flow metering.

Magnet Cooling with Stirling Cryogenics’ Helium Flow System

At Stirling Cryogenics, we offer an advanced cooling method that avoids sacrificial LN₂ and minimizes LHe consumption. Our system enables a helium gas flow cooldown from ambient to ~20 K, closely tracking the temperature of the superconducting magnet itself.

Key features of our method:

• We use a closed-loop Stirling helium gas cooling system, connecting via vacuum-jacketed (VJ) lines to the magnet’s LHe vessel inlet and outlet.
• A two-stage Stirling cryocooler extracts heat from the gas. During the cooldown:
  • From ~300 K down to 60 K, the first stage of the cryocooler handles the majority of heat removal.
  • Below 60 K, the second stage takes over, bringing the helium-gas flow down to ~20 K.

• A helium blower (CryoFan) circulates the cold gas through the magnet’s vessel, gradually reducing its temperature without thermal shock.
• The system can be configured flexibly: one-to-one (one cryocooler per magnet) or parallel configurations for multiple magnets.

Cooldown time to 20 K typically ranges from 1 to 3 days, depending on the thermal mass and allowable cooldown rate of the magnet.

Benefits of This Approach for Superconducting Magnet Cooling

• Reduced LHe consumption: Because most of the thermal load is removed before reaching ~20 K, the demand for LHe is significantly lowered.
• Elimination of nitrogen purge steps: No sacrificial LN₂ means no risk of residual nitrogen disrupting the cooling or triggering quench.
• Gentler thermal gradients: The temperature of the helium gas remains close to the magnet’s temperature, reducing thermal shock risks.
• Operational flexibility: Suitable for single magnets or multi-magnet arrays, with modular cryocooler deployment.
• Cleaner, simpler transitions: After reaching ~20 K, the system can be disconnected, and the final cooling to 4 K is either done with minimal LHe or an internal cryocooler (as commonly used in MRI systems).

Why Choose Stirling Cryogenics for Your Superconducting Magnet Project

Stirling Cryogenics specializes in cryogenic solutions that combine efficiency, safety, and custom design. Our expertise includes:

• Engineering bespoke helium cooling systems tailored to magnet geometry and thermal properties
• Integrating control systems for purging, vacuum pumping, and sequence control
• Delivering robust systems for long-term, stable magnet operation

Whether for research institutions, medical devices, particle accelerators, or industrial installations, we provide turn-key solutions for superconducting magnet cooling.

Contact us to explore a system design for your specific magnet requirements.

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