Superconducting Magnet Cooling

Newly built superconducting magnets will need to be cooled down from ambient temperature for their initial use and maintained at cryogenic temperatures during operation. For this initial cool-down, a superconducting magnet receives cryogenic cooling by filling its liquid helium (LHe) vessel with liquid nitrogen (LN2) to first reach 77 K. This LN2 is then pumped out, after which LHe is used for further cooling. However, this process is complicated and costly because the LN2 must be completely removed to prevent quenching. Furthermore, the subsequent cool-down from 77 K using LHe is inefficient and expensive, as valuable 4 K liquid is used to cool material already at a comparatively high 77 K temperature.

Magnet Cooling with Stirling Cryogenics

To avoid the use of sacrificial LN2 and LHe for cool-down, Stirling Cryogenics has developed a system that cools a magnet from 300 K to 20 K using a flow of helium gas that is close to the actual temperature of the magnet materials, eliminating the LN2, risk of thermal shock, and a large majority of sacrificial LHe. Utilizing vacuum-jacketed (VJ) lines, the Stirling Closed Loop Helium Gas Cooling System is connected to the inlet and outlets of the LHe vessel. The two-stage Stirling cryocooler is activated to produce cryogenic cooling, which is circulated to the vessel by a helium blower: a Stirling Cryogenics CryoFan. At start-up, the loop will be warm and will gradually decrease in temperature as energy is extracted from the magnet’s thermal mass.

In the temperature range from ambient to 60 K, the helium cooling is provided by the first stage Stirling heat exchanger. Since this concept operates with a working temperature near the actual magnet temperature and offers the highest cooling power, it makes the process very efficient. Below 60 K, after most of the thermal energy has been removed, the second stage of the Stirling cryocooler takes over, cooling further down to 20 K using the same cold helium loop. Depending on the thermal mass of the magnet, the cool-down to 20 K will take 1 to 3 days. This duration depends on the maximum allowable cool-down rate necessary to prevent thermal shock to the magnet.

Efficient Helium Cooling

Once pre-cooled to 20 K, the system is disconnected. An advantage of this method is that the cool-down is performed with helium gas, eliminating the need for a cleaning step to remove residual nitrogen. Since most of the thermal energy of the magnet is already removed at 20 K, the remaining cooling to 4 K consumes only a limited amount of LHe or can be accomplished by an internal cryocooler often implemented in MRI systems.

A system based on this concept can be configured in a one-to-one arrangement, where one Stirling cryocooler cools one magnet at a time. Alternatively, it can be built with multiple cryocoolers cooling multiple magnets in parallel. The choice of this setup depends on the logistical requirements and preferences of the customer.

All Stirling systems include a required valving and connection system, along with controls for coupling and decoupling the magnets. The operator needs only to physically connect the VJ line couplings, after which the system takes over and ensures purging, vacuum pumping, and filling of the helium loop in the correct sequence to avoid the risk of freezing or contaminating cold lines with air.

Stirling Cryogenics can design and offer a customized system for your specific project.

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