Newly build superconducting magnets will need to be cooled down from ambient for the first time, and to be kept cold while in use.
For such a first cool-down, a superconducting magnet is often cooled by filling its LHe vessel with LN2 to first reach 77K. This is then pumped out, after which LHe is used to cool down further.
This process is complicated and costly because the LN2 must be removed completely as it will induce quenching. The further cool-down from 77K with LHe is costly and not efficient since expensive 4K liquid is used to cool 77K material.
To avoid the use of sacrificial LN2 and LHe for a cool-down, Stirling Cryogenics has developed a system to cool a magnet from 300 to 20K using a flow helium gas that is close to the actual temperature of the magnet materials.
Using VJ lines, the Stirling Closed Loop Helium Gas Cooling System is connected to the in- and outlets to the LHe vessel. The two stage Stirling Cryogenerator is started to produce cooling power which is circulated to the vessel by the CryoFan. At start-up the loop will be still warm, then slowly dropping in temperature as more and more energy is removed from the magnets thermal mass.
In the temperature range from ambient to 60K, most of the cooling power will come from the first stage Stirling heat-exchanger. Since in this concept the working temperature is near actual magnet temperature, this will provide the highest cooling power, making this concept very efficient.
Below 60K, when most of the thermal energy is already removed, the second stage of the Stirling Cryogenerator takes over, cooling further down to 20K using the same cold helium loop.
Depending the thermal mass of the magnet, cool down to 20K will take 1-3 days which also depends on the maximum allowed cool down speed to ensure the magnet does not suffer from thermal shock.
Once pre-cooled to 20K, the system is disconnected. Advantage is that the cool-down has been done with helium gas, so no cleaning step to remove N2 is required. Since most of the energy of the thermal mass is already removed at 20K, the remaining step to 4K consumes only a limited amount of LHe or is done by MRI internal cryocooler.
A system according to the above concept can be built as one to one, in which one Stirling Cryogenerator cools one magnet at the time. Or can be built with multiple Cryogenerators cooling multiple magnets in parallel. Choice of this set-up depends on the logistic requirements and preferences of the customer.
All systems include an elaborate valving and connection system and control, for coupling and de-coupling of the magnets. The operator needs to physically connect 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 risk of freezing or polluting cold lines with air.
Stirling Cryogenics can design and offer a customized system for your specific project, please contact us to discuss your requirements.