Nuclear Demagnetization Magnets

Magnets for nuclear demagnetization are required to have a large volume at high fields into which the sample to be demagnetized (frequently a bundle of copper wires) is placed. A He³-He⁴ dilution refrigerator is generally used to cool the sample while it is in the magnetic field and before it is demagnetized. During the demagnetization process, the refrigerator is thermally decoupled from the sample by means of a superconductive heat switch.

To accomplish these operations, the high field magnet used to magnetize the copper wire bundle must be compensated so it will not affect the operation of the heat switch or switches and so it will not generate appreciable eddy currents in the mixing chamber of the dilution refrigerator. This field free region is also used for the ultra low temperature experiment itself.

The magnet system for this application is a high field magnet, a nulling coil, and a series of compensating coils mounted on an integral cylinder extending above the main magnet. The dilution refrigerator and the experiment itself are located inside the compensation coils where the field is typically reduced to less than 3 mT when the main field is at 8 T. In some cases, an additional highly homogeneous magnet is placed in the compensated region to permit the temperature to be measured using nuclear magnetic resonance techniques.

When the ultimate in low temperatures is required, two stages of demagnetization are employed. In this case, PrNi₅ is sometimes used in the first stage to achieve higher cooling capacities at higher temperatures. After the first stage is thermally decoupled from the dilution refrigerator and demagnetized, the second stage is decoupled and demagnetized to achieve the ultimate low temperatures.

The dilution refrigerator and magnet designs must be closely coordinated to assure that the proper low magnetic fields are achieved at the superconducting heat switches and that adequate space is available inside the magnet for the various refrigerator components.

AMI specializes in working closely with customers who wish to purchase the dilution refrigerator and magnet system from separate vendors. Our engineering staff carefully reviews customer provided drawings of the insert and designs the magnet system around this. Approval drawings are provided prior to production to ensure there is no mismatch when the final unit is integrated. By purchasing the magnet system separately many customers realized significant cost savings and get a system exactly tailored to their experiment.