A Desktop Cryogen Free Magnet for NMR and ESR

Cryogenic Ltd. have built cryogen free table top mini magnets which provide 2.5 to 5 Tesla but requiring only 1 kilowatt of input power, making high magnetic fields available independent of liquid helium or high power requirements. This may be compared to the 2 megawatts required for a 5 Tesla electromagnet in the 1960's or the 20 kW required for a 2.5 Tesla Iron electromagnet. The mini cryogen free magnet makes use of NbTi coil of 70 mm bore at 4 K running at 30 to 60 Amps. The exceptionally low heat-load of less than 25 milliwatts at 4K is obtained using current leads made from BISCCO tape conductor which carries the current from 60 to 4 K. The leads above 60K are made from brass. Use of a cryogen free "dry" magnet means that there are no cold seals to be a source of leaks. The cooling power is provided by a miniature Gifford McMahon (GM) refrigerator which has 100 mW cooling power at 4 K and 3 W at 60 K with an input power of just 1 kW. A comparison of conductor alternatives such as HTS, Copper, NbSn shows that NbTi at 4.2 K is the optimum choice