A Desktop Cryogen Free Magnet For NMR And ESR

Cryogenic Ltd. have built a miniature cryogen free magnet with 5 ppm uniformity at fields of 2.5 to 9 Tesla. The 2.5 Tesla magnet is cooled by a small 1-kilowatt air-cooled refrigerator to 4K without the use of liquid helium. The overall dimensions of the cryostat are 240 x 540mm. The workspace is a bore of 51mm. Recent developments in Cryogen Free technology have made it possible to achieve temperatures down to 3K without the use of liquid helium. They require only mains power and usually water-cooling of the compressor that drives the mechanical refrigeration. A resistive lead goes from room temperature to the 1st stage of the cooler. The 1st stage operates close to 50K, which is well below the transition temperature of BISCO high temperature superconductors, which are used to carry the current from the 50K stage to the 4K stage. These give a heat load of 25 milliwatt on the 4K stage to which the magnet is attached. In practice this magnet can be operated with current permanently in the leads or in persistent mode. The magnet has homogeneity of 5ppm over a 10mm dsv. This magnet demonstrates that high fields can be produced and maintained easily in compact and practical fashion without recourse to liquid helium.