Generation of the highest continuous magnetic fields

The generation of the highest magnetic fields is a true challenge to the magnet designer and a stimulus for the materials scientist. Record fields are achieved through a successful interplay between the precise understanding of the physics of magnets, the detailed knowledge and conscious use of the available materials, and a design that optimizes all the different parameters. We describe three technologies essential for the generation of the highest magnetic fields: coils wound from high-temperature superconductors (HTS) operated at liquid helium temperatures in a background field; high-power resistive magnets; and hybrid magnets, which combine the advantages of resistive and superconducting magnets. HTS show high current densities at elevated fields confirmed so far by measurements up to 33 T. In spite of the strain limitation of the conductor, insert coils up to 3.2 T have been built at different laboratories for a total field of 23.5 T. Recently a record field of 25 T using a 5 T Bi₂Sr₂CaCu₂O_x superconducting insert coil has been achieved at the NHMFL. For resistive magnets, dramatic improvements in magnet design, such as the Florida-Bitter magnet, and new materials, such as the microcomposite CuAg, have made it possible that magnetic fields can now be generated that exceed the fields that hybrid magnets produced 10 years ago. Today, 33 T are provided on a routine basis with resistive magnets. The highest continuous magnetic field ever of 45 T is obtained with a new generation of hybrid magnets. It is available for scientific research in our user facility.