From a technical point of view, magnetic fields can be regarded as “high” when they can no longer be produced with commercial products. The principal difficulty lies in a magnet’s limited capability to obtain, sustain and contain the magnetic energy that is stored in the field. Permanent magnets are thus limited to about 1 Tesla as the alignment of spins in their structure must be compensated by other, energetically more favorable, solid state interactions. Substantially higher fields can be obtained by using electric energy to drive a current through an electromagnet. The efficiency of converting electric into magnetic energy is, however, limited by alternative processes, all of which can ultimately destroy the magnet: the Lorentz force that arises from the crossing of current and magnetic field lines thus produces deformations that accumulate substantial amounts of mechanical energy ; passing a current through a resistive conductor furthermore creates heat ; and, in the particular case of superconducting materials, the available energy can nourish phase transitions that destroy the superconducting state. Commercial coils for the production of magnetic fields use mainly Niobium based superconductors cooled by liquid Helium at 4 Kelvin. The physical properties of these materials inhibit to produce a magnetic field beyond 27 T.
The role of high-field facilities worldwide is to overcome these problems and to provide magnetic fields well in excess of the commercially available limit for the scientific community. For this purpose different coil technologies are developed. Copper based conductors with optimized mechanical and electrical properties are used to produce these high field coils. Due to the Joule effect, the electrical power is dissipated into heat during the process.
Static magnetic fields of more than 30 T generally require resistive, or a combination of resistive and superconducting, magnets. The performance of the respective facilities depends on two factors: their capacity to design state-of-the-art magnets using advanced materials with optimized mechanical and electrical properties; and the development of a technical infrastructure that permits the electric compensation of dissipative losses, and the extraction of the generated heat from the magnet. The LNCMI Grenoble thus disposes of a 25 MW power supply and a hydraulic cooling system with 300 l/s flow rate to produce up to 37 T in a 34 mm room temperature bore.
Pulsed magnet facilities avoid excessive heating by limiting the duration of the current that passes through a magnet to the order of 10-100 ms. They require energy sources, usually capacitor banks, that can store and rapidly release the necessary energy. Conductors and additional reinforcement materials primarily have to cope with the increasingly severe mechanical constraints at fields beyond 60 T. The LNCMI Toulouse disposes of several capacitor banks with a total energy of 22 MJ and is capable to produce fields between 60 T in 28 mm and 90 T in 8 mm bore. Beyond the limit of ordinary pulsed fields, it also features a Megagauss installation that makes use of dynamic effects in exploding single-turn coils to produce 200 T on a microsecond time scale.