A quick tour of high-field magnet technology

Magnetic fields affect the electronic and magnetic properties of matter. Originally on a microscopic level, these changes give rise to macroscopic signature effects that researchers investigate with different experimental methods to understand how a material works. Examples are field-dependent changes of the electrical conductivity, shifting optical absorption edges or lines, magnetic phase transitions and many more. In addition, magnetic fields are used to manipulate objects in applications such as magnetic levitation, separation, projection, forming or welding.

As a rule, higher fields produce stronger effects but are also more difficult to generate and use. This is to some extent the result of a technical trade-off between the strength of a magnetic field and its duration.

Permanent magnets do not need any external energy source but are intrinsically limited to fields of less than 1 T. Electromagnets, on the other hand, require electrical energy to build up and maintain a magnetic field. Most convenient in this respect are superconducting magnets that do not produce waste heat. However, as superconductivity eventually breaks down, resistive and hybrid magnets are currently the only practical option to generate continuous fields between 24 and 45 T. These magnets require large technical infrastructures to evacuate waste heat and compensate the respective energy loss with tens-of-megawatt power supplies.

Fields between 45 and 100 T are no longer continuous. Here, a capacitor bank is discharged into a resistive magnet giving rise to a field pulse of less than 1 s. The coil buffers heat produced in the process and subsequently evacuates it during a much longer cool-down time. Larger currents give rise to higher fields, but generate more heat and thus require shorter pulses down to the order of a few milliseconds.

Apart from dissipation and heating, electromagnets also need to resist their very own magnetic forces. The strongest non-destructive magnets are thus exposed to gigapascal pressures, a level that even the most advanced reinforcement materials barely manage to contain. To enter the Megagauss regime (1 MG = 100 T), one thus reverts to simple disposable magnets that generate a field with even shorter duration during the initial stages of their inevitable destruction.

At the LNCMI, continuous, pulsed non-destructive and destructive magnet systems are operated and constantly improved to provide the highest possible fields for scientific research. In its facilities for continuous fields in Grenoble and pulsed fields in Toulouse, the LNCMI furthermore develops appropriate instrumentation to perform experiments in the respective field ranges.