Nondestructive pulsed magnetic fields at LNCMI-Toulouse
Team : Jérôme Béard, Julien Billette, Tristan Moraine, Thierry Lemaire, Enora Noël
At LNCMI, magnetic fields are produced by circulating a current in an electromagnet, a coil. It is a winding, generally of cylindrical symmetry, around a hole in which the intensity of the field will be maximum and where the physical measurement will take place. Up to now, a magnetic field whose value exceeds 45 Tesla can only be reached by the use of pulsed electromagnets. Beyond this value, it is no longer possible to extract the heat produced by the Joule effect, it is therefore necessary to circulate the current for a short time. The LNCMI offers on its Toulouse site a variety of pulsed coils adapted to a large number of experimental techniques up to 98.8 T. For information, the world record is 100.75 T established in 2012 by the National High Magnetic Field Laboratory of Los Alamos in the USA.
Non-destructive pulsed magnetic fields are produced by the discharge of capacitor bank in an electromagnet. A current of several tens of thousands of amperes flows in the coil for a time less than one second. The circulation of this strong current allows the production of an intense magnetic field of several tens of Tesla in the center of the coil. The combination of these strong currents and magnetic fields causes mechanical stresses of several gigapascals which must be contained by the materials constituting the winding. Due to these extreme mechanical stresses, the lifetime of the electromagnets, before their failure, is limited to a few hundred pulses.
Pulsed generators
The LNCMI on its Toulouse site has designed and use 5 pulsed generators made up of capacitor banks which can be charged at voltages up to 24 kV. They store an electrical energy of 1 to 21 megajoules and all allow the delivery of currents of several tens of thousands of amperes corresponding to an electrical power of the order of a gigawatt.
3D CAD view of one of the two 3 MJ generators during its design…
… and its construction. Each one of the three subparts contains one element of the generator (switch, crowbar, dump resistors).
Second generation of 14 MJ capacitor bank. Subparts developed for the 3 MJ generators have been duplicated to rationalize construction, maintain and automation. In the same room there is a 1 MJ generator based on the same techniques.
Transportable 1 MJ generator (the three cubes on the left) here in use at the Institut Laue-Langevin in Grenoble. This generator is also used at the European Synchrotron Radiation Facility in Grenoble for experiments combining X-rays and intense pulsed magnetic field. Until 2019 it was used to energize multi-coils magnets.
Switch room. High voltage/high current contactors distribute three generators in the 7 measurement sites of the laboratory.
Measurement site. The coil is cooled down to 77 K (-196 °C) in a liquid nitrogen dewar installed in a pit. The cell is reinforced to resist in case of violent coil failure.
Pulsed magnets
The LNCMI-Toulouse designs and produces all the non-destructive pulsed magnets used for high-magnetic field experiments.
Temporal profiles of the pulsed magnetic fields generated at the LNCMI-Toulouse. Magnets A, B and C are monolithic, i.e. formed with one coil connected to one generator. Magnets D, E and F are composed of multiple concentric coils. D and F are double coils and F, the ‘100 T Magnet’, is the first triple coil in the world. It has generated, in February 2017, 98.8 T and this value is the current European record.
