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Beyond 100 T : « Megagauss » fields

Beyond 100 Tesla ...

Magnetic fields in excess of 100 T can only be generated at the expense of a drastic reduction in pulse duration. Invariably, they also lead to the destruction of the coil which, however, doesn’t prevent the use of Megagauss magnetic fields (1 Megagauss = 100 Tesla) for scientific experiments.

The LNCMI Megagauss generator is one out of three platforms worldwide that are making use of capacitor-driven single-turn coils (STC) to produce fields in the 150 to 250 T range for scientific applications. Although still higher fields can be obtained with flux compression techniques, STCs have the advantage that the coil destruction does not affect the experimentally useful volume: Samples, cryostats and other equipment generally survive and experiments can therefore be performed reproducibly.

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Megagauss fields at the LNCMI

The LNCMI Megagauss generator has been designed and constructed in the mid-90’s at the Humboldt University Berlin from where it was transferred to Toulouse in 2006. It features a 200 kJ modular capacitor bank that can be charged up to 60 kV. Its record field amounts to 331 T obtained in a STC with 3 mm inner diameter.

For scientific experiments, fields between 150 and 260 T with a pulse duration of 6 μs can be generated in coils featuring 8 to 15 mm diameter. Low temperatures are available depending on coil diameter : measurements at liquid Helium temperature require a STC with at least 12 mm diameter, capable of producing close to 190 T. A coil with 8 mm diameter still permits measurements at liquid Nitrogen temperature while producing a field of 260 T. Experiments can in principle be performed once every half hour.

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Experimental techniques

Scientific experiments in STCs are challenging but feasible. The main handicaps are the short pulse duration, the induced electric fields near the coil and the electromagnetic noise caused by the discharge of the capacitor banc. Thanks to the tremendous evolution of fast electronics and opto-electronics throughout the past decades, a number of experimental techniques have nevertheless been adapted to STCs. At present, optical experiments using single-element detectors can be performed in several wavelength ranges between 400 nm and 11 μm. Another well-established technique are magnetization measurements making use of compensated pick-up coils.

Current developments at the LNCMI are mainly aiming at the improvement of optical measurements in two respects: an extension of the available wavelength range close to the THz-regime, and the use of detector arrays in the visible and near-infrared.

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Membres d’équipe / contact : O. Portugall, O. Drachenko, A. Miyata