Megagauss fields
Beyond 100 T, even the strongest existing magnets are no longer able to resist the concomitant magnetic forces. This does not mean that so-called Megagauss fields cannot be generated at all…
On a human timescale, a pulsed magnet yielding to the hoop stress caused by the field it generates, literally explodes. However, during the first instants of this explosion, arcs bridge the nascent gaps between conductor fragments and current temporarily continues to flow until the discharge is either complete or comes to a gradual stop. At the same time, inertia prevents the fragments from gathering speed, thereby keeping them close to their original position. For a very short laps of time, the current thus continues to flow almost as if the magnet were still intact. This phenomenon is used to generate transient fields well beyond 100 T in simple disposable coils that are destroyed in the process.
Depending on size, single-turn coils (STC) produce fields between 100 and 300 T on a microsecond timescale. Their operation requires the design of fast capacitor banks that can generate and inject currents of several megaampere with the necessary speed. By comparison, engineering the coil itself is extremely simple. Here, the principal challenge is to adjust the coil’s thickness such that it starts to expand significantly just after the maximum field has been achieved. In this way, fragments are projected away from the coil bore, thereby conserving experimental equipment in the center. Sometimes referred to as semi-destructive, the outward explosion makes STC an economic and versatile solution for experiments between 100 and 200 T requiring more than a single shot.
To use STC for scientific experiments, one has to cope with different consequences of the short pulse duration. Owing to the immense progress of fast electronics, data acquisition speed no longer represents a limitation in this respect. However, the inevitable use of gas discharge devices in Megagauss generators gives rise to transient electromagnetic disturbances (TED) whose effect on sensitive measurements has to be controlled with advanced filtering and screening techniques. Moreover, the rapidly changing magnetic field induces voltages in any conducting loop exposed to it. This effect is used to monitor the field with tiny pick-up coils, but has to be avoided otherwise.
The LNCMI operates one out of 2 currently active Megagauss installations for scientific applications where experiments are routinely performed up to 150 T. A close partnership is maintained with the other active player in the same area, the International Megagauss Science Laboratory (IMGSL) of the University of Tokyo in Kashiwa, Japan.
Helium bath and flow-type cryostats covering temperatures ranging from 4.2 to 300 K are available for all experiments.
Magnetization measurements are performed with compensated pick-up coils. Samples are typically 1 mm in diameter and slightly elongated. Conducting materials are preferably measured as powders in an epoxy matrix to limit the effect of eddy currents.
Optical absorption and Faraday rotation can be measured using different types of monochromatic sources in the visible, near and mid-infrared. Experiments are normally performed using a transmission setup in Faraday configuration. In the visible and near-infrared, samples should be large enough to cover the end of an optical fiber with 0.1 – 0.4 mm diameter and no thicker than 1 mm. Measurements in the mid-infrared are performed with a free beam requiring samples of at least 0.5 mm diameter. To fit into the existing cryostats, no sample should be larger than 2.5 mm in diameter.
A ground-breaking new experiment for electric transport measurements is in the final development stages and will be available soon.
