A peak field of 1200 T was generated by the electromagnetic flux-compression (EMFC) technique with a newly developed megagauss generator system.
Magnetic fields closely up to the turn-around peak were recorded by a reflection-type Faraday rotation magnetic-field optical-fiber probe.
The performance was analyzed and compared with data obtained by the preceding EMFC experiments to show a significant increase in the liner imploding speed of up to 5 km/s.
Magnetic fields are one of the fundamental properties of a physical environment.
They can be controlled with high precision and interact directly with electronic orbitals and spins; this makes them indispensable for research in areas of solid state physics such as magnetic materials, superconductors, semiconductors, strongly correlated electron materials, and other nanomaterials.
When a material is placed in a magnetic field of 1000 T, the Zeeman energy induced in the electrons becomes enormously high (as high as 1300 K), which corresponds to an energy far above room temperature, resulting in substantial effects on the electronic properties of the material.
A magnetic field of 1000 T corresponds to 0.8 nm cyclotron-orbit radius (magnetic length) of an electron; this is of the same order as a typical lattice constant.
Therefore, in these extreme conditions the Broch electron model, which is based on the atomic periodic potential, does not hold anymore.
Such ultra-high magnetic fields provide new opportunities for insights into material science and may allow us to develop a deeper understanding of novel physical concepts.
The generation of magnetic field is always accompanied by strong Maxwell stress; at 1000 T, this amounts to 4 × 1011 N/m2 (4 × 106 times atmospheric pressure). All aspects of the megagauss generator, including the coil and surrounding instruments, are designed to protect against the violent self-destruction of the magnetic coil, disregarding all of the reinforcement techniques accumulated so far for nondestructive pulsed magnets.
In order to use the high magnetic fields for reliable and precise physical measurements, it is crucial to generate them over a large volume to provide sufficient working space over the bore and length; hence, they must occupy at least one cubic centimeter.
So far, several methods for generating megagauss magnetic fields with this field volume have been developed for application to physical measurements.
A metal cylinder, called the “liner,” is used to compress the magnetic flux and generate megagauss magnetic fields.
Two main methods are used to implode the liner compressing the magnetic flux.
The first is based on explosively-driven flux-compression,1–3 which uses chemical explosives such as trinitrotoluene (TNT) to accelerate the liner. Experiments are only possible to be conducted at explosive-proving grounds or inside large bomb chambers.
The second is based on electromagnetic flux-compression (EMFC),4–7 which uses electromagnetic forces produced by powerful condenser bank units. Experiments can be performed indoor, in laboratories that can be housed in a conventional building.
Another technique uses imploding plasma to compress the magnetic flux very efficiently and is known as the plasma focus method.8
Alternatively, high-power laser is used as a source for producing an ultra-high magnetic field associated with a high-density electromagnetic field in its strong radiation.9–13
However, there are hurdles to their application due to the limitations resulting from the extremely short rise time (less than a few microseconds) and small volume (much less than one cubic centimeter).
In contrast to the flux-compression techniques mentioned above, a single-turn coil with a one-turn winding of a thin-copper plate can be used as a disposable magnet. Megagauss fields are generated by discharging a megaampere electric current from fast-condenser bank units; this is referred to as the single-turn coil method.
However, this technique is limited to a maximum magnetic field of approximately 300 T.14–16
The first report of magnetic fields exceeding 1000 T generated by explosively-driven flux-compression appeared in 1960.1
In 2001, a field of 2800 T in a volume having a diameter of 5 mm was reported as an absolute record; it was achieved by employing a “three-stage cascade liner” in which the liner degradation accompanying the implosion process was circumvented.2,3
All these explosively-driven flux-compression experiments must be performed outside. Therefore, it is difficult to precisely control or reproduce the experiments, which restricts their use in sophisticated and precise solid-state physics experiments.
On the other hand, electromagnetic flux-compression, known as EMFC, relies solely upon the electric energy stored in the condenser bank units, and is a highly controllable method for the indoor generation of megagauss magnetic fields.
This technique, utilizing the electromagnetic force, was first presented by Cnare in 1966, and he reported a peak field of 210 T from 0.136 MJ condenser banks.4 The principal mechanism is simple; a thin metal cylinder, called the “liner,” undergoes high-speed implosion accelerated by the magnetic force induced by a huge electric current about its circumference, and a magnetic flux (a seed magnetic field) initially generated in a large volume (∼500 cm3) is eventually compressed into a megagauss magnetic field.
Therefore, the magnet employed in the EMFC method is, in principle, composed of a single-turn primary outer coil, the liner (which is coaxially set inside the primary coil), and a pair of seed field coils set on either side of the primary coil.17–19
It is noted, however, that during implosion, under certain circumstances the magnetic flux that penetrates through the liner’s wall can be compressed and generates a very high magnetic field, even in the absence of a seed field.
This phenomenon (the so-called Cnare effect) was presented actually by Cnare in his seminal work.4
Over the last decade, new records have been set for the maximum magnetic field generated by EMFC, with notable progress made by the Institute for Solid State Physics, University of Tokyo.
A 730 T field was recorded using the newly designed primary coil, a high performance copper-lined (CL) steel primary coil.7 Using the same set-up for the condenser bank units as for EMFC, a peak field of 985 T has recently been announced, approaching 1000 T by simply adjusting the initial seed magnetic field.20
A new set-up for the EMFC megagauss generator system with improved performance has now been installed.
A peak field of 1200 T was recorded using the Faraday rotation magnetic field measurement,21 a substantial improvement on the previous record, for the new system using the same destructing magnet CL coil.
The present experimental results are discussed with regard to the previous EMFC experiments.7,20
The schematic view of the EMFC instrument is illustrated in Fig. 1(a).
The electric current discharged from the main condenser banks was concentrated on the collector plate through 480 high-voltage co-axial cables.
In order to reduce the electromagnetic stress induced by the huge electric current, the collector plate was divided in two parts.
It was then mechanically and electrically connected to the interface plate as shown in Fig. 1(b), where the primary coil was clamped by a hydraulic press (100 tons).
FIG. 1.(a) Schematic view of the EMFC megagauss generator. (b) Cross section of (a). (c) Copper-lined primary coil and a pair of seed field coils. [Reproduced with permission from Nakamura et al., Rev. Sci. Instrum. 84, 044702 (2013). Copyright 2013 AIP Publishing LLC.] In (a), one of the initial seed field coils is presented in the displaced position, for more visible view of the primary coil. The primary coil is covered by an anti-explosive block made of bulk iron-steel, which is omitted in the drawing (a).
THE EXPERIMENT
Here’s what happened when it was powered up to 3.2 megajoules for the experiment that produced the 1,200-tesla field:
