By Maja Bachmann
When discussing extremes in physics, this magazine edition would not be complete without taking a look at the most extreme and powerful magnets that exist. They enable physicists to study the properties of complex materials under extreme conditions and sometimes even new phases of matter are discovered!
The most intense magnets can reach up to 3’500’000 times the strength of the Earth’s natural magnetic field. This would easily bend a bar of steel and so one of the challenges in building these magnet systems is to find materials strong enough to withstand the enormous forces that are generated while using the magnet.
Let us take a step back and look at the different types of magnets that exist. They can be divided into two classes: permanent and non-permanent magnets, also called electromagnets. Permanent magnets come in all shapes and sizes, one might even be hanging on your fridge at home. They are made from materials that create their own persistent magnetic field. Their strength depends on the material they are made of and, as the name suggests, they cannot be switched off. A typical fridge magnet has a strength of about 0.005 Tesla. Tesla (or ‘T’) is the unit of magnetic fields and measures how strong the field of a magnet is. Researchers have discovered some very robust compounds for making permanent magnets, most notably Neodymium alloys, which can reach up to 1.5 Tesla.
The most common and versatile type of magnet is the electromagnet. Here the magnetic field is produced by an electric current, which can be switched on and off. If the current is run through a wire wound into a coil, it will create a concentrated magnetic field in its center. There are two ways to make an electromagnet more powerful: Either by increasing the number of turns per length or by passing a stronger current through the wire. However, thanks to the resistance, a large current passing through the wire will cause heating (just like in the glowing wire in the light bulb above you). This limits how much current can be put into a system before it ultimately melts.
To overcome the problem of resistive heating, an elegant solution is to use superconducting coils. Superconductors are a class of materials that, while very cold (typically below – 250 °C), can carry current without any resistance up to a certain current strength. In other words, if wires in a coil are made from a superconducting material, they will not experience heating as long as one stays below a certain, material specific threshold current. Because they can transport electricity without any losses, they are very energy efficient and can be found in nearly every condensed matter physics lab around the world. The field limit of pure superconducting magnets lies at the notable 27T.
But how can we reach even higher fields? It turns out that the way to go is a carefully designed resistive copper coil, which is cooled by a high-pressure water system and re-enforced by steel bars. Such resistive magnets can routinely reach 36T, but this brute force method comes at its price. It takes 20 MW (‘Mega Watt’) to reach these fields, enough to power a mid-sized town!
We can take a step further – the world-record holding MagLab Hybrid Magnet System (at U.S. National High Magnetic Field Laboratory) generates a persistent 45T field, and was built using a combination of a superconducting magnet and multiple resistive magnets stacked one into another, just like Russian matryoshka dolls. It was a great engineering challenge of many years to design and build this machine that hits the ultimate boundary of what the static magnetic fields can achieve.
This, however, is not the limit for the non-static fields! Even more powerful magnetic fields are achieved in the so-called pulsed magnet systems. Instead of having a continuously running electromagnet that needs to be cooled, here a huge amount of energy is sent through a coil in a fraction of a second, creating enormous fields of up to 100T. Clearly, this number of Teslas means a lot of energy and a tremendously high stress on the material the systems are made from. In fact, pulsed magnets rely on state-of-the-art materials research, pushing the ultimate boundary of tensile strength, as most materials would simply tear apart under these extreme conditions. But even so, the unavoidable fate of every pulsed magnet is its self-destruction after a couple hundred pulses.
Similar to the static case, the highest pulsed field is created when not merely one but up to seven coils are stacked one into another, so that the fields can add up to over 100T in the center of the coils. To power such a magnet, MagLab’s Pulsed Field Facility at Los Alamos, for example, uses a re-purposed nuclear power plant generator to create 1.2 Gigawatt of energy that is sent through the coils. This is an equivalent energy of 1000 tons of TNT.
Finally, no magnet is more extreme than destructive pulsed magnets! These magnets circumvent the problem of material robustness, and are designed to explode with every pulse. The intense magnetic field exists only as long as it takes a shock-wave to propagate through the magnet – and so the pulse lasts only a few microseconds. The highest intensity is achieved when the sample explodes and the explosion compresses the magnetic field into the sample – destroying the magnet itself after each pulse.
The main application of research in extremely high magnetic fields lies in material science and technology. Innovations are made through the identification of next-generation materials, crucial to the technological advancement of our world. Ranging from the more efficient solar cells, improved batteries and air-conditioning systems to the more powerful computers to the new means of fighting cancer, the applications are numerous and exciting.
In conclusion, we have learned about the most extreme magnets in the world, which are powerful enough to bend bars of steel and often end in self-destructive explosions – all in the name of physics. For the end, we should not forget to mention the strongest known magnetic field in the universe, which belongs to a magnetar. This is a type of neutron star with magnetic fields reaching up to 1011 T – the kind of fields that scientists on Earth can only dream about…
Fig.1 Horseshoe magnet, taken from: http://images.wisegeek.com/horseshoe-magnet.jpg Bar magnet, taken from: http://benjermcveigh.com/wp-content/uploads/2014/03/Bar_magnet1.jpg
Fig.2 Maja Bachmann, 2018
Fig.3 Electromagnet, taken from: https://www.dkfindout.com/uk/science/magnets/solenoids/
Fig.4 LANL 100T, taken from: https://img.newatlas.com/lanl-100-tesla-7.jpg