Sciencealert.com FIONA MACDONALD | 11 AUG 2016
Stronger than any magnetic field on Earth.
Researchers have proposed a new way to use lasers to generate magnetic fields that are at least one order of magnitude stronger than anything we can currently produce on Earth.
In nature, such super-strong fields only exist in space, and they could be they key to harnessing the clean power of nuclear fusion and modelling astrophysical processes in the lab.
It’s exciting stuff, but so far, physicists have only used theoretical calculations to show that the technique could work, and it hasn’t been experimentally verified as yet for a good reason – we currently don’t have lasers strong enough to test it out.
But on paper, the premise works, thanks to something known as the Faraday effect, which is the result of a strange interaction between light and a magnetic field.
It’s a little complicated, but basically the Faraday effect refers to the fact that if an electromagnetic wave, such as visible light, is travelling through a non-magnetic medium, then its polarisation plane will be rotating in the presence of a constant magnetic field.
To break that down a bit further, when light is polarised, it means all the light waves are vibrating in a single plane. But the angle of that plane can rotate.
And, because of the Faraday effect, as light passes through a medium, the polarisation plane will rotate according to a constant magnetic field.
What does any of that have to do with lasers? Well, the spin off of the Faraday effect is that, if you mess with the polarisation of the visible light travelling through a magnetic medium, it will generate a magnetic field.
The stronger the electromagnetic wave, the higher the magnetic field it can produce – so if you use really strong lasers, you should be able to produce a really badass field.
This is an idea physicists toyed around with back in the 1960s, but the reason it never went anywhere is because the Faraday effect also requires absorption to take place – something that usually happens through electrons colliding.
Once you get to a certain intensity of laser, the electrons become ultra-relativistic, which means they collide a whole lot less often, and conventional absorption eventually stops happening.
Because of this, researchers have assumed that a laser powerful enough to generate a super-strong magnetic field would also stop the absorption process from happening, which would void the Faraday effect.
But now researchers from Russia, Italy, and Germany have hypothesised that, at very high laser wave intensities, the absorption can be effectively provided by radiation friction, instead of electron collisions.
And this specific type of friction, on paper at least, can lead to the generation of a super-strong magnetic field.
According to the team’s calculations, a powerful enough laser would be able to produce magnetic fields with a strength of several giga-Gauss (Gauss is the unit used to measure magnetic fields).
To put that into perspective, a giga-Gauss is 109 Gauss, or 1,000,000,000 Gauss. The crazy strong magnetic field produced by an MRI machine can only get up to 70,000 Gauss, whereas the surface of a neutron star is around 1012 Gauss.
Magnetic fields that we can produce in the lab today max out at around 108 Gauss – and they struggle to efficiently control nuclear fusion for long periods of time, which is where this new technique would come in handy.
It would also allow researchers to recreate the crazy strong magnetic conditions in space inside the lab.
“A new research field – laboratory astrophysics – has emerged relatively recently, and now it is very fast-developing,” said one of the researchers, Sergey Popruzhenko from the Moscow Engineering Physics Institute in Russia. “Our work is of particular interest because it suggests new opportunities in this field.”
The challenge will be to experimentally test this new technique, to see if it works in real life, just as it does on paper. But while Popruzhenko predicts we’ll be able to do this in the “near future“, we need to wait until we have a laser powerful enough.
The good news is that three of them are now under construction as part of the European project, Extreme Light Infrastructure, being built in the Czech Republic, Romania, and Hungary, so we’re already making progress.
“These laser facilities will be capable of the intensities required for the generation of super-strong magnetic fields due to radiation friction and also for the observation of many other fundamental strong-field effects,” said Popruzhenko.
The research has been published in the New Journal of Physics.