Optical Rocket Boosts Electrons to Nearly the Speed of Light

A team of researchers from the University of Nebraska–Lincoln recently conducted an experiment where they were able to accelerate plasma electrons to close to the speed of light. This “optical rocket”, which pushed electrons at a force a trillion-trillion times greater than that generated by a conventional rocket, could have serious implications for everything from space travel to computing and nanotechnology.

When it comes to the future of space exploration and scientific research, it is clear that light will play a vital role. On the one hand, space agencies are investigating “optical communications” – sending information using lasers – to handle the increasing amounts of data missions will collect and send to Earth. Researchers and engineers, on the other hand, are looking to lasers to conduct microscopic manipulations of matter and optical computers.

However, one of the main challenges with these sorts of applications has been the size of the equipment involved. What it comes down to is the fact that conventional, high-energy lasers are generally big and expensive. As such, the ability to scale-down the process where light is used to accelerate particles would not only be a boon for researchers, it could also lead to countless new applications.

This is precisely what the team from UNL’s Extreme Light Laboratory (ELL) did using the laboratory’s Diocles Laser. This x-ray laser, which is ten-million times brighter than the sun, was used to focus rapid laser pulses on plasma electrons – a process known as wakefield acceleration (or electron acceleration). The study which describes their findings recently appeared in the Physical Review Letters.

Ordinarily, light exerts a tiny force wherever it is reflected, scattered or absorbed. While the force is exceedingly small, it can have a cumulative effect when it is focused properly and continuously.  During the experiment, the team found that light pulses caused electrons in the plasma to be pushed out of the path of the pulses, creating plasma waves in their wake.

The electrons also picked up additional acceleration from these “wakefield waves”, which brought them to ultra-relativistic speeds (i.e. close to the speed of light). As Donald Umstadter, the director of the Extreme Light Laboratory, explained in a Nebraska Today press release:

“This new and unique application of intense light can improve the performance of compact electron accelerators. But the novel and more general scientific aspect of our results is that the application of force of light resulted in the direct acceleration of matter.”

This new experiment effectively demonstrated the ability to control the initial phase of wakefield acceleration, which could improve the performance of compact electron accelerators. It is was significant in that it has numerous applications that were previously not possible, due to the enormous size of conventional electron accelerators.

One such application is known as an “optical tweezer”, a process where light is used to manipulate microscopic objects. Another possible application is the concept known as the “light sail” (aka. solar or photon cell), a method of space propulsion where a focused laser beam is used to accelerate a reflective sail to incredible speeds.

One such example of this is Breakthrough Starshot, a proposed spacecraft being developed by Breakthrough Initiatives – a non-profit organization founded by Russian billionaire Yuri Milner. Consisting of a nanocraft being towed by a sightsail, this spacecraft would rely on focused lasers to accelerate it to relativistic speeds (20% the speed of light). At this velocity, the craft would be able to make the journey to Alpha Centauri in just 20 years and could send back images of any exoplanets there (including Proxima b).

In the meantime, this experiment is likely to open up some serious research opportunities for particle physicists. The study was led by  Grigoroy Golovin, a postdoc researcher from the University of Nebraska-Lincoln’s (UNL) Extreme Light Laboratory (ELL), and included multiple scientists from the ELL and Shanghai Jiao Tong University.

Further Reading: Nebraska Today, Physical Review Letters