A team of researchers led have for the first time demonstrated in a lab setting that light can be used to stop high-speed electrons. The interactions of the particles were beyond the dynamics described by classical physics and showed that unexpected things happen when objects are moving at near light speeds. For the experiment, the researchers used intense laser pulses, and an electron beam, where the electrons were moving at speeds close to that of light. The researchers used the Astra Gemini laser, a high power ultrashort pulse system, at the Science and Technology Facilities Council’s Central Laser Facility in the United Kingdom. The findings of the experiment have been published in Physical Review X.
The ridiculous intensities required of the laser beam were achieved by highly focused targeting, and delivering the energy in a very short duration of time. The target was only a few micrometres wide, and the energy was delivered within 40 femtoseconds. The laser beam had the intensity of a billion million times brighter than the light at the surface of the Sun. To make the laser pulses strike the electron beam, the researchers made use of a technique known as laser wakefield acceleration. Another laser beam was fired at a gas, that got converted to plasma. As the laser beam travels through the gas, it creates a wave known as the wakefield. The electrons riding on this wave can reach very high energy levels over a short distance.
The photons reflected from objects moving close to the speed of light see an increase in their energy levels. In the experiment, the photons went from the visible part of the spectrum to high energy gamma rays. These intense flashes were what let the researchers know that the laser pulses had in fact, collided with the electron beam. Comparing the energies of the gamma rays with the energy of the electron beam after the collision, showed lower levels of electron energy, providing evidence that the beam of light had managed to stop the electrons. Professor Mattias Marklund, a co-author of the study, said “Testing our theoretical predictions is of central importance for us at Chalmers, especially in new regimes where there is much to learn. Paired with theory, these experiments are a foundation for high-intensity laser research in the quantum domain.”
Normally when the light hits an object, some of it scatters from the surface. When the high energy laser pulses hit the electron beam, the electrons radiated so much energy that they slowed down as if they had hit a sheet of lead, instead of just light. This is the first time that a phenomenon known as “radiation reaction” has been observed in laboratory conditions. Radiation reaction is believed to occur in some of the most intense interactions in the universe, including around black holes and quasars. Observing these interactions in a laboratory environment allows researchers to confirm theoretical predictions of high energy interactions. The team intends to prove the viability of the approach for investigating quantum theories, by using higher intensity laser pulses, or higher energy electron beams.