Stopping Electrons With Light

By hitting electrons with an ultra-intense laser, researchers have revealed dynamics that go beyond clas­sical physics and hint at quantum effects. Whenever light hits an object, some of the light scatters back from the surface of the object. However, if the object is moving extremely fast, and if the light is incre­dibly intense, strange things can happen. Electrons, for example, can be shaken so vio­lently that they actually slow down because they radiate so much energy.

Illustriation of a radiation reaction in the collision of a high-intensity laser pulse with a laser-wakefield accelerated electron beam. (Source: S. Mangles, ICL)

This radia­tion reaction is thought to occur around objects such as black holes and quasars. Being able to measure radiation reaction in the lab will therefore provide insights into processes that occur in some of the most extreme environ­ments in the universe. Radiation reaction is also interes­ting to physicists studying effects beyond classical physics, as the Maxwell’s equations that tradi­tionally define the forces acting on objects fall short in these extreme environ­ments. Now, a team of researchers led by Imperial College London have demon­strated radia­tion reaction in the lab for the first time.

They were able to observe this radiation reaction by colliding a laser beam one quadril­lion times brighter than light at the surface of the Sun with a high-energy beam of electrons. The experiment, which required extreme precision and exquisite timing, was achieved using the Gemini laser at the Science and Tech­nology Facilities Council’s Central Laser Facility in the UK. Photons that reflect from an object moving close to the speed of light have their energy increased. In the extreme condi­tions of this experi­ment, this shifts the reflected light from the visible part of the spectrum all the way up to high energy gamma rays. This effect let the researchers know when they had success­fully collided the beams.

Senior author of the study, Stuart Mangles from the Depart­ment of Physics at Imperial, said: “We knew we had been successful in colliding the two beams when we detected very bright high energy gamma-ray radiation. The real result then came when we compared this detec­tion with the energy in the electron beam after the col­lision. We found that these success­ful col­lisions had a lower than expected electron energy, which is clear evidence of radiation reaction.”

Study co-author Alec Thomas, from Lancaster Univer­sity and the Univer­sity of Michigan, added: “One thing I always find so fasci­nating about this is that the electrons are stopped as effectively by this sheet of light, a fraction of a hair’s breadth thick, as by something like a milli­metre of lead. That is extra­ordinary.” The data from the experiment also agrees better with a theoretical model based on the principles of quantum electro­dynamics, rather than Maxwell’s equations, poten­tially providing some of the first evidence of previously untested quantum models.

Mattias Marklund of Chalmers Univer­sity of Techno­logy, Sweden whose group were involved in 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.” However more experi­ments at even higher intensity or with even higher energy electron beams will be needed to confirm if this is true. The team will be carrying out these experiments in the coming year.

The team were able to make the light so intense in the current experi­ment by focussing it to a very small spot just a few micro­metres across and deli­vering all the energy in a very short duration just 40 femto­seconds long. To make the electron beam small enough to interact with the focussed laser, the team used a laser wakefield acce­leration. The laser wake­field technique fires another intense laser pulse into a gas. The laser turns the gas into a plasma and drives a wake­field, behind it as it travels through the plasma. Electrons in the plasma can surf on this wake and reach very high energies in a very short distance. (Source: ICL)

Reference: J. M. Cole et al.: Experimental evidence of radiation reaction in the collision of a high-intensity laser pulse with a laser-wakefield accelerated electron beam, Phys. Rev. X., online 7 february 2018; preprint arXiv: 1707.06821

Link: The John Adams Inst. for Accelerator Science, Imperial College London, London, UK

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