Electrons Accelerated by Laser in a Vacuum

Accelerating a free electron with a laser has been a longtime goal of solid-state physicists, and now two UCLA scientists have demonstrated the acceleration of electrons by a laser in a vacuum.

David Cline, a distinguished professor in the University of California, Los Angeles, department of physics and astronomy, and Xiaoping Ding, an assistant researcher at UCLA, have conducted research at Brookhaven National Laboratory in New York, establishing that an electron beam can be accelerated by a laser in free space. This feat has never been done at high energies and represents a significant breakthrough, Cline said, adding that it also may have implications for fusion as a new energy source.

Pictures taken from a spectrometer on Beam Line #1 at Brookhaven National Laboratory’s Accelerator Test Facility. Each row of two frames represents one snapshot-pair of laser on (on the right side) and laser off (on the left side) with unchanged configuration. One can see a clear increase from these pictures, proof that the laser accelerates the 20-meV electron beam in vacuum. Pictures of the beam momentum spread after the spectrometer taken with the laser off (left column) and the laser on (right column). The length of the beam image reveals the energy spread of the beam. The experiment recorded 30 shots. Twenty shots were high intensity and showed effects of the laser on/laser off difference. Four shot examples are shown here. Courtesy of UCLA.

A plane-wave laser cannot accelerate an electron in free space, according to the Lawson–Woodward theorem, posited in 1979. However, Yu-kun Ho, a professor at China's Fudan University in Shanghai, and his research group have proposed a concept of what physicists refer to as the capture–acceleration scenario to show that an electron can be accelerated by a tightly focused laser in a vacuum.

In the capture–acceleration scenario, the diffraction from a tightly focused laser changes not only the intensity distribution of the laser but also its phase distribution, which results in the field phase velocity being lower than the speed of light in a vacuum in some areas.

Thus, a channel that overlaps features of both strong longitudinal electric field and low–laser-phase velocity is created, and electrons can receive energy gain from the laser. The acceleration effect increases along with increasing laser intensity, Cline said. This channel for electrons may also find use in guiding an electron beam into a specific region of laser fusion applications, he said.

A possible application of this discovery is the use of laser plasma fusion to provide a new energy source for the US and other countries. The laser’s focus generates a natural channel that captures electrons and drives them into a pellet that explodes, by fusion, to produce excess energy, Cline said.

A proof-of-principle beam test for the novel vacuum acceleration will be conducted at Brookhaven National Laboratory’s Accelerator Test Facility (BNL-ATF) — one of the few facilities that can provide both a high-quality electron beam and a high-intensity laser beam for the beam test, Cline said.

Simulation research work and hardware designs have been done in accordance with BNL-ATF's experimental conditions. The simulation results predict that vacuum laser acceleration phenomena can be observed with ATF's diagnostic system.

Two papers detailing the research were published in Nuclear Instruments and Methods in Physics Research A (doi: 10.1016/j.nima.2012.09.053) and the Journal of Modern Physics (doi: 10.4236/jmp.2013.41001).

For more information, visit: www.ucla.edu