Accelerating a free electron with a laser has been a longtime goal of solid-state physicists. David Cline, a distinguished professor in the UCLA Department of Physics and Astronomy, and Xiaoping Ding, an assistant researcher at UCLA, have conducted research at Brookhaven National Laboratory in New York and have established that an electron beam can be accelerated by a laser in free space.
This has never been done before at high energies and represents a significant breakthrough, Cline said, adding that it also may have implications for fusion as a new energy source.
In free space, a plane-wave laser is unable to accelerate an electron, 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 be useful for other scientific endeavors, such as 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 U.S. and other countries. The focus of the laser generates a natural channel that can capture electrons and drive them into a pellet that explodes, by fusion, to produce excess energy, Cline said.
With federal funding from the U.S. Department of Energy, a project to carry out a proof-of-principle beam test for the novel vacuum acceleration at Brookhaven National Laboratory's Accelerator Test Facility (BNL-ATF) has been proposed and approved -- a collaboration among the UCLA Center for Advanced Accelerators, of which Cline is principal investigator, Ho's group and the Accelerator Test Facility team.
BNL-ATF is 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. Ho's group provides theoretical support. UCLA scientists -- Cline, Ding and Lei Shao, a former UCLA physics graduate student of Cline's -- are responsible for the whole experiment and the experimental data analysis.
Simulation research work and hardware design 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.
- L. Shao, D. Cline, X. Ding, Y.K. Ho, Q. Kong, J.J. Xu, I. Pogorelsky, V. Yakimenko, K. Kusche. Simulation prediction and experiment setup of vacuum laser acceleration at Brookhaven National Lab-Accelerator Test Facility. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2013; 701: 25 DOI: 10.1016/j.nima.2012.09.053
- David Cline, Lei Shao, Xiaoping Ding, Yukun Ho, Qing Kong, Pingxiao Wang. First Observation of Acceleration of Electrons by a Laser in a Vacuum. Journal of Modern Physics, 2013; 04 (01): 1 DOI: 10.4236/jmp.2013.41001
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