A proof-of-principle test has demonstrated that an electron beam can be accelerated by a laser in free space, a feat never before accomplished at such high energies. The breakthrough may have implications for laser plasma fusion as a new energy source. UCLA’s David Cline, principal investigator for the UCLA Center for Advanced Accelerators, and Xiaoping Ding carried out their beam test at Brookhaven National Laboratory (BNL) in Upton using the lab’s Accelerator Test Facility (ATF). The 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. Pictures taken from a spectrometer on Beam Line #1 at Brookhaven National Laboratory’s Accelerator Test Facility. Four shot examples are shown here: 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. 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. A clear increase from these pictures can be seen, proof that the laser accelerates the 20-meV electron beam in vacuum. In free space, a plane-wave laser is unable to accelerate an electron, according to the Lawson-Woodward theorem posited in 1979. A group in China, however, has proposed a concept in which an electron can be accelerated by a tightly focused laser in a vacuum. In this scenario, the diffraction from the laser changes not only the intensity distribution of the laser, but also its phase distribution, resulting in the field phase velocity being lower than the speed of light in a vacuum in some areas. This creates a channel that overlaps features of both strong longitudinal electric field and low-laser-phase velocity, and electrons can receive energy gain from the laser. The acceleration effect increases along with increasing laser intensity, Cline said. A possible application is the use of laser plasma fusion to provide a new energy source. 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, he added. 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. The simulation results predict that vacuum laser acceleration phenomena can be observed with ATF’s diagnostic system. Two papers on the research have been published, one in Nuclear Instruments and Methods in Physics Research A (doi: 10.1016/j.nima.2012.09.053), and the other in the Journal of Modern Physics (doi: 10.4236/jmp.2013.41001).