A plasma channel, more dense toward the edges, guides the laser and allows it to form high-quality electron beams. As the laser pulse travels from left to right, it excites a wake in the plasma, trapping and accelerating bunches of electrons to high energies.
Mile-long radio frequency electron accelerators and the light sources they provide may become a thing of the past, following recent developments in laser wakefield accelerators at the Council for the Central Laboratory of the Research Councils' (CCLRC) Rutherford Appleton Labora-tory (Oxon, United Kingdom), Laboratoire d'Optique Appliquée (La Garde Cedex, France), and the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley, CA). The developments could lead to cheaper, more compact accelerators for application in medicine, materials science, and homeland security and defense.
A team of researchers in the United Kingdom, led by Professor Karl Krushelnick from Imperial College, London, in collaboration with the CCLRC Rutherford Appleton Laboratory, the University of Strathclyde (Glasgow, Scotland), and the University of California, Los Angeles, have demonstrated the first laser plasma accelerator capable of producing electron beams with an energy spread of a few percent and resultant increased energy of more than 109 electrons above 80 MeV, paving the way for more compact, tunable high-brightness sources of electrons and radiation.
Laser wakefield acceleration was conceived more than a quarter century ago by Toshi Tajima and John Dawson. An intense laser pulse focused into a region of gas creates a plasma wave (the wakefield). Some of the electrons in the plasma are carried along by the laser wakefield. The plasma wave can generate electric fields of hundreds of GV/m, one thousand times greater than that of conventional radio frequency accelerators.
To date, the role of laser wakefield accelerators has been inhibited by extremely limited acceleration distances, due to a lack of controllable methods for extending the propagation length beyond the Rayleigh lengththe length over which the laser remains focused. Previous experiments relied on the plasma waves breaking to produce large numbers of energetic electrons. This process, however, has always produced a large, unwanted energy spread, due to a loss of coherency in the plasma wave during the wavebreaking. The resulting energy spread limits the potential of such systems. Careful control of the laser and plasma parameters by the Imperial College team has enabled the production of electron beams with a narrow energy spread and increased energymore than 109 electrons above 80 MeV. These results were achieved by controlling the injection of electrons into the wake to preserve the coherency of the plasma wave.
In a separate effort, scientists at the Lawrence Berkeley National Laboratory have developed a way to increase the acceleration distance by guiding the laser pulse across many Rayleigh lengths. The group fires a pulse through gas to create a "plasma wire," then heats the wire from the side with a second pulse, causing the plasma field to expand and become more dense at the center. A final 9 TW drive pulse creates the wakefield while maintaining focus across several Rayleigh lengths. Recent developments by this research team have demonstrated the ability to guide and control extremely intense laser beams over greater distances than ever before, producing high-quality electron beams.
The developments of both groups are "greatly advanced over their progress a few years ago," explains Alan Todd of Advanced Energy Systems (New York, NY). "Although the current issue of beam energy reproducibility and the average currents that can eventually be delivered means that this technology is still a long way from becoming commercially viable, there are several experimental points presented by the researchers which are extremely compelling."
Particles accelerated by the electric fields of laser-driven plasma waves in a few meters could, theoretically, reach the equivalent energies attained by mile-long machines of conventional accelerators. These recent developments pave the way for more compact, tunable high-brightness sources, offering the potential of efficient generation of high-power femtosecond x-rays, coherent terahertz and IR radiation.