... as Do Plasma Channels Created Using Three-Pulse Technique

Another laser wakefield approach also displays promise for the development of tabletop particle accelerators. The Sept. 30 issue of Nature features the findings of investigators at Lawrence Berkeley National Laboratory in Berkeley, Calif., the University of California, Berkeley, Technische Universiteit Eindhoven in the Netherlands, Tech-X Corp. in Boulder, Colo., and the University of Colorado at Boulder, who produced monoenergetic electron beams by means of a three-pulse technique that creates plasma channels in the target to guide the driving laser pulse.

A Ti:sapphire operating at 810 nm first ionized a supersonic hydrogen gas jet with a 15-mJ, 60-fs-long pulse. Next, the laser heated the plasma with a 150-mJ, 250-ps-long pulse, creating a hot plasma filament that expanded to generate a shock wave. Finally, the laser irradiated the induced plasma wave with a 500-mJ, 55-fs-long pulse focused into an 8.5-µm-diameter spot. The energy spectrum of the resulting electron beam was centered at 86 MeV and displayed a spread of 2 percent.

The researchers attribute the results to the fact that the technique effectively increased the propagation length of the driving laser pulse to 10 times the Rayleigh length. In previous experiments, increasing the laser spot size, which required greater laser powers, accomplished this. The three-pulse approach thus suggests that the development of tabletop accelerators need not be dependent solely on more powerful lasers.