In recent years, physicists have explored the possibility of using ultrashort laser pulses to accelerate electrons or positrons in the wake of waves generated in jets of ionized gases. Initial results suggested that this laser wakefield approach was promising, pointing to the potential of such laser setups to act as tabletop particle accelerators for applications in high-energy physics, biology and medicine. Unfortunately, this promise was undermined by the wide spread in the energy and in the angular divergence of the accelerated particles.

Now two research groups have reported that the proper control of laser/plasma interaction parameters can yield electron beams with a small energy spread and low angular divergence. They presented their findings in the Sept. 30 issue of Nature.

Scientists from Imperial College London, Rutherford Appleton Laboratory in Didcot, UK, the University of Strathclyde in Glasgow, UK, and the University of California, Los Angeles, investigated the electron beams produced when 0.5-J, 40-fs-long pulses of 800-nm radiation from a Ti:sapphire laser focused into a 25-µm-diameter spot interacted with supersonic jets of helium gas at various pressures (lower image). Adjusting the density of the generated plasma between 3 × 10¹⁸ and 5 × 10¹⁹ electrons per cubic centimeter, they discovered that plasma densities of less than 7 × 10¹⁸ electrons per cubic centimeter yielded no energetic electrons. Higher plasma densities, however, produced up to 100-MeV electrons in a beam with an angular divergence of less than 5°, and irradiating a density of 2 × 10¹⁹ electrons per cubic centimeter generated not only a tight beam, but also one with an energy spread as low as ±3 percent.

At high densities, they detected the same large energy spread observed in previous experiments. It is suggested that wave breaking occurs earlier and more violently at high plasma densities than at lower ones. Simulations of the laser/plasma interaction indicated that monoenergetic electron beams likely have been unobserved previously because plasma densities were too high.

Researchers from Centre National de la Recherche Scientifique in Palaiseau, France, Heinrich Heine Universität Düsseldorf in Germany and the Commissariat à l'Energie Atomique in Bruyères-le-Châtel, France, similarly investigated the relationship among laser pulse duration, plasma densities and the characteristics of the generated electron beam. In their experiments, they employed a Ti:sapphire laser, which emitted 1-J, 33-fs-long pulses of 820-nm radiation, focused into a 21-µm-diameter spot on a supersonic helium jet.

At a plasma density of 6 × 10¹⁸ electrons per cubic centimeter, they also observed a tightly focused, quasi-monoenergetic beam of 170-MeV electrons, with an angular divergence of 0.6° as well as an energy spread of 24 percent. Their simulations confirmed that the proper selection of laser and plasma parameters is necessary to produce a monoenergetic beam.