Conventional accelerators have to be hundreds of metres in length, or longer, to accelerate particles to energies of interest to particle physicists. In recent years, scientists have developed a variety of techniques, mostly based on laser-produced plasmas, that are able to achieve much higher acceleration gradients than conventional accelerators. This opens the possibility of significantly reducing the length of these machines. However, some of these techniques suffer from synchrotron radiation losses or poor beam quality, which will limit their appeal to particle physicists.

The new method demonstrated by the Stanford team involves using a laser beam with a longitudinal electric field component -- that is, an electric field component in the direction that the laser beam is travelling -- to accelerate electrons that are travelling in the same direction. The energy gained by the electron corresponds to the integral of the longitudinal electric field component over the distance along which the electron beam and the laser beam interact with each other. The device relies on accelerating the electrons in a vacuum rather than in the much more complicated environment of a plasma.

In free space, the phase velocity of the laser -- the speed at which light of a single wavelength would move -- does not match the velocity of the electrons, so there is no acceleration. However, Plettner and co-workers have now overcome this problem by placing a "boundary", made of gold-coated polymer tape, at the point where the beams interact (see figure). This limits the interaction between the beams and allows for a non-zero energy exchange between the two, which leads to electron acceleration.

"The initial and main motivation for this work is the possibility for developing particle accelerator technology that could reduce the length of existing linear accelerators by an order of magnitude," says Plettner. "This will lead to a compact high-luminosity lepton collider with the potential for collision energies of 1 TeV (1012 electron volts) and beyond." The new approach could also lead to the development of very compact coherent X-ray sources.