Surfing on a Laser's Wake

Particle accelerators are some of the biggest lab equipment around, with dimensions of kilometers. Now a group of researchers has tested a tabletop accelerator that uses powerful laser pulses to create plasma waves on which electrons can surf to higher speeds. The device, reported in the 3 August Physical Review Letters, is only a prototype, but it could ultimately lead to far more compact accelerators.

The basic principle is called the wakefield effect: When a laser beam ionizes a gas, it creates a wake like that of a ship as it passes through. To harness this wave, a team lead by François Amiranoff of the École Politechnique in Palaiseau, France, injected electrons at an energy of 3 million electron volts into the plasma along with the laser pulses. If an electron jumps in on the right part of the wave, it will "surf" the wave and accelerate.

To measure the acceleration, the researchers deflected the electrons with a strong magnet. The fastest, or most energetic, electrons had gained 50% in energy. That's still a million times less acceleration than in standard particle accelerators. The reason is that the plasma scatters the laser after only a few millimeters--too short a ride for electrons to gain much speed. "If we want to increase the energy, we will have to ... find ways to guide the laser beams in the plasma so that they remain focused," says Amiranoff.

If they succeed, tabletop accelerators may send electrons slamming into targets to generate high-energy x-rays for medicine and research, says Amiranoff. Donald Umstadter, a physicist at the University of Michigan, Ann Arbor, says that the experiment is an "important proof-of-principle test of one of the most promising concepts." However, he suspects that the low-power device won't replace hulking accelerators anytime soon.