A team led by Victor Malka and Jerome Faure added a twist to a breakthrough they and two other groups--from University College London and Lawrence Berkeley National Lab, respectively--announced in the fall of 2004 known as "plasma wakefield acceleration." This principle involves a plasma, in this case a stew of ionized helium atoms and electrons recently dissociated from them. Passing a laser pulse through helium gas generates the plasma as well as a wake from free electrons moving out of its trajectory. According to Tom Katsouleas, an electrophysicist at the University of Southern California who wrote an accompanying commentary in this week's Nature, this works "much in the manner of a speedboat passing through water." He adds that "the wake can gain such a large amplitude that it 'breaks.'" At this point, some of the loose electrons will move along with the wave, and then electric forces in the wake accelerate them to just short of the speed of light. As a result, a beam of electrons can attain an energy of 100 million electron volts within one millimeter, which Katsouleas claims is "1,000 times shorter than the length of a comparable conventional accelerator driven by radio-frequency waves." (Conventional accelerators, however, can boost electrons into energies in the trillions of electron volts.) Unfortunately, it also made for unstable beams; final energies could vary wildly.
Malka and Faure have corrected for this drawback by introducing a second laser pulse, which runs directly countercurrent to the first one. This beam triggers an injection of electrons that are trapped by the plasma wave. As Katsouleas puts it, this method allows the researchers to control "precisely the way the electrons 'surf' the plasma wave." As a result, the team created a beam in which all the electrons had the same energy. Besides the suddenly stable beam, Malka points out that the parameters of the final beam of accelerated electrons is completely adjustable--between a range of 50 to 250 mega-electron volts. "For example," he explains, "you can change the electron energy very easily, just by changing the delay between the two laser pulses." Physicists can also control the number of electrons in the beam simply by tuning the lasers."
Katsouleas cites the only disadvantage of the colliding pulse injection method: the second laser cannot inject as much charge as is made when only a single beam is able to self-inject electrons from the plasma. The amount of charge that can be accelerated is on the order of tens of picocoulombs, one tenth of that which would be possible if there were no second laser. He insists, however, that the charge is sufficient for most of this accelerator's stated uses, which extend from medicine to materials science to aerospace engineering. Malka states his team's next goal is to develop an X-ray beam from a free-electron laser, using this new high quality, high current technology. He thinks this development will result in increased funding and resources to get there. "The route is of course very long," he says about high field physics. "We progress step by step."