Wake Up, Little Surfers: Riding waves toward tabletop accelerators

Many physics discoveries of the past century have emerged from giant particle accelerators costing up to billions of dollars and sprawling over acres. Now, three independent research groups in the United States, France, and England have simultaneously passed a major milestone toward a laser-based electron accelerator that could fit inside a room and cost only a fraction of the price of a conventional machine.

The new work pushes forward an accelerator scheme that uses extremely brief and intense laser pulses (SN: 9/5/98, p. 157). As each pulse plows through a puff of gas, it pulls electrons from the gas into its wake. Over distances hardly more than the thickness of a coin, such laser “wakefield” accelerators can pump electrons to high energies, the scientists report.

Whereas previous experiments demonstrated the principle behind wakefield accelerators, the energy of the electrons wasn’t uniform enough for precision research, says Wim P. Leemans of the Lawrence Berkeley (Calif.) National Laboratory, leader of the team based in the United States.

That flaw has become a thing of the past. In back-to-back-to-back reports in the Sept. 30 Nature, each of the three teams reports finding virtually the same sweet spot of operating conditions. All the laser-driven apparatuses emit extremely brief, narrowly confined pulses of electrons in which all the particles in the pulse race at nearly the same speed and therefore have nearly the same energy.

“These results represent the most significant step so far for laser-based accelerators and should stimulate further advances in the near future,” says Thomas Katsouleas of the University of Southern California in Los Angeles in a commentary accompanying the reports.

The newly achieved electron pulses, which last only tens of femtoseconds, pack energy up to 1,000 times as densely as did electron packets produced by wakefield accelerators of the past, Katsouleas notes.

In any wakefield scheme, an intense laser pulse strips electrons from gas atoms and shoves those electrons ahead and sideways, much as a boat pushes waves off its bow and hull. That creates an electron-depleted wake. A wave of displaced electrons then pours back into the wake. As that electron wave surges forward, other electrons just ahead get propelled by repulsive electrostatic forces, effectively surfing the wave.

In the new experiments, each research team boosted the number of surfing electrons until the waves couldn’t take on any more. By tweaking how the laser pulses plow through the gas, the researchers preserved the waves long enough for every surfer to get a full ride.

The resulting uniform distribution of energy among electrons in the pulses bodes well for future efforts to create a compact series of wakefield accelerators to attain the enormous energies needed to explore the fundamental nature of matter and energy, says Victor Malka of École Nationale Supérieure de Techniques Avancées in Palaiseau, leader of the France-based team.

As promising as wakefield acceleration now looks, it can’t displace all conventional accelerator technology. Many of the world’s leading particle accelerators propel protons or ions, which are too massive for wakefield methods, notes English team member Stuart P.D. Mangles of the Imperial College London.

Still, he adds, the potential of 1,000-fold shrinkage for electron accelerators could open the way for a new generation of affordable, miniaturized accelerators. These may include synchrotrons that generate intense X rays for analyzing inanimate and biological materials, including cells and tissues, and much more powerful electron-positron colliders for probing fundamental questions about the universe.