Laser light traveling through plasma can produce a beam of electrons all with nearly the same energy, physicists report. The advance, reported in the 30 September issue of Nature, marks a key step toward developing practical high-energy particle accelerators that can fit into a room.
In recent decades, physicists have experimented with accelerating particles by shining extremely intense pulses of laser light into an ionized gas, or plasma. As the light pulse travels through the plasma, it pushes negatively charged electrons, creating a region of excess positive ions that trails behind the light. Other electrons then rush to that spot, forming a region of negative charge following close behind the positive one. Between these positively and negatively charged tag-along regions arises a huge wave-like electric field, known as a "wakefield," that still other electrons can surf to gain staggering amounts of energy in a very short distance. So far, however, this scheme has produced a spray of electrons with many energies, and experiments generally require tightly focused beams of a single energy.
Now, three groups have independently achieved beams of electrons with narrow energy ranges. The key to making such "monoenergetic" beams is to ensure that the wakefield propagates far enough for all the electrons on it to surf all the way from the crest to the foot of the wave, but not farther, report teams led by Wim Leemans of Lawrence Berkeley National Laboratory in California, Victor Malka of the French National Institute for Advanced Technologies in Palaiseau, and Karl Krushelnick of Imperial College London.
All three groups fired pulses of laser light lasting less than a ten-thousandth of a nanosecond and packing trillions of watts of power into jets of hydrogen or helium gas. Malka's and Krushelnick's groups used a relatively large laser spot, which ensures that the spot can travel 1 to 3 millimeters before spreading out and enables the electrons to surf just the right distance. Leeman's group used one laser pulse to create an ionized channel, which allowed the wakefield wave of a second pulse to travel farther through the plasma than it would otherwise. All groups found that they could produce compact bursts of electrons with energies around 100 mega-electron-volts in distances a thousand times shorter than a conventional accelerator could.
"For a while [monoenergetic beams] seemed like a distant goal," says Ilan Ben-Zvi, a physicist at Brookhaven National Laboratory in Upton, New York. "It was quite amazing that not just one, but three groups almost simultaneously were able to demonstrate this." Such accelerators could be useful for experiments in biology and materials science, although Ben-Zvi cautions that the technique won't soon replace conventional accelerators in colliders at high-energy physics labs, which must produce beams of many billions of electron-volts.