Table-top lasers could bring high-energy physics into the lab© NatureThe image of particle-physics machines tends to be of huge beasts snaking through kilometres of countryside and costing billions of dollars. But that is about to change.
Cheap table-top particle accelerators could bring high-energy physics into normal laboratories, now that several teams have cleared the major hurdle to shrinking these machines to a manageable size.
The new devices use laser pulses to drag electrons into tightly focused bursts, rather than relying on the varying electric fields that squeeze particles along lengthy tunnels in atom-smashing facilities such as that at CERN in Geneva, Switzerland. With lasers, the same acceleration can now be achieved over much shorter distances.
In focus
“These results represent the most significant step so far in laser-based accelerators.”
Thomas Katsouleas
University of Southern California, Los Angeles
Physicists have had high hopes for laser accelerators for more than a decade, but there have been two main obstacles. The beam of electrons produced by the devices always had a very wide range of energies, and they were physically spread into a broad fan. Both characteristics make the beams useless for physicists wanting to use them to probe the structure of matter.
Three teams have now overcome this problem, producing tightly focused beams of electrons within a very narrow energy range. All three report their results in this week's Nature1- 3.
"These results represent the most significant step so far in laser-based accelerators," says Thomas Katsouleas, an electrical engineer at the University of Southern California, Los Angeles.
A wake of electrons
The machines built by the three research groups are remarkably similar, because they all rely on the same advances in laser technology.
Stuart Mangles, a physicist from Imperial College London, UK, led one of the teams. His device fires extremely short laser pulses, each lasting just 40 femtoseconds (4 x 10-14 s), into a 1-millimetre-wide chamber of helium gas. The pulse blasts the helium atoms into a plasma, an energetic soup of free electrons and helium nuclei.
The laser pulse, like all electromagnetic radiation, is partly composed of a changing electric field. It's not powerful enough to bother the relatively heavy nuclei, but it pushes the electrons around just like a boat creating a wake of water behind it as it cuts through water. Pulled along in that wake is a tight packet of electrons travelling at just a fraction slower than the speed of light.
Mangles explains that the secret to the team's success was being able to tune the laser so that the trailing electrons actually enhance the pulse's intensity, creating a positive-feedback effect that gathers more and more electrons together until they are delivered in a single, concentrated burst as they emerge from the gas chamber. "The electrons eventually break like waves on a sea shore," he says.
Size of the future
The electrons each have about 100 MeV of energy - the same as if they were delivered by a 100-million-volt battery. Although this is a thousand times less energetic than particles shot out by the most powerful accelerators, it is similar to those being used to determine the structure of biological molecules. Mangles thinks this will be the first use for his new tool.
Accelerators for this kind of research, for example the Swiss Light Source linear accelerator in Villigen, are currently the size of an aircraft hangar. But Mangles is confident that, within two years, laser accelerators of the equivalent power but just two metres long will be commercially available, for around US$1.8 million each.
It'll take a bit longer to replace the most powerful machines, such as the Large Hadron Collider currently being built at CERN. This uses a 27-kilometre-long tunnel and will have cost $2.5 billion by the time it opens in July 2007.
But within 20 years or so, Mangles believes lasers could achieve the equivalent power with an accelerator just metres across.
In the meantime, if biologists and chemists can use these mini-accelerators for studying molecules, it will free up more time for physicists to use the really big machines to tackle fundamental subatomic problems that need higher energies.
University of Southern California, Los Angeles
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