Table-top accelerators make progress
Sep 29, 2004
The prospects for plasma-based particle accelerators have improved following breakthroughs by independent groups in the UK, US and France. Acceleration gradients thousands of times higher than those produced in conventional accelerators have already been demonstrated in laser-produced plasmas. However, it has only been possible to accelerate particles over distances of about 1 millimetre and the resulting beams have been of poor quality with a large energy spread. Now, three groups of physicists have developed a variety of techniques -- including the formation of channels and bubbles in the plasma -- to reduce the energy spread in the beam to just a few percent (Nature 431 535, 538 and 541).
Conventional particle accelerators have to be hundreds of metres long to accelerate electrons to energies in the GeV range in, for example, synchrotron radiation sources. The machines used to accelerate particles to energies in the TeV for particle physics are even longer. Laser-produced plasmas are promising candidates for next-generation "table-top" particle accelerators because they can support electric fields that are thousands of times greater than those that can be produced in conventional accelerators.
The "laser wakefield" accelerator exploits the radiation pressure of an intense laser pulse to displace the electrons in a laser-produced plasma, leaving a large electric field in its wake. In 2002, Victor Malka of the Ecole Polytechnique in Paris (ENSTA) and colleagues showed that electrons could be accelerated to energies of 200 MeV over distances as short as a millimetre by "surfing" such a wakefield.
However, the beams produced in this and other experiments were of poor quality because the electrons had a wide range of energies. Now, three groups -- including Malka's team in Paris, a group led by Wim Leemans at the Lawrence Berkeley National Laboratory in the US, and a group led by Karl Krushelnick of Imperial College in London -- have overcome this problem. By using variations on this approach the three teams have now produced monoenergetic beams.
Leemans and co-workers use pre-formed plasma channels to guide the laser beams over distances that are long compared to the natural diffraction distance of the laser beam. This diffraction would normally limit the distance over which particles surf the wake.
Krushelnick and colleagues employed a "forced" laser wake field approach, where the plasma wave actually "breaks" as the laser beam propagates through the plasma. This can lead to some of the electrons in the wave being "self-injected" into the wave. "It just so happens that when breaking first occurs, this bunch of electrons has a narrow energy spread, which is just what we want from our accelerator," says Stuart Mangles from Imperial.
Malka's group used their laser to create a "bubble" in the plasma, which traps and accelerates the electrons. "The main applications which will appear will probably be in radiobiology, medicine and chemistry," says Malka. "In addition, our electron source will be perfectly suited for use in compact synchrotrons and free-electron lasers."
All three groups now hope to reach GeV energies by accelerating the particles over longer distances.