A major challenge for laser-based particle accelerators is to generate a particle beam for acceleration in the first instance. Now Christopher Moore and colleagues at the Naval Research Laboratory in Washington have shown how to generate highly directional beams of electrons in ultrashort pulses. Moore and colleagues used a 2.5 terawatt laser to produce electron beams with energies up to 340 keV (Phys. Rev. Lett. 82 1688).
Physicists hope to develop new ways of accelerating particles because of the high cost and large size of traditional accelerators. Electrons in standard accelerators gain energy by surfing large electromagnetic (EM) waves contained by powerful magnetic fields. However it may be possible to adapt the intense EM waves generated when a laser pulse interacts with a plasma to create a ‘table-top’ accelerator.
The problem to date has been how to inject the electrons into the laser-based accelerator. Moore and colleagues have shown that lasers can be used for this process as well. The terawatt laser was tightly focussed onto a small volume of krypton in a vacuum chamber. The laser pulse rips up to 18 electrons off each atom in the gas. The amount of energy absorbed by the electrons is so large that some of them are ejected at 80% of the speed of light. Surprisingly, instead of scattering in all directions, the electrons shoot off in two tightly focussed beams.
However, laser-based accelerators are extremely demanding in terms of alignment and synchronization, and engineers are unlikely to be able to take advantage of these new techniques in the short term.
