Laser wakefield acceleration

Laser wakefield acceleration of monoenergetic electron beams has been achieved by three independent groups. When an intense laser pulse enters a gas or plasma, the laser’s electric field can accelerate the mobile electrons until the relatively stationary ions pull them back via the Coulomb force. The resulting plasma wave moves along in the wake of the laser pulse. Under the right conditions, electrons can surf the wave, but until now the accelerated electrons have had a wide spread of energies. However, the accelerating electrons can outpace the wave and all can achieve the same energy—provided the acceleration is stopped at the right moment. Groups led by Victor Malka (CNRS, France) and by Stuart Mangles (Imperial College London) exquisitely tuned their laser and plasma parameters to produce collimated electron beams of about 170 MeV and 70 MeV, respectively. Meanwhile, a group led by Wim Leemans (Lawrence Berkeley National Laboratory) used three lasers—two to bore a plasma channel and the third to produce wakefield acceleration—to avoid the quenching effects of diffraction and achieve an 80-MeV beam. The LBNL group says that their method is useful for lengthening the acceleration distance in a plasma. All three groups generated electron beams in a distance of 3 mm or less. (S. P. D. Mangles et al., Nature 431 , 535, 2004 http://dx.doi.org/10.1063/1.1797559 ; C. G. R. Geddes et al., Nature 431 , 538, 2004 http://dx.doi.org/10.1038/nature02900 ; J. Faure et al., Nature 431 , 541, 2004 http://dx.doi.org/10.1038/nature02963 .)