TRAPPED ELECTRON ACCELERATION BY A LASER-DRIVEN RELATIVISTIC PLASMA-WAVE

THE aim of new approaches for high-energy particle acceleration1 is to push the acceleration rate beyond the limit (approximately 100 MeV m- 1) imposed by radio-frequency breakdown in conventional accelerators. Relativistic plasma waves, having phase velocities very close to the s peed of light, have been proposed2-6 as a means of accelerating charge d particles, and this has recently been demonstrated7,8. Here we show that the charged particles can be trapped by relativistic plasma waves -a necessary condition for obtaining the maximum amount of energy theo retically possible for such schemes. In our experiments, plasma waves are excited in a hydrogen plasma by beats induced by two collinear las er beams, the difference in whose frequencies matches the plasma frequ ency. Electrons with an energy of 2 MeV are injected into the excited plasma, and the energy spectrum of the exiting electrons is analysed. We detect electrons with velocities exceeding that of the plasma wave, demonstrating that some electrons are 'trapped' by the wave potential and therefore move synchronously with the plasma wave. We observe a m aximum energy gain of 28 MeV, corresponding to an acceleration rate of about 2.8 GeV m-1.