Few femtosecond, few kiloampere electron bunch produced by a laser-plasma accelerator

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma(1) can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread(2-5), low emittance(6) and up to 1 GeV peak energy(7,8). Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection(9), contains a temporal feature that can be identified as a 15 pC, 1.4-1.8 fs electron bunch (root mean square) leading to a peak current of 3-4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources(10,11), and will benefit the realization of laboratory-scale free-electron lasers(12-14).