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 .