Temporal characteristics of monoenergetic electron beams generated by the laser wakefield acceleration

Pulse length of quasimonoenergetic electrons accelerated by the wakefield generated by 12 TW, 40 fs laser pulses in a gas jet is determined via spectral measurements using a bolometer to detect coherent transition radiation. A quasimonoenergetic electron beam with its mean energy E = 21 MeV, dispersion Delta E = 4 MeV, total charge q similar to 30 pC, and the geometrical emittance 0.07 pi mm mrad is generated with high reproducibility. The averaged duration of only the quasimonoenergetic electron bunches peaked around 20 MeV is 130 +/- 30 fs (FWHM), while it is 250 +/- 70 fs (FWHM) for electron bunches with quasimonoenergetic distributions peaked around 4 MeV, at a distance of 180 mm far from the gas jet because of relatively large electron energy spread. Pulse elongation of the electron bunch with the quasimonoenergetic distribution after 180 mm path is 60-220 fs (FWHM). Therefore, the initial duration of the electron bunch at the gas-jet rear is expected to be less than 100 fs (FWHM).