We have measured and evaluated the femtosecond bunch duration of electron bunches produced by a 12 TW 40 fs laser pulse focused into a He gas jet. Well reproducible electron bunches can be generated with a low-emittance and high-charge, which are very important characteristics for future time-resolved applications. The bunch duration of such electron bunches was determinded to be less than 2 ps (FWHM) at 340 mm downstream from the gas jet by spectral analysis of the coherent transition radiation. The effects of bunch elongation due to both the energy spread and the space charge of electron bunches were estimated, and this estimation is in good agreement with the experimental results. The feasibility for time-resolved applications is also discussed.
