Generating ultrarelativistic attosecond electron bunches with laser wakefield accelerators.

Femtosecond electron bunches with ultrarelativistic energies were recently generated by laser wakefield accelerators. Here we predict that laser wakefield acceleration can generate even attosecond bunches, due to a strong chirp of the betatron frequency. We show how the bunch duration scales with the acceleration parameters and that, after acceleration, the bunches can propagate over many tens of centimeters without a significant increase in duration.