All-optical Femtosecond Bunch Length Measurement of Laser-accelerated Electron Beams

Electron bunches accelerated in the wakefield of a laser beam are expected to have pulse lengths below a hundred femtoseconds. Such pulse lengths cannot be measured by established accelerator diagnostics. However, with laser-driven electron beam generation it becomes possible to perform an all-optical measurement of the electron pulse length on a single-shot basis.
We present the analysis of data taken in 2005 at the ASTRA laser. A 400 mJ, 45 fs laser pulse is focused to a spot size of 8 mum FWHM using an f/16 parabolic mirror to accelerate electrons in a gas jet of 2 mm length. Terahertz transition radiation created in the passage of these electrons through a 50 mum Al foil is focused onto a 200 mum thick ZnTe crystal and brought into overlap with part of the ASTRA pulse negatively chirped to 5 ps.
The Terahertz pulse induces transient birefringence in the crystal via the electro-optical Pockels effect while the ASTRA pulse probes the resulting change in polarization and therefore the temporal structure of the Terahertz pulse.
We will focus on the details of the data analysis and show that the observed temporal signal can clearly distinguish between the quasi-monoenergetic electron bunch and the quasi-thermal tail of the electron distribution in a manner which is consistent with the measured electron spectra.