Coherent undulator radiation from attosecond electron bunches produced by a laser wakefield accelerator

We present the first observation of coherent undulator radiation emitted by attosecond electron bunches from a laser wakefield accelerator, at the Scottish Centre for the Application of Plasma-based Accelerators. ∼10 pC electron bunches with 50 MeV to 140 MeV and normalised emittances of ≈0.3 πmmmrad, are produced by a self-injection on a density bump created by shocks of intersecting micro-jets of helium gas. A thin aluminium foil is used to reject the remaining laser light while allowing the ultra-short electron bunches to pass through the foil. Electrons are periodically deflected by a 14 period, 4 cm periodicity undulator that has a deflection parameter K = 0.6, and produce UV to visible wavelength coherent radiation. Coherence is confirmed by the spectrum the peak wavelength dependence on electron energy, and quadratic scaling of photon number with charge. Measurement of coherent transition radiation spectra from the foil show that electron bunches have RMS rising edges between 50 as and ≈1 fs, the RMS bunch length varies between 500 as and 1.5 fs. We observe intensity that vary over six orders of magnitude between 10 × 103 eV−1 to 10 × 109 eV−1, for charges of ≈1 pC to 80 pC