Low Emittance, High Brilliance Relativistic Electron Beams from a Laser-Plasma Accelerator

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than $1\ensuremath{\pi}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as $1.1\ifmmode\pm\else\textpm\fi{}0.1\ensuremath{\pi}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$ is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of $5\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{A}\text{ }{\mathrm{m}}^{\ensuremath{-}1}\text{ }{\mathrm{rad}}^{\ensuremath{-}1}$ makes it suitable for compact XUV FELs.