The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the “pepper-pot” method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as $(2.7\pm 0.9)\pi \mathrm{mm}$ mrad for $(55\pm 2)\mathrm{MeV}$ electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.

• Received 6 November 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.165006