Emittance and divergence of laser wakefield accelerated electrons

We apply the pepper-pot method to measure in a single shot the transverse emittance of quasimonoenergetic electrons with 20 MeV energy produced by laser wakefield acceleration (LWFA). The large divergence of LWFA beams (>1mrad typical) compared to conventional rf accelerator beams places additional restrictions on the pepper-pot design. The LWFA beam is found to have a normalized rms transverse emittance of ⋯N=2.3πmmmrad, with a shot-to-shot fluctuation of 17%. This emittance is comparable to state-of-the-art injectors for conventional linear accelerators. In addition, we examine the beam divergence of LWFA electrons. Simulations and theory indicate that an adiabatic reduction in the beam divergence occurs when the transition region of the downstream plasma density profile is comparable to the betatron period of the electron beam in the plasma accelerator.