Abstract
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 ($>1\text{ }\text{ }\mathrm{mrad}$ 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 ${ϵ}_{N}=2.3\ensuremath{\pi}\text{ }\text{ }\mathrm{mm}\text{ }\mathrm{mrad}$, 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.
Highlights
Laser wakefield acceleration offers the promising possibility of producing high energy electrons with high peak charge in compact setups far smaller than conventional accelerators
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)
We have measured the emittance of quasimonoenergetic electrons produced by laser wakefield acceleration utilizing injection at a steep density transition
Summary
Laser wakefield acceleration offers the promising possibility of producing high energy electrons with high peak charge in compact setups far smaller than conventional accelerators. The divergence of the individual beamlets is a convolution of the collective divergence of the entire beam with the emittance spread of the electrons. The slit width must be chosen small enough to reduce the contribution from the collective divergence This is an important issue in the case of laser wakefield produced electrons that have a large divergence >1 mrad. Case of a zero emittance beam, the beamlet size on the screen would be determined by the slit width and the collective divergence of the electrons. The magnification factor M accounts for the projected finite slit size increase at the screen due to the collective divergence of the electron beam. Each beamlet can still shift away from a perfectly linearly correlated beam through the beamlet divergence x0j This formalism can still account for nonlinear distortions to the beam phase space and the increased emittance that results.
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