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 ($>1\mathrm{mrad}$ 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 ${ϵ}_N=2.3\pi \mathrm{mm}\mathrm{mrad}$, 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.

Figure 1

Layout of the pepper-pot emittance diagnostic. Also shown is the evolution of the transverse phase space: before the mask (a), the beamlets cut out by the mask (b), after propagation in a drift (c), and the profile observed on the screen (d).

Figure 2

Example data from the emittance diagnostic. The measured normalized rms emittance for this shot is $1.6\pm 0.18\pi \mathrm{mm}\mathrm{mrad}$. Left: False color image at the emittance diagnostic phosphor showing the vertically stripped beamlets. Right: Vertically summed intensity across the horizontal field of view along with Gaussian fits to each beamlet. The inset shows the fit to a single peak. The mean energy for this shot was 17 MeV.

Figure 3

Variation of the geometric emittance versus mean energy of the electrons. Error bars indicate the standard deviation of the mean for each binned set of data. The data follow a $1/\gamma$ curve (red line) consistent with a constant normalized emittance of $2.3\pm 17\%\pi \mathrm{mm}\mathrm{mrad}$ from the laser wakefield acceleration for all shots.

Figure 4

Particle-in-cell simulation of the evolution of the beam angular spread at the downstream end of the plasma. Also shown is the plasma density roll-off given by an exponential function with $\sigma =30\mu \mathrm{m}$ starting at $z=340\mu \mathrm{m}$.

Figure 5

Left: Outcoupled LWFA beam divergence as a function of downstream plasma density taper starting from $n_0=3\times {10}^{19}{\mathrm{cm}}^{-3}$. Right: Evolution of the beam envelope derivative inside the plasma along the plasma profile of the nozzle used in the experiment. The derivative of the envelope settles to a final divergence of 4 mrad rms.