Intrinsic normalized emittance growth in laser-driven electron accelerators

Laser-based electron sources are attracting strong interest from the conventional accelerator community due to their unique characteristics in terms of high initial energy, low emittance, and significant beam current. Extremely strong electric fields (up to hundreds of $\mathrm{GV}/\mathrm{m}$) generated in the plasma allow accelerating gradients much higher than in conventional accelerators and set the basis for achieving very high final energies in a compact space. Generating laser-driven high-energy electron beam lines therefore represents an attractive challenge for novel particle accelerators. In this paper we show that laser-driven electrons generated by the nowadays consolidated TW laser systems, when leaving the interaction region, are subject to a very strong, normalized emittance worsening which makes them quickly unusable for any beam transport. Furthermore, due to their intrinsic beam characteristics, controlling and capturing the full beam current can only be achieved improving the source parameters.

Figure 1

Charge distribution vs energy (top) and current vs time (bottom) of the initial electron distribution as obtained with the PIC code ALaDyn.

Figure 2

Normalized transverse emittance along a transport line using magnetic quadrupoles: comparison between TSTEP simulations (red line), Eq. (4) (black line), and the $\gamma \epsilon$ approximation (blue line). Inset: Zoom of the initial drift.

Figure 3

Comparison of slice normalized emittance at the interaction point and after a 1 cm drift space (top) and slice energy spread (bottom). The number of slices in the longitudinal plane is 200.