Ultracold Electron Bunch Generation via Plasma Photocathode Emission and Acceleration in a Beam-Driven Plasma Blowout

Beam-driven plasma wakefield acceleration using low-ionization-threshold gas such as Li is combined with laser-controlled electron injection via ionization of high-ionization-threshold gas such as He. The He electrons are released with low transverse momentum in the focus of the copropagating, nonrelativistic-intensity laser pulse directly inside the accelerating or focusing phase of the Li blowout. This concept paves the way for the generation of sub-$\mu \mathrm{m}$-size, ultralow-emittance, highly tunable electron bunches, thus enabling a flexible new class of an advanced free electron laser capable high-field accelerator.

Figure 1

Results from a VORPAL [25] simulation show how an electron driver ionizes Li gas and generates a Li blowout with an electron density of $n_e(\mathrm{Li})=3.3\times {10}^{17}{\mathrm{cm}}^{-3}$, corresponding to a linear plasma wavelength of ${\lambda }_p(\mathrm{Li})\approx 60\mu \mathrm{m}$. The Ti:sapphire laser pulse with a duration of $\tau \approx 8\mathrm{fs}$ and $a_0=0.018$ is located at the end of the first half of the blowout at the electric field’s turning point, and has already ionized some He electrons, which are then trapped and accelerated.

Figure 2

Injection of He electrons at the beginning of the interaction. Snapshots (a) to (e) show $E$ generated by the Li blowout and the laser pulse, and the He electrons which are born inside the Li blowout due to ionization by the focused laser pulse, while (f) shows only $E_z$ and a lineout on axis, corresponding to (d).

Figure 3

Electric field and He bunch after $z=2.5\mathrm{mm}$ (a), current $J$ after $z=4.4\mathrm{mm}$ (b), $p_z-z$ phase space for three different acceleration times (c), and He electron energy spectrum at these times (d).