Two-Color Laser-Ionization Injection

A method is proposed to generate femtosecond, ultralow emittance ($\sim {10}^{-8}\mathrm{m}\mathrm{rad}$), electron beams in a laser-plasma accelerator using two lasers of different colors. A long-wavelength pump pulse, with a large ponderomotive force and small peak electric field, excites a wake without fully ionizing a high-$Z$ gas. A short-wavelength injection pulse, with a small ponderomotive force and large peak electric field, copropagating and delayed with respect to the pump laser, ionizes a fraction of the remaining bound electrons at a trapping wake phase, generating an electron beam that is accelerated in the wake.

Figure 1

Example of two-color laser-ionization injection using Kr gas. (a) The vector potentials of the pump pulse $a_0$ (with ${\lambda }_0=5\mu \mathrm{m}$) and injection pulse $a_1$ (with ${\lambda }_1=0.4\mu \mathrm{m}$) (black curves) and the normalized excited wakefield $e{E}_z/(m_e{k}_p{c}^2)$ (green dotted curve). $H_s-H_i$ is shown as a function of wake phase $\psi$ (blue dashed curve), and $H_s-H_i>0$ is the trapping condition for an ionized electron. (b) The electric fields of the pump pulse $E_0$ and injection pulse $E_1$ (black curves), normalized to the peak of $E_0$, and the fraction of ionization for ${\mathrm{Kr}}^{8+}\to {\mathrm{Kr}}^{9+}$ (red dashed curve).

Figure 2

(a) The electric field of the laser pulses (green curve is on-axis laser field profile), (b) the longitudinal wakefield (green curve is on-axis wakefield), and (c) the transverse phase space of the injected electrons, after the (mixed gas) ionization injection region. (d) The normalized transverse emittance ${\epsilon }_n$ (red solid curve) and trapped electron number $N$ (black dashed curve) versus the injection pulse amplitude $a_1$. See text for laser-plasma parameters.

Figure 3

(a) The normalized transverse emittance ${\epsilon }_n$, and the maximum energy $W_{\max }$ of the trapped electron beam versus acceleration distance $z$. (b) Electron beam energy spectra at $z=0.6\mathrm{mm}$, 1.2 mm, and 1.8 mm. The laser-plasma parameters are the same as in Fig. 2.