Ultrashort high energy electron bunches from tunable surface plasma waves driven with laser wavefront rotation

We propose to use ultrahigh intensity laser pulses with wave-front rotation (WFR) to produce short, ultraintense surface plasma waves (SPW) on grating targets for electron acceleration. Combining a smart grating design with optimal WFR conditions identified through simple analytical modeling and particle-in-cell simulation allows us to decrease the SPW duration (down to a few optical cycles) and increase its peak amplitude. In the relativistic regime, for $I{\lambda }_0^2=3.4\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2\mu {\mathrm{m}}^2$, such SPW are found to accelerate high charge (few 10 s of pC), high energy (up to 70 MeV), and ultrashort (few fs) electron bunches.

Figure 1

Interaction setup: the central laser wave fronts are shown at best focus ($t=0$) and striking the target ($t>0$). Due to WFR, setting the target at a distance $x_f$ from best focus leads to a “sliding focus” effect, with the maximum on-target intensity sliding in the $y$ direction at a velocity $v_{\mathrm{sl}}$. The upper left insert compares $v_{\mathrm{sl}}$ from Eq. (3) (solid lines) with measures from PIC simulations ( symbols) for $x_f=25{\lambda }_0$ ( black circles) and $x_f=50{\lambda }_0$ ( magenta triangles).

Figure 2

SPW magnetic field at the target surface for (a) $\mathrm{\Delta }\beta =0$ and (b) $\mathrm{\Delta }\beta =67\mathrm{mrad}$ with $a_0=0.1$ and $x_f=25{\lambda }_0$. (c) Maximum SPW field amplitude and (d) duration (FWHM) vs the WFR parameter $\mathrm{\Delta }\beta$ for $a_0=0.1$. Markers p1 and p2 indicate the cases shown in panels (a) and (b). (e) Maximum electron momentum along the surface ($p_y$) and (f) electron bunch duration (FWHM) vs $\mathrm{\Delta }\beta$ for $a_0=5$. In panels (c) to (f), $x_f=0$ (green triangles) and $x_f=25{\lambda }_0$ (black circles).

Figure 3

Electron phase-space (red dots) and SPW field amplitude (blue line, right scale) for $a_0=5$ at times, $t=t_0+10{\lambda }_0/c$ and $t=t_0+20{\lambda }_0/c$. [(a), (b)] $\mathrm{\Delta }\beta =0$ and [(c), (d)] $\mathrm{\Delta }\beta =67\mathrm{mrad}$. The gray vertical dashed line indicates the end of the grating and beginning of the flat region.

Figure 4

(a) Electron energy distribution in MeV as a function of the emission angle $\varphi ={tan}^{-1}(p_y/p_x)$ for $\mathrm{\Delta }\beta =67\mathrm{mrad}$ and $a_0=5$. (b) Electron energy distribution for $\mathrm{\Delta }\beta =67$ (red solid line), $\mathrm{\Delta }\beta =0$ (blue dashed line), and $\mathrm{\Delta }\beta =-40\mathrm{mrad}$ (green dotted line).