Optical Control of the Topology of Laser-Plasma Accelerators

We propose a twisted plasma accelerator capable of generating relativistic electron vortex beams with helical current profiles. The angular momentum of these vortex bunches is quantized, dominates their transverse motion, and results in spiraling particle trajectories around the twisted wakefield. We focus on a laser wakefield acceleration scenario, driven by a laser beam with a helical spatiotemporal intensity profile, also known as a light spring. We find that these light springs can rotate as they excite the twisted plasma wakefield, providing a new mechanism to control the twisted wakefield phase velocity and enhance energy gain and trapping efficiency beyond planar wakefields.

Figure 1

Pulsed LG beams (a)–(b) versus light springs (c)–(d). The upper graphs show the frequency dependence of the azimuthal mode index $\ell$ in the two cases. The lower graphs display the corresponding spatiotemporal intensity profiles of the pulses.

Figure 2

Twisted plasma wakefields driven by a light spring moving in a preformed plasma doped with nitrogen. (a) (right) Rainbow colors display the electric field of the light spring. (left) Blue-red isosurfaces show the twisted longitudinal electric field structure of the wake excited by this LS in the underdense plasma. These surfaces are not displayed for $x\ge 178$ to avoid hiding the other plots. (middle) Spheres in rainbow colors correspond to ionization injected electrons from the inner (6th-7th) shells of nitrogen. (b)–(c) Slices of the corresponding radial and azimuthal electric fields in the plasma. Field values are normalized to the cold wave breaking limit $E_0=m_{e}c{\omega }_p/e$.

Figure 3

Properties of relativistic vortex beams accelerated by twisted plasma waves. (a) $p_{\theta }-p_x$ phase space of vortex electron bunches in plasma waves with different OAM levels. The dashed lines show the prediction of Eq. (1) in each case. Note the negligible azimuthal momentum of trapped particles in the case where $\mathrm{\Delta }\ell =0$ (gray distribution). (b) Helical trajectories of a random sample of ionization injected nitrogen electrons forming a vortex beam with ${\ell }_p=1$. The trajectories are colored according to the propagation time. (c) Trajectories of two electrons [shown in blue and red in (b)] along the propagation time. (d)–(e) Three-dimensional vortex particle bunches colored according to the energy.

Figure 4

Effects of the LS azimuthal rotation. (a) Spatiotemporal intensity profile of a LS propagating in a plasma channel, at different times in the propagation. (b)–(c) Evolution of the accelerating fields at $y\simeq 9c/{\omega }_p$ and $z=0$, corresponding to the region around the radial maximum of the LS intensity, for a fixed ${\ell }_0>0$. The theoretical predictions of Eq. (2) are shown as black lines. In panel (b), the phase velocity of the wakefield is very close to $c>v_g$, due to the effect of the LS rotation. (d) $p_{\theta }-\mathrm{\Delta }{p}_x$ phase space in twisted plasma waves with different OAM levels. The theoretical predictions of Eq. (1) are given by the black dashed lines. Solid red and point black curves, respectively, show the spectra of accelerated particles.