Electron trapping and reinjection in prepulse-shaped gas targets for laser-plasma accelerators

A novel mechanism for injection, emittance selection, and postacceleration for laser wakefield electron acceleration is identified and described. It is shown that a laser prepulse can create an ionized plasma filament through multiphoton ionization and this heats the electrons and ions, driving an ellipsoidal blast-wave aligned with the laser-axis. The subsequent high-intensity laser-pulse generates a plasma wakefield which, on entering the leading edge of the blast-wave structure, encounters a sharp reduction in electron density, causing density down-ramp electron injection. The injected electrons are accelerated to $\sim 2\mathrm{MeV}$ within the blast-wave. After the main laser-pulse has propagated past the blast-wave, it drives a secondary wakefield within the homogenous background plasma. On exiting the blast-wave structure, the preaccelerated electrons encounter these secondary wakefields, are retrapped, and accelerated to higher energies. Due to the longitudinal extent of the blast-wave, only those electrons with small transverse velocity are retrapped, leading to the potential for the generation of electron bunches with reduced transverse size and emittance.

Figure 1

Electron beam energy deposition on a lanex screen, indicative of beam divergence (a) with prepulse, and (b) without. Note (a) is unfortunately saturated. Lines depict the relative signal and show both increased total signal (charge) and reduced divergence. Representative electron spectra with prepulse (c) and without (d). Spectra are imaged in the vertical direction corroborating (a) and (b).

Figure 2

(a) EPOCH simulations of hydrogen ionization caused by the prepulse. The blue line is an ionization fraction line-out along $X=0$ (upper X-axis). (b) Electron temperatures. Here the blue line is an electron temperature line-out along $X=0$ (upper X-axis).

Figure 3

(a) The blast-wave structure which evolves from the initial electron temperature profile induced by the laser pre-pulse. (b) A 1D slice of the density profile at $X=0$, showing the characteristic blast-wave structure. Note this figure has been labelled with X and Y for consistency with the EPOCH simulations, in these simulations Y is in fact the radial dimension in the cylindrical coordinate system employed.

Figure 4

(a) Lineouts along $y=0$, just before trapping occurs: mass density (black), electric field in X (blue), electric field gradient in X (red), the markers show the positions of the tracer particles, for these, the Y axis is position. (b) The same graph as (a) but at the time when the wake enters the blast-wave, and trapping occurs. The density drop induces a sharp peak in the electric field gradient, which traps the particles. (c) The mass density at the time of injection, markers show the tracer positions. (d) Charge density showing the wake structures and tracer positions.

Figure 5

Plasma parameters after the laser has propagated to $X=205\mu \mathrm{m}$. The blast-wave ends at $X=70\mu \mathrm{m}$. (a) The charge density distribution shows the wakefield structures created after the blast-wave; these wakes trap those electrons which are preaccelerated within the blast-wave. (b) This plot of cell-averaged particle kinetic energy shows multiple bunches are accelerated to higher energies by the multiple wakes trailing the laser. The laser is not shown, but is centered at $X=205\mu \mathrm{m}$. Only those electrons originating from the nitrogen atoms are shown. (c) As per (b) but with a density profile which only varies as a function of x. Here the lateral extent of the bunches is significantly increased from $\sim 2\mu \mathrm{m}$ to $10\mu \mathrm{m}$. (d) Blue arrows illustrate the initial propagation directions of those electrons injected at the blast-wave. The electrons are injected approximately perpendicularly to the blast-wave at a given location, so only those with small divergence (near $y=0$) enter the region of secondary injection.