Monoenergetic Electronic Beam Production Using Dual Collinear Laser Pulses

The production of monoenergetic electron beams by two copropagating ultrashort laser pulses is investigated both by experiment and using particle-in-cell simulations. By proper timing between guiding and driver pulses, a high-amplitude plasma wave is generated and sustained for longer than is possible with either of the laser pulses individually, due to plasma waveguiding of the driver by the guiding pulse. The growth of the plasma wave is inferred by the measurement of monoenergetic electron beams with low divergence that are not measured by using either of the pulses individually. This scheme can be easily implemented and may allow more control of the interaction than is available to the single pulse scheme.

Figure 1

Schematic of the experimental setup.

Figure 2

(a) Typical electron spectra with the drive pulse only, (b)–(d) quasimonoenergetic electron spectra with highest charge shown for various timings between the drive and guiding laser pulses. $n_e=1\times {10}^{19}{\mathrm{cm}}^{-3}$ and $E_{\mathrm{pulse}}<300\mathrm{mJ}$.

Figure 3

(a) Relative mean energy ${\overline{W}}_R$ as a function of the relative arrival time between the pulses ($\Delta t$), and (b) ${\overline{W}}_R$ averaged over clusters of shots taken at the same time. The black bars show the standard error in these shots. The inset of (a) shows expanded versions of the highlighted regions, showing the relative stability of ${\overline{W}}_R$ for certain timings, where the number labels indicate the number of shots in the cluster.

Figure 4

The normalized intensity $e^2{E}_0^2/m^2{c}^2{\omega }_0^2$ of driver pulses having propagated $1500\mu \mathrm{m}$ in a density of $n_e=1\times {10}^{19}{\mathrm{cm}}^{-3}$ with a separation of (a) $21\mu \mathrm{m}$ (well phased) and (b) $18\mu \mathrm{m}$ (not well phased). Contours are given at intervals of 0.25. The energy trapped in filament (b) is 62% of (a).

Figure 5

The transverse envelope as a function of propagation distance (a) driver without a guide pulse and (b) driver with appropriately timed guide pulse. (c) The transverse envelope of the guide pulse only for comparison. The transverse envelope was found by integrating the electric field of the pulse longitudinally ($z$). The graphs show (left) the intensity profiles in the transverse ($x$) direction and (right) the wakefield and pulse profiles along the axis of propagation. (d) The laser envelope, and (e) $\delta n_e/n_{e0}$ for the wake.