Observation of strong correlation between quasimonoenergetic electron beam generation by laser wakefield and laser guiding inside a preplasma cavity

We use a one-shot measurement technique to study effects of laser prepulses on the electron laser wakefield acceleration driven by relativistically intense laser pulses ($\lambda =790\mathrm{nm}$, $11\mathrm{TW}$, $37\mathrm{fs}$) in dense helium gas jets. A quasimonoenergetic electron bunch with an energy peak $\sim 11.5\mathrm{MeV}$ [$\Delta E∕E\sim 10\%$ (FWHM)] and with a narrow-cone angle $\left(0.04\pi \mathrm{mm}\mathrm{mrad}\right)$ of ejection is detected at a plasma density of $8\times {10}^{19}{\mathrm{cm}}^{-3}$. A strong correlation between the generation of monoenergetic electrons and optical guiding of the pulse in a thin channel produced by picosecond laser prepulses is observed. This generation mechanism is well corroborated by two-dimensional particle-in-cell simulations.

Figure 1

Experimental setup. A supersonic slit nozzle is magnified in the inset.

Figure 2

Typical sequential shadowgraph images of the plasma for a gas density of $4\times {10}^{19}{\mathrm{cm}}^{-3}$ (plasma density of $8\times {10}^{19}{\mathrm{cm}}^{-3}$) and a laser power of $11\mathrm{TW}$. (a) $1.5\mathrm{ps}$ before the main pulse, (b) the time where just the main pulse comes to the focus point, (c) $1.2\mathrm{ps}$ after t he main pulse, (d) $5.2\mathrm{ps}$ after the main pulse, and (e) the magnified image of the cavity region in (d).

Figure 3

Typical interferograms and schlieren pictures of the plasma for a density of $4\times {10}^{19}{\mathrm{cm}}^{-3}$ (plasma density of $8\times {10}^{19}{\mathrm{cm}}^{-3}$) and a laser power of $11\mathrm{TW}$. (a) at $5.2\mathrm{ps}$ after the main pulse corresponding to 2(d), (b) a magnified image of the cavity region in (a), and (c) typical schlieren pictures of the plasma corresponding to 2(d).

Figure 4

Typical images of electron deposition on the DRZ screen behind the magnetic defrector for a gas density of $4\times {10}^{19}{\mathrm{cm}}^{-3}$ (plasma density of $8\times {10}^{19}{\mathrm{cm}}^{-3}$) and of $11\mathrm{TW}$ laser pulse. (a) a 100% energy spread spectrum case; (b) a quasimonoenergetic spectrum case.

Figure 5

Typical energy spectra of accelerated electrons converted from the images of Fig. 4. (a) a 100% energy spread spectrum case; (b) a quasimonoenergetic spectrum case.

Figure 6

Typical shadowgraph images of the plasma overlapped with Thomson scattering of the laser light in the plasma at $5.2\mathrm{ps}$ after the main pulse for gas density $N=4\times {10}^{19}{\mathrm{cm}}^{-3}$ (plasma density $N_e=8\times {10}^{19}{\mathrm{cm}}^{-3}$) and a laser power of $11\mathrm{TW}$. (a) a 100% energy spread spectrum case; (b) a quasimonoenergetic spectrum case; (c) a magnified image of the cavity region in (b).

Figure 7

Plasma and laser parameters for a 2D-PIC simulation.

Figure 8

Spatial distributions of the normalized electron density at (a) $0.59\mathrm{ps}$, (b) $0.98\mathrm{ps}$, and (c) $1.37\mathrm{ps}$, respectively, after the laser injection into the plasma channel. Laser intensity is $I=1\times {10}^{19}\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 9

Illustration of the laser propagation in the channel obtained by the 2D-PIC simulation.

Figure 10

The energy spectrum of the accelerated electrons from the 2D-PIC simulation.