Laser-driven pointed acceleration of electrons with preformed plasma lens

The simultaneous laser-driven acceleration and angular manipulation of the fast electron beam are experimentally demonstrated. The bunch of multi-MeV energy charged particles is generated during the propagation of the femtosecond laser pulse through the near-critical plasma slab accompanied by plasma channeling. Plasma is formed by the controlled breakdown of a thin-tape target by a powerful nanosecond prepulse. The electron beam pointing approach is based on the refraction of a laser pulse in the presence of a strong radial density gradient in the breakdown of the tape with a small displacement of the femtosecond laser beam relative to the breakdown symmetry axis. A shift of several micrometers makes it possible to achieve beam deflection by an angle up to 10° with acceptable beam charge and spectrum conservation. This opens up opportunities for in situ applications for scanning objects with an electron beam and the multistage electron beam energy gain in consecutive laser accelerators without bulk magnetic optics for particles. Experimental findings are supported by numerical particle-in-cell calculations of laser-plasma acceleration and hydrodynamic simulations.

Figure 1

Experimental scheme: 1—Nd:YAG nanosecond prepulse, 2—adjustable mirror for Nd:YAG radiation, 3—femtosecond pulse, 4—tape target, 5—electron beam detector (Lanex scintillation screen and camera with lens), 6—camera with microobjective collecting the scattered light from plasma (perpendicular to observer), $D$—deviation of the Ti:Sa laser pulse axis relative to the Nd:YAG axis (target breakdown symmetry axis).

Figure 2

Electron beam stamps on the Lanex detector averaged (first column) and single shot (second column) as well as plasma scattered light signal (third column) at $D=0\mathrm{\mu }\mathrm{m}$ (first row), $5\mathrm{\mu }\mathrm{m}$ (second row), $10\mathrm{\mu }\mathrm{m}$ (third row), and $-10\mathrm{\mu }\mathrm{m}$ (forth row). Red arrow indicates the incident laser beam direction. The red lines mark the tape initial position.

Figure 3

Calculated distributions $(r,z)$ of the ions concentration in plasma of the PET tape.

Figure 4

Magnetic field of the laser pulse in units of $a_0$ at different 3D PIC simulation periods at the background of electron density in units of $n_{cr}$ (a), (c). Typical trajectories of particles with energy over 3 MeV at the end of simulation are showed by curves, where the color corresponds to particles energy. In (b), the wave-vectors diagram for (a) is demonstrated, where $k_0$ is the laser wave vector. Color bars for electron energy and density are identical in (a) and (c).

Figure 5

Comparison of magnetic field of the laser pulse in 3D and 2D at different simulations periods (a). Normalized angular distribution of particles with energy $E$ over 3 MeV (b) and energy spectra (c). In (b), the dashed experimental curve corresponds to Fig. 2.

Figure 6

Results of numerical 2D PIC calculations: normalized magnetic field in the box at the background of normalized electron density during propagation of a femtosecond pulse (left) and angular-energy spectra of accelerated electrons (right) with deviation $D=0$ (a), (b) and $10\mathrm{\mu }\mathrm{m}$ (c), (d). Green dots mark electrons with energies above 3 MeV. The blue dashed lines represent the results of ray tracing of a laser beam through an inhomogeneous plasma. Indicated color scales for electron density, magnetic field amplitude, and angular spectra are the same for $D=0$ and $10\mathrm{\mu }\mathrm{m}$.

Figure 7

Energy spectra of the electron beam at varied driver laser beam deviation $D$ from symmetry axis for the vector potential $a_0=1.5$ (a) and $a_0=2.2$ (b). The relative charge $Q$ of the electron beam for particles above 3 MeV in dependence on the $D$ (c).

Figure 8

Numerical and experimental data on the deflection of the electron beam (a) and the laser pulse (b) as a function of the femtosecond pulse deviation $D$ relative to the breakdown symmetry axis.

Figure 9

Simplified model of the implementation of electron beam deflection approach to the AWAKE experiment.