Pair filamentation and laser scattering in beam-driven QED cascades

We report the observation of longitudinal filamentation of an electron-positron pair plasma in a beam-driven QED cascade. The filaments are created in the “pair-reflection” regime, where the generated pairs are partially stopped and reflected in the strong laser field. The density filaments form near the center of the laser pulse and have diameters similar to the laser wavelength. They develop and saturate within a few laser cycles and do not induce sizable magnetostatic fields. We rule out the onset of two-stream instability or Weibel instability and attribute the origin of pair filamentation to laser ponderomotive forces. The small plasma filaments induce strong scattering of laser energy to large angles, serving as a signature of collective QED plasma dynamics.

Figure 1

Pair filamentation (green) and laser scattering (red) as a result of $e^{-}$-beam driven QED cascade. The laser is polarized in the $y$ direction and propagates to the $+x$ direction. The $e^{-}$ beam (not shown) propagates to the $-x$ direction. The snapshot is taken at $t=0.22\mathrm{ps}$.

Figure 2

Pair density $\left({\mathrm{cm}}^{-3}\right)$ at $t=0.18\mathrm{ps}$ (a), (b), $0.2\mathrm{ps}$ (c), (d), and $0.22\mathrm{ps}$ (e), (f), respectively. The blue curves show the laser intensity contours at $1,3,5,7\times {10}^{22}{\mathrm{Wcm}}^{-2}$ from outer to inner, respectively. The left column shows the $y=0$ cross section and the right column shows the $x=-2\mu \mathrm{m}$ cross section indicated by the vertical line on the left.

Figure 3

Pair momentum distributions in the transverse direction (left column) and longitudinal direction (right column) at $t=0.16\mathrm{ps}$ (a), (b), $0.18\mathrm{ps}$ (c), (d), and $0.2\mathrm{ps}$ (e), (f), respectively.

Figure 4

Magnetic field $B_x$ (a), (b), $B_y$ (c), (d), and $B_z$ (e), (f) in the $y=0$ cross section at $t=0.18\mathrm{ps}$ (left column), and $0.2\mathrm{ps}$ (right column), respectively.

Figure 5

Poynting vector (in unit ${\mathrm{Wm}}^{-2}$) $S_y$ in the $z=0$ cross section (a), (b) and $S_z$ in the $y=0$ cross section (c), (d) at $t=0.18\mathrm{ps}$ (a), (c), and $0.26\mathrm{ps}$ (b), (d), respectively.

Figure 6

Evolution of the peak laser intensity (blue solid) and total energy (red dashed).

Figure 7

The laser magnetic field $B_x$ (a), (b), $B_y$ (c), (d), and $B_z$ (e), (f) in the $y=0$ cross section without interacting with electron beam at $t=0.18\mathrm{ps}$ (left column), and $0.2\mathrm{ps}$ (right column), respectively.

Figure 8

Poynting vector (in unit ${\mathrm{Wm}}^{-2}$) $S_y$ in the $z=0$ cross section (a), (b) and $S_z$ in the $y=0$ cross section (c), (d) of the laser without interacting with electron beam at $t=0.18\mathrm{ps}$ (a), (c), and $0.26\mathrm{ps}$ (b), (d), respectively.

Figure 9

Evolution of the peak laser intensity (blue solid) and total energy (red dashed).