Collimated ultrabright gamma rays from electron wiggling along a petawatt laser-irradiated wire in the QED regime

Abstract

Even though high-quality X- and gamma rays with photon energy below mega-electron volt (MeV) are available from large-scale X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate bright gamma rays over 10 MeV. Recently, gamma rays with energies up to the MeV level were observed in Compton scattering experiments based on laser wakefield accelerators, but the yield efficiency was as low as [Formula: see text], owing to low charge of the electron beam. Here, we propose a scheme to efficiently generate gamma rays of hundreds of MeV from submicrometer wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. The wiggling is caused by the quasistatic electric and magnetic fields induced around the wire surface, and these are so high that even quantum electrodynamics (QED) effects become significant for gamma-ray generation, although the driving lasers are only at the petawatt level. Our full 3D simulations show that directional, ultrabright gamma rays are generated, containing [Formula: see text] photons between 5 and 500 MeV within a 10-fs duration. The brilliance, up to [Formula: see text] photons [Formula: see text] per 0.1% bandwidth at an average photon energy of 20 MeV, is second only to X-ray free electron lasers, while the photon energy is 3 orders of magnitude higher than the latter. In addition, the gamma ray yield efficiency approaches 10%-that is, 5 orders of magnitude higher than the Compton scattering based on laser wakefield accelerators. Such high-energy, ultrabright, femtosecond-duration gamma rays may find applications in nuclear photonics, radiotherapy, and laboratory astrophysics.

Keywords: high-energy density physics; high-energy high-brightness gamma ray; particle-in-cell simulation; strong field QED process; ultraintense laser matter interaction.

Fig. 1.

Schematic of the wire scheme. (A) Schematic: As a laser pulse propagates along a subwavelength wire and approaches its focusing plane (a distance behind the wire front to allow the electron to guide and accelerate), electrons along the wire surface are gradually accelerated with reduced divergent angles; meanwhile, the electrons are wiggled perpendicularly to the surface, which causes gamma rays emitted with increased photon energies and decreased divergent angles. (B) Chart of photon energy and brilliance (photons ${\mathrm{s}}^{-1}\hspace{0.17em}{\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}}^{-2}\hspace{0.17em}{\mathrm{m}\mathrm{m}}^{-2}$ per 0.1% bandwidth) of gamma rays generated from our wire scheme, XFEL, synchrotron radiation facilities, and betatron radiation and Compton scattering based on LWFA.

Fig. 2.

Generated gamma rays. 3D isosurfaces of (A) the laser field ($mc{\omega }_0/e$) and (B) gamma-ray photon density ($n_c$) at the time of 30 ${\tau }_0$ as well as the slices at the planes with respective peak values, where a $0.6\hspace{0.17em}\mathrm{\mu }\mathrm{m}$-wide wire is taken. Note that the laser pulse peak arrives at the focusing plane at about 30 ${\tau }_0$. (C) Angular distributions and (D) energy spectra of gamma rays emitted from the wire and a flat slab target, respectively.

Fig. 3.

Wiggling fields. 3D isosurfaces of (A) electrostatic and (C) magnetostatic fields ($mc{\omega }_0/e$), (B) electron density ($n_c$), and (D) current density ($ec{n}_c$) at the time of 30 ${\tau }_0$ as well as the slices at the planes with respective peak values, where they are obtained by temporally averaging $E_y$, $B_z$, $n_e$, and $J_x$, respectively, over one laser cycle. The corresponding one-dimensional distributions of these fields and densities at $x=21\hspace{0.17em}\mathrm{\mu }\mathrm{m}$ and $z=0.26\hspace{0.17em}\mathrm{\mu }\mathrm{m}$ are shown in E and F.

Fig. 4.

Trace of typical electrons. Evolution for an electron from the 0.6 $\mathrm{\mu }\mathrm{m}$ (A and C) and 0.3 $\mathrm{\mu }\mathrm{m}$ wires (B and D), respectively, is shown for the transverse position y ($\mathrm{\mu }m$), $E_y-B_z$ (units of $1000m_{e}c{\omega }_0/e$), divergence angle $\theta$ (units of $30^{○}$), energy $\epsilon$ (units of 5 GeV), and QED parameter $\chi$, where we plot $y+0.3$ in A, since the electron wiggles around $-0.3\hspace{0.17em}\mathrm{\mu }\mathrm{m}$.

Fig. 5.

Dependency of gamma-ray generation on laser powers and wire widths. Angular distributions of gamma rays with different wire widths (A) and under different laser powers (B), where “$\times 10$” in the legend means the photon number multiplied by a factor of 10. Energy conversion efficiency of the gamma rays versus wire widths (C) and laser powers (D). (E) Energy spectra of gamma rays at 50 ${\tau }_0$ under different laser powers. In A and C, the laser power is fixed at 2.5 PW. In B, D, and E, the wire width is fixed at 0.6 $\mathrm{\mu }\mathrm{m}$.

Fig. 6.

Generated electron beams. The number (units of $10^8$) of electrons with energies above 10 MeV as a function of ($\theta$, $y$, $\epsilon$) at 30 ${\tau }_0$, where Insets in each plot show number distributions at angles of $1^{○}$, $10^{○}$, and $12^{○}$ corresponding to curves in different colors. The left (A and C) and right columns (B and D) correspond to 0.6 $\mathrm{\mu }\mathrm{m}$ and 0.3 $\mathrm{\mu }\mathrm{m}$ wires, respectively. In A and C, the electron beam at the angle about $1^{○}$ is circled.