Extremely Dense Gamma-Ray Pulses in Electron Beam-Multifoil Collisions

Sources of high-energy photons have important applications in almost all areas of research. However, the photon flux and intensity of existing sources is strongly limited for photon energies above a few hundred keV. Here we show that a high-current ultrarelativistic electron beam interacting with multiple submicrometer-thick conducting foils can undergo strong self-focusing accompanied by efficient emission of gamma-ray synchrotron photons. Physically, self-focusing and high-energy photon emission originate from the beam interaction with the near-field transition radiation accompanying the beam-foil collision. This near field radiation is of amplitude comparable with the beam self-field, and can be strong enough that a single emitted photon can carry away a significant fraction of the emitting electron energy. After beam collision with multiple foils, femtosecond collimated electron and photon beams with number density exceeding that of a solid are obtained. The relative simplicity, unique properties, and high efficiency of this gamma-ray source open up new opportunities for both applied and fundamental research including laserless investigations of strong-field QED processes with a single electron beam.

Figure 1

Schematic setup. An ultrarelativistic electron beam sequentially collides with aluminum foils. At each beam-foil collision, a strong transverse force which focuses the electron beam and leads to copious gamma-ray emission is induced.

Figure 2

(a) Electron beam density, (b) transverse magnetic field, and (c) transverse electric field in the collision with a $0.5\mu \mathrm{m}$-thick aluminum foil. For comparison, the peak magnetic and electric beam self-fields are $3.1\times {10}^4\mathrm{T}$, and $9.4\times {10}^{12}\mathrm{V}/\mathrm{m}$, respectively. (d) Electron beam to radiation energy conversion efficiency $\eta$ as a function of ${\sigma }_{\perp }$ in the collision with one foil. The electron beam has 2 nC charge, 10 GeV energy, and ${\sigma }_{\parallel }=0.55\mu \mathrm{m}$. Black circles: 3D PIC simulations results; blue circles: reflected-field model predictions. (e) Same as in panel (d) but for ${\sigma }_{\perp }=0.55\mu \mathrm{m}$ and as a function of ${\sigma }_{\parallel }$.

Figure 3

Beam evolution. First column, initial electron beam density (a1), its magnetic (b1) and electric (c1) fields, and the initial photon density (d1). Second to sixth column, same quantities as in the first column but at the 3rd (a2)–(d2), the 6th (a3)–(d3), the 7th (a4)–(d4), the 12th (a5)–(d5), and the 16th (a6)–(d6) beam-foil interaction, respectively.

Figure 4

(a) Initial (black dashed line) and final (blue line) electron beam energy distribution. (b) Final photon spectrum. The inset displays $\eta$ as a function of the number of foils crossed by the electron beam.