  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.
