Quantum Radiation Reaction in Laser–Electron-Beam Collisions

It is possible using current high-intensity laser facilities to reach the quantum radiation reaction regime for energetic electrons. An experiment using a wakefield accelerator to drive GeV electrons into a counterpropagating laser pulse would demonstrate the increase in the yield of high-energy photons caused by the stochastic nature of quantum synchrotron emission: we show that a beam of ${10}^9$ 1 GeV electrons colliding with a 30 fs laser pulse of intensity ${10}^{22}\mathrm{W}{\mathrm{cm}}^{-2}$ will emit 6300 photons with energy greater than 700 MeV, $60\times$ the number predicted by classical theory.

Figure 1

Diagram of an experimental geometry that could demonstrate quantum radiation reaction and pair production. The GeV electrons decelerate in large fields at the laser focus, producing gamma rays that pass through a hole in the $f/2$ optic.

Figure 2

The number of photons with energy $>\hslash \omega$ for a 1 GeV electron incident on a linearly polarized, plane-wave laser pulse with Gaussian temporal profile (FWHM 30 fs) and peak intensity (a) ${10}^{21}$ (b) ${10}^{22}\mathrm{W}{\mathrm{cm}}^{-2}$. Inset (top): The fractional increase, due to straggling, in the number of photons with given minimum energy. Inset (bottom): The maximum $\eta$ experienced by a (c) continuously and (d) discontinuously radiating GeV electron incident on the same pulse with given intensity. The increase in going from (c) to (d) is responsible for the hardening of the photon spectrum.

Figure 3

Indicated along the colored lines are the numbers of photons per electron with energy greater than 100 (red line), 200 (green line), 400 (blue line), and 500 MeV (purple line) if the electrons radiate discontinuously (vertical axis) or continuously (horizontal axis). The interaction here is between a focused laser pulse of given peak intensity (FWHM 30 fs, waist $2.2\mu \mathrm{m}$) and a beam of 1 GeV electrons (radius $10\mu \mathrm{m}$ around the optical axis).

Figure 4

The number of Breit-Wheeler (red line) and trident (blue line) electron-positron pairs produced per 1 GeV electron colliding with a linearly polarized plane-wave laser pulse with Gaussian temporal profile (FWHM 30 fs) and given intensity. Inset: The fractional increase in the number of pairs for the same processes produced by treating radiation reaction probabilistically.

Figure 5

The final state energy distribution $dN/d{E}_e$ of a $10\mu \mathrm{m}$ radius beam of 1 GeV electrons incident on a pulse with given peak intensity. White, dashed lines are contours of constant $dN/d{E}_e$. The red line is the lowest energy that can be reached with classical radiation.