Transverse Spreading of Electrons in High-Intensity Laser Fields

We show that for collisions of electrons with a high-intensity laser, discrete photon emissions introduce a transverse beam spread that is distinct from that due to classical (or beam shape) effects. Via numerical simulations, we show that this quantum induced transverse momentum gain of the electron is manifest in collisions with a realistic laser pulse of intensity within reach of current technology, and we propose it as a measurable signature of strong-field quantum electrodynamics.

Figure 1

Simulation results for electrons with initial energy of 102.2 MeV ($\gamma =200$) colliding head-on with a 30 fs FWHM Gaussian plane-wave field of intensity $a_0=150$ and wavelength $\lambda =0.8\mu \mathrm{m}$ propagating in the positive $z$ direction. LL [Eq. (1)], red lines; sample of five QED (simulated) electrons, blue lines: (a) trajectories; (b) energies of electrons in (a). The electrons are timed such that they would enter the peak of the pulse at $t=z=0$, were they not affected by the field.

Figure 2

Sample of 20 simulated electron trajectories in a paraxial Gaussian laser beam of intensity $a_0=150$ ($9.5\times 10^{22}\mathrm{W}{\mathrm{cm}}^{-2}$), wavelength of 800 nm, waist radius of $10\mu \mathrm{m}$ and duration 35 fs (dashed line shows laser profile). Initial electron energies distributed around 255 MeV (${\gamma }_0=500$) according to a Gaussian of FWHM 0.52 MeV. Electrons incident at 10 degree angle. (a) classical (LL); (b) QED electrons.

Figure 3

Simulation results for a bunch of 1000 electrons initially spatially distributed according to a Gaussian of $2.5\mu \mathrm{m}$ FWHM and satisfying an initial energy distribution centred around 255 MeV (${\gamma }_0=500$) according to a Gaussian of FWHM 0.52 MeV. Laser parameters as Fig. 2. Upper panels: (a) final electron opening angles $\theta \equiv \arctan (x/z)$, (b) final electron energies. (Thin, red lines: classical (LL); thick, blue lines: QED). Lower panels: (c) opening angles and (d) energies of the emitted photons for the QED simulation.