Ultrarelativistic electron states in a general background electromagnetic field

Charged particle beam transport in a flying focus pulse with orbital angular momentum

We demonstrate the capability of flying focus (FF) laser pulses with ℓ=1 orbital angular momentum (OAM) to transversely confine ultrarelativistic charged particle bunches over macroscopic distances while maintaining a tight bunch radius. A FF pulse with ℓ=1 OAM creates a radial ponderomotive barrier that constrains the transverse motion of particles and travels with the bunch over extended distances. As compared with freely propagating bunches, which quickly diverge due to their initial momentum spread, the particles cotraveling with the ponderomotive barrier slowly oscillate around the laser pulse axis within the spot size of the pulse. This can be achieved at FF pulse energies that are orders of magnitude lower than required by Gaussian or Bessel pulses with OAM. The ponderomotive trapping is further enhanced by radiative cooling of the bunch resulting from rapid oscillations of the charged particles in the laser field. This cooling decreases the mean-square radius and emittance of the bunch during propagation.

Sauter-Schwinger Effect for Colliding Laser Pulses

Via a combination of analytical and numerical methods, we study electron-positron pair creation by the electromagnetic field A(t,r)=[f(ct-x)+f(ct+x)]e_{y} of two colliding laser pulses. Employing a generalized Wentzel-Kramers-Brillouin approach, we find that the pair creation rate along the symmetry plane x=0 (where one would expect the maximum contribution) displays the same exponential dependence as for a purely time-dependent electric field A(t)=2f(ct)e_{y}. The prefactor in front of this exponential does also contain corrections due to focusing or defocusing effects induced by the spatially inhomogeneous magnetic field. We compare our analytical results to numerical simulations using the Dirac-Heisenberg-Wigner method and find good agreement.

Quantum Limitation to the Coherent Emission of Accelerated Charges

Accelerated charges emit electromagnetic radiation. According to classical electrodynamics, if the charges move along sufficiently close trajectories they emit coherently; i.e., their emitted energy scales quadratically with their number rather than linearly. By investigating the emission by a two-electron wave packet in the presence of an electromagnetic plane wave within strong-field QED, we show that quantum effects deteriorate the coherence predicted by classical electrodynamics even if the typical quantum nonlinearity parameter of the system is much smaller than unity. We explain this result by observing that coherence effects are also controlled by a new quantum parameter which relates the recoil undergone by the electron to the width of its wave packet in momentum space.

Exact Classical and Quantum Dynamics in Background Electromagnetic Fields

Analytic results for (Q)ED processes in external fields are limited to a few special cases, such as plane waves. However, the strong focusing of intense laser fields implies a need to go beyond the plane wave model. By exploiting Poincaré symmetry and superintegrability we show how to construct, and solve without approximation, new models of laser-matter interactions. We illustrate the method with a model of a radially polarized (TM) laser beam, for which we exactly determine the classical orbits and quantum wave functions. Including in this way the effects of transverse field structure should improve predictions and analyses for experiments at intense laser facilities.

Nonlinear Breit-Wheeler Pair Production in a Tightly Focused Laser Beam

The only available analytical framework for investigating QED processes in a strong laser field systematically relies on approximating the latter as a plane wave. However, realistic high-intensity laser beams feature much more complex space-time structures than plane waves. Here, we show the feasibility of an analytical framework for investigating strong-field QED processes in laser beams of arbitrary space-time structure by determining the energy spectrum of positrons produced via nonlinear Breit-Wheeler pair production as a function of the background field in the realistic assumption that the energy of the incoming photon is the largest dynamical energy in the problem. A numerical evaluation of the angular resolved positron spectrum shows significant quantitative differences with respect to the analogous result in a plane wave, such that the present results will be also important for the design of upcoming strong laser facilities aiming at measuring this process.

Quantum Radiation Reaction: From Interference to Incoherence

We investigate quantum radiation reaction in laser-electron interactions across different energy and intensity regimes. Using a fully quantum approach which also accounts exactly for the effect of the strong laser pulse on the electron motion, we identify in particular a regime in which radiation reaction is dominated by quantum interference. We find signatures of quantum radiation reaction in the electron spectra which have no classical analogue and which cannot be captured by the incoherent approximations typically used in the high-intensity regime. These signatures are measurable with presently available laser and accelerator technology.