Diagnosis of ultrafast ultraintense laser pulse characteristics by machine-learning-assisted electron spin

The rapid development of ultrafast ultraintense laser technology continues to create opportunities for studying strong-field physics under extreme conditions.However,accurate determination of the spatial and temporal characteristics of a laser pulse is still a great challenge,especially when laser powers higher than hundreds of terawatts are involved.In this paper,by utilizing the radiative spin-flip effect,we find that the spin depolarization of an electron beam can be employed to diagnose characteristics of ultrafast ultraintense lasers with peak intensities around 10^(20)–10^(22) W/cm^(2).With three shots,our machine-learning-assisted model can predict,simultaneously,the pulse duration,peak intensity,and focal radius of a focused Gaussian ultrafast ultraintense laser(in principle,the profile can be arbitrary)with relative errors of 0.1%–10%.The underlying physics and an alternative diagnosis method(without the assistance of machine learning)are revealed by the asymptotic approximation of the final spin degree of polarization.Our proposed scheme exhibits robustness and detection accuracy with respect to fluctuations in the electron beam parameters.Accurate measurements of ultrafast ultraintense laser parameters will lead to much higher precision in,for example,laser nuclear physics investigations and laboratory astrophysics studies.Robust machine learning techniques may also find applications in more general strong-field physics scenarios.

Generation of highly-polarized high-energy brilliant γ-rays via laser-plasma interaction

The generation of highly polarized high-energy brilliantγ-rays via laser–plasma interaction is investigated in the quantum radiation-reaction regime.We employ a quantum electrodynamics particle-in-cell code to describe spin-resolved electron dynamics semiclassically and photon emission and polarization quantum mechanically in the local constant field approximation.As an ultrastrong linearly polarized(LP)laser pulse irradiates a near-critical-density(NCD)plasma followed by an ultrathin planar aluminum target,the electrons in the NCD plasma are first accelerated by the driving laser to ultrarelativistic energies and then collide head-on with the laser pulse reflected by the aluminum target,emitting brilliant LPγ-rays via nonlinear Compton scattering with an average polarization of about 70%and energy up to hundreds of MeV.Suchγ-rays can be produced with currently achievable laser facilities and will find various applications in high-energy physics and laboratory astrophysics.

Generation of arbitrarily polarized GeV lepton beams via nonlinear Breit-Wheeler process

Generation of arbitrarily spin-polarized electron and positron beams has been investigated in the single-shot interaction of high-energy polarized r-photons with an ultraintense asymmetric laser pulse via nonlinear Breit-Wheeler pair production.We develop a fully spin-resolved semi-classical Monte Carlo method to describe the pair creation and polarization.In the considered general setup,there are two sources of the polarization of created pairs:the spin angular momentum transfer from the polarized parent-photons,as well as the asymmetry and polarization of the driving laser field.This allows to develop a highly sensitive tool to control the polarization of created electrons and positrons.Thus,dense GeV lepton beams with average polarization degree up to about 80%,adjustable continuously between the transverse and longitudinal components,can be obtained by our all-optical method with currently achievable laser facilities,which could find an application as injectors of the polarized e^(+)e^(-)collider to search for new physics beyond the Standard Model.