arXiv subjects

Karen Z. Hatsagortsyan

Publications and source records attributed to Karen Z. Hatsagortsyan.

At least 37 records · Page 2Linked to original sources

arXiv2022

Electron polarization in ultrarelativistic plasma current filamentation instabilities

Plasma current filamentation of an ultrarelativistic electron beam impinging on an overdense plasma is investigated, with emphasis on radiation-induced electron polarization. Particle-in-cell simulations provide the classification and in-depth analysis of three different regimes of the current filaments, namely, the normal filament, abnormal filament, and quenching regimes. We show that electron radiative polarization emerges during the instability along the azimuthal direction in the momentum space, which significantly varies across the regimes. We put forward an intuitive Hamiltonian model to trace the origin of the electron polarization dynamics. In particular, we discern the role of nonlinear transverse motion of plasma filaments, which induces asymmetry in radiative spin flips, yielding an accumulation of electron polarization. Our results break the conventional perception that quasi-symmetric fields are inefficient for generating radiative spin-polarized beams, suggesting the potential of electron polarization as a source of new information on laboratory and astrophysical plasma instabilities.

physics.plasm-ph↗

arXiv2022

Reconciling conflicting approaches for the tunneling time delay in strong field ionization

Several recent attoclock experiments have investigated the fundamental question of a quantum mechanically induced time delay in tunneling ionization via extremely precise photoelectron momentum spectroscopy. The interpretations of those attoclock experimental results were controversially discussed, because the entanglement of the laser and Coulomb field did not allow for theoretical treatments without undisputed approximations. The method of semiclassical propagation matched with the tunneled wavefunction, the quasistatic Wigner theory, the analytical R-matrix theory, the backpropagation method, and the under-the-barrier recollision theory are the leading conceptual approaches put forward to treat this problem, however, with seemingly conflicting conclusions on the existence of a tunneling time delay. To resolve the contradicting conclusions of the different approaches, we consider a very simple tunneling scenario which is not plagued with complications stemming from the Coulomb potential of the atomic core, avoids consequent controversial approximations and, therefore, allows us to unequivocally identify the origin of the tunneling time delay.

quant-ph↗

arXiv2022

Deciphering in situ electron dynamics of ultrarelativistic plasma via polarization pattern of emitted gamma photons

Understanding and interpretation of the dynamics of ultrarelativistic plasma is a challenge, which calls for the development of methods for \textit{in situ} probing the plasma dynamical characteristics. We put forward a new method, harnessing polarization properties of $γ$-photons emitted from a non-pre-polarized plasma irradiated by a circularly polarized pulse. We show that the angular pattern of $γ$-photon linear polarization is explicitly correlated with the dynamics of the radiating electrons, which provides information on the laser-plasma interaction regime. Furthermore, with the $γ$-photon circular polarization originating from the electron radiative spin-flips, the plasma susceptibility to quantum electrodynamical processes is gauged. Our study demonstrates that the polarization signal of emitted $γ$-photons can be a versatile information source, which would be beneficial for the research fields of laser-driven plasma, accelerator science, and laboratory astrophysics.

physics.plasm-ph↗

arXiv2022

Nondipole Coulomb sub-barrier ionization dynamics and photon momentum sharing

The nondipole under-the-barrier dynamics of the electron during strong-field tunneling ionization is investigated, examining the role of the Coulomb field of the atomic core. The common analysis in the strong field approximation is consequently generalised to include the leading light-front non-dipole Coulomb corrections and demonstrates the counter-intuitive impact of the sub-barrier Coulomb field. Despite its attractive nature, the sub-barrier Coulomb field increases the photoelectron nondipole momentum shift along the laser propagation direction, involving a strong dependence on the laser field. The scaling of the effect with respect to the principal quantum number and angular momentum of the bound state is found. We demonstrate that the signature of Coulomb induced sub-barrier effects can be identified in the asymptotic photoelectron momentum distribution via a comparative study of the field-dependent longitudinal momentum shift for different atomic species with state-of-the-art experimental techniques of mid-infrared lasers.

physics.atom-ph↗

arXiv2022

Sub-cycle time-resolved nondipole dynamics in tunneling ionization

The electron nondipole dynamics in tunneling ionization in an elliptically polarized laser field is investigated theoretically using a relativistic Coulomb-corrected strong-field approximation (SFA) based on the eikonal approximation of the Klein-Gordon equation. We calculate attoclock angle-resolved light-front momentum distributions at different ellipticities of the laser field in quasistatic and nonadiabatic regimes and analyze them with an improved Simpleman model. The nondipole correlations between longitudinal and transverse momentum components are examined. Deviations of the photoelectron momentum distribution calculated via SFA with respect to the available experimental results as well as with the improved Simpleman model are discussed and interpreted in terms of nonadiabatic as well as Coulomb effects in the continuum and under-the-barrier. The favorable prospects of an experimental observation are discussed.

physics.atom-ph↗

arXiv2022

Electron spin- and photon polarization-resolved probabilities of strong-field QED processes

A derivation of fully polarization-resolved probabilities is provided for high-energy photon emission and electron-positron pair production in ultrastrong laser fields. The probabilities resolved in both electron spin and photon polarization of incoming and outgoing particles are indispensable for developing QED Monte Carlo and QED-Particle-in-Cell codes, aimed at the investigation of polarization effects in nonlinear QED processes in ultraintense laser-plasma and laser-electron beam interactions, and other nonlinear QED processes in external ultrastrong fields, which involve multiple elementary processes of a photon emission and pair production. The quantum operator method introduced by Baier and Katkov is employed for the calculation of probabilities within the quasiclassical approach and the local constant field approximation. The probabilities for the ultrarelativistic regime are given in a compact form and are suitable to describe polarization effects in strong laser fields of arbitrary configuration, rendering them very well suited for applications.

hep-ph↗

arXiv2022

All-optical ultrafast spin rotation for relativistic charged particle beams

An all-optical method of ultrafast spin rotation is put forward to precisely manipulate the polarization of relativistic charged particle beams of leptons or ions. In particular, laser-driven dense ultrashort beams are manipulated via single-shot interaction with a co-propagating moderate temporally asymmetric (frequency-chirped or subcycle THz) laser pulse. Using semi-classical numerical simulations, we find that in a temporally asymmetrical laser field, the spin rotation of a particle can be determined from the flexibly controllable phase retardation between its spin precession and momentum oscillation. An initial polarization of a proton beam can be rotated to any desired orientation (e.g., from the common transverse to the more useful longitudinal polarization) with extraordinary precision (better than 1\%) in tens of femtoseconds using a feasible frequency-chirped laser pulse. Moreover, the beam qualities, in terms of energy and angular divergence, can be significantly improved in the rotation process. This method has potential applications in various areas involving ultrafast spin manipulation, like laser-plasma, laser-nuclear and high-energy particle physics.

physics.plasm-ph↗

arXiv2022

Helicity transfer in strong laser fields via the electron anomalous magnetic moment

Electron beam longitudinal polarization during the interaction with counterpropagating circularly-polarized ultraintense laser pulses is investigated, while accounting for the anomalous magnetic moment of the electron. Although it is known that the helicity transfer from the laser photons to the electron beam is suppressed in linear and nonlinear Compton scattering processes, we show that the helicity transfer nevertheless can happen via an intermediate step of the electron radiative transverse polarization, phase-matched with the driving field, followed up by spin rotation into the longitudinal direction as induced by the anomalous magnetic moment of the electron. With spin-resolved QED Monte Carlo simulations, we demonstrate the consequent helicity transfer from laser photons to the electron beam with a degree up to 10%, along with an electron radial polarization up to 65% after multiple photon emissions in a femtosecond timescale. This effect is detectable with currently achievable laser facilities, evidencing the role of the leading QED vertex correction to the electron anomalous magnetic moment in the polarization dynamics in ultrastrong laser fields.

physics.plasm-ph↗

arXiv2021

Retrieving transient magnetic fields of ultrarelativistic laser plasma via ejected electron polarization

Interaction of an ultrastrong short laser pulse with non-prepolarized near-critical density plasma is investigated in an ultrarelativistic regime, with an emphasis on the radiative spin polarization of ejected electrons. Our particle-in-cell simulations show explicit correlations between the angle resolved electron polarization and the structure and properties of the transient quasistatic plasma magnetic field. While the magnitude of the spin signal is the indicator of the magnetic field strength created by the longitudinal electron current, the asymmetry of electron polarization is found to gauge the island-like magnetic distribution which emerges due to the transverse current induced by the laser wave front. Our studies demonstrate that the spin degree of freedom of ejected electrons could potentially serve as an efficient tool to retrieve the features of strong plasma fields.

physics.plasm-ph↗

arXiv2021

Photon polarization effects in polarized electron-positron pair production in a strong laser field

Deep understanding of photon polarization impact on pair production is essential for the efficient creation of laser driven polarized positron beams, and demands a complete description of polarization effects in strong-field QED processes. We investigate, employing fully polarization resolved Monte Carlo simulations, the correlated photon and electron (positron) polarization effects in multiphoton Breit-Wheeler pair production process during the interaction of an ultrarelativistic electron beam with a counterpropagating elliptically polarized laser pulse. We showed that the polarization of e^-e^+ pairs is degraded by 35\%, when the polarization of the intermediate photon is resolved, accompanied with an approximately 13\% decrease of the pair yield. Moreover, the polarization direction of energetic positrons in small angle region is reversed, which originates from the pair production of hard photons with polarization parallel with electric field.

physics.plasm-ph↗

arXiv2021

The role of reflections in the generation of a time delay in strong field ionization

The problem of time delay in tunneling ionization is revisited. The origin of time delay at the tunnel exit is analysed, underlining the two faces of the concept of the tunnelling time delay: the time delay around the tunnel exit and the asymptotic time delay at a detector. We show that the former time delay, in the sense of a delay in the peak of the wavefunction, exists as a matter of principle and arises due to the sub-barrier interference of the reflected and transmitted components of the tunneling electronic wavepacket. We exemplify this by describing the tunnelling ionization of an electron bound by a short-range potential within the strong field approximation in a "deep tunnelling" regime. If sub-barrier reflections are extracted from this wavefunction, then the time delay of the peak is shown to vanish. Thus, we assert that the disturbance of the tunnelling wavepacket by the reflection from the surface of the barrier causes a time delay in the neighbourhood of the tunnel exit.

physics.atom-ph↗

arXiv2021

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

Generation of arbitrarily spin-polarized lepton (here refer in particular to electron and positron) beams has been investigated in the single-shot interaction of high-energy polarized $γ$ photons with an ultraintense asymmetric laser pulse via nonlinear Breit-Wheeler (BW) pair production. We develop a fully spin-resolved semi-classical Monte Carlo method to describe the pair creation and polarization in the local constant field approximation. In nonlinear BW process the polarization of created pairs is simultaneously determined by the polarization of parent $γ$ photons, the polarization and asymmetry of scattering laser field, due to the spin angular momentum transfer and the asymmetric spin-dependent pair production probabilities, respectively. In considered all-optical method, dense GeV lepton beams with average polarization degree up to about $80\%$ (adjustable between the transverse and longitudinal components) can be obtained with currently achievable laser facilities, which could be used as injectors of the polarized $e^{+}e^{-}$ collider to search for new physics beyond the Standard Model.

physics.plasm-ph↗

arXiv2021

Generation of quasi-monoenergetic proton beams via quantum radiative compression

Dense high-energy monoenergetic proton beams are vital for wide applications, thus modern laser-plasma-based ion acceleration methods are aiming to obtain high-energy proton beams with energy spread as low as possible. In this work, we put forward a quantum radiative compression method to post-compress a highly accelerated proton beam and convert it to a dense quasi-monoenergetic one. We find that when the relativistic plasma produced by radiation pressure acceleration collides head-on with an ultraintense laser beam, large-amplitude plasma oscillations are excited due to quantum radiation-reaction and the ponderomotive force, which induce compression of the phase space of protons located in its acceleration phase with negative gradient. Our three-dimensional spin-resolved QED particle-in-cell simulations show that hollow-structure proton beams with a peak energy $\sim$ GeV, relative energy spread of few percents and number $N_p\sim10^{10}$ (or $N_p\sim 10^9$ with a $1\%$ energy spread) can be produced in near future laser facilities, which may fulfill the requirements of important applications, such as, for radiography of ultra-thick dense materials, or as injectors of hadron colliders.

physics.plasm-ph↗

arXiv2020

Construction of Dirac spinors for electron vortex beams in background electromagnetic fields

Exact solutions of the Dirac equation, a system of four partial differential equations, are rare. The vast majority of them are for highly symmetric stationary systems. Moreover, only a handful of solutions for time dependent dynamics exists. Given the growing number of applications of high energy electron beams interacting with a variety of quantum systems in laser fields, novel methods for finding exact solutions to the Dirac equation are called for. We present a method for building up solutions to the Dirac equation employing a recently introduced approach for the description of spinorial fields and their driving electromagnetic fields in terms of geometric algebras. We illustrate the method by developing several stationary as well as non-stationary solutions of the Dirac equation with well defined orbital angular momentum along the electron's propagation direction. The first set of solutions describe free electron beams in terms of Bessel functions as well as stationary solutions for both a homogeneous and an inhomogeneous magnetic field. The second set of solutions are new and involve a plane electromagnetic wave combined with a generally inhomogeneous longitudinal magnetic field. Moreover, the developed technique allows us to derive general physical properties of the dynamics in such field configurations, as well as provides physical predictions on the self-consistent electromagnetic fields induced by the dynamics.

quant-ph↗

arXiv2020

Sub-barrier pathways to Freeman resonances

The problem of Freeman resonances [R. R. Freeman \textit{et al.}, Phys. Rev. Lett. \textbf{59}, 1092 (1987)] when strong field ionization is enhanced due to the transient population of excited states during the ionization, is revisited. An intuitive model is put forward which explains the mechanism of the intermediate population of excited states during nonadiabatic tunneling ionization via the under-the-barrier recollision and recombination. The theoretical model is based on perturbative strong-field approximation (SFA), where the sub-barrier bound-continuum-bound pathway is described in the second-order SFA, while the further ionization from the excited state by an additional perturbative step. The enhancement of ionization is shown to arise due to the constructive interference of contributions into the excitation amplitudes originating from different laser cycles. The applied model provides an intuitive understanding of the electron dynamics during a Freeman resonance in strong-field ionization, as well as means of enhancing the process and possible applications to related processes.

physics.atom-ph↗

arXiv2020

Stochasticity in radiative polarization of ultrarelativistic electrons in an ultrastrong laser pulse

Stochasticity effects in the spin (de)polarization of an ultrarelativistic electron beam during photon emissions in a counterpropoagating ultrastrong focused laser pulse in the quantum radiation reaction regime are investigated. We employ a Monte Carlo method to describe the electron dynamics semiclassically, and photon emissions as well as the electron radiative polarization quantum mechanically. While in the latter the photon emission is inherently stochastic, we were able to identify its imprints in comparison with the new developed semiclassical stochasticity-free method of radiative polarization applicable in the quantum regime. With an initially spin-polarized electron beam, the stochastic spin effects are seen in the dependence of the depolarization degree on the electron scattering angle and the electron final energy (spin stochastic diffusion). With an initially unpolarized electron beam, the spin stochasticity is exhibited in enhancing the known effect of splitting of the electron beam along the propagation direction into two oppositely polarized parts by an elliptically polarized laser pulse. The considered stochasticity effects for the spin are observable with currently achievable laser and electron beam parameters.

physics.plasm-ph↗

arXiv2020

High-energy gamma-photon polarization in nonlinear Breit-Wheeler pair production and gamma-polarimetry

The interaction of an unpolarized electron beam with a counterpropagating ultraintense linearly polarized laser pulse is investigated in the quantum radiation-dominated regime. We employ a semiclassical Monte Carlo method to describe spin-resolved electron dynamics, photon emissions and polarization, and pair production. Abundant high-energy linearly polarized gamma photons are generated intermediately during this interaction via nonlinear Compton scattering, with an average polarization degree of more than 50%, which further interacting with the laser fields produce electron-positron pairs due to nonlinear Breit-Wheeler process. The photon polarization is shown to significantly affect the pair yield by a factor beyond 10%. The considered signature of the photon polarization in the pair's yield can be experimentally identified in a prospective two-stage setup. Moreover, the signature can serve also for the polarimetry of high-energy high-flux gamma photons with a resolution well below 1% with currently achievable laser facilities.

physics.plasm-ph↗

arXiv2019

Polarized ultrashort brilliant multi-GeV $γ$-rays via single-shot laser-electron interaction

Generation of circularly-polarized (CP) and linearly-polarized (LP) $γ$-rays via the single-shot interaction of an ultraintense laser pulse with a spin-polarized counterpropagating ultrarelativistic electron beam has been investigated in nonlinear Compton scattering in the quantum radiation-dominated regime. For the process simulation a Monte Carlo method is developed which employs the electron-spin-resolved probabilities for polarized photon emissions. We show efficient ways for the transfer of the electron polarization to the high-energy photon polarization. In particular, multi-GeV CP (LP) $γ$-rays with polarization of up to about 95\% can be generated by a longitudinally (transversely) spin-polarized electron beam, with a photon flux at a single shot meeting the requirements of recent proposals for the vacuum birefringence measurement in ultrastrong laser fields. Such high-energy, high-brilliance, high-polarization $γ$-rays are also beneficial for other applications in high-energy physics, nuclear physics, and laboratory astrophysics.

physics.plasm-ph↗