arXiv subjects

A. V. Arefiev

Publications and source records attributed to A. V. Arefiev.

34 records · Page 2Linked to original sources

arXiv2019

Forward sliding-swing acceleration: electron acceleration by high-intensity lasers in strong plasma magnetic fields

A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasi-static Mega Tesla-level azimuthal magnetic field. The transverse laser field efficiently accelerates electrons in the presence of such a field that confines the transverse motion and deflects the electrons in the forward direction, establishing the novel forward-sliding swing acceleration mechanism. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electrons' dynamics by a simplified model analytically, specifically deriving simple relations between the current, the particles' initial transverse momenta and the laser's field strength classifying the energy gain. We confirm the model's predictions by numerical simulations, indicating Mega ampere-level threshold currents and energy gains two orders of magnitude higher than achievable without the magnetic field.

physics.plasm-ph↗

arXiv2018

High-angle Deflection of the Energetic Electrons by a Voluminous Magnetic Structure in Near-normal Intense Laser-plasma Interactions

The physics governing electron acceleration by a relativistically intense laser are not confined to the critical density surface, they also pervade the sub-critical plasma in front of the target. Here, particles can gain many times the ponderomotive energy from the overlying laser, and strong fields can grow. Experiments using a high contrast laser and a prescribed laser pre-pulse demonstrate that development of the pre-plasma has an unexpectedly strong effect on the most energetic, super-ponderomotive electrons. Presented 2D particle-in-cell simulations reveal how strong, voluminous magnetic structures that evolve in the pre-plasma impact high energy electrons more significantly than low energy ones for longer pulse durations and how the common practice of tilting the target to a modest incidence angle can be enough to initiate strong deflection. The implications are that multiple angular spectral measurements are necessary to prevent misleading conclusions from past and future experiments.

physics.plasm-ph↗

arXiv2018

The Unexpected Role of Evolving Longitudinal Electric Fields in Generating Energetic Electrons in Relativistically Transparent Plasmas

Superponderomotive-energy electrons are observed experimentally from the interaction of an intense laser pulse with a relativistically transparent target. For a relativistically transparent target, kinetic modeling shows that the generation of energetic electrons is dominated by energy transfer within the main, classically overdense, plasma volume. The laser pulse produces a narrowing, funnel-like channel inside the plasma volume that generates a field structure responsible for the electron heating. The field structure combines a slowly evolving azimuthal magnetic field, generated by a strong laser-driven longitudinal electron current, and, unexpectedly, a strong propagating longitudinal electric field, generated by reflections off the walls of the funnel-like channel. The magnetic field assists electron heating by the transverse electric field of the laser pulse through deflections, whereas the longitudinal electric field directly accelerates the electrons in the forward direction. The longitudinal electric field produced by reflections is 30 times stronger than that in the incoming laser beam and the resulting direct laser acceleration contributes roughly one third of the energy transferred by the transverse electric field of the laser pulse to electrons of the super-ponderomotive tail.

physics.plasm-ph↗

arXiv2018

Highly collimated electron acceleration by longitudinal laser fields in a hollow-core target

The substantial angular divergence of electron beams produced by direct laser acceleration is often considered as an inherent negative feature of the mechanism. The divergence however arises primarily because the standard approach relies on transverse electron oscillations and their interplay with the transverse electric fields of the laser pulse. We propose a conceptually different approach to direct laser acceleration that leverages longitudinal laser electric fields that are present in a tightly focused laser beam. A structured hollow-core target is used to enhance the longitudinal fields and maintain them over a distance much longer than the Rayleigh length by guiding the laser pulse. Electrons are injected by the transverse laser electric field into the channel and then they are accelerated forward by the pulse, creating an electron current. The forces from electric and magnetic fields of this electron population compensate each other, creating a favorable configuration without a strong restoring force. We use two-dimensional particle-in-cell simulations to demonstrate that a low divergence energetic electron beam with an opening angle of less than 5$^\circ$ can be generated in this configuration. Most of the energy is transferred to the electrons by the longitudinal laser electric field and, given a sufficient acceleration distance, super-ponderomotive energies can be realized without sacrificing the collimation.

physics.plasm-ph↗

arXiv2017

Laser-driven strong magnetostatic fields with applications to charged beam transport and magnetized high energy-density physics

Powerful laser-plasma processes are explored to generate discharge currents of a few $100\,$kA in coil targets, yielding magnetostatic fields (B-fields) in excess of $0.5\,$kT. The quasi-static currents are provided from hot electron ejection from the laser-irradiated surface. According to our model, describing qualitatively the evolution of the discharge current, the major control parameter is the laser irradiance $I_{\mathrm{las}}λ_{\mathrm{las}}^2$. The space-time evolution of the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and by proton-deflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targets through resistive diffusion. We applied it in experiments of laser-generated relativistic electron transport into solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at $60 \,\mathrm{μm}$ depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetized high-energy density physics investigations, related to laser-generated secondary sources of radiation and/or high-energy particles and their transport, to high-gain fusion energy schemes and to laboratory astrophysics.

physics.plasm-ph↗

arXiv2017

Parametric decay of plasma waves near the upper-hybrid resonance

An intense X wave propagating perpendicularly to dc magnetic field is unstable with respect to a parametric decay into an electron Bernstein wave and a lower-hybrid wave. A modified theory of this effect is proposed that extends to the high-intensity regime, where the instability rate $γ$ ceases to be a linear function of the incident-wave amplitude. An explicit formula for $γ$ is derived and expressed in terms of cold-plasma parameters. Theory predictions are in reasonable agreement with the results of the particle-in-cell simulations presented in a separate publication.

physics.plasm-ph↗

arXiv2016

Kinetic simulations of X-B and O-X-B mode conversion and its deterioration at high input power

Spherical tokamak plasmas are typically overdense and thus inaccessible to externally-injected microwaves in the electron cyclotron range. The electrostatic electron Bernstein wave (EBW), however, provides a method to access the plasma core for heating and diagnostic purposes. Understanding the details of the coupling process to electromagnetic waves is thus important both for the interpretation of microwave diagnostic data and for assessing the feasibility of EBW heating and current drive. While the coupling is reasonably well-understood in the linear regime, nonlinear physics arising from high input power has not been previously quantified. To tackle this problem, we have performed one- and two-dimensional fully kinetic particle-in-cell simulations of the two possible coupling mechanisms, namely X-B and O-X-B mode conversion. We find that the ion dynamics has a profound effect on the field structure in the nonlinear regime, as high amplitude short-scale oscillations of the longitudinal electric field are excited in the region below the high-density cut-off prior to the arrival of the EBW. We identify this effect as the instability of the X wave with respect to resonant scattering into an EBW and a lower-hybrid wave. We calculate the instability rate analytically and find this basic theory to be in reasonable agreement with our simulation results.

physics.plasm-ph↗

arXiv2016

Beyond the ponderomotive limit: direct laser acceleration of relativistic electrons in sub-critical plasmas

We examine a regime in which a linearly-polarized laser pulse with relativistic intensity irradiates a sub-critical plasma for much longer than the characteristic electron response time. A steady-state channel is formed in the plasma in this case with quasi-static transverse and longitudinal electric fields. These relatively weak fields significantly alter the electron dynamics. The longitudinal electric field reduces the longitudinal dephasing between the electron and the wave, leading to an enhancement of the electron energy gain from the pulse. The energy gain in this regime is ultimately limited by the superluminosity of the wave fronts induced by the plasma in the channel. The transverse electric field alters the oscillations of the transverse electron velocity, allowing it to remain anti-parallel to laser electric field and leading to a significant energy gain. The energy enhancement is accompanied by development of significant oscillations perpendicular to the plane of the driven motion, making trajectories of energetic electrons three-dimensional. Proper electron injection into the laser beam can further boost the electron energy gain.

physics.plasm-ph↗

arXiv2015

Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

The irradiation of few nm thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse. The targets decompress to near and lower than critical densities plasmas extending over few micrometers, i.e. multiple wavelengths. The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam. Experiments at the GHOST laser system at UT Austin using such targets measured non-Maxwellian, peaked electron distribution with large bunch charge and high electron density in the laser propagation direction. These results are reproduced in 2D PIC simulations using the EPOCH code, identifying Direct Laser Acceleration (DLA) as the responsible mechanism. This is the first time that DLA has been observed to produce peaked spectra as opposed to broad, maxwellian spectra observed in earlier experiments. This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.

physics.plasm-ph↗

arXiv2015

Highly-efficient source of collimated multi-MeV photons driven by radiation reaction of an electron beam in a self-generated magnetic field

The rapid development of high brilliance X-ray radiation sources is revolutionizing physics, chemistry, and biology research through their novel applications. Another breakthrough is anticipated with the construction of next-generation laser facilities which will operate at intensities beyond $10^{23}$ $\mathrm{W/cm^2}$, leading to higher yield, shorter wavelength radiation sources. We use numerical simulations to demonstrate that a source of collimated multi-MeV photons with conversion efficiency comparable to the one expected for these facilities is achievable at an order of magnitude lower in intensity, within reach of the existing facilities. In the optimal setup, the laser pulse irradiates a bulk solid-density target, heating the target electrons and inducing relativistic transparency. As the pulse then propagates, it generates a beam of energetic electrons which in turn drives a strong azimuthal magnetic field. This field significantly enhances the radiation reaction for the electrons, yielding tens of TW of directed MeV photons for a PW-class laser.

physics.plasm-ph↗

arXiv2015

Spontaneous emergence of non-planar electron orbits during direct laser acceleration by a linearly polarized laser pulse

An electron irradiated by a linearly polarized relativistic intensity laser pulse in a cylindrical plasma channel can gain significant energy from the pulse. The laser electric and magnetic fields drive electron oscillations in a plane making it natural to expect the electron trajectory to be flat. We show that strong modulations of the relativistic $γ$-factor associated with the energy enhancement cause the free oscillations perpendicular to the plane of the driven motion to become unstable. As a consequence, out of plane displacements grow to become comparable to the amplitude of the driven oscillations and the electron trajectory becomes essentially three-dimensional, even if at an early stage of the acceleration it was flat. The development of the instability profoundly affects the x-ray emission, causing considerable divergence of the radiation perpendicular to the plane of the driven oscillations, while also reducing the overall emitted energy.

physics.plasm-ph↗

arXiv2015

Kinetic simulations of X-B and O-X-B mode conversion

We have performed fully-kinetic simulations of X-B and O-X-B mode conversion in one and two dimensional setups using the PIC code EPOCH. We have recovered the linear dispersion relation for electron Bernstein waves by employing relatively low amplitude incoming waves. The setups presented here can be used to study non-linear regimes of X-B and O-X-B mode conversion.

physics.plasm-ph↗

arXiv2015

Novel Aspects of Direct Laser Acceleration of Relativistic Electrons

We examine the impact of several factors on electron acceleration by a laser pulse and the resulting electron energy gain. Specifically, we consider the role played by: 1) static longitudinal electric field; 2) static transverse electric field; 3) electron injection into the laser pulse; and 4) static longitudinal magnetic field. It is shown that all of these factors lead, under certain conditions, to a considerable electron energy gain from the laser pulse. In contrast with other mechanisms such as wakefield acceleration, the static electric fields in this case do not directly transfer substantial energy to the electron. Instead, they reduce the longitudinal dephasing between the electron and the laser beam, which then allows the electron to gain extra energy from the beam. The mechanisms discussed here are relevant to experiments with under-dense gas jets, as well as to experiments with solid-density targets involving an extended pre-plasma.

physics.plasm-ph↗

arXiv2014

New mechanisms for generating super-ponderomotive electrons in laser-irradiated targets

It is shown that static longitudinal and transverse electric fields can significantly alter electron acceleration by a long laser beam in a sub-critical plasma, enabling generation of super-ponderomotive electrons. The role of the plasma fields in this regime is not to directly transfer substantial energy to the electron, but rather to reduce the axial dephasing rate between the electron and the laser beam. The reduced dephasing in both cases leads to a subsequent enhancement of the axial momentum and total electron energy. These mechanisms can be relevant to experiments with solid-density targets where a sub-critical plasma layer occurs as a result of a considerable prepulse.

physics.plasm-ph↗

arXiv2013

Observation of Self-Sustaining Relativistic Ionization Wave Launched by Sheath Field

We present experimental evidence supported by simulations of a relativistic ionization wave launched into surrounding gas by the sheath field of a plasma filament with high energy electrons. Such filament is created by irradiating a clustering gas jet with a short pulse laser ($\sim$115 fs) at a peak intensity of $5 \times 10^{17}$ W/cm$^2$. We observe an ionization wave propagating radially through the gas for about 2 ps at 0.2-0.5 $c$ after the laser has passed, doubling the initial radius of the filament. The gas is ionized by the sheath field, while the longevity of the wave is explained by a moving field structure that traps the high energy electrons near the boundary, maintaining a strong sheath field despite the significant expansion of the plasma.

physics.plasm-ph↗

arXiv2013

Generation of 'Super-Ponderomotive' Electrons due to a non-Wakefield interaction between a Laser Pulse and a Longitudinal Electric Field

It is shown that electrons with momenta exceeding the `free electron' limit of $m_eca_0^2/2$ can be produced when a laser pulse and a longitudinal electric field interact with an electron via a non-wakefield mechanism. The mechanism consists of two stages: the reduction of the electron dephasing rate $γ-p_x/m_ec$ by an accelerating region of electric field and electron acceleration by the laser via the Lorentz force. This mechanism can, in principle, produce electrons that have longtudinal momenta that is a significant multiple of $m_eca_0^2/2$. 2D PIC simulations of a relatively simple laser-plasma interaction indicate that the generation of super-ponderomotive electrons is strongly affected by this `anti-dephasing' mechanism.

physics.plasm-ph↗