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.
It is demonstrated that targets with a broken rotational symmetry may facilitate generation of a strong axial (poloidal) magnetic field. An intense laser beam irradiating such a target creates intense electron currents carrying vorticity and producing strong spontaneous magnetic fields. Combined with a laser electron acceleration, such targets may be used for generation and guiding of magnetized, collimated particle or plasma beams.
Three dimensional FLASH magneto-hydrodynamics(MHD) modeling is carried out to interpret the OMEGA laser experiments of strongly magnetized, highly collimated jets driven by a ring of 20 OMEGA beams. The predicted optical Thomson scattering spectra and proton images are in good agreement with a subset of the experimental data. Magnetic fields generated via the Biermann battery term are amplified at the boundary between the core and the surrounding of the jet. The simulation predicts multiple axially aligned magnetic flux ropes with alternating poloidal component. Future applications of the holl…
A numerical investigation on the spatiotemporal evolution of an electron beam, externally injected in a plasma in the presence of a plasma wake field, is carried out. The latter is driven by an ultra-short relativistic axially-symmetric femtosecond electron bunch. We first derive a novel Poisson-like equation for the wake potential where the driving term is the ultra-short bunch density, taking suitably into account the interplay between the sharpness and high energy of the bunch. Then, we show that a channel is formed longitudinally, through the externally injected beam while experiencing the…
Using two-dimensional (2D) and three-dimensional (3D) kinetic simulations, we examine the impact of simulation dimensionality on the laser-driven electron acceleration and the emission of collimated $γ$-ray beams from hollow micro-channel targets. We demonstrate that the dimensionality of the simulations considerably influences the results of electron acceleration and photon generation owing to the variation of laser phase velocity in different geometries. In a 3D simulation with a cylindrical geometry, the acceleration process of electrons terminates early due to the higher phase velocity of …
Based on the already existing analytical theory of the strongly-nonlinear wakefield (which is called "bubble") in transversely inhomogeneous plasmas, we study particular behavior of non-loaded (empty) bubbles and bubbles with accelerated bunches. We obtain an analytical expression for the shape of a non-loaded bubble in a general case and verify it with particle-in-cell (PIC) simulations. We derive a method of calculation of the acceleration efficiency for arbitrary accelerated bunches. The influence of flat-top electron bunches on the shape of a bubble is studied. It is also shown that it is …
Plasma wakefield acceleration in the blowout regime is particularly promising for high-energy acceleration of electron beams because of its potential to simultaneously provide large acceleration gradients and high energy transfer efficiency while maintaining excellent beam quality. However, no equivalent regime for positron acceleration in plasma wakes has been discovered to-date. We show that after a short propagation distance, an asymmetric electron beam drives a stable wakefield in a hollow plasma channel that can be both accelerating and focusing for a positron beam. A high charge positron…
A scheme to generate highly collimated $γ$-ray pulse is proposed for the production of muon and electron pairs in $γ-γ$ collider. The $γ$-ray pulse, with high conversion efficiency, can be produced as the result of electron phase-locked acceleration in longitudinal electric field through the interaction between an ultra-intense laser pulse and a narrow tube target. Numerical simulation shows that 18\% energy of a 10-PW laser pulse is transferred into the forward $γ$-rays in a divergence angle less than $ 3^\circ$. The $γ$-ray pulse is applied in $γ-γ$ collider, in which muon pairs can be produ…
A scheme of wake field generation for positron acceleration using hollow or donut shaped electron driver beams is studied. An annular shaped, electron free region forms around a hollow driver beam creating a favorable region (longitudinal field is accelerating and transverse field is focusing and radially linear) for positron acceleration. Accelerating gradients of the order of 10 GV/m are produced by a hollow electron beam driver with FACET like parameters. The peak accelerating field increases linearly with the total charge in the beam driver while the axial size of the favorable region ($\s…
Plasma-based electron and positron wakefield acceleration has made great strides in the past decade. However one major challenge for its applications to coherent light sources and colliders is the relatively large energy spread of the accelerated beams, currently at a few percent level. This energy spread is usually correlated with particle position in the beam arising from the longitudinal chirp of the wakefield amplitude. Therefore a dechirper is highly desirable for reducing this spread down to $\sim0.1\%$ level, while at the same time for maintaining the emittance of the accelerated beam. …
With the development of ultra intense laser technology, MeV ions from the laser foil interaction have been obtained by different mechanisms, such as target normal sheath acceleration, radiation pressure acceleration, collisionless shock acceleration, breakout afterburner, and a combination of different mechanisms. These energetic ion beams can be applied in fast ignition for inertial confinement fusion, medical therapy, and proton imaging. However, these ions are mainly accelerated in the laser propagation direction, and the ion acceleration in an azimuthal orientation is scarcely mentioned. H…
Supersonic plasma outflows driven by multi-beam, high-energy lasers, such as Omega and NIF, have been and will be used as platforms for a variety of laboratory astrophysics experiments. Here we propose a new way of launching high density and high velocity, plasma jets using multiple intense laser beams in a hollow ring formation. We show that such jets provide a more flexible and versatile platform for future laboratory astrophysics experiments. Using high resolution hydrodynamic simulations, we demonstrate that the collimated jets can achieve much higher density, temperature and velocity when…
It is shown that well collimated mono-energetic ion beams with a large particle number can be generated in the hole-boring radiation pressure acceleration regime by using an elliptically polarized laser pulse with appropriate theoretically determined laser polarization ratio. Due to the $\bm{J}\times\bm{B}$ effect, the double-layer charge separation region is imbued with hot electrons that prevent ion pileup, thus suppressing the double-layer oscillations. The proposed mechanism is well confirmed by Particle-in-Cell simulations, and after suppressing the longitudinal double-layer oscillations,…
Using two-dimensional particle-in-cell simulations, we examine how an externally applied strong magnetic impacts proton acceleration in laser-irradiated solid-density targets. We find that a kT-level external magnetic field can sufficiently inhibit transverse transport of hot electrons in a flat laser-irradiated target. While the electron heating by the laser remains mostly unaffected, the reduced electron transport during proton acceleration leads to an enhancement of maximum proton energies and the overall number of energetic protons. The resulting proton beam is much better collimated compa…
Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed. The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was directional along the laser propagation path, with the highest energy ions being collimated to a cone of less than 10 degrees. 2D particle-in-cell simulations have been used to investigate the acceleration mechanism. The time varying magnetic field associated with the fast electron current provides a contribution to the accelerating electric field as well as providing …
The production of a highly-polarized positron beam via nonlinear Breit-Wheeler processes during the interaction of an ultraintense circularly polarized laser pulse with a longitudinally spin-polarized ultrarelativistic electron beam is investigated theoretically. A new Monte Carlo method employing fully spin-resolved quantum probabilities is developed under the local constant field approximation to include three-dimensional polarizations effects in strong laser fields. The produced positrons are longitudinally polarized through polarization transferred from the polarized electrons by the mediu…
We propose a simple laser-driven electron acceleration scheme based on tightly focused radially polarized laser pulses for the production of femtosecond electron bunches with energies in the few-hundreds-of-keV range. In this method, the electrons are accelerated forward in the focal volume by the longitudinal electric field component of the laser pulse. Three-dimensional test-particle and particle-in-cell simulations reveal the feasibility of generating well-collimated electron bunches with an energy spread of 5% and a temporal duration of the order of 1 fs. These results offer a route toward…
Mega-Gauss Plasma Jet Creation Using a Ring of Laser Beams
Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）；Rice University（莱斯大学）；Laboratory for Laser Energetics（激光能学实验室）；Massachusetts Institute of Technology（麻省理工学院）；University of Chicago（芝加哥大学）；General Atomics（通用原子能公司）
A new electron acceleration mechanism is identified that develops when a relativistically intense laser irradiates the wedge of an over-dense plasma. This induces a diffracted electromagnetic wave with a significant longitudinal electric field that accelerates electrons from the plasma over long distances to relativistic energies. Well collimated, highly-charged (nC) electron bunches with energies up to 100's MeV are obtained using a laser beam with $I λ_0^2 =3.5\times 10^{19}\,{\rm W μm^2/cm^2}$. Multi-dimensional particle-in-cell simulations, supported by a simple analytical model, confirm t…
We report on experimental studies of divergence of proton beams from nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser with high contrast. Proton beams with extremely small divergence (half angle) of 2 degree are observed in addition with a remarkably well-collimated feature over the whole energy range, showing one order of magnitude reduction of the divergence angle in comparison to the results from micrometer thick targets. We demonstrate that this reduction arises from a steep longitudinal electron density gradient and an exponentially decaying transverse profil…
We study hollow plasma channels with smooth boundaries for laser-driven electron acceleration in the bubble regime. Contrary to the uniform plasma case, the laser forms no optical shock and no etching at the front. This increases the effective bubble phase velocity and energy gain. The longitudinal field has a plateau that allows for mono-energetic acceleration. We observe as low as 10^{-3} r.m.s. relative witness beam energy uncertainty in each cross-section and 0.3% total energy spread. By varying plasma density profile inside a deep channel, the bubble fields can be adjusted to balance the …
A new geometrical perturbation scheme is developed in order to calculate the electromagnetic fields produced by charged sources in prescribed motion moving in a non-straight perfectly conducting beam pipe. The pipe is regarded as a perturbed infinitely long hollow right-circular cylinder. The perturbation maintains the pipe's circular cross-section while deforming its axis into a planar space-curve with, in general, non-constant curvature. Various charged source models are considered including a charged bunch and an off-axis point particle. In the ultra-relativistic limit this permits a calcul…
Using fully three-dimensional particle-in-cell simulations, we show that readily available femtosecond laser systems can stably generate proton beams with hundred MeV energy and low spread at $\sim1\%$ level by parallel irradiation of a tens of micrometers long plasma plate. As the laser pulse sweeps along the plate, it drags out a huge charge ($\sim$100 nC) of collimated energetic electrons and accelerates them along the plate surface to superponderomotive energies. When this dense electron current arrives at the rear end of the plate, it induces a strong electrostatic field. Due to the exces…
Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocity-space polar angle of 52 degrees). Transport simulations with the PIC-based diverge…
We examine the impact of the ion dynamics on laser-driven electron acceleration in an initially empty channel irradiated by an ultra-high intensity laser pulse with $I > 10^{22}$ W/cm$^2$. The negative charge of the accelerated electrons inside the channel generates a quasi-static transverse electric field that causes gradual ion expansion into the channel. Once the ions fill the channel, the pinching force from the quasi-static magnetic field generated by the accelerated electrons becomes uncompensated due to the reduction of the quasi-static transverse electric field. As a result there are t…
A solid density target irradiated by a high-intensity laser pulse can become relativistically transparent, which then allows it to sustain an extremely strong laser-driven longitudinal electron current. The current generates a filament with a slowly-varying MT-level azimuthal magnetic field that has been shown to prompt efficient emission of multi-MeV photons in the form of a collimated beam required for multiple applications. This work examines the feasibility of using an x-ray beam from the European XFEL for the detection of the magnetic field via the Faraday rotation. Post-processed 3D part…
