Laser Acceleration Research Articles

Extreme nonlinear dynamics in vacuum laser acceleration with a crossed beam configuration

A relatively simple model problem where a single electron moves in two relativistically strong obliquely intersecting plane wave-packets is studied using a number of different numerical solvers. It is shown that, in general, even the most advanced solvers are unable to obtain converged solutions for more than about 100 fs in contrast to the single plane wave problem, and that some basic metrics of the orbit show enormous sensitivity to the initial conditions. At a bare minimum, this indicates an unusual degree of nonlinearity, and may well indicate that the dynamics of this system are chaotic.

Effects of hole-boring and relativistic transparency on particle acceleration in overdense plasma irradiated by short multi-PW laser pulses

Propagation of short and ultraintense laser pulses in a semi-infinite space of overdense hydrogen plasma is analyzed via fully relativistic, real geometry particle-in-cell (PIC) simulations including radiation friction. The relativistic transparency and hole-boring regimes are found to be sensitive to the transverse plasma field, backward light reflection, and laser pulse filamentation. For laser intensities approaching I ∼ 1024 W/cm2, the direct laser acceleration of protons, along with ion Coulomb explosion, results in their injection into the acceleration phase of the compressed electron wave at the front of the laser pulses. The protons are observed to be accelerated up to 10–20 GeV with densities around a few times the critical density. The effect qualitatively depends on initial density and laser intensity, disappearing with the initial density increase or intensity decrease.

Science of High Energy, Single-Cycled Lasers

With the advent of the Thin Film Compression, high energy single-cycled laser pulses have become an eminent path to the future of new high-field science. An existing CPA high power laser pulse such as a commercially available PW laser may be readily converted into a single-cycled laser pulse in the 10PW regime without losing much energy through the compression. We examine some of the scientific applications of this, such as laser ion accelerator called single-cycle laser acceleration (SCLA) and bow wake electron acceleration. Further, such a single-cycled laser pulse may be readily converted through relativistic compression into a single-cycled, X-ray laser pulse. We see that this is the quickest and very innovative way to ascend to the EW (exawatt) and zs (zeptosecond) science and technology. We suggest that such X-ray laser pulses have a broad and new horizon of applications. We have begun exploring the X-ray crystal (or nanostructured) wakefield accelerator and its broad and new applications into gamma rays. Here, we make a brief sketch of our survey of this vista of the new developments.

Direct laser acceleration of electrons in the plasma bubble by tightly focused laser pulses

We present an analytical theory that reveals the importance of the longitudinal laser electric field in the course of the resonant acceleration of relativistic electrons by a tightly confined laser beam. It is shown that this laser field component always counteracts the transverse one and effectively decreases the final energy gain of electrons via the direct laser acceleration (DLA) mechanism. This effect is demonstrated by carrying out particle-in-cell simulations of the DLA of the electrons injected into the accelerating phase of the plasma wake. It is shown that the electron energy gain from the wakefield is substantially compensated by the quasiresonant energy loss to the longitudinal laser field component. The analytically obtained scalings and estimates are in good agreement with the results of the numerical simulations.

Numerical investigations into a THz-driven dielectric accelerator with a Bragg reflector

Abstract Using THz pulses to generate an accelerating field in a dielectric loaded accelerator structure is a promising way to mitigate the disadvantages of RF and Laser accelerator structures. This paper proposes an energy efficient side-coupling dielectric single-grating structure accompanied with a multi-layer Bragg reflector that is designed for THz pulse operation. To show the comparative advantages of the new design, we also numerically investigate a single-side coupling THz-driven dual-grating structure which is composed of Silicon Dioxide (SiO2). Simulation results show that the reflector structure effectively manipulates the THz field compared to dual-grating structure, not only boosts the field strength in the accelerating channel, but also optimizes the field distribution and generates a periodic field reversal to achieve better phase synchronicity for relativistic particles, thereby increasing the accelerating gradient by more than 40%. It is also found that the energy-recycling capability of a Bragg reflector highly depends on the operating THz pulse duration and beam channel width. Choosing a longer pulse duration and setting a narrower beam channel width with an optimum pillar height can increase the single pulse efficiency by more than 70% compared to the bare dual-grating structure.

Ultrashort Optical Pulses and Their Generation in Resonant Media (Scientific Summary)

Our recent studies of methods for obtaining ultrashort light pulses and analysis of the impact of ultrashort optical pulses on classical and quantum micro-objects with the identification of the determining role of the degree of unipolarity of pulses (maximum for strictly unipolar pulses) in the efficiency of direct laser acceleration of charged particles and excitation of atoms and molecules have been reviewed. Special attention has been paid to the coherent mode locking regime in lasers, which is implemented when the laser pulse duration is less than the relaxation time of the working transition in media with resonance amplification and absorption (laser analog of the phenomenon of self-induced transparency). Experimental data on coherent mode locking in a titanium-sapphire laser with a cell with rubidium vapor at self-induced transparency in rubidium have been presented. The interaction of ultrashort pulses in resonant media has also been analyzed.

Reconfigurable Photonic Circuit for Controlled Power Delivery to Laser-Driven Accelerators on a Chip

Dielectric laser acceleration (DLA) represents a promising approach to building miniature particle accelerators on a chip. However, similar to conventional RF accelerators, an automatic and reconfigurable control mechanism is needed to scale DLA technology towards high energy gains and practical applications. We present a system providing control of the laser coupling to DLA using integrated optics and introduce a novel component for power distribution using a reconfigurable mesh of Mach-Zehnder interferometers. We show how such a mesh may be sequentially and efficiently tuned to optimize power distribution in the circuit and find that this strategy has favorable scaling properties with respect to size of the mesh.

Well collimated MeV electron beam generation in the plasma channel from relativistic laser-solid interaction

Efficient direct electron acceleration in the plasma channel with injection through the breaking of plasma waves generated by parametric instabilities was demonstrated experimentally and reproduced in the 2D3V PIC simulations. The electron bunch was produced using the specific plasma profile containing arbitrary sharp, ∼0.5λ, gradient at the vicinity of 0.1–0.5 critical density and a long tail of a tenuous preplasma. Such a preplasma profile was formed by an additional nanosecond laser pulse with intensity of 5 × 1012 W cm−2. In the case of optimal preplasma parameters femtosecond laser pulse with an intensity of 5 × 1018 W cm−2 and an energy of 50 mJ generates a collimated electron bunch having divergence of 50 mrad, exponential spectrum with the slope of ∼2 MeV and charge of tens of pC. The charge was confirmed measuring neutron yield from Be(g, n) photonuclear reaction with threshold of 1.7 MeV. By the contrast, a ring-like electron beam with divergency of 300 mrad and significantly lower charge is generated if the prepulse intensity drops to 5 × 1011 W cm−2. The 2D PIC simulations confirmed beamed electron’s acceleration in the plasma channel (so-called direct laser acceleration). This channel is formed in a long tail of teneous preplasma by the laser pulse specularly reflected from the arbitrary sharp gradient. The ring-like electron beam was attributed to the longer gradient case enlarging divergence of the reflected laser beam, preventing channel’s formation and electron acceleration by the so-called vacuum laser acceleration, or VLA. We also showed that injected electrons appeared from the wave breaking of plasma waves of hybrid SRS-TPD instability for the both gradients. Electrons received an initial momentum from this breaking to be effectively injected into the plasma channel.

Some consequences of theories with maximal acceleration in laser–plasma acceleration

In this paper, we consider classical electrodynamic theories with maximal acceleration and some of their phenomenological consequences for laser–plasma acceleration. It is shown that in a recently proposed higher-order jet theory of electrodynamics, the maximal effective acceleration reachable by a consistent bunch of point-charged particles being accelerated by the wakefield is damped for bunches containing large number of charged particles. We argue that such a prediction of the theory is falsifiable. In the case of Born–Infeld kinematics, laser–plasma acceleration phenomenology provides an upper bound for the Born–Infeld parameter b. Improvements in the beam qualities will imply stronger constraints on b.

Anomalous Photon-Induced Near-Field Electron Microscopy.

We reveal the classical and quantum regimes of free electron interaction with radiation, common to the general variety of radiation sources (e.g., a Smith-Purcell radiation), the dielectric laser accelerator, and photo-induced near-field electron microscopy (PINEM). Modeling the electron with initial conditions of a coherent quantum electron wave packet, its topology in phase space uniquely defines a universal distinction of three interaction regimes: point-particle-like acceleration, a quantum wave function (PINEM), and a newly reported regime of anomalous PINEM (APINEM). The quantum interference beat of APINEM is capable of improving the spectral resolution of postselective electron microscopy. The particle-wave duality transition between regimes reveals the history-dependent nature of quantum electron interaction with light.

Tens GeV positron generation and acceleration in a compact plasma channel

We propose a compact scheme for high-density and high-energy positron generation. In the scheme, the generation, compression and acceleration of positrons are encompassed in a plasma channel irradiated by an ultra intense laser. In about 20 μm, positrons are accelerated with their energy beyond 20 GeV, and are compressed with a maximal density of 4.1nc. The obtained positron beam with a wave-like density profile is accelerated via the direct laser acceleration.

Enhanced ion acceleration in the ultra-intense laser driven magnetized collisionless shocks

The effect of an externally applied magnetic field on the ion acceleration by laser-driven collisionless shocks is examined by means of multi-dimensional particle-in-cell simulations. For the interaction of ultra-intense sub-picosecond laser pulses with the near-relativistic critical-density plasma, the longitudinal transport of the laser generated fast electrons are significantly inhibited by the kilo-Tesla (kT) level transverse magnetic field, resulting in a thermal pressure which significantly exceeds the laser radiation pressure in the hot electron accumulation region. As a result, the accumulated plasma expands into the vacuum and leads to acceleration of a supersonic plasma flow in the opposite direction through the rocket effect, which streams into the target and drives a supercritical magnetized collisionless shock. In comparison with the case without the external magnetic field, where an electrostatic collisionless shock can be driven, the energy flux of the shock accelerated quasi-monoenergetic ion beam is considerably increased by an order of magnitude due to the strength enhancement of the magnetized shock.

Enhanced photoneutron production by intense picoseconds laser interacting with gas-solid hybrid targets

Experiments to enhance photoneutron production from the Ta (γ,xn) reaction with 100 TW picosecond laser pulses irradiating on gas-solid hybrid targets have been performed on the XingGuangIII laser facility at the Laser Fusion Research Center (LFRC) in Mianyang. The so-called gas-solid hybrid target was composed of a 1 mm thick N2 gas jet and a 2 cm thick Ta block. Picosecond laser pulses with intensities up to 1019 W/cm2 first interact with the tenuous gas to enhance the yields of high energy electrons through the direct laser acceleration (DLA) mechanism, and then through bremsstrahlung and subsequent (γ, n) reactions in a Ta converter, photoneutrons were enhanced effectively and the total number of neutrons was optimized by changing the gas density. A maximum neutron yield of 4 × 107/shot over 4π was achieved which is 200 times higher than that in the direct laser-solid interaction shot in our experiment. The spectrum of photoneutrons was measured which is in agreement with Monte Carlo (MC) simulation. From the size of the reaction area and the neutron pulse duration inferred from simulation, the corresponding flux was calculated to be 1.2 × 1016 n/cm2/s.

Dielectric laser electron acceleration in a dual pillar grating with a distributed Bragg reflector.

We report on the efficacy of a novel design for dielectric laser accelerators by adding a distributed Bragg reflector (DBR) to a dual pillar grating accelerating structure. This mimics a double-sided laser illumination, resulting in an enhanced longitudinal electric field while reducing the deflecting transverse effects when compared to single-sided illumination. We improve the coupling efficiency of the incident electric field into the accelerating mode by 57%. The 12μm long structures accelerate sub-relativistic 28keV electrons with gradients of up to 200 MeV/m in theory and 133 MeV/m in practice. This Letter shows how lithographically produced nano-structures help to make novel laser accelerators more efficient.

Relativistic laser driven electron accelerator using micro-channel plasma targets

We present an experimental demonstration of the efficient acceleration of electrons beyond 60 MeV using micro-channel plasma targets. We employed a high-contrast, 2.5 J, 32 fs short pulse laser interacting with a 5 μm inner diameter, 300 μm long micro-channel plasma target. The micro-channel was aligned to be collinear with the incident laser pulse, confining the majority of the laser energy within the channel. The measured electron spectrum showed a large increase in the cut-off energy and slope temperature when compared to that from a 2 μm flat Copper target, with the cutoff energy more than doubled and the total energy in electrons >5 MeV enhanced by over 10 times. Three-dimensional particle-in-cell simulations confirm efficient direct laser acceleration enabled by the novel structure as the dominant acceleration mechanism for the high energy electrons. The simulations further reveal the guiding effect of the channel that successfully explains preferential acceleration on the laser/channel axis observed in experiments. Finally, systematic simulations provide scalings for the energy and charge of the electron pulses. Our results show that the micro-channel plasma target is a promising electron source for applications such as ion acceleration, Bremsstrahlung X-ray radiation, and THZ generation.

A Simple Model for the Fields of a Chirped Laser Pulse With Application to Electron Laser Acceleration

A simple model is introduced for the fields of a chirped laser pulse. Dynamics of laser-acceleration of a single electron by the fields of a pulse, with a $\sin^4$ envelope, is investigated. Multi-GeV energy gains from interaction with pulses of peak intensity $I_0\sim10^{20}$ W/cm$^2$, are reported.

Design of a plasmonic metasurface laser accelerator with a tapered phase velocity for subrelativistic particles

A metallic metasurface-based laser-driven particle accelerator for subrelativistic particles is proposed and studied theoretically. The metasurface consists of a nonperiodic array of nanoslits which focuses the field of the driving laser, utilizing the phenomenon of extraordinary plasmonic transmission, to maximize the acceleration gradient. In order to account for the actual change in the particles' velocity during their propagation through the structure, the separation between successive slits is not constant but rather optimized according to the expected trajectory of the particles. The metasurface laser accelerator (MLA) is designed for an ultrafast driving laser source operating at $2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelength. An approximate analytical model verified by particle tracking simulations predicts a net average acceleration with a normalized acceleration gradient of 1.34 times the incident laser field. Compared to other laser-driven accelerator designs, the MLA provides substantially higher efficiency, due to the field enhancement associated with nanoantennas, and relaxed fabrication challenges (especially for subrelativistic particles). It is found that the output particle beam is microbunched, suggesting the possibility of using a short MLA structure as a prebuncher to improve the initial capture efficiency in a subsequent longer MLA device. The impact of space-charge effects is also studied, and the loaded gradient and optimal bunch charge are estimated.

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 (DLA) 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 consider a conceptually different approach to DLA 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, generating an electron current. We show that 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° 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.

Two-stage laser acceleration of high quality protons using a tailored density plasma

A new scheme of proton acceleration from a laser-driven near-critical-density plasma is proposed. Plasma with a tailored density profile allows a two-stage acceleration of protons. The protons are pre-accelerated in the laser-driven wakefields, and are then further accelerated by the collisionless shock, launched from the rear side of the plasma. The shock has a small transverse size, and it generates a strong space-charge field, which defocuses protons in such a way, that only those protons with the highest energies and low energy spread remains collimated. Theoretical and numerical analysis demonstrates production of high-energy proton beams with few tens of percents energy spread, few degrees divergence and charge of few nC. This scheme indicates the efficient generation of quasi-monoenergetic proton beams with energies up to several hundreds of MeV with PW-class ultrashort lasers.

Silicon nitride waveguide as a power delivery component for on-chip dielectric laser accelerators.

We study the weakly guided silicon nitride waveguide as an on-chip power delivery solution for dielectric laser accelerators (DLAs). We focus on the two main limiting factors on the waveguide network for DLAs: the optical damage and nonlinear characteristics. The typical delivered fluence at the onset of optical damage is measured to be ∼0.19 J/cm2 at a 2μm central wavelength and 250fs pulse width. This damage fluence is lower than that of the bulk Si3N4 (∼0.65 J/cm2), but higher than that of bulk silicon (∼0.17 J/cm2). We also report the nonlinearity-induced spectrum and phase variance of the output pulse at this pulse duration. We find that a total waveguide length within 3mm is sufficient to avoid significant self-phase modulation effects when operating slightly below the damage threshold. We also estimate that one SiNx waveguide can power 70μm silicon dual pillar DLAs from a single side, based on the results from the recent free-space DLA experiment.