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With the help of three-dimensional particle-in-cell simulations, we study the electron dynamics in radiation pressure acceleration of a mass-limited foil target. When a mass-limited foil is irradiated by a circularly polarized laser pulse, the wing parts of the laser overtake the foil from the foil boundaries. In the laser propagation direction, electrons are significantly accelerated by the laser radiation pressure. In the laser polarization direction, electrons are subjected to a strong radial space-charge force and oscillate in the overtaking laser field. Under their combined actions, electrons execute a betatron-like oscillation and a spatially helix electron bunch is thus formed. A simple analytical model is proposed for interpreting the high electron energies and large oscillating amplitudes. It is shown that, when the electron oscillation frequency coincides with the laser frequency as witnessed by the electron, betatron-like resonance occurs, resulting in hundreds of MeV electron generation. [ABSTRACT FROM AUTHOR]

Controlled electron injection into a laser-driven wakefield at a well defined space and time is reported based on particle-in-cell simulations. Key novel ingredients are an underdense plasma target with an up-ramp density profile followed by a plateau and a fairly large laser focus diameter that leads to an essentially one-dimensional (1D) regime of laser wakefield, which is different from the bubble (complete blowout) regime occurring for tightly focused drive beams. The up-ramp profile causes 1D wave breaking to occur sharply at the up-ramp-plateau transition. As a result, it generates an ultrathin (few nanometer, corresponding to attosecond duration), strongly overdense relativistic electron sheet that is injected and accelerated in the wakefield. A peaked electron energy spectrum and high charge (~nC) distinguish the final sheet. [ABSTRACT FROM AUTHOR]

Relativistic laser pulse propagation into homogeneous plasmas has been investigated as a function of plasma density. At first, the propagation features are compared systematically between relativistic transparency (RT) and hole-boring (HB). Paramountly, a considerably broad intermediate regime, namely the incomplete HB regime, has been found between the RT regime and the HB regime for an extremely intense circularly polarized (CP) pulse. In this regime HB proceeds in collaboration with RT, resulting in a much faster propagation speed and a higher cut-off energy of fast ions than in the classic HB regime. Similarly to the classic HB regime, formulae are presented to model the laser propagation and the ion acceleration according to the modified momentum flux balance in this incomplete HB regime. The simulations give the density boundary between this incomplete HB regime and the classic HB regime for CP pulses, which is crucial for estimating the maximum mean ion energy and the maximum conversion efficiency that can be achieved by the classic HB acceleration at a given laser intensity. For linear polarization (LP) the propagation mechanism apparently undergoes a transition in time between these two regimes. A detailed comparison between LP and circular polarization is made for these phenomena. [ABSTRACT FROM AUTHOR]

A new mechanism of harmonics generation is found in ultraintense laser interactions with plasmas, where radiation reaction effects dominate. The radiation reaction results in trapped electrons which oscillate in the laser field and emit high energy photons. Such emission event happens with a frequency spectrum containing double or high order components of the drive laser, inducing second and higher order harmonics emission. Although the intensity of such harmonics is too weak for practical applications, they could be used as a specific diagnostic to detect the radiation reaction effects in the upcoming experimental studies of the extremely intense laser pulse interacting with plasmas. [ABSTRACT FROM AUTHOR]

The feasibility of several novel ultrafast x-ray sources has been demonstrated through the interaction between laser and a micro-structured target. However, the resulting photon flux is still deficient for some applications. Here, we proposed a new method to yield high-flux synchrotron radiation by adopting compact hollow plasma waveguide. We verified the method theoretically and numerically. Linearly polarized laser pulses propagating in the waveguide are mainly converted into the electromagnetic mode HE11 with a superluminal phase speed of 1.02c when the pulse is coupled with the waveguide. The mode fields lead to a strong oscillating force and short oscillation period for energetic electrons, which are accelerated via the longitudinal acceleration mechanism. Then, the short x-ray beam is generated with high yield of 1011–1012. The corresponding photon flux reaches 3.5 × 1021 photons s−1 · 0.1% bandwidth. The high-flux source can be even used for single shot ultrafast imaging or the ultrafast x-ray diffraction and absorption studies. [ABSTRACT FROM AUTHOR]

It is shown that electron injection into a laser-driven plasma bubble can be manipulated by applying an external magnetic field in the presence of a plasma density gradient. The down-ramp of the density-tailored plasma locally reduces the plasma wave phase velocity, which triggers injection. The longitudinal magnetic field dynamically induces an expanding hole in the electron density distribution at the rear of the wake bubble, which reduces the peak electron velocity in its vicinity. Electron injection is suppressed when the electron velocity drops below the phase velocity, which depends on the size of the density hole. This enables the start and end of electron injection to be independently controlled, which allows generation of sub-femtosecond electron bunches with peak currents of a few kilo-Ampere, for an applied magnetic field of ∼10 Tesla. [ABSTRACT FROM AUTHOR]

A method for accurate measurements of the size and electron density of open-air plasmas by optical imaging is developed. The plasma size is determined by the intensified charge coupled device (ICCD) imaging and is related to the plasma inductance. The plasma density is then derived from the plasma inductance in open air. The electron densities measured by the ICCD imaging agree well with the reliable Stark broadening method, in stark contrast with the commonly used current–voltage I– method. These shortcomings of the I– method arise because of its heavy reliance on electron mobility values which are uncertain in complex gas mixtures such as air. This work thus presents a new way of using the ICCD imaging to determine the plasma size and electron density and as such contributes to the development of next-generation plasma diagnostic methods. [ABSTRACT FROM AUTHOR]

The dynamics of three-dimensional (3D) compression of ultrashort intense laser pulses in plasma is investigated theoretically and numerically. Starting from the slowly-varying envelope model, we derive equations describing the spatiotemporal evolution of a short laser pulse towards the singularity, or collapse, based on the variational method. In particular, the laser and plasma conditions leading to spherical compression are obtained. 3D particle-in-cell simulations are carried out to verify these conditions, which also enable one to examine the physical processes both towards and beyond the pulse collapse. Simulations suggest that the laser pulse can be spherically compressed down to a minimum size of the order of the laser wavelength, the so called lambda-cubic regime. The compression process develops over twice as fast in simulation than is predicted by the envelope model, due to the simplified nature of the latter. The final result of this process is pulse collapse, which is accompanied with strong plasma density modulation and spectrum broadening. The collapse can occur multiple times during the laser pulse propagation, until a significant part of the pulse energy is dissipated to electron acceleration by the laser ponderomitve force. It is also shown that a strong external DC magnetic field applied along the laser propagation direction can enhance the rate of compression for circularly-polarised laser pulses, when compared to an unmagnetised plasma, allowing access to strong compression and focusing in the low-density and low-amplitude regime. [ABSTRACT FROM AUTHOR]

The effect of an external transverse magnetic field on ionization injection of electrons in a laser wakefield accelerator (LWFA) is investigated by theoretical analysis and particle-in-cell simulations. On application of a few tens of Tesla magnetic field, both the electron trapping condition and the wakefield structure changes significantly such that injection occurs over a shorter distance and at an enhanced rate. Furthermore, beam loading is compensated for, as a result of the intrinsic trapezoidal-shaped longitudinal charge density profile of injected electrons. The nonlinear ionization injection and consequent compensation of beam loading lead to a reduction in the energy spread and an enhancement of both the charge and final peak energy of the electron beam from a LWFA immersed in the magnetic field. [ABSTRACT FROM AUTHOR]

Two-dimensional particle-in-cell simulations have been performed to study electron-positron pair production and cascade development in single ultra-relativistic laser interaction with solid targets. The spatiotemporal distributions of particles produced via QED processes are illustrated and their dependence on laser polarizations is investigated. The evolution of particle generation displays clear QED cascade characters. Studies show that although a circularly polarized laser delays the QED process due to the effective ion acceleration, it can reduce the target heating and confine high-energy charged particles, which leads to deeper QED cascade order and denser pair plasma production than linearly polarized lasers. These findings may benefit the understanding of the coming experimental studies of ultra-relativistic laser target interaction in the QED dominated regime. [ABSTRACT FROM AUTHOR]

A new mechanism of harmonics generation (HG) induced by quantum electrodynamics (QED) effects in extreme intense laser foil interaction is found and investigated by particle-in-cell (PIC) simulations. When two laser pulses with identical intensities of are counter-incident on a thin foil target, harmonics emission is observed in their reflected electromagnetic waves. Such harmonics radiation is excited due to transversely oscillating electric currents coming from the vibration of QED effect generated pairs. The effects of laser intensity and polarization were studied. By distinguishing the cascade depth of generated photons and pairs, the influence of QED cascades on HG was analyzed. Although the current HG is not an efficient way for radiation source applications, it may provide a unique way to detect the QED processes in the near future ultra-relativistic laser solid interactions. [ABSTRACT FROM AUTHOR]

The generation of ultrashort high-power light sources in the mid-infrared (mid-IR) to terahertz (THz) range is of interest for applications in a number of fields, from fundamental research to biology and medicine. Besides conventional laser technology, photon deceleration in plasma wakes provides an alternative approach to the generation of ultrashort mid-IR or THz pulses. Here, we present a photon deceleration scheme for the efficient generation of ultrashort mid-IR or THz pulses by using an intense driver laser pulse with a relatively short wavelength and a signal laser pulse with a relatively long wavelength. The signal pulse trails the driver pulse with an appropriate time delay such that it sits at the front of the second wake bubble that is driven by the driver pulse. Owing to its relatively long wavelength, the signal pulse will be subjected to a large gradient of the refractive index in the plasma wake bubble. Consequently, the photon deceleration in the plasma wake becomes faster and more efficient for signal pulses with longer wavelengths. This greatly enhances the capacity and efficiency of photon deceleration in the generation of ultrashort high-power light sources in the long-wavelength IR and THz spectral ranges.

Pickup ring of energetic electrons found in relativistic magnetic reconnection (MR) driven by two relativistic intense femtosecond laser pulses is investigated by particle simulation in 3D geometry. Magnetic reconnection processes and configurations are characterized by plasma current density distributions at different axial positions. Two helical structures associated with the circular polarization of laser pulses break down in the reconnection processes to form a current sheet between them, where energetic electrons are found to pile up and the outflow relativistic electron jets are observed. In the field line diffusion region, electrons are accelerated to multi-MeV with a flatter power-law spectrum due to MR. The development of the pickup ring of energetic electrons is strongly dependent upon laser peak intensities. [ABSTRACT FROM AUTHOR]

We examine the interaction between intense laser pulses and strongly magnetised plasmas in the weakly relativistic regime. An expression for the electron Lorentz factor coupling both relativistic and cyclotron motion nonlinearities is derived for static magnetic fields along the laser propagation axis. This is applied to predict modifications to the refractive index, critical density, group velocity dispersion and power threshold for relativistic self-focusing. It is found that electron quiver response is enhanced under right circularly-polarised light, decreasing the power threshold for various instabilities, while a dampening effect occurs under left circularly-polarised light, increasing the power thresholds. Derived theoretical predictions are tested by one- and three-dimensional particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

The development of tabletop intense terahertz (THz) radiation sources is extremely important for THz science and applications. This paper presents our measurements of intense THz radiation from relativistic laser–plasma interactions under different experimental conditions. Several THz generation mechanisms have been proposed and investigated, including coherent transition radiation (CTR) emitted by fast electrons from the target rear surface, transient current radiation at the front of the target, and mode conversion from electron plasma waves (EPWs) to THz waves. The results indicate that relativistic laser plasma is a promising driver of intense THz radiation sources. [ABSTRACT FROM AUTHOR]

A terahertz radiation enhancing scheme, in which a linearly polarized weakly relativistic laser pulse irradiates a target consisting of two parallel thin-solid layers with a certain gap, is proposed and studied by using two-dimensional particle-in-cell simulations. The radiation is known to be produced by laser-produced hot electrons via mechanisms such as coherent transition radiation at the target surfaces. Under optimized conditions, the energy conversion efficiency of terahertz radiation can be as high as 3.3%, which is nearly 1.5 times higher than that obtained with a single-layer target with the same drive laser. This is mainly due to the enhanced hot electron generation with moderate energy via multiple reflections of the laser pulse between the two target layers. The radiation has two peaks close to 30° from the target surface, which are more collimated than that with the single-layer target. The dependence of the terahertz radiation on a variety of target parameters is given, which can control the terahertz spectrum and radiation efficiency and thus provide guidance for experimental investigations. Moreover, both the coherent transition radiation and antenna radiation models are applied to explain the angular distributions of the terahertz emission found in the simulations.

We present a laser-assisted electron injection scheme for beam-driven plasma wakefield acceleration. The laser is collinear with the driver and triggers the injection of hot electrons into the plasma wake by interaction with a thin solid target. We present a baseline case using the AWAKE Run 2 parameters and then perform variations on key parameters to explore the scheme. It is found that the trapped witness electron charge may be tuned by altering laser parameters, with a strong dependence on the phase of the wake upon injection. Normalized emittance settles at the order of micrometres and varies with witness charge. The scheme is robust to misalignment, with a 1/10th plasma skin-depth offset (20 μm for the AWAKE case) having a negligible effect on the final beam. The final beam quality is better than similar existing schemes, and several avenues for further optimization are indicated. The constraints on the AWAKE experiment are very specific, but the general principles of this mechanism can be applied to future beam-driven plasma wakefield accelerator experiments.

Polarized electron beam production via laser wakefield acceleration in pre-polarized plasma is investigated by particle-in-cell simulations. The evolution of the electron beam polarization is studied based on the Thomas–Bargmann–Michel–Telegdi equation for the transverse and longitudinal self-injection, and the depolarization process is found to be influenced by the injection schemes. In the case of transverse self-injection, as found typically in the bubble regime, the spin precession of the accelerated electrons is mainly influenced by the wakefield. However, in the case of longitudinal injection in the quasi-1D regime (for example, F. Y. Li et al., Phys. Rev. Lett. 110, 135002 (2013)), the direction of electron spin oscillates in the laser field. Since the electrons move around the laser axis, the net influence of the laser field is nearly zero and the contribution of the wakefield can be ignored. Finally, an ultra-short electron beam with polarization of $99\%$ can be obtained using longitudinal self-injection.

Spiking electron beam driven plasma waves with novel laser-driven underdense photocathodes can produce electron witness bunches with extreme brightness. The development of such hybrid systems and their potential as future plasma-based accelerators and compact yet high performance light sources is discussed. [ABSTRACT FROM AUTHOR]

Broadband lasers have been proposed as future drivers of inertial confined fusion (ICF) to enhance the laser–target coupling efficiency via the mitigation of various parametric instabilities. The physical mechanisms involved have been explored recently, but are not yet fully understood. Here, stimulated Raman scattering (SRS) as one of the key parametric instabilities is investigated theoretically and numerically for a broadband laser propagating in homogeneous plasma in multidimensional geometry. The linear SRS growth rate is derived as a function of scattering angles for two monochromatic laser beams with a fixed frequency difference δω. If δω/ω0 ∼ 1%, with ω0 the laser frequency, these two laser beams may be decoupled in stimulating backward SRS while remaining coupled for sideward SRS at the laser intensities typical for ICF. Consequently, side-scattering may dominate over backward SRS for two-color laser light. This finding of SRS transition from backward to sideward SRS is then generalized for a broadband laser with a few-percent bandwidth. Particle-in-cell simulations demonstrate that with increasing laser bandwidth, the sideward SRS gradually becomes dominant over the backward SRS. Since sideward SRS is very efficient in producing harmful hot electrons, attention needs to be paid on this effect if ultra-broadband lasers are considered as next-generation ICF drivers.

Sunlight-like lasers that have a continuous broad frequency spectrum, random phase spectrum, and random polarization are formulated theoretically. With a sunlight-like laser beam consisting of a sequence of temporal speckles, the resonant three-wave coupling that underlies parametric instabilities in laser–plasma interactions can be greatly degraded owing to the limited duration of each speckle and the frequency shift between two adjacent speckles. The wave coupling can be further weakened by the random polarization of such beams. Numerical simulations demonstrate that the intensity threshold of stimulated Raman scattering in homogeneous plasmas can be doubled by using a sunlight-like laser beam with a relative bandwidth of ∼1% as compared with a monochromatic laser beam. Consequently, the hot-electron generation harmful to inertial confinement fusion can be effectively controlled by using sunlight-like laser drivers. Such drivers may be realized in the next generation of broadband lasers by combining two or more broadband beams with independent phase spectra or by applying polarization smoothing to a single broadband beam.

A pairwise nuclear fusion algorithm for particle-in-cell simulations for arbitrarily weighted macro-particles at relativistic energies is proposed. When comparing with a recent study by Higginson et al. [J. Comput. Phys. 388, 439 (2019)], our method is also fitted with the widely used Coulomb scattering algorithm by Nanbu and Yonemura [J. Comput. Phys. 145, 639 (1998)] and Sentoku and Kemp [J. Comput. Phys. 227, 6846 (2008)] and can be implemented into a particle-in-cell simulation code with marginal efforts. The algorithm is benchmarked in situations with like-particles, unlike-particles, thermonuclear plasmas, and beam–target fusion. It is shown that only 50–100 macro-particles per cell are needed for the repeatability of fusion yields around 1%.

We report systematic studies of laser-driven proton beams produced with micrometer-thick solid targets made of aluminum and plastic, respectively. Distinct effects of the target materials are found on the total charge, cutoff energy, and beam spot of protons in the experiments, and these are described well by two-dimensional particle-in-cell simulations incorporating intrinsic material properties. It is found that with a laser intensity of 8 × 1019 W/cm2, target normal sheath acceleration is the dominant mechanism for both types of target. For a plastic target, the higher charge and cutoff energy of the protons are due to the greater energy coupling efficiencies from the intense laser beams, and the larger divergence angle of the protons is due to the deflection of hot electrons during transport in the targets. We also find that the energy loss of hot electrons in targets of different thickness has a significant effect on the proton cutoff energy. The consistent results obtained here further narrow the gap between simulations and experiments.