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A computational study is presented on laser wakefield acceleration (LWFA) in bubble regime with the use of ultrashort laser pulse propagating in an under‐dense plasma. The Particle‐In‐Cell simulations are performed to investigate the bubble wakefield acceleration of electrons realized by the incidence of an intense laser beam on cold, under‐dense plasma in two‐dimensional geometry. Different simulations are carried out and the results are compared for the beams with trapezoidal and Gaussian temporal pulse profiles having almost equal but slightly different energy contents. Focus is given to plasma density modulation, wakefield strength, electrons self‐injection, energy spectrum of accelerated electrons, the effect of an external longitudinal magnetic field and the study of pump depletion length and dephasing length in bubble regime with respect to these laser pulse profiles. Two limiting cases of the trapezoidal pulse, that is, triangular and rectangular pulses, are also discussed for better understanding of the role of steepness and plateau region in the laser pulse profile to the bubble wakefield acceleration. Since down ramp density gradient plays a crucial role for the generation of high‐quality electron beam in plasma wakefield acceleration as well as in LWFA, three different adjustments on the down ramp length determining three different density gradients are discussed for uncovering the role of trapezoidal laser pulse in LWFA. [ABSTRACT FROM AUTHOR]

The emission of artificial electron beams causes an active spacecraft charging effect that poses challenges for electron beam sounding experiments. Plasma contactors have been suggested as a potential solution to this issue based on theoretical and simulation studies, but the impact of these contactors on spacecraft potentials in complex space environments remains unexplored. This paper investigates the fast active charging effect of spacecraft under the action of plasma contactors in the geostationary Earth orbit environment by using a two‐dimensional Particle‐in‐Cell model. We analyze and compare the effects of the "ion emission" process of the plasma contactor and the "electron collection" process of the background plasma on the spacecraft potential at different beam and environment parameters, as well as the coupling effects when both processes occur simultaneously. Our findings indicate that the "ion emission" and "electron collection" processes are the primary factors in determining the spacecraft potential in relatively thin and dense plasma environments, respectively. When the two processes occur together, the contactor plasma cloud increases the "electron collection" return current by expanding the range of the positive potential area around the spacecraft, while the dense background plasma mediates and provides the return current to ensure that the plasma contactor can effectively suppress the active charging effect even after the ion sheath is present. These results suggest that a reasonable adjustment of the contactor parameters based on the relationship between the electron beam and the background environment can effectively extend the normal emission time of the electron beam. Key Points: When the total current of "ion emission" and "electron collection" exceeds the beam current, the spacecraft reaches peak potentialDuring the "ion emission" process, the diffusion of the space plasma cloud raises the "electron collection" return currentBy acting as a medium and supplying return current, dense plasma suppresses the upward trend of spacecraft potential following an ion sheath [ABSTRACT FROM AUTHOR]

Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness. Plain Language Summary: Ion scale current sheets forming sufficiently far from Earth are necessary to explain its stretched magnetic field reconfiguration on the night side. However, isotropic plasmas form magnetic fields that inflate with distance from Earth and cannot reproduce the observed stretched geometry. We present kinetic simulations of current sheets that inflate more gradually with distance due to slight field‐aligned anisotropy of the ion species. Their formation is provided by a special population of suprathermal ions with figure‐of‐eight orbits. We find that the resulting current sheets are stable over a long time scale and have a thickness comparable to the size of these orbits. Key Points: Two‐dimensional ion‐scale current sheets stretched way beyond the isotropic limit are reproduced in particle‐in‐cell simulationsWeak ion anisotropy and agyrotropy substantially modify the current density and the isotropic force balanceIon‐scale current sheets are stable in spite of the fact that they are longer compared to isotropic sheets with similar thickness [ABSTRACT FROM AUTHOR]

Magnetized high energy density physics offers new opportunities for observingmagnetic field-related physics for the first time in the laser-plasma context.We focus on one such phenomenon, which is the ability of a laser-irradiatedmagnetized plasma to amplify a seed magnetic field. We performed a series offully kinetic 3D simulations of magnetic field amplification by apicosecond-scale relativistic laser pulse of intensity $4.2\times 10^{18}$W/cm$^2$ incident on a thin foil. We observe axial magnetic field amplificationfrom an initial 0.1 kT seed to 1.5 kT over a volume of several cubic microns,persisting hundreds of femtoseconds longer than the laser pulse duration. Themagnetic field amplification is driven by electrons in the return currentgaining favorable orbital angular momentum from the seed magnetic field. Thismechanism is robust to laser polarization and delivers order-of-magnitudeamplification over a range of simulation parameters.

Tunable broadband terahertz (THz) sources with power at the gigawatt level are desirable for many applications. A scheme to generate such THz emission by off‐axially injecting a weakly relativistic ultrashort laser into a parabolic plasma channel is presented. By utilizing two‐dimensional particle‐in‐cell simulation, it is demonstrated that there are two major physical mechanisms involved, i.e., linear mode conversion from laser wakefields and the electromagnetic waveguide mode excitation inside the plasma channel. The two radiation modes can lead to linearly polarized and radially polarized THz emissions, respectively, with distinct frequency spectra and spatial distributions. It is found that they predominate alternatively with the change of the plasma channel length. For a given plasma channel, one can switch the radiation modes by adjusting the injection position and the injection angle of the laser pulse. In particular, the radiation mode of the linear mode conversion can produce THz pulses with the peak amplitude of sub‐GV cm−1 with the energy conversion efficiency ≈10−3, even though the peak power of the incident laser is just at the terawatt level. The scheme provides a powerful THz source with tunable intensity, spatial distributions, spectra, and polarizations by simply adjusting the injection conditions of incident laser pulses. [ABSTRACT FROM AUTHOR]

Abstract Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness.

The guidance and stable propagation of laser pulses over many Rayleigh lengths are crucial for the plasma electron acceleration in the laser wakefield accelerators. Using plasma channels with specific characteristics can lead to the proper guidance of the laser pulse. Here, quasi-three-dimensional particle-in-cell (PIC) simulations are performed to investigate the guidance of Bessel–Gaussian pulse (BGP) of zeroth order and super-Gaussian pulse (SGP) of 3rd and 4th orders in an axially and radially inhomogeneous plasma channel. The effects of the channel radius and depth, the laser wavelength and initial spot size, and the plasma channel inhomogeneity on the guidance of the laser pulse are also examined. The results indicate that the guidance of a laser pulse in the plasma channel depends on the pulse profile, and under certain conditions, the pulses can be guided with the least variation of spot size in the inhomogeneous plasma channel. It is shown that the channel depth and the initial laser spot size are very effective in pulse guiding, as the values of these parameters increase, the pulse guidance is done better. In addition, the results show that the guidance of laser pulse is dependent on the type of plasma inhomogeneity represented by three different kinds of initial conditions, as considering the nonlinear-axial inhomogeneity in the parabolic plasma channel can lead to more convergence than the axially homogeneous and linear-axially plasma density profiles. [ABSTRACT FROM AUTHOR]

Transport of energetic electrons in the heliosphere is governed by resonant interaction with plasma waves, for electrons with sub-GeV kinetic energies specifically with dispersive modes in the whistler regime. In this paper, particle-in-cell simulations of kinetic turbulence with test-particle electrons are performed. The pitch-angle diffusion coefficients of these test particles are analyzed and compared to an analytical model for left-handed and right-handed polarized wave modes.

Quantitative predictions from synthetic radiation diagnostics often have to consider all accelerated particles. For particle-in-cell (PIC) codes, this not only means including all macro-particles but also taking into account the discrete electron distribution associated with them. This paper presents a general form factor formalism that allows to determine the radiation from this discrete electron distribution in order to compute the coherent and incoherent radiation self-consistently. Furthermore, we discuss a memory-efficient implementation that allows PIC simulations with billions of macro-particles. The impact on the radiation spectra is demonstrated on a large scale LWFA simulation.

In capacitively coupled plasmas (CCPs) operated under various pressure conditions in semiconductor manufacturing processes, the transition in discharge characteristics is observed depending on local or nonlocal electron kinetics. A two‐dimensional (2D) particle‐in‐cell (PIC) simulation is required because the one‐dimensional PIC method cannot capture the effect of device structures and because fluid models cannot treat the self‐consistent change in the energy distributions. Using a 2D PIC simulation parallelized with a graphics processing unit, we investigated the transition of electron kinetics for the variation of the driving voltage, gas pressure, the gap distance between upper and lower electrodes, and the sidewall condition. Moreover, the mechanism for electron energy relaxation in a CCP is elucidated to develop a method to improve the process uniformity. [ABSTRACT FROM AUTHOR]

Electron co-extraction and suppression by a transverse magnetic fields is studied within a two dimensional particle-in-cell numerical model of surface ionisation ion source with beveled extraction opening. A novel approach of data presentation is proposed, based on the fact that dependences of co-extracted current on the filter strength could be approximated by four parameters only, describing e.g. initial electron current value and cut-off B value. In the paper the influence of extraction system geometry is considered – it is shown that the cut-off B value increases with the size of the opening in the extraction electrode, while the inclination of the extraction opening walls does not play any significant role. It is demostrated that the most of electron is eliminated by hiting the extraction electrode walls, however up to 30 % of electrons were lost by encountering the extraction channel walls due to the modification of their trajectories by the filter field. The influence of the magnetic filter field placement is also investigated – the center of the filter field has to be no further than 2 mm form the extraction channel orifice in order to achieve minimal values of the cut-off field (~20 mT). The possibly low extraction voltages are preferable, as the ammount of co-extracted electrons grows rapidly with Vext resulting in e.g. threefold increase of cut-off parameter value when extraction voltage is changed from 1 kV up to 10 kV. Within the considered model the filter field does not have any significant influecne on extracted H- current. [ABSTRACT FROM AUTHOR]

The acceleration of super-heavy ions from an ultra-thin lead target irradiated by a femtosecond laser pulse with an intensity in the range of ∼1022â€"1023 W cmâˆ'2 was investigated using an advanced 2D3V particle-in-cell code. It is shown that by properly selecting the laser pulse parameters, it is possible to produce a practically single-charge Pb ion beam with multi-GeV ion energies and the laser-to-ions energy conversion efficiency approaching 30%. At the laser intensity of 1023 W cmâˆ'2, Pb ions with the charge state Z = 72 carry over 90% of the total energy of all ions, while the peak intensity and peak fluence of the Pb+72 ion beam are at least two orders of magnitude higher than for other types of ions. In addition, the Pb+72 ion beam is more compact and has a smaller angular divergence than those for other types of ions. The above properties of the Pb+72 ion beam mean that further energy-efficient purification of the beam from other types of ions is possible, even in simple ion transport and selection systems. [ABSTRACT FROM AUTHOR]

Collisionless plasmas in space often evolve into turbulence by exciting an ensemble of broadband electromagnetic and plasma fluctuations. Such dynamics are observed to operate in various space plasmas such as in the solar corona, the solar wind, as well as in the Earth and planetary magnetospheres. Though nonlinear in nature, turbulent fluctuations in the kinetic range (small wavelengths of the order of the ion inertial length or smaller) are believed to retain some properties reminiscent of linear-mode waves. In this paper we discuss what we understand, to the best of our ability, was Peter Gary’s view of kinetic-range turbulence. We call it the Gary picture for brevity. The Gary picture postulates that kinetic-range turbulence exhibits two different channels of energy cascade: one developing from Alfvén waves at longer wavelengths into kinetic Alfvén turbulence at shorter wavelengths, and the other developing from magnetosonic waves into whistler turbulence. Particle-in-cell simulations confirm that the Gary picture is a useful guide to reveal various properties of kinetic-range turbulence such as the wavevector anisotropy, various heating mechanisms, and control parameters that influence the evolution of turbulence in the kinetic range.

Full-3D particle-in-cell Monte Carlo simulation of a new scheme of electron radiotherapy based on electron acceleration by high-power femtosecond laser pulse propagating in plasma of sub-critical density in the relativistic self-trapping regime (V. Yu. Bychenkov et al., Plasma Phys. Control. Fusion 61, 124004 (2019)) was carried out. Based on the results of simulation of distribution of energy deposited by electron bunches accelerated in such high-efficiency regime, it is demonstrated that a laser facility of TW class is capable of providing therapy of deep soft-tissue lesions in soft biotissue and this approach has a number of advantages relative to traditional methods of beam therapy. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulations were used to study the dependence of maximum energy of ions (protons) accelerated from low-density targets on the duration of the accelerating femtosecond laser pulse at its given total energy which, in particular, is important due to the latest achievements in the so-called postcompression of laser pulses to very short durations. It was shown that an optimum pulse length exists, which leads to maximum proton energy acquired in the regime of their maximum efficient synchronized acceleration by the "slow light" (A. V. Brantov et al., Phys. Rev. Lett. 116, 085004 (2016) [1]) and that the ultra shortening of the laser pulse does not lead to the increase in the maximum ion energy despite the increased intensity of laser light. [ABSTRACT FROM AUTHOR]

Transport of energetic electrons in the heliosphere is governed by resonant interaction with plasma waves, for electrons with sub-GeV kinetic energies specifically with dispersive modes in the whistler regime. In this paper, particle-in-cell simulations of kinetic turbulence with test-particle electrons are performed. The pitch-angle diffusion coefficients of these test particles are analyzed and compared to an analytical model for left-handed and right-handed polarized wave modes. [ABSTRACT FROM AUTHOR]

A scheme to generate magnetized relativistic plasmas in a laboratory setting is proposed. It is based on the interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron-thick foils or films. By means of Particle-In-Cell simulations, it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target, building up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath, a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity, whereas the plasma pressure is lower than the magnetic pressure and the electron gyroradius is lower than the plasma dimension. This scheme can be utilized to study astrophysical extreme phenomena such as relativistic magnetic reconnection in laboratory. [ABSTRACT FROM AUTHOR]

Electrostatic solitary waves (ESWs) are ubiquitously observed in magnetic reconnection. In this study, two‐dimensional (2‐D) particle‐in‐cell (PIC) simulations are performed to investigate the characteristics of ESWs in asymmetric magnetic reconnection. ESWs with bipolar structures of the parallel electric field can be generated near the separatrices only on the magnetosphere side, and propagate to reconnection downstream direction along the magnetic field. These structures corresponding to electron phase‐space holes (electron holes) can cross both the electron outflow and inflow channels although their main part is located in the electron outflow. When there is no guide field, the ESWs are generated by electron two‐stream instability. When there is a guide field, the amplitude of the ESWs are different on the right and left side of the X line. On the left side, the ESWs are weaker and generated by the electron two‐stream instability. On the right side, the ESWs are stronger, and there are two kinds of ESWs with distinct phase speed. The faster one is generated by electron two‐stream instability, while the slower one is generated by Buneman instability. Key Points: ESWs corresponding to electron holes appear near the separatrices only on the magnetosphere side during asymmetric reconnectionWhen there is no guide field, ESWs are generated by electron two‐stream instabilityWhen there is a guide field, ESWs with distinct phase speed are generated by electron two‐stream instability and Buneman instability [ABSTRACT FROM AUTHOR]

Mining of substorm magnetic field data reveals the formation of two X‐lines preceded by the flux accumulation at the tailward end of a thin current sheet (TCS). Three‐dimensional particle‐in‐cell simulations guided by these pre‐onset reconnection features are performed, taking also into account weak external driving, negative charging of TCS and domination of electrons as current carriers. Simulations reveal an interesting multiscale picture. On the global scale, they show the formation of two X‐lines, with stronger magnetic field variations and inhomogeneous electric fields found closer to Earth. The X‐line appearance is preceded by the formation of two diverging electron outflow regions embedded into a single diverging ion outflow pattern and transforming into faster electron‐scale reconnection jets after the onset. Distributions of the agyrotropy parameters suggest that reconnection is provided by ion and then electron demagnetization. The bulk flow and agyrotropy distributions are consistent with MMS observations. Plain Language Summary: The process of sudden changes of the magnetic field topology on the night side of the magnetosphere, a global magnetic bubble shielding our planet from the flow of solar wind particles, is simulated for the first time using several important observational constraints. They include the magnetic flux accumulation, formation of a thin current layer with the thickness comparable to the ion gyroradius, solar wind driving electric field, negative charging of the layer and domination of electrons as current carriers. Simulations resolve ion and electron motions beyond their fluid approximation and reveal interesting embedded structure of electron and ion watersheds preceding the topology change. Key Points: Onset of reconnection is reproduced in three‐dimensional particle‐in‐cell simulations guided by magnetometer data mining and local plasma observationsIt is preceded by the formation of two diverging electron outflow regions embedded into a single diverging ion outflow patternReconnection is provided by ion and then electron demagnetization consistent with MMS data [ABSTRACT FROM AUTHOR]

The head-on collision between electrostatic shocks is studied via multi-dimensional particle-in-cell simulations. A strong magnetic field develops after the interaction, which causes the shock velocities to drop significantly. This transverse magnetic field is generated by the Weibel instability, which is driven by pressure anisotropies due to longitudinal electron heating while the shocks approach each other. The possibility to explore the physics underpinning the shock collision in the laboratory with current laser facilities is discussed.

A scheme to generate magnetized relativistic plasmas in a laboratory setting is proposed. It is based on the interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron-thick foils or films. By means of Particle-In-Cell simulations, it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target, building up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath, a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity, whereas the plasma pressure is lower than the magnetic pressure and the electron gyroradius is lower than the plasma dimension. This scheme can be utilized to study astrophysical extreme phenomena such as relativistic magnetic reconnection in laboratory.

Based on particle-in-cell simulations, we propose to generate sub-nanosecond pulses of narrowband terahertz radiation with tens of MW power using unique properties of kiloampere relativistic (2 MeV) electron beams produced by linear induction accelerators. Due to small emittance of such beams, they can be focused into millimeter and sub-millimeter spots comparable in sizes with the wavelength of THz radiation. If such a beam is injected into a plasma, it becomes unstable against the two-stream instability and excites plasma oscillations that can be converted to electromagnetic waves at the plasma frequency and its harmonics. It is shown that several radiation mechanisms with high efficiency of power conversion (∼1%) come into play when the radial size of the beam–plasma system becomes comparable with the wavelength of the emitted waves.

We report on the concept of an innovative laser-driven plasma accelerator for polarized proton (or deuteron) beams with a kinetic energy up to several GeV. In order to model the motion of the particle spins in the plasmas, these have been implemented as an additional degree of freedom into the Particle-in-Cell simulation code VLPL. For the experimental realization, a polarized HCl gas-jet target is under construction, where the degree of proton polarization is determined with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM/Shanghai. [ABSTRACT FROM AUTHOR]

Sub-10 attosecond pulses (APs) with half-cycle electric fields provide exceptional options to detect and manipulate electrons in the atomic timescale. However, the availability of such pulses is still challenging. Here, we propose a method to generate isolated sub-10 attosecond half-cycle pulses based on a cascade process naturally happening in plasma. A backward AP is first generated by shooting a moderate overdense plasma with a one-cycle femtosecond pulse. After that, an electron sheet with the thickness of several nanometers is formed and accelerated forward by the electrostatic field. Then this electron sheet goes through unipolar perturbations driven by the tail of the first-stage AP instead of the initial laser pulse. As a result, a half-cycle sub-10 AP is cascadedly produced in the transmission direction. Two-dimensional particle-in-cell simulations indicate that an isolated half-cycle pulse with the duration of 7.3 attoseconds can be generated from the cascaded scheme. Apart from a one-cycle driving pulse, such a scheme also can be realized with a commercial 100 TW 25 fs driving laser by shaping the pulse with a relativistic plasma lens in advance.

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field B _seed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, B _gen ∼ −2 πB _seed Δ x / λ _0 (in the large spot size limit), where Δ x is the target thickness and λ _0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μ m-thick laser-irradiated target with a kilotesla-level seed magnetic field.

Magnetized high energy density physics offers new opportunities for observing magnetic field-related physics for the first time in the laser–plasma context. We focus on one such phenomenon, which is the ability of a laser-irradiated magnetized plasma to amplify a seed magnetic field. We performed a series of fully kinetic 3D simulations of magnetic field amplification by a picosecond-scale relativistic laser pulse of intensity 4.2 × 10 ^18 W cm ^−2 incident on a thin overdense target. We observe axial magnetic field amplification from an initial 0.1 kT seed to 1.5 kT over a volume of several cubic microns, persisting hundreds of femtoseconds longer than the laser pulse duration. The magnetic field amplification is driven by electrons in the return current gaining favorable orbital angular momentum from the seed magnetic field. This mechanism is robust to laser polarization and delivers order-of-magnitude amplification over a range of simulation parameters.

Laser-driven ion sources are approaching the requirements for several applications in materials and nuclear science. Relying on compact, table-top, femtosecond laser systems is pivotal to enable most of these applications. However, the moderate intensity of these systems ( I ≲ 10 ^19 W cm ^−2 ) could lead to insufficient energy and total charge of the accelerated ions. The use of solid foils coated with a nanostructured near-critical layer is emerging as a promising targeted solution to enhance the energy and the total charge of the accelerated ions. For an appropriate theoretical understanding of this acceleration scheme, a realistic description of the nanostructure is essential, also to precisely assess its role in the physical processes at play. Here, by means of 3D particle-in-cell simulations, we investigate ion acceleration in this scenario, assessing the role of different realistic nanostructure morphologies, such as fractal-like foams and nanowire forests. With respect to a simple flat foil, the presence of a nanostructure allows for up to a × 3 increase of the maximum ion energy and for a significant increase of the conversion efficiency of laser energy into ion kinetic energy. Simulations show also that the details of the nanostructure morphology affect both the maximum energy of the ions and their angular distribution. Furthermore, combined 3D particle-in-cell and Monte Carlo simulations show that if accelerated ions are used for neutron generation with a beryllium converter, double-layer nanostructured targets allow to greatly enhance the neutron yield. These results suggest that nanostructured double-layer targets could be an essential component to enable applications of hadron sources driven by compact, table-top lasers.

This contribution presents results of a Particle-in-Cell simulation of ion beam acceleration via the interaction of a petawatt 25 fs laser pulse of high intensity (up to ~1021 W/cm2) with thin hydrocarbon (CH) and erbium hydride (ErH3) targets of equal areal mass density (of 0.6 g/m2). A special attention is paid to the effect that the laser pulse polarization and the material composition of the target have on the maximum ion energies and the number of high energy (>10 MeV) protons. It is shown that both the mean and the maximum ion energies are higher for the linear polarization than for the circular one. A comparison of the maximum proton energies and the total number of protons generated from the CH and ErH3 targets using a linearly polarized beam is presented. For the ErH3 targets the maximum proton energies are higher and they reach 50 MeV for the laser pulse intensity of 1021 W/cm2. The number of protons with energies higher than 10 MeV is an order of magnitude higher for the ErH3 targets than that for the CH targets.

Abstract Beam quality preservation in the external injection scheme is one of the key missing milestones towards staging of plasma-wakefield accelerators, a prerequisite for their utilisation in particle physics or other applications requiring high energy beams. This topic will be studied at FLASHForward, a unique beam-driven plasma wakefield acceleration facility currently under construction at DESY (Hamburg, Germany), in the frame of the FLASHForward X-2 experiment. High-quality 1 GeV-class electron beams from the free-electron laser FLASH with μ m-emittances, kA-scale currents, and less than 100 fs durations will be utilised to generate driver–witness pairs by using a mask in a dispersive section. In this contribution the physics case and the current status of the FLASHForward X-2 experiment are reviewed. The experimental installation is described, with a focus on the electron beamline. Electron beam dynamics and particle-in-cell simulations are presented. [ABSTRACT FROM AUTHOR]

Abstract Quantitative predictions from synthetic radiation diagnostics often have to consider all accelerated particles. For particle-in-cell (PIC) codes, this not only means including all macro-particles but also taking into account the discrete electron distribution associated with them. This paper presents a general form factor formalism that allows to determine the radiation from this discrete electron distribution in order to compute the coherent and incoherent radiation self-consistently. Furthermore, we discuss a memory-efficient implementation that allows PIC simulations with billions of macro-particles. The impact on the radiation spectra is demonstrated on a large scale LWFA simulation. [ABSTRACT FROM AUTHOR]

A spacecraft‐charging mitigation scheme necessary for the operation of a high‐power electron beam in the low‐density magnetosphere is analyzed. The scheme is based on a plasma contactor, that is, a high‐density charge‐neutral plasma emitted prior to and during beam emission and its ability to emit high ion currents without strong space‐charge limitations. A simple theoretical model for the transient of the spacecraft potential and contactor expansion during beam emission is presented. The model focuses on the contactor ion dynamics and is valid in the limit when the ion contactor current is equal to the beam current. The model is found in very good agreement with particle‐in‐cell simulations over a large parametric study that varies the initial expansion time of the contactor, the contactor current, and the ion mass. The model highlights the physics of the spacecraft‐charging mitigation scheme, indicating that the most important part of the dynamics is the evolution of the outermost ion front, which is pushed away by the charge accumulated in the system by the beam. The model can be also used to estimate the long‐time evolution of the spacecraft potential. For a short contactor expansion (0.3‐ or 0.6‐ms helium plasma or 0.8‐ms argon plasma, both with 1‐mA current) it yields a peak spacecraft potential of the order of 1–3 kV. This implies that a 1‐mA relativistic electron beam would be easily emitted by the spacecraft. Key Points: A theoretical model of spacecraft‐charging mitigation for the emission of a relativistic electron beam by a magnetospheric spacecraft is presentedThe model is in good agreement with particle‐in‐cell simulationsThe model predicts the long‐time value of the spacecraft potential, indicating that a relativistic beam would be easily emitted [ABSTRACT FROM AUTHOR]

The orientation and stability of the reconnection x line in asymmetric geometry is studied using three‐dimensional (3‐D) particle‐in‐cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2‐D simulations indicate that this x line orientation maximizes the reconnection rate. The divergence of the nongyrotropic pressure tensor breaks the frozen‐in condition, consistent with its 2‐D counterpart. We then design 3‐D simulations with one dimension being short to fix the x line orientation but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. The development of oblique structure easily leads to turbulence inside small periodic systems. Key Points: The orientation of asymmetric reconnection x line is studied using large 3‐D particle‐in‐cell simulationsCompanion 2‐D simulations indicate that the 3‐D system selects a state of, or at least near, the maximal reconnection rateThe system tends to radiates secondary oblique tearing modes when the primary x line is forced to misalign with this optimal orientation [ABSTRACT FROM AUTHOR]

The recent advancement of high-intensity lasers has made all-optical Compton scattering become a promising way to produce ultrashort brilliant γ -rays in an ultra-compact system. However, so far achieved Compton γ -ray sources are limited by low conversion efficiency and spectral intensity. Here we present a highly efficient gamma photon emitter obtained by irradiating a high-intensity laser pulse on a miniature plasma device consisting of a plasma lens and a plasma mirror. This concept exploits strong spatiotemporal laser-shaping process and high-charge electron acceleration process in the plasma lens, as well as an efficient nonlinear Compton scattering process enabled by the plasma mirror. Our full three-dimensional particle-in-cell simulations demonstrate that in this novel scheme, brilliant γ -rays with very high conversion efficiency (higher than 10 ^−2 ) and spectral intensity (∼10 ^9 $\mathrm{photons}/0.1 \% \mathrm{BW}$ ) can be achieved by employing currently available petawatt-class lasers with intensity of 10 ^21 W cm ^−2 . Such efficient and intense γ -ray sources would find applications in wide-ranging areas.

The particle-in-cell (PIC) method was developed to investigate microscopic phenomena, and with the advances in computing power, newly developed codes have been used for several fields, such as astrophysical, magnetospheric, and solar plasmas. PIC applications have grown extensively, with large computing powers available on supercomputers such as Pleiades and Blue Waters in the US. For astrophysical plasma research, PIC methods have been utilized for several topics, such as reconnection, pulsar dynamics, non-relativistic shocks, relativistic shocks, and relativistic jets. PIC simulations of relativistic jets have been reviewed with emphasis placed on the physics involved in the simulations. This review summarizes PIC simulations, starting with the Weibel instability in slab models of jets, and then focuses on global jet evolution in helical magnetic field geometry. In particular, we address kinetic Kelvin-Helmholtz instabilities and mushroom instabilities.

In this study, we investigate the interaction of jets with their environment at a microscopic level, which is a key open question in the study of relativistic jets. Using small simulation systems during past research, we initially studied the evolution of both electron–proton and electron–positron relativistic jets containing helical magnetic fields, by focusing on their interactions with an ambient plasma. Here, using larger jet radii, we have performed simulations of global jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities, such as the Weibel instability, the kinetic Kelvin–Helmholtz instability (kKHI) and the mushroom instability (MI). We found that the evolution of global jets strongly depends on the size of the jet radius. For example, phase bunching of jet electrons, in particular in the electron–proton jet, is mixed with a larger jet radius as a result of the more complicated structures of magnetic fields with excited kinetic instabilities. In our simulation, these kinetic instabilities led to new types of instabilities in global jets. In the electron–proton jet simulation, a modified recollimation occurred, and jet electrons were strongly perturbed. In the electron–positron jet simulation, mixed kinetic instabilities occurred early, followed by a turbulence-like structure. Simulations using much larger (and longer) systems are required in order to further thoroughly investigate the evolution of global jets containing helical magnetic fields. [ABSTRACT FROM AUTHOR]

The paper presents results of two-dimensional particle-in-cell simulations of ion beam acceleration at the interactions of a 130-fs laser pulse of intensity in the range 1021-1023 W/cm², predicted for the Extreme Light Infrastructure lasers, with thin hydrocarbon (CH) or erbium hydride (ErH3) targets. A special attention is paid to the effect of the laser pulse intensity and polarization (linear, circular) on the proton energy spectrum, the proton beam spatial distribution and the proton pulse shape and intensity. It is shown that for the low laser intensities (~1021 W/cm²) considerably higher proton beam parameters (proton energies, beam intensities) are achieved for the ErH3 target for both polarizations and the effect of polarization on the beam parameters is significant (higher parameters are achieved for the linear polarization). However, for the highest, ultra-relativistic intensities (~1023W/cm²) higher proton beam parameters are attained for the CH target and the effect of polarization on these parameters is relatively low. In this case, for both polarizations quasimonoenergetic proton beams are generated from the CH target of the mean proton energy ~2 GeV and dEp/Ep ≈≈ 0.3 for the linear polarization and dEp/Ep ≈≈ 0.2 for the circular one. At the highest laser intensities also the proton pulse peak intensities are higher for the CH target and for both polarizations they reach values well above 1021W/cm². In the paper, the properties of proton beam generation indicated above are discussed in detail and a physical explanation of the observed effects is done. [ABSTRACT FROM AUTHOR]

In the wake of the intense effort made for the experimental CILEX project, numerical simulation campaigns have been carried out in order to finalize the design of the facility and to identify optimal laser and plasma parameters. These simulations bring, of course, important insight into the fundamental physics at play. As a by-product, they also characterize the quality of our theoretical and numerical models. In this paper, we compare the results given by different codes and point out algorithmic limitations both in terms of physical accuracy and computational performances. These limitations are illustrated in the context of electron l aser w ake f ield a cceleration (LWFA). The main limitation we identify in state-of-the-art P article- I n- C ell (PIC) codes is computational load imbalance. We propose an innovative algorithm to deal with this specific issue as well as milestones towards a modern, accurate high-performance PIC code for high energy particle acceleration. [ABSTRACT FROM AUTHOR]

Recent experiments carried out at the Italian National Research Center, National Optics Institute Department in Pisa, are showing interesting results regarding maximum proton energies achievable with sub-100 TW laser systems. While laser systems are being continuously upgraded in laboratories around the world, at the same time a new trend on stabilizing and making ion acceleration results reproducible is growing in importance. Almost all applications require a beam with fixed performance, so that the energy spectrum and the total charge exhibit moderate shot to shot variations. This result is surely far from being achieved, but many paths are being explored in order to reach it. Some of the reasons for this variability come from fluctuations in laser intensity and focusing, due to optics instability. Other variation sources come from small differences in the target structure. The target structure can vary substantially, when it is impacted by the main pulse, due to the prepulse duration and intensity, the shape of the main pulse and the total energy deposited. In order to qualitatively describe the prepulse effect, we will present a two dimensional parametric scan of its relevant parameters. A single case is also analyzed with a full three dimensional simulation, obtaining reasonable agreement between the numerical and the experimental energy spectrum. [ABSTRACT FROM AUTHOR]

Using two-dimensional particle-in-cell simulations, the properties of a proton beam generated from a thin erbium hydride target irradiated by a 25 fs laser pulse of intensity ranging from 1020 to 1021 W/cm2 are investigated and compared with the features of a proton beam produced from a hydrocarbon (CH) target. It is shown that in case of using the hydride target the mean proton energy and the number of high-energy (>10 MeV) protons as well as the peak proton pulse intensity can be higher by a factor ~10 than the ones obtained from the CH target. [ABSTRACT FROM AUTHOR]