Conditions of structural transition for collisionless electrostatic shock.

Laser-driven collisionless electrostatic shock formation and the subsequent ion acceleration have been studied in near critical density plasmas. Particle-in-cell simulations show that both the speed of laser-driven collisionless electrostatic shock and the energies of shock-accelerated ions can be greatly enhanced due to fast laser propagation in near critical density plasmas. However, a response time longer than tens of laser wave cycles is required before the shock formation in a near critical density plasma, in contrast to the quick shock formation in a highly overdense target. More important, we find that some ions can be reflected by the collisionless shock even if the electrostatic potential jump across the shock is smaller than the ion kinetic energy in the shock frame, which seems against the conventional ion-reflection condition. These anomalous ion reflections are attributed to the strong time-oscillating electric field accompanying the laser-driven collisionless shock in a near critical density plasma.

A novel scheme for generating high-flux neutron beams via collisionless shock acceleration is proposed, where a subpicosecond laser pulse interacts with a practical gas cell target composed of uniform deuteron gas and a solid foil covering the front of the cell. When the heated foil expands into the low-density gas, a strong electrostatic field induced by hot electrons rolls up the gas deuterons, compressing them into a high-density peak while accelerating them to supersonic speeds. This process would spontaneously launch a strong collisionless shock wave, without the need for a specific plasma profile. Copious upstream deuterons will be continuously reflected and accelerated to high energies. Once the laser pulse penetrates the thin foil due to relativistic transparency, the shock velocity and the reflected deuteron energies are further enhanced. By combining two-dimensional particle-in-cell and three-dimensional Monte Carlo simulations, we demonstrate that by utilizing a LiF catcher target, a high-flux collimated neutron beam with yield per unit solid angle exceeding 10 10 n sr − 1 is generated via 7 L i ( d , x n ) reactions driven by a subpicosecond laser pulse at an intensity of 8.8 × 10 19 W cm − 2 , six times higher than that from a typical foil target without deuterium gas. This simple approach overcomes the challenges of controlling the density profiles in conventional ablation schemes, paving the way for laser-driven compact neutron sources.

Collisionless electrostatic shock (CES) generation and subsequent ion acceleration in laser plasma interaction are studied numerically by particle-in-cell simulations. Usually a CES is composed of a high ion density spike surrounded by a bipolar electric field. Ions in front of it can be either submerged or reflected by the shock front. The submerged ions experience few oscillations before becoming part of the shock itself, while the reflected ions are accelerated to twice the shock speed. The effects of the target thickness, density, ion mass, preplasma conditions, as well as the laser intensity on the shock generation are examined. Simulations show that such shocks can be formed in a wide range of laser and target conditions. The characteristic of the shock propagation through a plane interface between two targets with different properties is also investigated. These results are useful for future experimental studies of shock generation and acceleration.

Interplanetary shocks accelerate charged particles and have significant space weather effects. The widely accepted categorization of shocks uses the upstream plasma beta (βu), the shock angle (θBn), and the shock Mach number (M, i.e., the fast magnetosonic Mach number). In addition, two critical Mach numbers (Mc) classify shocks based on whether shock dissipation is provided by electron resistivity and conductance alone or also requires ion viscosity. The two critical Mach numbers were defined in studies by Edmiston and Kennel and by Kennel in the 1980s. The corresponding two transition points from subcritical to supercritical are referred to as critical Mach numbers Mc (EK84) and Mc(K87), respectively. In this study, we use the critical Mach numbers and magnetic overshoot as identifiers of supercritical shocks. When the ratio of M/Mc ≥ 1, and/or an overshoot presents, a shock is categorized as a supercritical shock. We found that in more than 80% of supercritical shocks (with M/Mc(K87) ≥ 1), in addition to the regular ion reflection around the magnetic foot, there is another group of ion reflections: energetic ion reflections (EIRs). Typically, regularly reflected ions range from ~1–4 keV, staying at or around the foot, while EIR ions range from ~4–25 keV (or higher), running minutes upstream of the shock, perhaps as a result of ion backstreaming and escaping. EIR is a newly discovered feature observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) at supercritical interplanetary shocks, occurring even when θBn > 60°, which is excluded by simulations due to a strong suppression of the backstreaming reflection in such circumstances. The close correlation between EIR occurrence and the critical Mach number Mc(K87) revealed that the conceptual ion viscosity in the magnetohydrodynamics (MHD)‐derived criticality has a root in kinetic microphysics.

Particle acceleration, magnetic field generation, and emission in relativistic shocks

Collisionless shocks are considered as one of the most efficient phenomena for the electron acceleration in space and astrophysical plasmas, yet the detailed acceleration process and heating mechanism remain unsolved. Various acceleration and heating mechanisms have so far been proposed and discussed to explain the spacecraft measurements in situ at Earth’s bow shock, involving the large-scale electromagnetic fields, plasma turbulence, and wave-particle interactions. A key element of the acceleration and heatingmechanisms is the spatial scale of the shock transition with respect to the gyro-radius of the electrons [1]. In the simplest picture, since the shock transition occurs on a spatial scale of ion gyro-radius, the electrons behave as magnetized at the shock transition. The cross-shock potential (electrostatic potential drop across the shock transition) plays two distinct roles in electron dynamics [2]. First, the cross-shock potential reduces a certain amount of the upstream flow kinetic energy (mostly carried by the ions due to their mass); Second, the kinetic energy is used to heat the electrons (DC heating). On the other hand, the electrons can also be scattered (or demagnetized) by high-amplitude, randomly-oscillating electromagnetic fields such as turbulence or strong inhomogeneities (AC heating). Space turbulence can potentially scatter the electrons randomly leaving upstream or downstream of the shock ramp region. For a quasiperpendicular shock, the electrons become scattered in a non-adiabatic sense perpendicular to the mean magnetic field (Figure 1). In this paper, the adiabaticity in collisionless shocks is addressed to the studies based on particlein-cell-simulation (PIC). Two important parameters enter a PIC simulation carried out for the study of a collisionless shock: the mass ratio mp/me between ions and electrons and the ratio of the electron plasma frequency to gyrofrequency ωpe/ce, see e.g., [3]. Due to the computational limitations, the usage of the realistic values for the both parameters is not yet possible. The common way to overcome this problem is to find a compromise for the two parameters that can mostly optimize the purpose of the study. The impact of using low frequency ratio of the electron plasma frequency to gyrofrequency in numerical simulations has been discussed in Krasnoselskikh et al. [4]: the electric field is considerably overestimated than it should be for a comparison with the Earth bow shock. Here, I address the question, “What is the consequence of using a smaller frequency ratio upon the adiabatic motion of the electrons”?

Energy dissipation in collisionless shocks is a key mechanism in various astrophysical environments. Its nonlinear nature complicates analytical understanding and necessitates particle-in-cell (PIC) simulations. This study examines the impact of reducing the ion-to-electron mass ratio (m r ), to decrease computational cost, on energy partitioning in one spatial and three velocity-space dimension PIC simulations of strong, nonrelativistic, parallel electron–ion collisionless shocks using the SHARP code. We compare simulations with a reduced mass ratio (m r = 100) to those with a realistic mass ratio (m r = 1836) for shocks with high ( M A = 21.3 ) and low ( M A = 5.3 ) Alfvén Mach numbers. Our findings show that the mass ratio significantly affects particle acceleration and thermal energy dissipation. At high M A , a reduced mass ratio leads to more efficient electron acceleration and an unrealistically high ion flux at higher momentum. At low M A , it causes complete suppression of electron acceleration, whereas the realistic mass ratio enables efficient electron acceleration. The reduced mass ratio also results in excessive electron heating and lower heating in downstream ions at both Mach numbers, with slightly more magnetic field amplification at low M A . Consequently, the electron-to-ion temperature ratio is high at low M A due to reduced ion heating and remains high at high M A due to increased electron heating. In contrast, simulations with the realistic m r show that the ion-to-electron temperature ratio is independent of the upstream magnetic field, a result not observed in reduced m r simulations.

Shock waves propagating in collisionless heliospheric and astrophysical plasmas have been studied extensively over the decades. One prime motivation is to understand the nonthermal particle acceleration at shocks. Although the theory of diffusive shock acceleration (DSA) has long been the standard for cosmic-ray acceleration at shocks, plasma physical understanding of particle acceleration remains elusive. In this review, we discuss nonthermal electron acceleration mechanisms at quasi-perpendicular shocks, for which substantial progress has been made in recent years. The discussion presented in this review is restricted to the following three specific topics: The first is stochastic shock drift acceleration (SSDA), which is a relatively new mechanism for electron injection into DSA. The basic mechanism, related in-situ observations and kinetic simulations results, and how it is connected with DSA will be discussed. Second, we discuss shock surfing acceleration (SSA) at very high Mach number shocks relevant to young supernova remnants (SNRs). While the original proposal under the one-dimensional assumption is unrealistic, SSA has now been proven efficient by a fully three-dimensional kinetic simulation. We discuss the multidimensional nature of SSA and its role in electron injection. Finally, we discuss the current understanding of the magnetized Weibel-dominated shock. It is essentially a magnetized shock in which the reflected-gyrating ions dominate the formation of the shock structure but with a substantial magnetic field amplification by the ion-Weibel instability. Spontaneous magnetic reconnection of self-generated current sheets within the shock structure is an interesting consequence of Weibel-generated strong magnetic turbulence. Although the exact condition for active magnetic reconnection has not been clarified, we argue that high Mach number shocks with both Alfvén and sound Mach numbers exceeding 20–40 will likely behave as a Weibel-dominated shock. Despite a number of interesting recent findings, the relative roles of SSDA, SSA, and magnetic reconnection for electron acceleration at collisionless shocks and how the dominant particle acceleration mechanisms change depending on shock parameters remain to be answered.

We demonstrate how to tune the main ion acceleration mechanism in laser-plasma interactions to collisionless shock acceleration, thus achieving control over the final ion beam properties (e. g. maximum energy, divergence, number of accelerated ions). We investigate this technique with three-dimensional particle-in-cell simulations and illustrate a possible experimental realisation. The setup consists of an isolated solid density target, which is preheated by a first laser pulse to initiate target expansion, and a second one to trigger acceleration. The timing between the two laser pulses allows to access all ion acceleration regimes, ranging from target normal sheath acceleration, to hole boring and collisionless shock acceleration. We further demonstrate that the most energetic ions are produced by collisionless shock acceleration, if the target density is near-critical, ne ≈ 0.5 ncr. A scaling of the laser power shows that 100 MeV protons may be achieved in the PW range.

Structural phase transformation in crystalline gallium arsenide is studied with the isothermal-isobaric molecular-dynamics method. The pair-distribution functions and bond-angle distributions indicate that the fourfold-coordinated zinc-blende structure transforms into a sixfold-coordinated orthorhombic structure under a pressure of 22 GPa. The reverse transformation, from an orthorhombic to a zinc-blende structure, shows hysteresis, and is observed at a pressure of \ensuremath{\cong}10 GPa. Molecular-dynamics results are in good agreement with experiments. The energy barrier associated with this structural transformation is estimated to be \ensuremath{\sim}0.06 eV.

Communities are undergoing a fundamental transformation in structure. This change in social organization has implications for group formation, loyalty strength, and the flow of information and resources through social networks. Both the former and new structures have unique advantages as well as particular weaknesses. This paper explores both sets of community structures, and examines how various technologies facilitate the transformation by increasing access to diverse human capital. It also examines the uses of this model in sustainable development projects.

The release of a hot and dense drifted plasma into a background ambient magnetoplasma is analyzed with the help of one-dimensional hybrid and full particle electromagnetic simulations, in conditions approaching those commonly met in laboratory experiments. The overall study is mainly focused on the early times of the interaction between both plasmas and for a strictly perpendicular expansion in the magnetic field. Two successive regimes are evidenced: a “transitory” regime, where a characteristic interface builds up, followed by a second regime, where a shock forms. Our purpose in this study is to analyze in detail the differences inherent to each code, and to quantify their impact on the corresponding simulation results. Several difficulties are identified throughout this comparison, since different assumptions are used within each code; a method based both on the best time/space scale lengths fit and on a judicious choice of plasma and numerical parameters is proposed in order to overcome these difficulties. The present results and a similar comparative method may be easily extended when analyzing the dynamics of the interface expansion building up in laboratory plasmas (during experiments of laser–target interactions), in space plasma (active experiments), and when applied to open questions related to collisionless shocks.

Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulæ, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.

A kinetic entropy diagnostic was systematically developed for fully kinetic collisionless particle-in-cell (PIC) simulations in Liang et al., Phys. Plasmas 26, 082903 (2019). Here, we first show that kinetic entropy can be used to quantitatively evaluate numerical dissipation in the PIC simulation. Assuming numerical effects can be treated using a relaxation time approximation collision operator, the rate of increase of the kinetic entropy is related to the kinetic entropy. The effective collision frequency due to numerical effects is then easy to evaluate in a collisionless PIC simulation. We find an effective collision frequency of approximately a tenth of the ion cyclotron frequency. This could have important implications for collisionless PIC simulation studies of magnetic reconnection, plasma turbulence, and collisionless shocks. Then, we analyze the uncertainty of the local kinetic entropy density at different locations as a function of the chosen velocity space grid. We find that although the numerically obtained kinetic entropy density varies significantly for small or large velocity space grids, there is a range for which the kinetic entropy density is only weakly sensitive to the velocity space grid. Our analysis of the uncertainty suggests a velocity space grid close to the thermal velocity is optimal, and the uncertainty introduced is significantly less than the physical change in kinetic entropy density.

An approximate method for the account of compressibility, which was successfully applied earlier for airfoil with subsonic-type edge, was used for airfoil with supersonic-type edge in the present paper. Calculations of the critical Mach number of free-stream flow around circular-arc airfoil are considered. This method allows defining the parameters of a compressible fluid through the flow characteristics of an incompressible fluid. The method to account of compressibility of a flow does not depend on mathematical model of calculation of incompressible flow. Calculations of the velocity field for incompressible flow on the basis of Theory Function of Complex Variable (TFCV) have been accomplished. It has been shown that the value of critical Mach number decreases with increasing the relative thickness of the circular-arc airfoil.