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Magnetized collisionless shocks drive particle acceleration broadly in space and astrophysics. We perform the first large-scale particle-in-cell simulations with realistic laboratory parameters (density, temperature, and velocity) to investigate the magnetized shock in head-on colliding plasmas with an applied magnetic field of tens of Tesla. It is shown that a perpendicular collisionless shock is formed with about fourfold density jump when two pre-magnetized flows collide. This shock is also characterized by rapid increase of neutron yield, triggered by the beam-beam nuclear reactions between injected deuterons and ones reflected by the shock. Distinct from the shocks arising from the interaction of injected flows with a magnetized background, the self-generated magnetic field in this colliding plasmas experiences a significant amplification due to the increasing diamagnetic current, approximately 30 times of upstream magnetic field. Moreover, we find that ions, regardless of whether they pass through or are reflected by the shock, can gain energy by the shock surfing acceleration, generating a power-law energy spectrum. In addition, we also demonstrate that the shock mediated only by filamentation instability cannot be generated under the prevailing unmagnetized experimental parameters. These results provide a direct connection of astrophysical field amplification to the magnetized shock formation and nonthermal ion generation.

Compared to the D-T reaction, the neutron-free proton-boron (p-$^{11}$B) fusion has garnered increasing attention in recent years. However, significant Bremsstrahlung losses pose a formidable challenge in p-$^{11}$B plasmas in achieving $Q>1$ in thermal equilibrium. The primary aim of this study is to corroborate Todd H. Rider's seminal work in the 1997 Physics of Plasmas, who investigated the feasibility of sustaining p-$^{11}$B fusion under non-thermal equilibrium conditions. Employing a series of simulations with new fusion cross-section, we assessed the minimum recirculating power that must be recycled to maintain the system's non-thermal equilibrium and found that it is substantially greater than the fusion power output, aligning with Rider's conclusions, whether under the conditions of non-Maxwellian electron distribution or Maxwellian electron distribution, reactors reliant on non-equilibrium plasmas for p-$^{11}$B fusion are unlikely to achieve net power production without the aid of highly efficient external heat engines. However, maintaining the ion temperature at 300 keV and the Coulomb logarithm at 15, while increasing the electron temperature beyond 23.33 keV set by Rider, leads to diminished electron-ion energy transfer and heightened Bremsstrahlung radiation. When the electron temperature approaches approximately 140 keV, this progression ultimately leads to a scenario where the power of Bremsstrahlung loss equals the power of electron-ion interactions, yet remains inferior to the fusion power. Consequently, this results in a net gain in energy production.

Ablative Rayleigh-Taylor instability (ARTI) and nonlocal heat transport are the critical problems in laser-driven inertial confinement fusion, while their coupling with each other is not completely understood yet. Here the ARTI in the presence of nonlocal heat transport is studied self-consistently for the first time theoretically and by using radiation hydrodynamic simulations. It is found that the nonlocal heat flux generated by the hot electron transport tends to attenuate the growth of instability, especially for short wavelength perturbations. A linear theory of the ARTI coupled with the nonlocal heat flux is developed, and a prominent stabilization of the ablation front via the nonlocal heat flux is found, in good agreement with numerical simulations. This effect becomes more significant as the laser intensity increases. Our results should have important references for the target designing for inertial confinement fusion., Comment: 8 pages, 5 figures

Electromagnetic turbulence and ion kinetics in counter-streaming plasmas hold great significance in laboratory astrophysics, such as turbulence field amplification and particle energization. Here, we quantitatively demonstrate for the first time how electromagnetic turbulence affects ion kinetics under achievable laboratory conditions (millimeter-scale interpenetrating plasmas with initial velocity of $2000\ \mathrm{km/s}$, density of $4 \times 10^{19}\ \mathrm{cm}^{-3}$, and temperature of $100\ \mathrm{eV}$) utilizing a recently developed high-order implicit particle-in-cell code without scaling transformation. It is found that the electromagnetic turbulence is driven by ion two-stream and filamentation instabilities. For the magnetized scenarios where an applied magnetic field of tens of Tesla is perpendicular to plasma flows, the growth rates of instabilities increase with the strengthening of applied magnetic field, which therefore leads to a significant enhancement of turbulence fields. Under the competition between the stochastic acceleration due to electromagnetic turbulence and collisional thermalization, ion distribution function shows a distinct super-Gaussian shape, and the ion kinetics are manifested in neutron yields and spectra. Our results have well explained the recent unmagnetized experimental observations, and the findings of magnetized scenario can be verified by current astrophysical experiments., Comment: Accepted by Phys. Rev. Lett. on 12 Mar

The normal modes, i.e., the eigen solutions to the dispersion relation equation, are the most fundamental properties of a plasma, which also of key importance to many nonlinear effects such as parametric and two-plasmon decay, and Raman scattering. The real part indicates the intrinsic oscillation frequency while the imaginary part the Landau damping rate. In most of the literatures, the normal modes of quantum plasmas are obtained by means of small damping approximation (SDA), which is invalid for high-$k$ modes. In this paper, we solve the exact dispersion relations via the analytical continuation (AC) scheme, and, due to the multi-value nature of the Fermi-Dirac distribution, reformation of the complex Riemann surface is required. It is found that the change of the topological shape of the root locus in quantum plasmas is quite different from classical plasmas, in which both real and imaginary frequencies of high-$k$ modes increase with $k$ in a steeper way than the typical linear behaviour as appears in classical plasmas. As a result, the temporal evolution of a high-$k$ perturbation in quantum plasmas is dominated by the ballistic modes.

Value of cross section is a fundamental parameter to depict the transport of charged particles in matters. Due to masses of orders of magnitude higher than electrons and convenience of realistic calculation, the cross section of elastic nuclei-nuclei collision is usually treated via classical mechanics. The famous Bohr criterion was firstly proposed to judge whether the treatment via classical mechanics is reliable or not. Later, Lindhard generalized the results of Coulomb to screening potentials. Considering the increasing importance of detailed ion-ion interactions under modern simulation codes in inertial confinement fusion (ICF) researches, the validation of classical transport cross section for ion-ion interactions in a big range of parameter space is certainly required. In this work, the transport cross sections via classical mechanics under repulsive Yukawa potential are compared with those via quantum mechanics. Differences of differential cross sections are found with respect to scattering angles and velocities. Our results generally indicate that the classical picture fails at the cases of both low and high velocities, which represent a significant extension of the famous Bohr criterion and its generalized variations. Furthermore, the precise validation zones of classical picture is also analysed in this work. This work is of significant importance for benchmarking the modern ion-kinetic simulation codes in ICF researches, concerning the stopping power of $\alpha$ particles in DT fuels, ion-ion friction and viscous effects in the formation of kinetic shocks.

The proton-boron (p-$^{11}$B) reaction is regarded as the holy grail of advanced fusion fuels, since the primary reaction produces three $\alpha$ particles with few neutrons and induced radio-activities from second order reactions. Compared to the Deuterium-Tritium reaction a much higher reaction temperature is required. Moreover, bremsstrahlung energy losses due to the high nuclear charge of boron deem it seemingly apparent than a fusion reactor based on Deuterium-Tritium plasma in equilibrium is to say the least very difficult.It is becoming more appealing to collide intense laser beams or accelerated proton beams with a boron target to produce p-$^{11}$B reactions. The fusion yield of p-$^{11}$B reactions is closely related to proton beam parameters and boron target conditions such as density, temperature, and ingredients. Quantum degeneracy will increase fusion yields by reducing the stopping power of injected protons. In this work, we suggest a high-efficiency scheme for beam-target p-$^{11}$B fusions via injecting a MeV proton beam into a highly compressed quantum degenerated boron target. Such a boron target can be achieved via quasi-isentropic compression of solid boron by using precisely shaped laser pulses. Our results indicate that for densities ranging from $10^3$ to $10^4\rho_s$, where $\rho_s$ is the density of solid boron, contributions of bound and free electrons to the stopping of protons can be completely disregarded and dramatically reduced respectively. The result is an increase in fusion yield by orders of magnitude. Furthermore, in order to achieve multiplication factor $F$ greater than one, with $F$ defined as the ratio of output fusion energy to the energy of injected protons, it is found there exits a minimum possible density of boron target, which is $2.15 \times 10^4 \rho_s$ when the kinetic energy of injected protons is $0.8$ MeV.

Electron beam polarization in the bubble regime of the interaction between a high-intensity laser and a longitudinally pre-polarized plasma is investigated by means of the Thomas-Bargmann-Michel-Telegdi equation. Using a test-particle model, the dependence of the accelerated electron polarization on the bubble geometry is analyzed in detail. Tracking the polarization dynamics of individual electrons reveals that although the spin direction changes during both the self-injection process and acceleration phase, the former has the biggest impact. For nearly spherical bubbles, the polarization of electron beam persists after capture and acceleration in the bubble. By contrast, for aspherical bubble shapes, the electron beam becomes rapidly depolarized, and the net polarization direction can even reverse in the case of a oblate spheroidal bubble. These findings are confirmed via particle-in-cell simulations., Comment: 8pages, 4 figures

The production of polarized proton beams with multi-GeV energies in ultra-intense laser interaction with targets is studied with three-dimensional Particle-In-Cell simulations. A near-critical density plasma target with pre-polarized proton and tritium ions is considered for the proton acceleration. The pre-polarized protons are initially accelerated by laser radiation pressure before injection and further acceleration in a bubble-like wakefield. The temporal dynamics of proton polarization is tracked via the T-BMT equation, and it is found that the proton polarization state can be altered both by the laser field and the magnetic component of the wakefield. The dependence of the proton acceleration and polarization on the ratio of the ion species is determined, and it is found that the protons can be efficiently accelerated as long as their relative fraction is less than 20%, in which case the bubble size is large enough for the protons to obtain sufficient energy to overcome the bubble injection threshold., Comment: 6 pages, 6 figures

The plasma density grating induced by intersecting intense laser pulses can be utilized as an optical compressors, polarizers, waveplates and photonic crystals for the manipulation of ultra-high-power laser pulses. However, the formation and evolution of the plasma density grating are still not fully understood as linear models are adopted to describe them usually. In this paper, two nonlinear theoretical models are presented to study the formation process of the plasma density grating. In the first model, a nonlinear analytical solution based on the fluid equations is presented while in the second model a particle-mesh method is adopted to investigate the kinetic effects. It is found that both models can describe the plasma density grating formation at different stages, well beyond the linear growth stage. More importantly, the second model can reproduce the phenomenon of "ion wave-breaking" of plasma density grating, which eventually induces the saturation of plasma density grating. Using the second model, the saturation time of the plasma density grating is obtained as a function of laser intensity and plasma density, which can be applied to estimate the lifetime of the plasma density grating in experiments. The results from these two nonlinear models are verified using particle-in-cell simulations., Comment: 15pages, 5figures

A method is proposed to control the transverse motion and X-ray emission of an electron beam in a laser driven wakefield by tuning the laser spatial chirp. The dispersion of a transversely chirped laser pulse and the transverse nonuniform refractive index of the plasma channel result in transverse laser centroid oscillations, which leads to periodic transverse oscillations of the laser-driven wake. Electrons accelerated inside the wake also undergo transverse oscillations making wiggler like motion. Both the oscillation period and amplitude can be controlled by tuning the laser chirp, the pulse duration or the plasma channel width. As a result, the far field spectral distributions of the X-ray emission can be flexibly manipulated., Comment: 8 pages, 5figures

We propose a scheme to mapping electromagnetic fields structure in plasma by using a spin polarized relativistic electron beam. Especially by using Particle-in-Cell (PIC) and electron spin tracing simulations, we have successfully reconstructed a plasma wakefield from the spin evolution of a transmitted electron beam. Electron trajectories of the probe beam are obtained from PIC simulations, and the spin evolutions during the beam propagating through the fields are calculated by a spin tracing code. The reconstructed fields illustrate the main characters of the original fields, which demonstrates the feasibility of fields detection by use of spin polarized relativistic electron beams.

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 $\sim 10$ Tesla., Comment: 10 pages, 4 figures

Coherent transition radiation (CTR) from relativistic electron beam interaction with an overdense plasma foil is investigated by making use of two-dimensional particle-in-cell simulations. Well-defined single electron beam either of uniform profile or having substructures is considered for various beam-plasma parameters. The main purpose is to mimic the complicated beam-plasma conditions that is often found, for example, in intense laser plasma interactions. Key properties of the CTR concerning their temporal, angular and spectral profiles are identified. Several saturation effects due to the beam energy, size and foil density are found for the CTR energy, and the dependences vary for different spectral components such as in the Terahertz (THz) and optical range. The detailed substructure of the beam also affects greatly the radiation generation, leading to distinctive high harmonic components. Electrons with kinetic energy from sub MeV to tens of GeV are explored. For few MeV electron beams, the effects of the foil plasma on the beam dynamics and associated CTR generation, resembles closely the CTR from hot electrons produced in intense laser-plasma interactions. These results may find important applications in beam diagnostics either in laser-plasma based acceleration or conventional accelerators. They may also be employed to design novel THz radiation sources using tunable electron beams., Comment: 6 pages, 9 figures

Terahertz radiation generation from liquid water has long been considered to be impossible due to strong absorption. A few very recent works reported terahertz generation from water, but the mechanism is not clear and the efficiency demands to be enhanced. We show experimentally that strong single-cycle terahertz radiation with field strength of $\rm 0.2~MV cm^{-1}$ is generated from a water line/column of $\sim 200 \mu m$ in diameter irradiated by a mJ femtosecond laser beam. This strength is 100-fold higher than that produced from air. We attribute the mechanism to the laser-ponderomotive-force-induced current with the symmetry broken around the water-column interface. This mechanism can explain our following observations: the radiation can be generated only when the laser propagation axis deviates from the column center; the deviation determines its field strength and polarity; it is always p-polarized no matter whether the laser is p- or s-polarized. This study provides a simple and efficient scheme of table-top terahertz sources based on liquid water., Comment: 4 figures

The ion beam bunching in a cascaded target normal sheath acceleration is investigated by theoretical analysis and particle-in-cell simulations. It is found that a proton beam can be accelerated and bunched simultaneously by injecting it into the rising sheath field at the rear side of a laser-irradiated foil target. In the rising sheath field, the ion phase rotation may take place since the back-end protons of the beam feels a stronger field than the front-end protons. Consequently, the injected proton beam can be compressed in the longitudinal direction. At last, the vital role of the ion beam bunching is illustrated by the integrated simulations of two successive stages in a cascaded acceleration., Comment: 15 pages, 7 figures

We propose a hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma. In the first stage, a sub-relativistic proton beam can be generated by the radiation pressure acceleration in the intense laser interaction with the solid foil. In the second stage, this sub-relativistic proton beam is further accelerated by the laser wakefield driven by the same laser pulse in a near-critical-density background plasma with a decreasing density profile. The propagating velocity of the laser front and the phase velocity of the excited wakefield wave are effectively lowered at the beginning of the second stage. By decreasing the background plasma density gradually from near critical density along the laser propagation direction, the wake travels faster and faster while it accelerates the protons. Consequently, the dephasing between the protons and the wake is postponed, and an efficient wakefield proton acceleration is achieved. This hybrid laser-driven proton acceleration scheme can be realized by using ultrashort laser pulses at the peak power of 10 PW for the generation of multi-GeV proton beams., Comment: 20 pages, 8 figures

With electron beam durations down to femtoseconds and sub-femtoseconds achievable in current state-of-the-art accelerators, longitudinal bunch length diagnostics with resolution at the attosecond level are required. In this paper, we present such a novel measurement device which combines a high power laser modulator with an RF deflecting cavity in the orthogonal direction. While the laser applies a strong correlated angular modulation to a beam, the RF deflector ensures the full resolution of this streaking effect across the bunch hence recovering the temporal beam profile with sub-femtosecond resolution. Preliminary measurements to test the key components of this concept were carried out at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory recently, the results of which are presented and discussed here. Moreover, a possible application of the technique for novel accelerator schemes is examined based on simulations with the particle-tracking code elegant and our beam profile reconstruction tool., Comment: EAAC'17 conference proceedings

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., Comment: 15 pages, 7 figures

Multistage coupling of laser-wakefield accelerators is essential to overcome laser energy depletion for high-energy applications such as TeV level electron-positron colliders. Current staging schemes feed subsequent laser pulses into stages using plasma mirrors, while controlling electron beam focusing with plasma lenses. Here a more compact and efficient scheme is proposed to realize simultaneous coupling of the electron beam and the laser pulse into a second stage. A curved channel with transition segment is used to guide a fresh laser pulse into a subsequent straight channel, while allowing the electrons to propagate in a straight channel. This scheme benefits from a shorter coupling distance and continuous guiding of the electrons in plasma, while suppressing transverse beam dispersion. With moderate laser parameters, particle-in-cell simulations demonstrate that the electron beam from a previous stage can be efficiently injected into a subsequent stage for further acceleration, while maintaining high capture efficiency, stability, and beam quality., Comment: 6 pages, 4 figures

It is shown by three-dimensional QED particle-in-cell simulation that as a laser pulse of 2.5 PW and 20 fs propagates along a sub-wavelength-wide solid wire, directional synchrotron $\gamma-$rays along the wire surface can be efficiently generated. With 8\% energy conversion from the pulse, the $\gamma-$rays contains $10^{12}$ photons between 5 and 500 MeV within 10 fs duration, corresponding to peak brilliance of $10^{27}$ photons ${\rm s^{-1}~ mrad^{-2}~ mm^{-2}}$ per 0.1\% bandwidth. The brilliance and photon energy are respectively 2 and 3 orders of magnitude higher than the highest values of synchrotron radiation facilities. The radiation is attributed to the generation of nC, GeV electron beams well guided along the wire surface and their wiggling motion in strong electrostatic and magnetostatic fields induced at the high-density-wire surface. In particular, these quasistatic fields are so strong that QED effects already play a significant role for the $\gamma-$ray radiation. With the laser power $P_0$ ranging from 0.5 PW to 5 PW available currently, this scheme can robustly produce $\gamma-$rays peaked at $1^\circ$ with few-mrad divergence and the photon energy and number roughly scales with $P_0$ and $P_0^{3/2}$, respectively. Our scheme embraces both the merits of high directionality comparable to those based upon laser wakefield acceleration and high charge comparable to those based upon laser-solid interaction., Comment: 6 pages, 4 figures

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at $\omega_2/\omega_1=$1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of $\omega_2/\omega_1=$1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism., Comment: 6 pages, 3 figures

A cascaded ion acceleration scheme is proposed by use of ultrashort laser-irradiated microtubes. When the electrons of a microtube are blown away by intense laser pulses, strong charge-separation electric fields are formed in the microtube both along the axial and along the radial directions. By controlling the time delay between the laser pulses and a pre-accelerated proton beam injected along the microtube axis, we demonstrate that this proton beam can be further accelerated by the transient axial electric field in the laser-irradiated microtube. Moreover, the collimation of the injected proton beam can be enhanced by the inward radial electric field. Numerical simulations show that this cascaded ion acceleration scheme works efficiently even at non-relativistic laser intensities, and it can be applied to injected proton beams in the energy range from 1 to 100 MeV. Therefore, it is particularly suitable for cascading acceleration of protons to higher energy., Comment: 13 pages, 4 figures

High-energy-density electron-positron pair plasma production and its dynamics in a thin foil illuminated by two counter-propagating laser pulses are investigated through multi-dimensional particle-in-cell simulations. We compare the production of electron-positron pairs and gamma-photons via quantum electrodynamics processes in the relativistic transparent and opaque regimes, and find that the target transparency can significantly enhance the electron-positron pair production due to the formation of stable standing wave (SW). An optimum foil density of 200 - 280 n_c (n_c is the laser critical density) is found for enhancing electron-positron pair production when laser intensity reaches a few 10e23 W/cm2. At such foil density, laser energy conversion to electron-positron pairs is approximately four times higher than at foil density of 710n_c, whereas laser energy conversion to gamma-photons keeps almost the same. Consequently, high dense electron-positron plasma with a maximum intensity above 10e20 W/cm2 is produced. Modulation dynamics of created pair plasmas is further observed when target foil becomes transparent. It is shown that stable SWs formed directly by two counter-propagating lasers, not only trap the created electron-positron pairs to their nodes, but also modulate periodically average energy and phase-space and angular distributions of trapped particles. However, similar trapping and modulation effects become obscure in the opaque regime due to the absence of stable SW field., Comment: 7 pages, 7 figures

The radiation pressure of next generation ultra-high intensity ($>10^{23}$ W/cm$^{2}$) lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these laser pulses with matter. Here we show that these effects may lead to the production of an extremely dense ($\sim10^{24}$ cm$^{-3}$) pair-plasma which absorbs the laser pulse consequently reducing the accelerated ion energy and energy conversion efficiency by up to 30-50\% \& 50-65\%, respectively. Thus we identify the regimes of laser-matter interaction where either ions are efficiently accelerated or dense pair-plasmas are produced as a guide for future experiments.

We investigate how next-generation laser pulses at 10 PW $-$ 200 PW interact with a solid target in the presence of a relativistically underdense preplasma produced by amplified spontaneous emission (ASE). Laser hole boring and relativistic transparency are strongly restrained due to the generation of electron-positron pairs and $\gamma$-ray photons via quantum electrodynamics (QED) processes. A pair plasma with a density above the initial preplasma density is formed, counteracting the electron-free channel produced by the hole boring. This pair-dominated plasma can block the laser transport and trigger an avalanche-like QED cascade, efficiently transfering the laser energy to photons. This renders a 1-$\rm\mu m$-scalelength, underdense preplasma completely opaque to laser pulses at this power level. The QED-induced opacity therefore sets much higher contrast requirements for such pulse in solid-target experiments than expected by classical plasma physics. Our simulations show for example, that proton acceleration from the rear of a solid with a preplasma would be strongly impaired., Comment: 5 figures

It is shown by particle-in-cell simulation that intense circularly polarized (CP) laser light can be contained in the cavity of a solid-density circular Al-plasma shell for hundreds of light-wave periods before it is dissipated by laser-plasma interaction. A right-hand CP laser pulse can propagate almost without reflection into the cavity through a highly magnetized overdense H-plasma slab filling the entrance hole. The entrapped laser light is then multiply reflected at the inner surfaces of the slab and shell plasmas, gradually losing energy to the latter. Compared to that of the incident laser, the frequency is only slightly broadened and the wave vector slightly modified by appearance of weak nearly isotropic and homogeneous higher harmonics.

Laser-driven collisonless 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 strongly time-oscillating electric field accompanying laser-driven collisionless shock in a near critical density plasma., Comment: 9 figures,\

We demonstrate an intense broadband terahertz (THz) source based on the interaction of relativistic-intensity femtosecond lasers with aligned copper nanorod array targets. For copper nanorod targets with length 5 \mu m, a maximum 13.8 times enhancement in the THz pulse energy (in $\leq$ 20 THz spectral range) is measured as compared to that with a thick plane copper target under the same laser conditions. A further increase in the nanorod length leads to a decrease in the THz pulse energy at medium frequencies ($\leq$ 20THz) and increase of the electromagnetic pulse energy in the high-frequency range (from 20 - 200 THz). For the latter, we measure a maximum energy enhancement of 28 times for the nanorod targets of length 60 \mu m . Particle-in-cell simulations reveal that THz pulses are mostly generated by coherent transition radiation of laser produced hot electrons, which are efficiently enhanced with the use of nanorod targets. Good agreement is found between the simulation and experimental results., Comment: 14 pages, 5 figure

We develop the particle-in-cell (PIC) code KLAPS to include the photon generation via the Compton scattering and electron-positron creation via the Breit-Wheeler process due to quantum electrodynamics (QED) effects. We compare two sets of existing formulas for the photon generation and different Monte Carlo algorithms. Then we benchmark the PIC simulation results., Comment: 8 pages, 5 figures

Significant progress has been made towards laser-driven fusion ignition via different schemes, including direct and indirect central ignition, fast ignition, shock ignition, and impact ignition schemes. However, to reach ignition conditions, there are still various technical and physical challenges to be solved for all these schemes. Here, our multi-dimensional integrated simulation shows that the fast-ignition conditions could be achieved when two 2.8 petawatt heating laser pulses counter-propagate along a 3.5 kilotesla external magnetic field. Within a period of 5 picoseconds, the laser pulses heat a nuclear fuel to reach the ignition conditions. Furthermore, we present the parameter windows of lasers and magnetic fields required for ignition for experimental test., Comment: 25 pages, 5 figures, 1 table

Laser-driven ion accelerators have the advantages of compact size, high density, and short bunch duration over conventional accelerators. Nevertheless, it is still challenging to simultaneously enhance the yield and quality of laser-driven ion beams for practical applications. Here we propose a scheme to address this challenge via the use of emerging multi-petawatt lasers and a density-modulated target. The density-modulated target permits its ions to be uniformly accelerated as a dense block by laser radiation pressure. In addition, the beam quality of the accelerated ions is remarkably improved by embedding the target in a thick enough substrate, which suppresses hot electron refluxing and thus alleviates plasma heating. Particle-in-cell simulations demonstrate that almost all ions in a solid-density plasma of a few microns can be uniformly accelerated to about 25% of the speed of light by a laser pulse at an intensity around 1022 W/cm2. The resulting dense block of energetic ions may drive fusion ignition and more generally create matter with unprecedented high energy density., Comment: 18 pages, 4 figures

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world's first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected to sample the wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented., Comment: 7 pages, 7 figures, 2 tables

This report describes the conceptual steps in reaching the design of the AWAKE experiment currently under construction at CERN. We start with an introduction to plasma wakefield acceleration and the motivation for using proton drivers. We then describe the self-modulation instability --- a key to an early realization of the concept. This is then followed by the historical development of the experimental design, where the critical issues that arose and their solutions are described. We conclude with the design of the experiment as it is being realized at CERN and some words on the future outlook. A summary of the AWAKE design and construction status as presented in this conference is given in [1]., Comment: 15 pages, 24 figures, 1 table, 111 references, 121 author from 36 organizations

A new scheme for bright hard x-ray emission from laser wakefield electron accelerator is reported, where pure nitrogen gas is adopted. Intense Betatron x-ray beams are generated from ionization injected K-shell electrons of nitrogen into the accelerating wave bucket. The x-ray radiation shows synchrotron-like spectrum with total photon yield 8$\times$10$^8$/shot and $10^8$ over 110keV. In particular, the betatron hard x-ray photon yield is 10 times higher compared to the case of helium gas under the same laser parameters. Particle-in-cell simulation suggests that the enhancement of the x-ray yield results from ionization injection, which enables the electrons to be quickly accelerated to the driving laser region for subsequent betatron resonance. Employing the present scheme,the single stage nitrogen gas target could be used to generate stable high brightness betatron hard x-ray beams.

It is shown, by simulation and theory, that circularly or elliptically polarized terahertz radiation can be generated when a static magnetic (B) field is imposed on a gas target along the propagation direction of a two-color laser driver. The radiation frequency is determined by $\sqrt{\omega_p^2+{\omega_c^2}/{4}} + {\omega_c}/{2}$, where $\omega_p$ is the plasma frequency and $\omega_c$ is the electron cyclotron frequency. With the increase of the B field, the radiation changes from a single-cycle broadband waveform to a continuous narrow-band emission. In high-B-field cases, the radiation strength is proportional to $\omega_p^2/\omega_c$. The B field provides a tunability in the radiation frequency, spectrum width, and field strength., Comment: 6 pages, 5 figures

The generation of fast ion beams in the hole-boring radiation pressure acceleration by intense laser pulses has been studied for targets with different ion components. We find that the oscillation of the longitudinal electric field for accelerating ions can be effectively suppressed by using a two-ion-species target, because fast ions from a two-ion-species target are distributed into more bunches and each bunch bears less charge. Consequently, the energy spread of ion beams generated in the hole-boring radiation pressure acceleration can be greatly reduced down to 3.7% according to our numerical simulation., Comment: Accepted for publication in Laser and Particle Beams on 7th October 2014

An integrated simulation approach fully based upon particle-in-cell (PIC) model is proposed, which involves both fast particle generation via laser solid-density plasma interaction and transport and energy deposition of the particles in extremely high density plasma. It is realized by introducing two independent systems in a simulation, where the fast particle generation is simulated by a full PIC system and the transport and energy deposition computed by a second PIC system with a reduced field solver. Data of the fast particles generated in the full PIC system are copied to the reduced PIC system in real time as the fast particle source. Unlike a two-region approach, which takes a single PIC system and two field solvers in two plasma density regions, respectively, the present one need not match the field-solvers since the reduced field solver and the full solver adopted respectively in the two systems are independent. A simulation case is presented, which demonstrates that this approach can be applied to integrated simulation of fast ignition with real target densities., Comment: 11 pages, 13 figures

A new parameter regime of laser wakefield acceleration driven by sub-petawatt femotsecond lasers is proposed, which enables the generation of relativistic electron mirrors further accelerated by the plasma wave. Integrated particle-in-cell simulation including the mirror formation and Thomson scattering demonstrates that efficient coherent backscattering up to keV photon energy can be obtained with moderate driver laser intensities and high density gas targets., Comment: 4 pages, 3 figures, submitted to Applied Phyics Letters