arXiv subjects
Publications and source records attributed to X. T. He.
A response to commenter Ke Lan's comment on our paper published in Nature Communications (2023)14:5782 by J. Yan et al
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.
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.
The interaction between the supersonic jet and background can influence the process of star formation, and this interaction also results in a change of the jet's velocity, direction and density through shock waves. However, due to the limitations of current astronomical facilities, the fine shock structure and the detailed interaction process still remain unclear. Here we investigate the plasma dynamics under different collision states through laser-driven experiments. A double-shock structure is shown in the optical diagnosis for collision case, but the integrated self-emitting X-ray characteristic is different. For solid plastic hemisphere obstacle, two-layer shock emission is observed, and for the relatively low-density laser-driven plasma core, only one shock emission is shown. And the plasma jets are deflected by $50 ^{\circ}$ through the interaction with the high-density background in both cases. For collisionless cases, filament structures are observed, and the mean width of filaments is roughly the same as the ion skin depth. High-energy electrons are observed in all interaction cases. We present the detailed process of the shock formation and filament instability through 2D/3D hydrodynamic simulations and particle-in-cell simulations respectively. Our results can also be applied to explain the shock structure in the Herbig-Haro (HH) 110/270 system, and the experiments indicate that the impact point may be pushed into the inside part of the cloud.
The proton-boron (p-$^{11}$B) reaction is regarded as the holy grail of advanced fusion fuels, since the primary reaction produces three $α$ 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 $10^4ρ_s$, where $ρ_s$ 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 $2.15 \times 10^4 ρ_s$ when the kinetic energy of injected protons is $0.8$ MeV.
Magnetic reconnection, breaking and reorganization of magnetic field topology, is a fundamental process for rapid release of magnetic energy into plasma particles that occurs pervasively throughout the universe. In most natural circumstances, the plasma properties on either side of the reconnection layer are asymmetric, in particular for the collision rates that are associated with a combination of density and temperature and critically determine the reconnection mechanism. To date, all laboratory experiments on magnetic reconnections have been limited to purely collisional or collisionless regimes. Here, we report a well-designed experimental investigation on asymmetric magnetic reconnections in a novel hybrid collisional-collisionless regime by interactions between laser-ablated Cu and CH plasmas. We show that the growth rate of the tearing instability in such a hybrid regime is still extremely large, resulting in rapid formation of multiple plasmoids, lower than that in the purely collisionless regime but much higher than the collisional case. In addition, we, for the first time, directly observe the topology evolutions of the whole process of plasmoid-dominated magnetic reconnections by using highly-resolved proton radiography.
The study of parametric instabilities has played a crucial role in understanding energy transfer to plasma and, with that, the development of key applications such as inertial confinement fusion. When the densities are between $0.108n_c\lesssim n_e\lesssim 0.138n_c$ and the electron temperature is $T_e=$2.5 keV, anomalous hot electrons with kinetic energies above 100 keV are generated. Here, a new electron acceleration mechanism - the anti-Stokes Langmuir decay instability cascade of forward stimulated Raman scattering - is investigated. This mechanism not only explains anomalous energetic electron generation in indirectly driven inertial confinement fusion experiments (and, with that, future mitigation strategies for experiments on the National Ignition Facility), it also provides a new way of accelerating electrons to higher energy for applications such as novel X-ray sources.
A new $α$-emitting isotope $^{214}$U, produced by fusion-evaporation reaction $^{182}$W($^{36}$Ar, 4n)$^{214}$U, was identified by employing the gas-filled recoil separator SHANS and recoil-$α$ correlation technique. More precise $α$-decay properties of even-even nuclei $^{216,218}$U were also measured in reactions of $^{40}$Ar, $^{40}$Ca with $^{180, 182, 184}$W targets. By combining the experimental data, improved $α$-decay reduced widths $δ^2$ for the even-even Po--Pu nuclei in the vicinity of magic neutron number $N=126$ were deduced. Their systematic trends are discussed in terms of $N_{p}N_{n}$ scheme in order to study the influence of proton-neutron interaction on $α$ decay in this region of nuclei. It is strikingly found that the reduced widths of $^{214,216}$U are significantly enhanced by a factor of two as compared with the $N_{p}N_{n}$ systematics for the $84 \leq Z \leq 90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the $π1f_{7/2}$ and $ν1f_{5/2}$ spin-orbit partner orbits, which is supported by a large-scale shell model calculation.
A new regime in the interaction of a two-colour ($ω$,$2ω$) laser with a nanometre-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses - even in the case of multi-cycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e. capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the transition cycle, a dense, nanometre-scale electron bunch is accelerated to relativistic velocities and emits a single, strong attosecond pulse with a peak intensity approaching that of the laser field.
Growth rate and gain of SBS considering the reduced Landau damping due to particle trapping has been proposed to predict the growth and average level of SBS reflectivity. Due to particle trapping, the reduced Landau damping has been taken used of to calculate the gain of SBS, which will make the simulation data of SBS average reflectivity be consistent to the Tang model better. This work will solve the pending questions in laser-plasma interaction and have wide applications in parametric instabilities.
A model of the fluid nonlinear frequency shift of ion acoustic waves (IAWs) in multi-ion species plasmas is presented, which considers the effect of ion temperature. Because the thermal ion exists in plasmas in inertial confinement fusion (ICF) and also solar wind, which should be considered in nonlinear frequency shift of IAWs. However, the existing models [Berger et al., Physics of Plasmas 20, 032107 (2013); Q. S. Feng et al., Phys. Rev. E 94, 023205 (2016)] just consider the cold ion fluid models. This complete theory considering multi-ion species and thermal ions will calculate the frequency of the large amplitude nonlinear IAWs more accurately, especially the slow mode with high ion temperature, which will have wide application in space physics and inertial confinement fusion.
The strong-coupling mode, called quasimode, will be excited by stimulated Brillouin scattering (SBS) in high-intensity laser-plasma interaction. And SBS of quasimode will compete with SBS of fast mode (or slow mode) in multi-ion species plasmas, thus leading to a low-frequency burst behavior of SBS reflectivity. The competition of quasimode and ion-acoustic wave (IAW) is an important saturation mechanism of SBS in high-intensity laser-plasma interaction. These results give a clear explanation to the low-frequency periodic burst behavior of SBS and should be considered as a saturation mechanism of SBS in high-intensity laser-plasma interaction.
The energy band structures caused by self-energy shifting that results in bound energy levels broadening and merging in warm dense aluminum and beryllium are observed. An energy band theory for warm dense matter (WDM) is proposed and a new code based on the energy band theory is developed to improve the traditional density functional method. Massive data of the equation of state and transport coefficients for WDM in medium and low Z have been simulated. The transition from fully degenerate to partially degenerate (related to WDM) and finally to non-degenerate state is investigated using the Lorenz number varying with the degeneracy parameter, and the lower and upper parameter boundaries for WDM are achieved. It is shown that the pressure ionization results in the Wiedemann-Franz law no longer available for WDM.
The composition of the astrophysical relativistic jets remains uncertain. By kinetic particle-in-cell simulations, we show that the baryon component in the jet, or the so-called baryon loading effect (BLE), heavily affects relativistic jets transport dynamics in the interstellar medium. On the one hand, with the BLE, relativistic jets can transport in a much longer distance, because jet electrons draw a significant amount of energy from jet baryons via the Buneman-induced electrostatic waves and the Weibel-mediated collisionless shock; on the other hand, the jet electron phase space distribution may transform from a bottom-wide-single-peak structure to a center-wide-multiple-peak one by increasing the BLE, which largely influences the observed jet morphology. Implications for related astrophysical studies are also discussed.
The kinetic effects of magnetic fields on the transport of relativistic jet in the intergalactic medium remain uncertain, especially for their perpendicular component. By particle-in-cell simulations, we find that when only jet electrons are fully magnetized, they are directly deflected by the magnetic field, but jet protons are mainly dragged by collective charge-separation electric field. However, when both electrons and protons are fully magnetized, the contrary is the case. Their balance tremendously distorts the jet density and electromagnetic fields, leading to enormous energy exchange between different species and fields. As a result, the electron spectrum energy distribution (SED) gets reshaped and the power law slope of the SED decreases as the magnetic field strength increases. In other words, we may infer that magnetic fields around the relativistic jet play a crucial role in shaping the observed SED.
Scaling laws of ion acceleration in ultrathin foils driven by radiation pressure of intense laser pulses are investigated by theoretical analysis and two-dimensional particle-in-cell simulations. Considering the instabilities are inevitable during laser plasma interaction, the maximum energy of ions should have two contributions: the bulk acceleration driven by radiation pressure and the sheath acceleration in the moving foil reference induced by hot electrons. A theoretical model is proposed to quantitatively explain the results that the cutoff energy and energy spread are larger than the predictions of "light sail" model, observed in simulations and experiments for a large range of laser and target parameters. Scaling laws derived from this model and supported by the simulation results are verified by the previous experiments.
Production of the huge longitudinal magnetic fields by using an ultraintense laser pulse irradiating a solenoid target is considered. Through three-dimensional particle-in-cell simulations, it is shown that the longitudinal magnetic field up to ten kilotesla can be observed in the ultraintense laser-solenoid target interactions. The finding is associated with both fast and return electron currents in the solenoid target. The huge longitudinal magnetic field is of interest for a number of important applications, which include controlling the divergence of laser-driven energetic particles for medical treatment, fast-ignition in inertial fusion, etc., as an example, the well focused and confined directional electron beams are realized by using the solenoid target.
We report the experimental generation of highly energetic carbon ions up to 48 MeV per nucleon by shooting double-layer targets composed of well-controlled slightly underdense plasma (SUP) and ultrathin foils with ultra-intense femtosecond laser pulses. Particle-in-cell simulations reveal that carbon ions residing in the ultrathin foils undergo radiation pressure acceleration and long-time sheath field acceleration in sequence due to the existence of the SUP in front of the foils. Such an acceleration scheme is especially suited for heavy ion acceleration with femtosecond laser pulses. The breakthrough of heavy ion energy up to multi-tens of MeV/u at high-repetition-rate would be able to trigger significant advances in nuclear physics, high energy density physics, and medical physics.