In indirect-drive inertial confinement fusion (ICF), a hohlraum serves the purpose of converting laser energy into thermal x-ray energy. This process involves the interaction of low-density ablated plasmas, which can give rise to weakly collisional shocks characterized by the Knudsen number Kn on the order of 1. The Knudsen number highlights the importance of kinetic effects. Preliminary investigations have demonstrated that the kinetic effects associated with weakly collisional shocks significantly impact the efficiency of the ICF process. The study explores the development and structural properties of weakly collisional shocks in hohlraums, focusing on ion mixing and separation in multicomponent plasmas, based on large-scale kinetic simulations. Key findings include the behavior of ions and electrons, which is different from strongly collisional shocks in hydrodynamic theory. The influence of charge-to-mass ratios on ion species separation results in two subshocks and deviations in hydrogen ion concentration from hydrodynamic predictions. The effects of weakly collisional shocks on electron and ion distributions are crucial for understanding laser energy coupling and improving diagnostics. Increased collision frequency could transition shock wave physical behavior from kinetically dominated to fluid-dominated. It further clarifies the impact of these shocks on electron and ion kinetics, providing valuable insights for future research in ICF and astrophysics.

Detailed investigation of the x-ray drive on the capsule at the center of the hohlraum is crucial to solve the "drive deficit" problem in inertial confinement fusion. A pioneering work for probing the drive flux at the hohlraum center by simultaneously measuring the re-emitted flux and shock velocity was reported [X. Xie et al., Phys. Rev. Lett. 128, 075001 (2022)0031-900710.1103/PhysRevLett.128.075001]. However, in the last experiment only two discrete shock velocity values can be obtained, as the streaked optical pyrometer was employed. In this work, we present characterization of the time-dependent x-ray drive on the capsule by simultaneously measuring the time-dependent localized re-emitted flux and the time-dependent shock velocity. The time-dependent localized re-emitted flux was measured by the space-resolving flux detector, while the time-dependent shock velocity was obtained with the velocity interferometer system for any reflector. Two-dimensional radiation hydrodynamic simulations revealed that the time history of the re-emitted flux as well as the shock velocity can be well reproduced. This technique presented a way for the determination of the x-ray drive at the center of the cylindrical hohlraum as well as other novel hohlraums.

The outflow velocity of expanding deuterium-tritium (DT) fuel can become significant after the bangtime, approaching local ion thermal velocity when the inertial confinement fusion (ICF) hotspot surpasses the ignition threshold. Under such conditions, the conventional assumption of an isotropic target-particle velocity distribution becomes questionable and the influence of bulk fluid motion on alpha-particle transport should be examined. In this paper we derive an analytical expression of the charged-particle stopping power in plasmas with arbitrary bulk fluid motion. We then apply this formalism to alpha-particle transport in an ICF hotspot. The results suggest that incorporating the background fluid velocity into the alpha-particle transport simulation results in a greater fraction of alpha-particle energy being transferred directly to the D and T ions, with a corresponding decrease in the energy deposited to electrons. These results indicate a potentially important correction to alpha-energy deposition in the post-bangtime hotspot.

In indirect-drive inertial confinement fusion, there are significant non-equilibrium kinetic effects that deviate from hydrodynamic descriptions in the region of relatively high temperature and low density. Crucially, the complex interfacial kinetic process can alter the plasma properties at the interface between the high- Z hohlraum inner wall plasma and the low-density fill-gas plasma, thereby affecting laser transport and energy deposition processes. The kinetic collisionless shock wave can form at the wall-gas interface. In this work, we investigated the multi-ion-species effects during the formation and propagation of the collisionless shock driven by the expansion of high- Z gold (Au) plasma into the rarefied deuterium–hydrogen gas mixture using one-dimensional implicit Particle-in-Cell simulations. The results reveal that the spatio-temporal evolution of the electrostatic potential approaches a steadily propagating two-step rise pattern due to its interplay with the plasma mixtures. The downstream ion-species stratification attributed to electro-diffusion was explored and found to increase the shock velocity compared to the single-ion species cases under identical initial temperature and electron densities. Additionally, the characteristic two-step rise potential structure would induce the anomalous velocity distributions of reflected suprathermal ions relevant to experiments. A quantitative relationship is established between shock characteristics and the velocity distribution discrepancies of reflected H and D ion species. These findings advance the understanding of electrostatic collisionless shock physics in multi-ion-species plasmas and provide critical insights for assessing the hohlraum plasma conditions.

The Richtmyer–Meshkov instability (RMI) is a key mechanism triggering interfacial mixing in high-energy-density plasmas, with shock strength strongly influencing its evolution. In this work, we demonstrate that the growth of the RMI in plasmas does not increase monotonically with shock strength, as predicted by classical idealized theory, but instead exhibits a critical shock strength that maximizes the instability growth. This behavior is revealed through hydrodynamic simulations performed with the FLASH code (Fryxell et al 2000 Astrophys. J. Suppl. Ser. 131 273), which incorporates a multi-species transport module (Vold et al 2017 Phys. Plasmas 24 042702) using species-dependent coefficients computed from a unified transport theory (Simakov and Molvig 2016 Phys. Plasmas 23 032115); (Simakov and Molvig 2016 Phys. Plasmas 23 032116). Simulation results show that perturbation amplitude and vortex structures exhibit a non-monotonic dependence on shock strength, with a critical shock strength corresponding to maximal growth, whereas molecular mixing increases monotonically. Decoupled simulations reveal that increasing shock strength naturally introduces a competition between stronger shock compression and acceleration versus suppression by viscosity and mass diffusion, leading to the critical behavior of amplitude. A semi-empirical amplitude model, fitted from a limited number of simulations, is also proposed to rapidly estimate the critical shock strength. Similarly, vortex structures exhibit a critical shock strength due to the interplay between shock-induced compression and baroclinic vorticity deposition versus viscous damping. In contrast, molecular mixing is dominated by enhanced mass diffusion, leading to a monotonic increase of the mixing layer with Mach number. These findings reveal that careful adjustment of shock strength can control the relative contributions of instability-driven mixing and diffusion-driven mixing. This may provide guidance for optimizing laser drive conditions to improve implosion performance in inertial confinement fusion.

The shock-bubble interaction in multi-species plasmas has been investigated via the hybrid fluid-particle-in-cell (PIC) method. Compared with the passive and active scalars used in hydrodynamic simulations, the shock-induced multi-ionic interpenetration is captured by first-principles PIC treatment of multi-species ions, which reveals a dual mixing mechanism involving both ion kinetic effects and plasma diffusion. The results show a mixed area about two times the initial bubble, causing a transformation from a nearly 'chunk mixing' state to approaching an 'atomic scale mixing' state. The momentum transport property is significantly changed in the mixed plasmas, showing an increase of the plasma viscosity that is involved self-consistently via the multi-ionic collisions. Shock-induced vortices are found to be dissipated persistently by the mixing-increased viscosity, indicating a decrease of the Reynolds number. Present results provide a better understanding of the mixing feature and hydrodynamic evolution during the shock propagation in inhomogeneous medium in inertial confinement fusion implosions.

In this study, we propose a novel composite carbon nanowire target with a tapered structure to enhance the acceleration of ion by the space-charge field. The numerical study based on two-dimensional particle-in-cell simulations has been carried out, and the physical mechanisms behind the acceleration of C6+ to energies much higher than those of a laser interacting with a normal nanowire target are reported. When the ultra-intense laser interacts with the tapered nanowire target, not only are the electrons accelerated more effectively, resulting in an enhanced sheath field on the back of the target, but also a strong forward quasi-static electric field is generated inside the target. Thus, there is a phenomenon that the C6+ ions are accelerated to higher energies in two stages by the quasi-static electrostatic fields inside and behind the target. In addition, in order to investigate the most efficient case of C6+ ion acceleration, the particle-in-cell simulations were also carried out using different laser intensities and target parameters.

Backscattering due to laser plasma instabilities (LPIs) presents a risk in the laser-driven inertial confinement fusion. Generally, it is assumed that the backscattering of laser beams in the same cone is identical in hohlraum physics studies. In the experiments performed at SG-100kJ laser facility, we find that the backscattering of laser beams in the same cone are quite different. Our investigation reveals the main reason for this phenomenon is that the laser beams in the same cone obtain different power from their neighbor beams via crossed-beam energy transfer (CBET) depending on their polarizations. The dependence of multi-beam CBET on laser polarization arrangement is confirmed in a specially designed experiment. These findings are crucial for understanding the backscattering, CBET, energy deficit and the azimuthal drive asymmetry in cylindrical hohlraums.

Broadband lasers are anticipated to play a pivotal role in future direct-drive inertial confinement fusion experiments as an advanced driving scheme for suppressing laser-plasma instabilities (LPIs). However, the nonlinearities associated with broadband LPI remain incompletely understood. In this work, we conducted numerical simulations to investigate the nonlinearities of broadband laser-driven back-stimulated Raman scattering across various plasma densities. Our results demonstrate that, in the fluid nonlinearity-dominated regime, broadband lasers significantly reduce backscattered light and suppress Langmuir wave decay instability. In the kinetic nonlinearity-dominated regime, broadband lasers induce kinetic inflation and amplify the frequency shift of electron plasma waves. Additionally, we found that the intensity fluctuations of broadband lasers enhance electron trapping across a wide range of plasma densities, leading to the generation of more hot electrons.

Impact flashes on the moon are caused by high-speed collisions of celestial bodies with the lunar surface. The study of the impacts is critical for exploring the evolutionary history and formation of the Moon, and for quantifying the risk posed by the impacts to future human activity. Although the impacts have been monitored from the Earth by a few projects in past 20 years, the events occurring on the lunar far side have not been explored systematically so far. We here present an end-to-end image simulator dedicated to detecting and monitoring the impacts from space, which is useful for future mission design. The simulator is designed for modularity and developed in the Python environment, which is mainly composed of four components: the flash temporal radiation, the background emission, the telescope and the detector used to collect and measure the radiation. Briefly speaking, with a set of input parameters, the simulator calculates the flash radiation in the context of the spherical droplet model and the background emission from the lunar surface. The resulting images are then generated by the simulator after considering a series observational effects, including the stray light, transmission of the instrument, point spread function and multiple kinds of noise caused by a CCD/CMOS detector. The simulator is validated by comparing the calculation with the observations taken on the ground. The modular design enables the simulator to be improved and enhanced by including more complex physical models in the future, and to be flexible for other future space missions.

The nonlinear evolution and energy transfer process of broadband laser-driven stimulated Raman scattering (SRS) near 1/4 critical density (n(c)) have been investigated by particle-in-cell simulations. We find that in this regime, where wave-wave interactions dominate the evolution of SRS, laser reflectivity increases at moderate bandwidth (Delta omega = 0.6%) and subsequently decreases as the bandwidth becomes larger. And at moderate bandwidth, the bursts of SRS lead to increased trapping of super-hot electrons (>50 keV) by electron plasma wave (EPW). We also find that the EPW undergoes a mode-coupling process: under the density modulations introduced by the ion-acoustic wave (k similar to 2k(0)) generated via Langmuir decay instability (LDI), the energy of the SRS-generated EPW (k similar to k(0)) is transferred to a coupled-EPW at k similar to 3k(0), which has a lower phase velocity. The coupled-EPWs trap a significant number of electrons in the 10-50 keV range, resulting in heating of the background plasma. Both moderate and large bandwidths are effective in suppressing the LDI and mode-coupling process, thereby reducing the trapping of electrons in the 10-50 keV range and limiting the increase in plasma temperature.

The overtaking and coalescence of plasma shocks are important scientific topics in inertial confinement fusion design. We present the comparative study of the impact of ion kinetic effects on the overtaking and coalescence of two shocks in deuterium (D) plasmas of various densities with both hybrid fluid-particle-in-cell simulations and hydrodynamic simulations. It is found that the energetic ions escaping from 2nd shock can prevent the compression of the fuel layer, by accelerating and heating upstream plasmas. In the strong-collision case (rho(D)>rho(th )= 30mgcc(-1)), the coalescent density is reduced by about 8% due to the decreased shock Mach number resulting from energetic ion deposition in the upstream region. In the weak-collision case (rho(D)

The effect of ion mixing on the growth of single-mode Richtmyer–Meshkov instability (RMI) at the carbon-hydrogen plasma interface is investigated using hybrid fluid-PIC simulations. It is observed that ion mixing primarily suppresses RMI growth by influencing the Atwood number and ionic kinematic viscosity, particularly for high wavenumber. By considering the effects of Atwood number and ionic kinematic viscosity, an analytical model for the growth of single-mode RMI at the plasma interface is developed, which demonstrates good agreement with data obtained from hybrid fluid-PIC simulations. This study offers new perspectives on the development of RMI and provides valuable references for future experimental and theoretical research.

The large time-scale evolution of two counter-propagating shock waves in the hohlraum has been investigated using both the hybrid fluid-PIC model and hydrodynamic model. It has been found that shock waves compress and heat gas plasma through a series of interactions. This makes the plasma a high-temperature, high-density platform that is stable for hundreds of picoseconds, which may lead to a significant enhancement of stimulated Raman scattering by reducing the Landau damping. Additionally, the interaction between shock wave and the wall/gas interface causes significant mixing of gas ions with wall plasma, resulting substantial differences in density and temperature profile between the hybrid fluid-PIC and hydrodynamic simulations. These insights not only deepen our understanding of shock dynamics in hohlraum environments but also improve the precision of plasma-condition predictions for inertial confinement fusion experiments.

Hybrid fluid-PIC (particle-in-cell) simulations aimed at a better understanding of the effect of ion diffusion and viscosity on the evolution of Richtmyer–Meshkov instability (RMI) under high temperature and high density conditions are described. The use of the hybrid fluid-PIC method, which treats ions as the traditional particle-in-cell method and electrons as a massless fluid, is motivated by the difficulty of handling material mixing in the context of commonly used fluid simulations. The superdiffusive behavior observed at the plasma interface is primarily attributed to the kinetic effect, which exerts a considerable influence on the evolution of the RMI. Applying time-varying viscosity and diffusion corrections to the analytical model of Carlès and Popinet results in a high degree of alignment between the simulation outcomes and the theoretical predictions. These discussions contribute to a more detailed understanding of the physics of ion mixing and its effect on the evolution of RMI in the inertial confinement fusion implosion.

In modeling the charged alpha particle transport in hot-spot plasmas of inertial confinement fusion, the energy-losing rate is a major concern in the Monte Carlo simulations of alpha particle transport of a radiative-hydrodynamic code. However, the traditionally used energy stopping-power only describes the averaged energy-losing rate of the incident charged particles, whereas the variance of the energy exchange with the background particles is generally ignored. In this paper, the variance of charged particle collisions is studied by both analytical derivation and Monte Carlo simulations. An expression of the divergence of the charged particle energy-losing rate is given for the first time, which can be directly used for practical estimations. It indicates that when the areal density of the target particles along the incident particle path length is low, the divergence of the lost energy would be much larger than the average value, and the traditionally used energy stopping-power would be no longer sufficient to describe the charged particle Coulomb collisions. It helps to obtain a more comprehensive understanding about the charged particle transport in plasmas.

We examine electron kinetic effects in broadband-laser-driven back-stimulated Raman scattering (BSRS) bursts using particle-in-cell simulations. These bursts occur during the nonlinear stage, causing reflectivity spikes and generating large numbers of hot electrons. Long-duration simulations are performed to observe burst events, and a simplified model is developed to eliminate the interference of the broadband laser’s random intensity fluctuations. Using the simplified model, we isolate and characterize the spectrum of electron plasma waves. The spectrum changes from a sideband structure to a turbulence-like structure during the burst. A significant asymmetry in the spectrum is observed. This asymmetry is amplified and transferred to electron phase space by high-intensity broadband laser pulses, leading to violent vortex-merging and generation of hot electrons. The proportion of hot electrons increases from 6.76% to 14.7% during a single violent burst event. We demonstrate that kinetic effects profoundly influence the BSRS evolution driven by broadband lasers.

Proton radiography is a widely used experimental method to diagnose the electric and magnetic (EM) fields in high-energy-density plasmas. In proton radiography, the probe protons are typically assumed to be deflected only by the EM fields, whereas the Coulomb scattering caused by the charged particles in the target plasmas is generally ignored. However, at high plasma densities, the presence of Coulomb scattering could reduce the proton flux perturbations recorded on the detector and influence the inversion of the EM fields from experiments. In this paper, a theoretical model is developed for the first time to describe the proton flux distribution on the detector when the EM field deflections and Coulomb scattering coexist in deflecting the probe proton trajectories. Our theory indicates that the Coulomb scattering could decrease the signal contrast of the probed EM fields, which is determined not only by the strengths of the EM field deflections and Coulomb scattering but also by the spatial gradient of the EM fields. Monte Carlo simulations are also conducted to validate our theoretical model. It would be helpful to interpret the proton radiography experiments quantitatively.

The impact of capsule imperfections is one of the challenges in achieving reproducible, high-gain ignition of inertial confinement fusion (ICF). The nonuniformities of capsule imperfections, which create instability seeds during the shock propagation, may cause significant performance degradation. A systematic study of the propagation of collisional shock wave in highly nonuniform plasmas is carried out using a hybrid fluid-PIC code, which enables the analysis of electromagnetic fields, ion mixing, and plasma viscosity self-consistently. During shock propagation, it interacts with multiple bubbles and significant material mixing in the nonuniform plasmas are observed. Consequently, the average viscosity of the post-shock plasma in the nonuniform case increases to 1 similar to 2 times that of the uniform case. These results provide a better understanding of the possible influence of capsule imperfections in ICF implosions.