ABSTRACT. I will discuss recent experiments at high energy laser systems to mimic planetary and stellar interiors in the lab. Via nanosecond dynamic compression in combination with XFEL probing, we can access exotic chemistry and the formation of unusual structures. With shorter pulses giving higher intensities and using spectrally resolved X-ray scattering, we can probe matter at conditions similar to the envelopes of white dwarfs. Experiments at the National Ignition Facility finally provide access to the deep interiors of small stars. X-ray scattering and absorption measurements give unprecedented insights into these peculiar states of matter.

ABSTRACT. Diamond possesses exceptional physical properties due to its remarkably strong carbon-carbon bonding, leading to significant resilience to structural transformations at very high pressures and temperatures. Very recently, a substantial effort has been directed to investigate carbon at megabar pressures and thousands of Kelvins to provide data for developing models of planetary interiors for carbon-rich exoplanets as well as to optimize diamond capsules for inertial confinement fusion experiments. Despite several experimental attempts, synthesis and recovery of the theoretically predicted post-diamond BC8 phase remains elusive. Through quantum accurate, multi-million atom molecular dynamics (MD) simulations, we have uncovered the extreme metastability of diamond at very high pressures, significantly exceeding its range of thermodynamic stability. We predict the post-diamond BC8 phase to be experimentally accessible only within a narrow high pressure-temperature region of the carbon phase diagram. The diamond to BC8 transformation proceeds through pre-melting followed by BC8 nucleation and growth in the metastable carbon liquid. We design a double-shock compression pathway to achieve BC8 synthesis, which is currently being explored in theory-inspired Discovery Science experiments at National Ignition Facility.

ABSTRACT. The mantles of icy planets are composed of ices formed for a variety of small molecules, including water, ammonia and methane. Under the extreme conditions found within these bodies, the methane converts to more complex hydrocarbons and ultimately the carbon precipitates as diamond. The pressure, and hence planetary depth, at which this process occurs has implication for their internal processes, including internal heating, convection and potentially magnetic field generation. There is substantial disagreement between statically compressed laser heated diamond anvil cells (DACs) and laser driven dynamic compression studies on the conditions of diamond formation from hydrocarbons. Recent results from X-ray pump-probe experiments on DAC compressed hydrocarbons at the European X-ray Free Electron Laser reveal the origin of these discrepancies to be kinetic and shed light on the carbon-hydrogen demixing process.

ABSTRACT. Calculation of material properties from ab inito simulations along Jupiter [1] and Brown Dwarf adiabats [2] have been subject of earlier studies. However, accurate models of Saturn’s interior are still very challenging. A recent study by Mankovich and Fortney on Jupiter and Saturn models was based on a single physical model [3] which predicts a strongly differentiated helium distribution in Saturn’s deep interior, resulting in a helium-rich shell above a diffuse core.

We focus on the calculation of material properties of matter at P-T conditions along the Saturn model proposed by Mankovich and Fortney [4]. The dissociation of hydrogen as well as the onset of the helium-rich layer have profound impact on material properties: Dissociation of hydrogen triggers the metallization of the hydrogen sub-system and the band gap of the system closes. However, helium is still an insulator under all the conditions of the model [5,6]. The onset of the helium-rich layer in the deep interior therefore again changes the properties of the mixture: Molecular hydrogen dominates the outer atmosphere, followed by a layer of mainly metallic hydrogen in the interior, followed again by a layer of helium-dominated insulating material above the core. We present results on thermodynamic and transport properties of a hydrogen-helium-water mixture that closely resembles the element distribution of the Saturn model. We discuss implications of the results on our understanding of Saturn’s interior and evolution.

[1] French et al., Astrophys. J. Suppl. Ser., 202, 5 (2012). [2] Becker et al., Astron. J., 156, 149 (2018). [3] Mankovich and Fortney, Astrophys. J., 889, 51 (2020). [4] Preising et al., Astrophys. J. Suppl. Ser., 269, 47 (2023). [5] Monserrat et al., Phys. Rev. Lett., 112, 055504 (2014). [6] Preising and Redmer, Phys. Rev. B, 102, 224107 (2020).

ABSTRACT. Highlights from research done on the National Ignition Facility (NIF) laser through the Discovery Science program will be presented. Plasma nuclear reactions relevant to stellar nucleosynthesis and nuclear reactions in high energy astrophysical scenarios are being studied. [1] Equations of state (EOS) at very high pressures (0.1-100 TPa or 1-1000 Mbar) relevant to planetary cores, brown dwarf interiors, and white dwarf envelopes are being measured on NIF, and show that the level of ionization can significantly affect the compressibility of the sample. [2-6] Studies of Rayleigh-Taylor instabilities in planar and cylindrical geometries at high Reynolds number, relevant to supernovae explosions and ICF implosions, are being investigated. [7-12] Relativistically hot plasmas [13,14] and target-normal sheath acceleration (TNSA) of protons [15-17] are also being studied on the NIF ARC laser. Experiments to study magnetic reconnection at high energy densities are underway. [18] High velocity, low density interpenetrating plasmas that generate collisionless astrophysical shocks, magnetic fields, bursts of neutrons, and that accelerate particles relevant to cosmic ray generation are also being studied on NIF. [19-21] And NIF experiments have demonstrated strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas. [22] A selection from these results will be presented and a path forward suggested.

References: [1] M. Gatu Johnson, PoP 24, 041407 (2017); and PoP 25, 056303 (2018). [2] T. Döppner, PRL 121, 025001 (2018). [3] A.L. Kritcher, Nature 584, 51 (2020). [4] A. Lazicki, Nature 589, 532 (2021). [5] R.F. Smith, Nature 511, 330 (2014). [6] R.F. Smith, Nature Astron. 2, 452 (2018). [7] C.C. Kuranz, Nature Commun. 9, 1564 (2018). [8] J.P. Sauppe, PRL 124, 185003 (2020). [9] S. Palaniyappan, PoP 27, 047208 (2020). [10] A. Casner, PoP 22, 056302 (2015). [11] A. Casner, PPCF 60, 014012 (2018). [12] D.A. Martinez, PRL 114, 215004 (2015). [13] G.J. Williams, PRE 101, 031201 (2020). [14] G.J. Williams, PRE 103, L031201 (2021). [15] D. Mariscal, PoP 26, 043110 (2019). [16] R.A. Simpson, PoP 28, 013108 (2021). [17] N. Iwata, PRR 3, 023193 (2021). [18] W. Fox, PRL, submitted (2024). [19] Steve Ross, PRL 118, 185003 (2017). [20] F. Fiuza, Nature Physics 16, 916 (2020). [21] D.P. Higginson, PoP 26, 012113 (2019). [22] J. Meinecke, Sci. Advances 8, eabj6799 (2022).

ABSTRACT. We discuss the results of computer simulations of reactive turbulence for conditions expected to exist in outer layers of massive white dwarfs during advanced stages of their evolution. We consider a scenario in which a weakly compressible turbulence in an initially quiescent, low-density, self-heated plasma is driven on large scales presumably by an approaching, Rayleigh-Taylor-unstable flame front.

We probe a parameter space of this problem by obtaining a series of models systematically varying characteristics of turbulence, and in magnetized models, also the initial strength of the magnetic field. The ultimate outcome of our models is a deflagration-to-detonation transition (DDT) due to the Zel'dovich reactivity gradient mechanism [1]. For the mechanism to be viable in the context of powering Type Ia supernova explosions, a DDT delay time must be sufficiently short compared to the SN Ia explosion timescale. The two critical problem parameters controlling the DDT delay time is the fuel ignition time and compressibility of turbulence. The delay time decreases with the ignition time, as expected, and, for a given turbulence Mach number, somewhat counterintuitively, with the compressibility of turbulence. We also find some initial evidence of magnetic field participating in the DDT preconditioning process.

Because DDT-relevant scales are orders of magnitude smaller than numerical resolution of current SN Ia explosions models, we are developing a suitable DDT subgrid scale model (SGS). We use a data-driven approach in which a Zel'dovich DDT condition, as described by the Khokhlov inequality [2], is parameterized in terms of turbulence fluctuations using a neural network. We briefly discuss a process of constructing a one-dimensional version of our ML-based DDT SGS, and its ability to correctly identify DDT in numerical simulations of SN Ia explosions.

[1] Y. Zel'dovich. Regime classification of an exothermic reaction with nonuniform initial conditions. Combustion and Flame, 39(2):211–214, 1980.

[2] A. M. Khokhlov. Mechanisms for the initiation of detonations in the degenerate matter of supernovae. Astronomy and Astrophysics, 246(2):383–396, 1991.

ABSTRACT. Laboratory astrophysics can provide insight into some astrophysical objects or processes, which are often observed from great distances under uncontrolled and unknown conditions. For an experiment to be well-scaled to an astrophysical process, several specific conditions must be considered, including key governing equations, specific spatial and temporal scaling, and global dynamics. In some cases, these conditions can be met using high-energy-density experimental facilities, such as, high-energy lasers or pulsed power devices. Experiments conducted at the National Ignition Facility are relevant to SN1993J, a red supergiant, core-collapse supernova. We focused on the Rayleigh-Taylor instable interface between the forward and reverse shocks in SN1933J. Here the forward shock is moving into the low-density circumstellar medium and is highly radiative. In the scaled experiments, a hohlraum drive creates a blast wave structure in a mm-scale target with a decrease in density at a perturbed interface. After the blast wave moves into the lower density material, the perturbation with grow due to hydrodynamics instabilities and the shock becomes radiative. We have detected the evolution of the interface structure under these conditions and will show the resulting experimental and radiation hydrodynamics simulation data. We found that significant energy fluxes from radiation and thermal heat conduction affect the hydrodynamics growth at the interface. Such effects are not currently included in astrophysical models but will have significant effects on the interface structure. We compare our experimental results with radiation hydrodynamics simulations and theoretical radiative shock models .

ABSTRACT. The nature of stellar feedback mechanisms (i.e., injection of energy and momentum into the interstellar medium by exploding supernovae) is one of the key outstanding challenges in the galaxy formation field. Recent advances in the field of astrophysical feedback strongly suggest that cosmic rays (CRs) — high-energy ions produced, e.g., in supernova shocks — may be crucially important for our understanding of cosmological galaxy formation and evolution. The appealing features of CRs are their relatively long cooling times, relatively strong dynamical coupling to plasma, and energy density comparable to that in turbulent motions and magnetic fields. CRs may, therefore, play an essential role in controlling feedback and driving galactic-scale outflows. However, the strength of CR feedback depends very sensitively on the dynamical coupling of CRs to the plasma, i.e., on CR transport mechanisms. I will briefly discuss forward modeling efforts to explain the intensity profiles, multiwavelength spectra, and synchrotron polarization observations of galactic-scale winds using simulations that incorporate CR physics including transport processes. A complete understanding of CR transport will require significant advancements in plasma physics, testing models against astronomical observations, and laboratory astrophysics experiments. I will present a brief overview of the state-of-the-art of CR feedback field emphasizing fundamental results and highlighting key outstanding challenges.

ABSTRACT. Weakly magnetized shock waves are paramount to a large diversity of environments, including supernova remnants, blazars, and binary-neutron-star mergers. Understanding the distribution of energy between electrons and ions within these astrophysical shock waves spanning a wide spectrum of velocities is a long-standing challenge. In this study, we present a unified model for the downstream electron temperature within unmagnetized shock waves, encompassing velocities from Newtonian to extreme relativistic regimes. Heating results from an ambipolar electric field generated by the difference in inertia between electrons and ions, coupled with rapid electron scattering in the decelerating turbulence. Based on large-scale Particle-In-Cell simulations supporting analytical models, our findings demonstrate that the electron temperature consistently represents 10% of the incoming ion kinetic energy in the shock front frame.

ABSTRACT. High Mach number astrophysical plasmas can create collisionless shocks via plasma instabilities and turbulence that are responsible for magnetic field generations and cosmic ray acceleration. With the advent of high-power lasers, laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the mechanisms of shock formation by plasma instabilities and self-generated magnetic fields. A series of experiments were conducted on Omega and the National Ignition Facility to observe: the filamentary Weibel instability that seeds microscale magnetic fields [1, 2]; collisionless shock formation seen by an abrupt ~4x increase in density and ~6x increase in temperature; and electron acceleration distributions that deviated from the thermal distributions [3]. In addition to the case of collisionless shock formation under unmagnetized initial condition, shock formation under magnetized environment is also being studied [4]. Experimental results along with theoretical interpretations aided by particle-in-cell simulations will be discussed.

[1] H.-S. Park et al., HEDP 8, 38 (2012) [2] C. Huntington et al., Nature Physics 11, 173 (2015) [3] F. Fiuza et al., Nature Physics, 16, 916 (2020) [4] E. Tubman et al., in preparation (2024)

* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

ABSTRACT. High Mach number shocks are ubiquitous in astrophysics. At the low densities typical of astrophysical plasmas, ion-ion collisional mean free path lengths are often larger than the observed shock fronts. These shocks must therefore be "collisionless", mediated not by collisions but instead by electro-magnetic fields seeded and amplified through the growth of plasma instabilities. Magnetic fields seeded and amplified by these collisionless shocks are thought to play a key role in the acceleration of cosmic rays. We report on laboratory experiments using the Omega and NIF lasers to investigate the formation of high Mach number collisionless shocks between pairs of interpenetrating laser-generated plasma flows. Omega experiments diagnosed using proton radiography showed the formation, growth, and merger of the filamentary magnetic field structures associated with the Weibel instability in the collisionless regime [1]. Novel experiments using Thomson scattering allowed quantitative measurements to be made of current and magnetic field modulations [2]. Initial, higher-energy experiments using the NIF laser indicated enhanced heating of the interacting flows compared to that expected from PIC modelling, indicating the possible onset of collisionless shock formation [3]. In the most recent experiments, a significant enhancement of density and temperature was observed using the new NIF Thomson scattering diagnostic. This data, along with enhanced neutron and energetic electron emission, is consistent with the observation of a fully formed collisionless shock [4]. The most recent experiments have focused on determining the ratio of the ion and electron temperatures in the shocked plasma, revealing details of the ion kinetics in the shock.

* Prepared by LLNL under Contract DE-AC52-07NA27344.

[1] C. M. Huntington et al., Nature Physics, 11, 173 (2015). [2] G.F. Swadling et al., Phys. Rev. Lett. 124, 215001 (2020). [3] J. S. Ross et al., Phys. Rev. Lett., 118, 185003 (2017). [4] F. Fiuza et al., Nature Physics 16, 916–920 (2020).

ABSTRACT. Magnetized, collisionless shocks are observed throughout the universe including within supernova remnants and at Earth’s magnetopause. These shocks have the potential to accelerate particles to far greater energies than many other astrophysical processes and may provide a source of high-energy cosmic rays. One challenge, yet to be addressed, is determining the exact mechanism of the energy dissipation by these shock waves. Designing a laboratory-based experiment that can create and investigate these shocks is advantageous for testing different theories and developing our knowledge of this phenomena.

This talk will present results from a platform using the Omega laser facility. A gas jet and MIFED assembly provide a pre-ionized, pre-magnetized background plasma, through which a shock wave is launched. We diagnose the effect of the background magnetic fields on the shock formation using Thomson scattering. This allow us to measure the plasma conditions, as well as identify where the background and shock piston material are spatially located. In addition, we diagnose the evolving electromagnetic field structures using proton probing. These diagnostics have led to the clear identification of the shock’s development stages as well as the initial separation of the shock piston and background material. The results assist in benchmarking particle-in-cell codes and hydrodynamic models as well as interpreting measurements from spacecrafts, to gain a better understanding of the underlying physics.

ABSTRACT. Central to laboratory astrophysics is the investigation of similarity properties of astrophysical flows and the development of scaling laws that serve as a bridge between astrophysical and laboratory plasmas [1]. Cooling flows are common in astrophysical environments and now at laboratory scales [2]. In these environments, various instabilities can develop, such as the Chevalier-Imamura cooling instability [3] or Falle instability (Catastrophic cooling) [4], which underlie luminosity oscillations observed in different accreting binary stars [5,6]. We discuss the possibility of reproducing these radiation flows in the laboratory and focus on the limits of radiation hydrodynamical scaling laws with respect to microscopic physics. Recently, a new approach has been developed to generalize dimensional analysis: mapping theory. The results of this theoretical program open up new possibilities for reproducing radiation astrophysical flows. These transformations have been successfully applied to extreme X-ray bursts around neutron stars [7]. During these explosions, intense radiative supersonic waves are produced and propagate in the surrounding accretion disk. Through the new theoretical approach presented in this work, we demonstrate the possibility of replicating such phenomena in the laboratory using powerful lasers like the National Ignition Facility and the Laser MegaJoule.

[1] Ryutov et al., Astrophys. J., 518, 821 (1999) [2] Markwick et al., MNRAS (2024) [3] Chevalier & Imamura Astrophys. J., 261, 543 (1982) [4] Falle, MNRAS, 195, 1011 (1981) [5] Busschaert et al., Astronom. Astrophys., 579, A25 (2015) [6] Van Box Som et al., MNRAS, 473, 3158 (2018) [7] Tranchant et al., Astrophys. J., 936, 14 (2022)

ABSTRACT. Magnetic dynamos are a ubiquitous way of growing, maintaining and structuring magnetic fields across many scales in the Universe. Turbulent, or small-scale dynamos, which power the turbulent components of the velocity and magnetic field, provide the reservoir of magnetic energy for large-scale dynamos through the electromotive force. Turbulent dynamos have been largely studied both theoretically and numerically in the incompressible regime, which has only limited application in both astrophysics and in lab experiments, where there can be strong gas density fluctuations and shocked gas. In this talk I will highlight some of the latest results in compressible (and supersonic) turbulent dynamo numerical calculations and theory, highlighting both the similarities and differences that the supersonic turbulent dynamo has with the incompressible turbulent dynamo.

ABSTRACT. Astrophysical plasmas, such as in collisionless shocks in gamma-ray bursts, and high-energy-density laboratory plasmas often have large-amplitude, sub-Larmor-scale electromagnetic fluctuations excited by various kinetic-streaming or anisotropy-driven instabilities. The Weibel (or the filamentation) instability is particularly important because it can rapidly generate strong magnetic fields, even in the absence of seed fields. Particles propagating in collisionless plasmas with such small-scale magnetic fields undergo stochastic deflections similar to Coulomb collisions, with the magnetic pitch-angle diffusion coefficient representing the effective ``collision'' frequency. We show that this effect of the plasma ``quasi-collisionality'' can strongly affect the growth rate and evolution of the Weibel instability in the deeply nonlinear regime. This result is especially important for understanding cosmic-ray-driven turbulence in an upstream region of a collisionless shock of a gamma-ray burst or a supernova. We demonstrate that the quasi-collisions caused by the fields generated in the upstream suppress the instability slightly but do not shut it down completely.

ABSTRACT. Stellar models require accurate thermonuclear reaction rates to predict the nuclear power production and dynamic evolution of these systems. Direct measurement of nuclear reaction rates in thermonuclear plasmas is challenging because these conditions are difficult to produce and diagnose. Still, there are physics issues such as plasma electron-screening or other plasma-nuclear effects that are present in stellar cores but not in terrestrial accelerator experiments.

Laser-based inertial confinement fusion (ICF) implosions produce extremely dense, hot plasmas that provide a path to study reactions in these thermonuclear conditions. However, ICF experiments have significant challenges not found in accelerator experiments. For example, the complex temporal and spatial evolution of these systems can make absolute cross-section measurements difficult and quite challenging to model. In this talk, we show that these issues can be overcome and ICF implosions can be used to make nuclear measurements in some specific circumstances.

In particular, the method of yield ratios is used to infer 2H(d,n)3He and 3H(t,2n)4He astrophysical S-factors by observing the 2H(d,n)3He and 3H(t,2n)4He yields relative to 3H(d,n)4He, in gas-filled implosions, using the 3H(d,n)4He reactivity as a reference. The resulting data shows excellent agreement with evaluations and prior accelerator data bolstering confidence in this method.

This technique is now being explored as a candidate for a future plasma-electron-screening experiment to attempt to observe enhancements to reaction rates in the presence of plasma electrons. Ongoing work to that end, will be shown.

*This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-768962

ABSTRACT. Laser-based fusion energy is a revolutionary solution for a sustainable energy future. While, reliance on deuterium-tritium (D-T) fuel holds societal implementation challenges, such as resource availability and environmental concerns, including tritium leakage and neutron-induced activation. Our study explores alternative fusion fuels’ viability through the proton-boron (p-11B) reaction. We have proposed a novel fuel geometry: a hollow spherical proton-boron configuration, contrasting the traditional pitcher-catcher setup. This geometry facilitates the acceleration and collision of protons and borons on the inner surface towards the center of the spherical fuel, creating a high-temperature, high-density proton-boron plasma core. We deployed multiple diagnostic techniques to investigate the p-11B fusion process. Alpha particles emitted from the p-11B reaction were measured using a Thomson parabola spectrometer and a solid-state track detector (CR-39), complemented by a machine learning-based track detection method for enhanced precision. This approach significantly surpasses alpha particle detection accuracy and analysis efficiency compared to the previous method. Additionally, the gamma-ray energy and half-life of byproducts of p-10B and p-11B reactions were measured with a Ge semiconductor detector; this measurement is crucial for assessing the reaction's frequency and efficiency. Experimental findings revealed that the hollow spherical-shell geometry induced more p-11B reactions than the flat-plate configuration, highlighting its advantageous feature. Yet, the reaction efficiency under the current conditions is lower than that of the pitcher-catcher approach, attributed to the insufficient focusing intensity of the LFEX laser for energetic proton acceleration in a blow-off plasma. The upgrade of the LFEX laser system involving implementation of new deformable mirrors aims to enhance the LFEX laser's focusing intensity to a peak of 1 x 10^20 W/cm^2 in 2024, expecting a significant boost in the p-11B reaction efficiency with hollow spherical fuels. Some of the diagnostic used in this study come from laboratory astrophysics techniques. Refining these diagnostics through p-11B fusion study could contribute to deepen the understanding of staller elemental synthesis and interior high-energy events.

ABSTRACT. The study of intense ion beam generation and transport in varying conditions of temperature and density is important for a variety of applications including high-yield neutron sources [1], exotic isotope creation [2], and Fast Ignition (FI) of Inertial Fusion Energy (IFE) [3]. The ion beams generated in high intensity short pulse laser-matter interactions provide unique means to isochorically heat targets. We have carried out a series of experiments on the Omega EP laser to measure and characterize proton energy deposition. We for the first time used both time-resolved and time-integrated emission x-ray spectroscopy to investigate the bulk heating dynamics of a proton-heated target. The EP laser (450-900 J, 5-10 ps) was focused onto a cone-enclosed partial hemisphere to generate and focus an intense proton beam onto a 10 µm- or 25 µm-thick solid copper sample. Experimental data shows that the cone structure provides enhanced focusing of proton beam for efficient heating of the target. The streaked x-ray spectrometer diagnosed the Cu Kα1 and Kα2 line emissions with a spectral resolution of <2 eV and a time resolution of ~2 ps, allowing to resolve temperature-dependent shifts of the lines; these correspond to sample temperatures up to ∼50 eV within ∼35 ps, according to atomic kinetics simulations. We compared these results with hybrid-PIC simulations and found consistent temperature evolution, when accounting for the temporal spreading of the proton beam as it traverses the cone.

[1] D. P. Higginson et al., “Transition to efficient, unsuppressed bulk-target ion acceleration via high-fluence laser irradiation”, Phys. Rev. Research 4, 033113 (2022). [2] F. Hannachi et al., “Prospects for nuclear physics with lasers” Plasma Phys. Controlled Fusion 49, B79 (2007). [3] M. Roth et al., “Fast ignition by intense laser-accelerated proton beams” Phys. Rev. Lett. 86 436 (2001).

The experiment was conducted at the OMEGA Laser Facility with the beam time through the National Laser Users’ Facility (NLUF) under the auspices of the U.S. DOE/NNSA by the University of Rochester’s Laboratory for Laser Energetics under Contract DE-NA0003856. This material is based upon work supported by the U.S. DOE/NNSA Award Number DE-NA0004147, and by the University of California San Diego under contract DE-NA0003943 (NLUF).

ABSTRACT. We perform a particle-in-cell study of the propagation of a relativistic laser beam through a plasma that is near its relativistically modified critical density. In this regime, the modification of the index of refraction of the plasma by the intensity spatial profile in a Gaussian beam causes focusing to occur, often to greater intensities than achievable without the plasma optics. This can help usher in new regimes of laser-plasma interactions where more exotic effects like pair production can begin to occur. We first perform studies with semi-infinite pulses and track the changing focal properties of the plasma; we explore this evolution’s dependencies on various plasma parameters. Following this, we perform more realistic ultra-short pulse simulations and measure the optical properties of the plasma throughout the propagation of the pulse to better understand this mechanism on the timescales characteristic of these systems.

ABSTRACT. Hawking radiation is one of the most striking consequences of combining quantum mechanics with gravity. However it is unlikely to be observable around astrophysical black holes. Instead, many related effects, especially Unruh radiation for accelerated observers, are more likely to have observationally verifiable signals. We explain how Unruh radiation manifests in the dynamics of highly accelerated electrons and its relationship to age-old problem of classical radiation reaction. We discuss the signatures of Unruh radiation in the highest gradient accelerators available in the lab: wakefield accelerators.

ABSTRACT. Radiative hydrodynamics models the coupling between the dynamics of a hypersonic hot plasma and the radiation it produces or external radiation. Almost every numerical codes use simplified models, that are in most cases either limited or wrong. To accurately model the photon transport the HADES code was specifically developed [2, 3, 4]. Such a code is preferable for studying astrophysical objects, in which optically intermediate regions are still poorly modeled, yet commonly encountered within such phenomena. Indeed, it has already been used in previous simplified versions.

This code solves the general equations of radiative hydrodynamics in the 2D case and uses the M1 model for radiation transfer [1]. Moreover, the M1-multigroup model is employed to accurately represent the spectral behavior of light, involving the partitioning of the electromagnetic spectrum into groups. This enables the simulation of photon transport within each group [6].

Radiative shock have undergone extensive numerical investigation, notably through the utilization of codes such as CRASH, FLASH, HERACLES, Hyades. . . Nevertheless, despite this extensive analysis, the complete impact of photon frequency dependence remains a topic requiring further investigation. Only HERACLES, with its detailed treatment of photon transport, has undertaken simulations accounting for this factor, employing the M1-multigroup method [8].

Thanks to a machine learning approach, a field of development of artificial intelligence, we have implemented an innovative computation of the closure relation in the M1-multigroup model [5]. Then we have been able to renew the study of the influence of the spectral behaviour of photons, using up to 100 groups, with a precision that has never been exhibited before. Our findings reveal substantial disparities in the academic shock structure compared to prior simulations. We are excited to unveil these results at the upcoming HEDLA conference.

References

[1] Dubroca, B. and Feugeas, J.-L., 1999, CRAS - Series I - Mathematics, vol. 329, p. 915-920 [2] Michaut, C., Nguyen, H.C. and Di Menza, L., 2011, ASS, vol. 336, p. 175–181 [3] Michaut, C., Di Menza, L., Nguyen, H.C., Bouquet, S.E. and Mancini, M., 2017, HEDP, vol. 22, p. 77-89 [4] Nguyen, H.C., 2011, Simulation de modèles hydrodynamiques et de transfert radiatif intervenant dans la description d’écoulements astrophysiques, PhD thesis, Paris 11 [5] Radureau, G. and Michaut, C., New computational method for multigroup radiative hydrodynamics using Artificial Intelligence: optimisation of the Eddington factor calculation, abstract submitted to this conference [6] Turpault, R., 2005, JQSRT, vol. 94, p. 357-371 [7] Vaytet, N.M.H., Audit, E., Dubroca, B. and Delahaye, F., 2011, JQSRT, vol. 112, p. 1323-1335 [8] Vaytet, N.M.H., González, M., Audit, E. and Chabrier, G., 2013, JQSRT, vol. 125, p. 105-122