Session BO07: HED: Laboratory Astrophysics I

Live

Hot, massive stars emit an abundance of photons with energies that ionize the surrounding interstellar medium. Gas clouds near the star respond to the irradiation in a variety of ways. If the cloud is optically thick to incident photons, then the photons will deposit at the cloud edge and drive a shock into the cloud. Further star formation may result. If the cloud is optically thin, then the photons deposit throughout the cloud, causing cloud heating, expansion and dissipation. Lengthy timescales of evolution make single cloud observations impractical. Simulations and laboratory astrophysics experiments are needed to fully understand these interactions. We replicate these phenomena using optical depth-scaled laboratory astrophysics experiments. A laser-irradiated, thin-gold foil represents the star. A primarily carbon foam sphere represents the cloud. Cloud responses - hydrodynamic limits - are chosen by careful selection of sphere parameters, such as composition, density and diameter. A preliminary comparison between optically thick experimental data and a simple analytic model is presented. This work is funded by the U.S. DOE NNSA Center of Excellence under Cooperative Agreement number DE-NA0003869, and the NLUF Program, grant number DE-NA0002719, and through the LLE, University of Rochester by the NNSA/OIFC under Cooperative Agreement No. DE-NA0003856. This work is funded by the LLNL under subcontract B614207.   

Live

Shock reflections are common features in astrophysical systems, for example in the internal structure of~protostellar~jets. We present experimental results from a laboratory study of oblique shock reflection in magnetised,~high density plasmas.~Shocks are produced by placing multiple obstacles into the supersonic, super-Alfv\'{e}nic~outflow from an ablating wire array z-pinch on the MAGPIE pulsed power facility.~Magnetic field pile-up can be controlled by altering the obstacle material and orientation with respect to the magnetic field.~ We compare experiments both with and without magnetic field pile-up and discuss the differences in shock reflection geometry.~In the presence of magnetic field pile-up, the compression ratio~is determined by the strong (5T) advected magnetic field.~The temperature of the shocked flow is consistent with an adiabatic compression of both the ions and electrons across the shock, which is caused by two fluid effects at the shock front.   

Live

This talk reports on a new experimental platform, that uses 50T externally imposed B-fields, producing unique plasma conditions, with both strongly magnetized electrons (e) and ions (i). The first set of experiments produced e and i Hall parameters of \textasciitilde 40 and \textasciitilde 5 respectively, based on the experimentally measured temperatures, convergence, and fuel composition, and \textasciitilde 7MG B-field. The field is flux compressed in these implosions because the B-field diffusion time (\textasciitilde 10$^{\mathrm{-6}}$s) is much longer than the implosion time (\textasciitilde ns). We observe, for the first time, that these high B-fields increased the (P2) anisotropy in implosions. Suppression of thermal transport by the strongly magnetized electrons is the primary mechanism for this effect. This platform opens-up opportunities for studies of (i)-Knudsen number reduction and (e)-thermal transport suppression in strongly magnetized HED plasmas.   

Live

In experiments performed with the OMEGA EP laser system, magnetic field generation in double ablation fronts was observed. Proton radiography measured the strength, spatial profile, and temporal dynamics of self-generated magnetic fields as the target material was varied between plastic (CH), aluminum, copper, and gold. Two distinct regions of magnetic field are generated in mid-Z targets -- one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced key aspects of the experiment, including field generation and double ablation front formation.   

Live

We present the design of a novel all-optical platform to magnetize laser-driven cylindrical implosions at the OMEGA facility and characterize them via X-ray line emission to investigate the effects of magnetization. A pair of capacitor-coil targets driven by OMEGA beams is expected to produce a seed B-field of \textasciitilde 50 T along the cylinder. The cylindrical targets are filled with Ar-doped D$_{\mathrm{2}}$ gas and symmetrically imploded using a 36-beam 15~kJ, 1.5 ns laser drive. Proton radiography and magnetic B-dot probes will be used to characterize the seed B-field. The 2-D numerical simulations performed with the MHD code GORGON predict a compressed B-field \textgreater 10 kT at stagnation, with $\rho $ \textgreater 1~g/cm$^{\mathrm{3\thinspace }}$and T$_{\mathrm{e}}$ \textgreater 1 keV. This magnetic field is strong enough to impact the hydrodynamic behavior and alter the characteristic conditions of the compressed core throughout the implosion. Initial results of the platform will be discussed and compared to our modeling predictions.   

Live

Collective Thomson-scattering experiments were used at the Omega Laser Facility to measure the heat-wave propagation in plasmas where an external magnetic field was scaled to 36 T in a 2-mm-diam gas-jet plasma. At the highest fields, the magnetic-field pressure was equal to the plasma pressure ($\beta \approx $ 1). These results are being used to study the limitations of thermal-transport models used in current hydrodynamic codes. Initial experimental and simulation results will be presented.   

Live

We apply a strong magnetic field to modify the expansion of a relativistic high energy density plasma into a neutral gas environment. Energy transport during plasma expansion into neutral gas is enhanced relative to plasma expansion into vacuum via a long-lasting relativistic ionization wave launched by the sheath electric field of the expanding plasma, which may be undesirable for applications. In this work, we use 1D kinetic simulations to examine the impact of an applied magnetic field on the propagation of the ionization wave. We find that an experimentally relevant 100 T-level magnetic field is capable of stopping the ionization wave propagation. In addition, we will discuss the condition for the ionization wave to stop in terms of the physical scales associated with the ionization wave and the applied magnetic field.   

Live

We demonstrate that the irradiation of a thin opaque target with an embedded seed magnetic field by a relativistically intense laser pulse can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the rear target surface. This magnetic field generation is a kinetic effect associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We present a simple predictive scaling for this phenomenon and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. For kilotesla-level seed fields, the strong seed and surface-generated magnetic fields can have a pronounced impact on application-relevant plasma dynamics, including ion acceleration from $\mu$m-thick targets.   

Abstract Withdrawn

First-principles kinetic simulations are used to investigate magnetic field generation processes in expanding ablated plasmas relevant to laser-driven foils and hohlraums. Strong filamentary magnetic filaments are found to grow in the corona of single expanding plasma plumes; such filaments are observed to outcompete Biermann-battery generation at sufficiently large laser focal radius, reaching saturation values of $\sim$ 100 T at National Ignition Facility-like conditions. The filamentary fields result from the ion-Weibel instability driven by relative counter-streaming between the ablated ions and a sparse background population, which could result from a gas prefill in a hohlraum or laser pre-pulse. The filamentation is robust with the inclusion of collisions and grows on a timescale of 100 ps, with a wavelength on the scale of 100-250 $\mu$m, over a wide range of background population densities; the instability also gives rise to coherent density oscillations. These results are of particular interest to inertial confinement fusion experiments, where such field and density perturbations can modify heat-transport as well as laser propagation and absorption.   

Live

Collisionless shocks are ubiquitous in astrophysical plasmas and are known to be important in magnetic field amplification and in the acceleration of both high-energy electrons and protons (cosmic rays). While the theory of diffusive shock acceleration (DSA) is well established, the details of particle injection into DSA remain a long-standing puzzle, particularly for electrons. Very recently, laser-driven high-energy-density experiments at the National Ignition Facility (NIF) have observed for the first time high-Mach number Weibel-mediated collisionless shocks and the associated nonthermal electron acceleration. We will discuss results from large-scale particle-in-cell simulations of counter-streaming plasma flows for the conditions of the NIF experiments. This study reveals that electrons can be effectively injected by multiple scatterings in small-scale turbulence produced within the shock front via a first order Fermi mechanism. We will present detailed analysis of the characteristic diffusion properties and energization associated with this mechanism, and discuss its relevance to electron injection in young supernova remnant shocks.   

Live

Laboratory laser experiments offer a novel approach to studying magnetized collisionless shocks, and a common method in recent experiments is to drive shocks using a laser-ablated piston plasma. However, current experimental capabilities are still limited to spatio-temporal scales on the order of shock formation, making it challenging to distinguish piston and shock dynamics. We present quasi-1D particle-in-cell simulations [1] of piston-driven, magnetized collisionless shock formation using the code PSC, which includes a model of laser-driven plasmas that can be well-matched to experimental conditions. The simulations cover a range of upstream and downstream parameters, and yield several robust signatures of shock formation that can provide a reference for experiments where traditional characterizations of shocks, such as the Rankine-Hugoniot jump conditions, are not easily applied. The results indicate that there are three key timescales in the evolution of piston-driven shocks, from the formation of a shock precursor to the development of a well-defined downstream region, which can be identified through measurements of ion velocity distributions or careful observations of density and temperature profiles. [1] Schaeffer, et al., Phys. Plasmas 27, 042901 (2020).   

Live

The study of astrophysically relevant collisionless shocks has largely been constrained to numerical and theoretical investigations until now. Recent experiments at OMEGA and the NIF have shown that electromagnetic collisionless shocks can be generated in the laboratory. The OMEGA experiments created magnetic piston generated collisionless shocks which are distinct from Weibel mediated shocks. This was achieved by colliding a supersonic plasma flow with a gas bag filled with H2 and enclosed by a thin CH shell. The explosion of the gas bag shell upon collision is not well understood and is currently too difficult to probe. Using a gas jet instead of a gas bag removes the complication of CH shell. These gas jet experiments offer a simpler system to study. Here, we present results from these gas jet experiments at OMEGA. A supersonic plasma flow intercepts the gas jet H2 gas puff, providing physical insight into the shock formation. This work is supported in part by the DOE U.S. DOE, the MIT/NNSA CoE, and NLUF.   

Live

X-ray opacities of hot dense matter are essential to radiation-hydrodynamic models of stars, other astrophysical objects, inertial confinement fusion, and other high-energy-density experiments. Opacity experiments on the National Ignition Facility (NIF) seek to replicate and extend results from the Sandia Z facility, which often diverge from theory. Promising initial measurements on NIF at temperatures \textasciitilde 150 eV and electron densities \textasciitilde 7x10$^{\mathrm{21}}$/cm$^{\mathrm{3}}$ also revealed areas for improvement. Improvements in backlighting, spatial resolution and signal uniformity, and reduction of backgrounds will be summarized. Recent data from Al:Mg samples will be presented in this context.   

Live

The soft x-ray Opacity Spectrometer (OpSpec) used on the National Ignition Facility (NIF) has recently incorporated an elliptically shaped crystal. The original OpSpec used two convex cylindrical crystals for time-integrated measurements of point-projection absorption spectra from 540 to 2100eV. However, with the convex geometry, the low-energy portion of the spectrum suffered from high backgrounds due to scattered x-rays as well as reflections from alternate crystal planes. The use of an elliptically shaped crystal allows an acceptance aperture at the crossover focus between the crystal and the detector, which reduces background and eliminates nearly all reflections from alternate crystal planes. The current elliptical design is an improvement from the convex cylindrical design, but has a usable energy range from 900 to 2100eV. In addition, OpSpec is currently used on 18 NIF shots per year, in which both crystals are typically damaged beyond reuse, so efficient production of 36 crystals/year is required. Design efforts to improve the existing system focus on mounting reliability, reducing crystal strain to increase survivability between mounting and shot time, and extending the energy range of the instrument down to 500 eV. The elliptical design, results, and future options are presented.