Session UO4: Laboratory Astrophysics

OMEGA Laser--Driven Hydrodynamic Plasma Jet Experiments with Relevance to Astrophysics

Using the University of Rochester's OMEGA laser, experimental techniques have been developed to study plasma jets. A charge-coupled-device (CCD) detector was configured to measure the jet evolution with a high signal-to-noise ratio compared with previous film detectors. The evolution of experimental supersonic plasma jets was observed over many dynamical times. Double-pulsed jets looked similar to single-pulsed jets at times long compared to the pulse separation. The bow-shock profiles of the experimental jets matched the predictions of an astrophysical energy-driven jet model. These jet experiments extend the applicable regime of impulsive, energy-driven jet simulations to density contrasts greater than 1. The experimental jets were observed under controlled conditions during earlier stages of development than astrophysical jets can be observed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.   

Laboratory Studies of High Mach Number Shock Collisions with Foils and Density Discontinuities.

The dynamics of strong shocks, blast waves and radiative blast waves play a key role in astrophysical objects such as nebulae and supernova remnants. Our understanding of these complex systems is underpinned by numerical simulations, however despite decades of work modeling of such phenomena remains extremely challenging. The interaction of strong shocks with discontinuities and reflecting surfaces represents a particularly demanding scenario against which to test rad-hydrocodes. As a result we have been developing scaled laboratory experiments to provide high quality data for code benchmarking, and to aid our physical insight. We report on experimental and numerical investigations into the interaction of strong shocks and blast waves with solid obstructions and density discontinuities. Shocks were driven by focusing a high intensity 750fs laser into a near atmospheric density atomic cluster medium at 10E17 W/cm\^{}2. Placing a solid foil in the gas stream used to create clusters produced a hydrodynamic bow shock, allowing us to investigate both shock-foil and shock-density discontinuity interactions. Shock evolution was followed as a function of time with an optical probe via Schlieren and interferometric imaging techniques. Numerical modeling of our experimental test system was carried out using the 3D magnetoresistive hydrocode GORGON.   

3D Tomographic imaging of colliding cylindrical blast waves

The interaction of strong shocks {\&} radiative blast waves is believed to give rise to the turbulent, knotted structures commonly observed in extended astrophysical objects. Modeling these systems is however extremely challenging due to the complex interplay between hydrodynamics, radiation and atomic physics. As a result we have been developing laboratory scale blast wave collision experiments to provide high quality data for code benchmarking, {\&} to improve our physical understanding. We report on experimental {\&} numerical investigations of the collision dynamics of counter propagating strong ($>$Mach 50) cylindrical thin-shelled blast waves driven by focusing intense laser pulses into an extended medium of atomic clusters. In our test system the blast wave collision creates strongly asymmetric electron density profiles, precluding the use of Abel inversion methods. In consequence we have employed a new tomographic imaging technique, allowing us to recover the full 3D, time framed electron density distribution. Tomography {\&} streaked Schlieren imaging enabled tracking of radial {\&} longitudinal mass flow {\&} the investigation of Mach stem formation as pairs of blast waves collided. We have compared our experimental system to numerical simulations by the 3D magnetoresistive hydrocode GORGON.   

Spike Extensions in Rayleigh-Taylor, Decelerating-Interface Experiments

This presentation discusses experiments well-scaled to the blast wave driven explosion phase of SN1987A. These experiments, performed at the Omega Laser facility, use $\sim $ 5kJ of laser energy to create a blast wave similar to those in supernovae. The blast wave crosses a perturbed interface with a density drop and produces Rayleigh-Taylor instability (RTI) growth. By performing experiments with more complex, three-dimensional initial conditions, we hope to observe the effect their complexity has on RTI growth. Recent advancements in x-ray backlighting have greatly improved the resolution of our x-ray radiographic images. These images show some mass extending beyond the spike tips. This presentation will discuss the amount of mass in these spike extensions. This research was sponsored by the NNSA through DOE Research Grants DE-FG52-07NA28058, DE-FG52-04NA00064, and other grants and contracts.   

Supersonic plasma jet interaction with gases and plasmas at the PALS laser facility

The interaction of supersonic plasma jets with dense gases and plasmas has been studied experimentally and theoretically. Under suitable conditions on the laser intensity, spot radius and target atomic number, a radiative jet can be launched from a simple planar target with a 100 J laser pulse [Ph. Nicolai et al, Phys. Plasmas 13, 062701 (2006)]. A typical copper jet has a velocity around 500 km/s, a Mach number greater than 10, a density around 10$^{18}$ cm$^{-3}$, a length of a few millimeters and a radius of 0.5 mm. The interaction of such a jet with Ar and He gas puffs at different pressures has been studied by using various optical and x-ray diagnostics. Qualitative estimates and numerical simulations with a 2D radiation hydrodynamic code allow to explain a sequence of physical processes during the interaction, which include the collision of two plasmas, shock propagation and radiation cooling. Variations in the atomic number and pressure of a target plasma allow us to control the role of radiative and kinetic processes in the jet evolution.   

Laser-triggered millimeter-scale collimated plasma jets in crossed electric and magnetic fields.

Some physical aspects of astrophysical jets can be scaled to laboratory experiments using magneto hydrodynamic scaling laws. We present a laser plasma-triggered jet experiment where we produce a millimeter-scale collimated outflows from a cylindrically symmetric electrode configuration. The electrode design consists of a grounded plane with a $\sim $1 cm diameter hole and a wire aligned normally to this plane, with its tip placed at the center of the hole. A rapid discharge is formed between the wire and ground plane when a laser pulse hits an aluminum target placed above the electrodes, creating a plasma which closes the circuit. The resulting current and corresponding magnetic field give rise to a plasma jet. The jets were 0.1-0.3 cm wide, about 2 cm in length, had velocities of $\sim$20 km/s and an estimated plasma density of less than 10$^{17}$ particles/cm$^3$. To study the effects of magnetic fields on jet evolution, we have embedded the plasma in axially directed permanent magnetic fields with strength up to 0.4 Tesla. We have measured the evolution of the jet over a duration of $\sim$1 $\mu$s with nanosecond resolution using a fast ICCD camera and interferometry. Under certain conditions the jets also form helical structures due to kink instabilities and the onset is characterized.   

Experimental Study of Episodic Magnetically Driven Radiatively Cooled Plasma Jets

Previous experiments on the 1MA MAGPIE generator have successfully showed the formation of magnetically driven radiatively cooled plasma jets which are relevant to the launching of astrophysical jets. The jets in these experiments are driven by the pressure of the toroidal magnetic field produced by the current, which leads to the formation of a ``magnetic tower'' structure. This scenario is characterized by the formation of a magnetic ``bubble'' surrounding a collimated plasma jet on axis. A modification of this experimental configuration, in which radial wire array is replaced by radial metallic foil, results in the formation of episodic magnetic tower outflows which emerge periodically on timescales of $\sim$30ns. The subsequent magnetic bubbles propagate with higher velocities (increasing from $\sim$100km/s to $\sim$300km/s) and interacting with previous eruptions leading to the formation of shocks. This experimental setup also allows the study of the interaction of episodic outflows with an ambient medium. This research was supported by the EU JETSET network and the NNSA under DOE Cooperative Agreement DE-FC03-02NA00057.   

Photon-photon scattering in vacuum and astophysical plasmas

We present for the first time the nonlinear dynamics of quantum electrodynamic (QED) photon splitting in an electron-positron plasma that is held in a super-strong magnetic field. Such plasmas exist in magnetars, and may also arise in the next generation laser-plasma experiments. By using a QED corrected Maxwell equation, we derive a set of equations that show the existence of nonlinear couplings between electromagnetic (EM) waves due to nonlinear plasma currents and QED polarization and magnetization effects. Numerical analyses of our coupled nonlinear EM wave equations reveal the possibility of a new decay interaction, as well as new features of energy exchange among the three EM modes that are nonlinearly interacting in a magnetized pair plasma. Applications of our investigation to astrophysical settings, such as magnetars, are pointed out.   

Line intensity enhancements in both stellar and laser-plasma coronal X-ray spectra due to opacity effects

The intensity of optically thin transitions increase linearly with optical depth. As one might expect an optically thick line to increase less quickly than linearly, the thick to thin ratio is normally thought to decrease with increasing optical depth. However, for systems in coronal equilibrium, this is not necessarily the case, and this ratio can have enhancements that are a function of plasma geometry and viewing angle. Here we consider the X-ray spectra for a number of late-type active stars, obtained with the Reflection Grating Spectrometer on the XMM-Newton satellite. Both flare and quiescent spectra are considered, and intensity ratios studied which involve the Fe XVII 15.01 {\AA} and 16.78 {\AA} transitions. We consider a large dataset for a number of stars, and in particular the case of EV Lac, where the 15.01 {\AA} line exhibits an enhancement in intensity over the optically thin value, which we interpret in terms of a geometry consistent with a largely planar feature on the surface of the star being observed at an angle of order 45 degrees. We show that such enhancements due to opacity should also be observable in laser-produced plasmas of specific geometry.   

Iron plasma transmission measurements at temperatures above 150 eV

Measurements of iron plasma transmission at 156 $\pm$ 6 eV electron temperature and 6.9 $\pm$ 1.7 $\times 10^{21}$ cm$^{-3}$ electron density are reported over the 800-1800 eV photon energy range. The temperature is more than twice that in prior experiments, permitting the first direct experimental tests of absorption features critical for understanding solar interior radiation transport. Detailed line-by-line opacity models are in excellent agreement with the data. Applications may require simplified models employing different approximations and the work described here provides a new ability to estimate the accuracy compromises that result. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under contract No. DE-AC04-94AL85000.   

Radiative cooling in relativistic collisionless shocks. Can simulations and experiments probe relevant GRB physics?

We address the question of whether PIC simulations and laboratory laser-plasma experiments can (or will be able to, in a near future) model realistic gamma-ray burst (GRB) shocks. For this, we compare the radiative cooling time, $t_{\rm cool} $, of relativistic electrons in the shock magnetic fields to the microscopic dynamical time of collisionless relativistic shocks, $\omega_{pp}^{-1}$. We have obtained that for $t_{\rm cool}\omega_{pp}^{-1}\la$~few hundred, the electrons cool efficiently at or near the shock jump and are capable of emitting away a large fraction of the shock energy. Such shocks are well-resolved in the existing 2D PIC simulations, therefore the microscopic structure can be studied in details, whereas the spectral power of the emitted radiation can also be directly obtained from simulations and compared with observational data. The conditions in the GRB shocks are almost identical to those in laser-produced plasmas; thus, such GRB-like plasmas can be created and studied in laboratory experiments using the presently available Petawatt-scale laser facilities.   

Shock-processing of astrophysical dust grains

We are developing a new capability to carry out experiments on the shock processing of astrophysical dust grains and cratering of space hardware due to hypervelocity interplanetary dust particle (IDP) impacts. A 527 nm, 200 J laser launches a shock through a target consisting of a 25 $\mu $m plastic ablator followed by a 200 $\mu $m, 100 mg/cm$^{3}$ foam. Dust grains embedded near the rear surface of the foam experience pressures of $\sim $200 kbar in a $<$50 ps spike, simulating astrophysical pressure conditions in grain-grain collisions. A first experiment shows acceleration of 5 $\mu $m diameter Al$_{2}$O$_{3}$ dust grains to several km/s as measured by particle image velocimetry using a double-pulsed probe laser that is Mie-scattered off the grains. The grains were allowed to impact high-purity Cu foils where they caused abundant cratering, similar to what is seen on recovered space hardware after exposure to IDPs. The cratering is currently being studied in scanning electron microscopes and a preliminary analysis will be presented.