Session PO7: Magnetized HED Plasmas and Laborary Astrophysics

Time-dependence of laser-driven magnetic field generation for long laser drive conditions

Magnetized high energy density (HED) laser experiments provide a way to test reduced physics models for heat and particle transport in plasmas as well as the growth of instabilities.  Laser coils provide an all-optical B-field generation technique which uses a laser-drive region to source the current to a conducting loop and achieve B-fields of up to 1 kilo-Tesla.  Recent experiments on the Omega-EP laser have used proton deflection to measure the temporal evolution of laser-coil fields for laser pulse lengths up to 10 ns.  The experiments show the B-field increasing during the laser pulse and then decaying after laser turn-off.  Existing models show agreement with some of the data but may require refinements to achieve a more comprehensive description of the measurements.  We will discuss the laser-coil experiments and the status of the models.   

Diagnostics, modeling and applications of magnetized jet creation using a ring of laser beams

Using 20 OMEGA laser beams in a hollow ring configuration to irradiate a flat plastic target, we created supersonic, well collimated plasma jets with self-generated megagauss magnetic fields. The jet was diagnosed by optical Thomson scattering, X-ray framing camera and proton radiography. Three dimensional FLASH MHD modeling is carried out to interpret the experimentally created jets. Multiple axially aligned magnetic flux ropes with alternating poloidal component was predicted and observed. The jets we created have a number of unique properties that will allow us to study important actrophyical physical processes and the effects of anisotropic heat conduction.

Plasma jet formation from laser ablation of thin aluminium film in a transverse magnetic field

Plasma jets are often observed near accretion disks and in laser plasma experiments. We perform a 2D nonlinear MHD simulation to model the experimentally observed plasma jet formation following the laser ablation of a thin aluminium film in an external transverse magnetic field. In a recent experiment, the laser-ablated plasma can expand into a bubble structure both with and without an external magnetic field, and that a jet is formed at the top of the bubble only when there is an applied transverse magnetic field. The initial laser-produced plasma condition is prepared using the radiation-hydrodynamics code FLASH. The subsequent plasma evolution in an external magnetic transverse field is simulated using the initial-value MHD code NIMROD. We find that the plasma expansion is squeezed by the transverse magnetic field on both sides of the bubble to form a jet structure in the center, which may explain why in experiment the jet can only be observed in a transverse magnetic field. This may help understand the jet formation in both astrophysical and laboratory systems.   

An Experiment to Observe Photoionization Fronts in the Laboratory

An experiment at the Omega-60 laser investigates radiation front propagation in a regime where the radiation transport is non-diffusive yet the initial target is many mean free paths in extent. The atomic physics at the front is such that photoionization is the dominant heating mechanism there. This type of front is relevant to star-forming regions and the age of reionization of the universe. In the experiment, a thin Au foil irradiated with a laser energy flux of ~10^14 W/cm^2, creates soft x-rays source that are incident on a N gas cell. The N is doped 1% Ar to enable absorption spectroscopy measurements. We will show preliminary results from upcoming experiments.   

Neon photoionized plasma experiments at Z and Zebra

We discuss two experiments to study the atomic kinetics in astrophysically relevant photoionized plasmas. Both experiments employ the intense x-ray flux of a wire-array Z-pinch to heat and backlight neon photoionized plasmas. The Z Machine produces up to 200 TW of x-ray power in less than 10 ns. A cm-scale gas cell is placed near the pinch and filled with atom number densities of 10¹⁷ to 4x10¹⁸ cm^-3. Windows along the line of sight thru the cell permit an x-ray absorption measurement showing K-shell line absorption from Be- to H-like neon ions, revealing a highly ionized neon plasma. At the Zebra generator, the x-ray flux is lower power but longer in duration. The gas is positioned closer to the pinch as a super-sonic gas jet with densities of 10¹⁸ cm^-3. Absorption spectra here also show K-shell line absorption but from C- to He-like neon ions. Analysis of the spectra in both cases yields ion areal densities and charge state distributions that can be compared with simulation results from atomic kinetics codes. The electron temperature extracted from Li-like ion level population ratios can be used to test heating models of photoionized plasmas.   

Photoionization of Super Sonic Gas Jet Targets Utilizing the 1MA Zebra Generator.

In astrophysical objects such as x-ray binary systems, active galactic nuclei, and black hole accretion disks a high-intensity broadband x-ray flux produces photoionized plasmas. Laboratory photoionized plasma experiments enable systematic studies relevant to astrophysics and provide data to test theory and benchmark modeling codes. An experimental platform has been developed to study photoionized plasmas via the photoionization of supersonic gas jets, using a 1MA class pulsed power generator. The gas jet targets are irradiated and ionized by a 25ns-duration x-ray flux produced by the implosion of a wire array. Neon, argon, and nitrogen gas jets have been investigated. Laser and x-ray diagnostics were used to probe the neutral gas jet as well as the photoionized plasma. Specifically, Mach-Zehnder interferometry at 266nm, air wedge interferometry at 266 and 532nm, and shadowgraphy at 266, 532, and 1064nm. The data from these measurements allowed us to determine the average ionization, charged state distribution, and map the topology of the neutral and ionized gas jets. Interferometry measured electron densities ~ 10¹⁸ - 10¹⁹cm^-3. X-ray spectroscopy showed neon photoionized plasmas with L-shell ions.