Currently, Forrest Doss is investigating the formation of structure in radiatively collapsed shocks. In the high-energy-density regime, radiative transport becomes significant, and the shocked layer undergoes a collapse to high densities due to the emission of large fractions of its thermal energy as radiation.
This work has identified a previously unobserved feature in systems with strong radiative transport. As the speed of a radiating shock increases, the corresponding radiation becomes strong enough to significantly heat the upstream shock tube wall. Beyond a threshold speed, discussed further below, the upstream radiation is sufficient to vaporize the tube material some distance ahead of the primary shock. This drives a secondary, radially converging shock, which we name a wall shock. The resulting observed structure can be used as a diagnostic of the dynamics.
This work also seeks to explain the apparent onset of instability in radiating shock experiments, and how it may be connected to the astrophysical case of the thin shell instability of a decelerating shock studied theoretically by Vishniac, among others. The production of both theory and experiments to investigate this structure are included in this research.
Recent theories suggest that the radiation signature of gamma ray bursts may be the result of the interaction of ultrarelativistic electrons, ejected from supernova shocks, with small-scale magnetic fields*. These tiny "tangled" magnetic fields are thought to be created by the two-stream filamentation instability, or Weibel Instability, of the beaming electrons. As the charged particles propagate, local density perturbations form lines of current, which create magnetic fields within the beam. These fields act to pinch the areas of higher electron density, forming filaments of characteristic diameter c/wp, where c is the speed of light and wp is the electron plasma frequency. Using the Hercules laser facility at the University of Michigan, we are conducting an experiment to create an electron beam by the laser wakefield technique, produce such filaments by passing the electron beam through another plasma, and image the resulting structure. Analysis of the beam structure will be compared with theory and simulation and will provide direction for future investigation of gamma ray burst signatures.
*Medvedev MV., Loeb A. Generation of Magnetic Fields in the Relativistic Shock of Gamma-Ray-Burst Sources. Astrophys.J. 526 (1999) 697-706
This research was sponsored by the National Science Foundation through Grant PHY-0114336 and by NNSA Stewardship Sciences Academic Alliances through DOE Research Grant DE-FG52-04NA00064.
SUBSONIC AND SUPERSONIC SHEAR FLOWS IN LASER DRIVEN HIGH-ENERGY-DENSITY PLASMAS
E.C. Harding, R.P. Drake, O.A. Hurricane, J.F. Hansen, Y. Aglitskiy, T. Plewa, B.A. Remington, H.F. Robey, J.L. Weaver, A.L. Velikovich, R.S. Gillespie, M.J. Bono, M.J. Grosskopf, C.C. Kuranz, A. Visco
Shear flows appear in many high-energy-density (HED) and astrophysical systems, yet few laboratory experiments have been carried out to study their evolution in these extreme environments. The distinction between subsonic and supersonic shear flows is important since the compressibility of the flow can influence the development of the shear layer. Most shear flows containing steep velocity gradients are Kelvin-Helmholtz (KH) unstable if the wavelength of the interface perturbation is much greater than the scale length of the velocity gradient. Until now the KH instability, in HED experiments, has been primarily observed in Rayleigh-Taylor unstable systems as a secondary instability. The shear layer generated between the high-density spikes and low-density bubbles causes the spike tips to grow into mushroom shaped structures. In this way the KH instability generates smaller length scales that allow the flow to dissipate energy and transition to a turbulent state. Understanding this transition is important since mixing rates and diffusivities can increase by several orders of magnitude in the turbulent regime.
We present two dedicated shear flow experiments that produced subsonic and supersonic shear layers in HED plasmas. In the subsonic case the Omega laser was used to drive a shock wave along a rippled plastic interface (single mode with = 400 m and amplitude = 30 m), which subsequently rolled-upped into large KH vortices that were accompanied by bubble-like structures. This was the first HED experiment to generate well-resolved KH vortices and observe their evolution. Interestingly the origins of bubble-like structures, which appeared as distinct, bright regions in the x-ray radiographs, are unknown. In a separate experiment, the Nike laser was used to drive a supersonic flow of Al plasma (Mach ~ 2-3) along a low-density foam surface seeded with a ripple (single mode with = 300 m and amplitude = 15 m). Unlike the subsonic case, detached shocks developed around the ripples in response to the supersonic Al flow. Both the Omega and Nike experiments have implemented novel target geometries that can be used for future investigations of the transition to turbulence in HED systems.
This research was sponsored by the Naval Research Laboratory through contract NRL N00173-06-1-G906, the NNSA Stewardship Sciences Academic Alliances through DOE Research Grant DE-FG52-04NA00064, the DOE NNSA under the Predictive Science Academic Alliance Program by grant DE-FC52-08NA28616, and under the National Laser User Facility by grant DE-FG03-00SF22021.
Carlos Di Stefano is continuing previous work done by Carolyn Kuranz in which laser energy is used to create a planar blast wave in a plastic disk. The disk is consequently accelerated into a lower-density foam, inducing the Rayleigh-Taylor instability. These experiments are intended to help understand the evolution of supernovae, and for this reason are well-scaled to the helium/hydrogen interface during the explosion phase of SN1987A. Current work seeks to refine diagnostic techniques as well as further examine features of the system's unstable behavior.