Development of a Radiative-Hydrodynamics Testbed Using the Petawatt Laser Facility

Many of the conditions believed to underlie astrophysical phenomena have been difficult to achieve in a laboratory setting. For example, models of supernova remnant evolution rely on a detailed understanding of the propagation of shock waves with gigabar pressures at temperatures of 1 keV or more, at which radiative effects can be important. Current models of gamma-ray bursts posit a relativistically expanding plasma fireball with copious production of electron-positron pairs, a difficult scenario to verify experimentally. However, a new class of lasers, such as the Petawatt Laser, is capable of producing focused intensities greater than 1020 W cm-2, at which such relativistic effects can be observed and even dominate the laser-target interaction. We report here on the development of a testbed using the Petawatt Laser to study the evolution of strong, radiative shock waves. There is ample evidence in observational data from supernova remnants of the aftermath of the passage of radiative shock or blast waves. In the early phases of supernova remnant evolution, the radially expanding shock wave expands nearly adiabatically since it is traveling at a very high velocity as it begins to sweep up the surrounding interstellar gas. A Sedov-Taylor blast wave solution can be applied to this phase when the mass of interstellar gas swept up by the blast greatly exceeds the mass of the stellar ejecta, or a self-similar driven wave model can be applied if the ejecta play a significant role. As the mass of the swept-up material begins to greatly exceed the mass of the stellar ejecta, the evolution transitions to a radiative phase wherein the remnant can be modeled as an interior region of low-density, high-pressure gas surrounded by a thin, spherical shell of cooled, dense gas with a radiative shock as its outer boundary, the pressure-driven snowplow. Until recently it has not been feasible to devise laboratory experiments wherein shock waves with initial pressures in excess of several hundred megabars and temperatures approaching 1 keV are achieved in order to validate the models of the expanding blast wave launched by a supernova in both of its phases of evolution. This new experiment was designed to follow the propagation of a strong blast wave launched by the interaction of an intense short-pulse laser with a solid target. This blast wave is generated by the irradiation of the front surface of a layered, solid target with ~400 J of 1 μm laser radiation in a 20 ps pulse focused to a ~50 μm diameter spot, which produces an intensity in excess of 1018 W cm-2. These conditions approximate a point explosion, and a blast wave that has an initial pressure of several hundred megabars and that decays as it travels approximately radially outward from the interaction region is predicted to be generated. We have utilized streaked optical pyrometry of the blast front to determine its time of arrival at the rear surface of the target. Applications of a self-similar Taylor-Sedov blast wave solution allows the amount of energy deposited to be estimated. By varying the parameters of the laser pulse that impinges on the target, pressures on the order of 1 Gbar with initial temperatures in excess of 1 keV are achievable. At these temperatures and densities radiative processes are coupled to the hydrodynamic evolution of the system. Short-pulse lasers produce a unique environment for the study of coupled radiation hydrodynamics in a laboratory setting.