Development of a radiative-hydrodynamics testbed using the Petawatt Laser facility

Many of the conditions believed to underlie astrophysical phenomena have be en difficult to achieve in a laboratory setting. For example, models of sup ernova remnant evolution rely on a detailed understanding of the propagatio n of shock waves with gigabar pressures at temperatures of 1 keV or more, a t which radiative effects can be important. Current models of gamma-ray bur sts posit a relativistically expanding plasma fireball with copious product ion of electron-positron pairs, a difficult scenario to verify experimental ly. However, a new class of lasers, such as the Petawatt Laser, is capable of producing focused intensities greater than 10(20) W cm(-2), at which suc h relativistic effects can be observed and even dominate the laser-target i nteraction. We report here on the development of a testbed using the Petawa tt Laser to study the evolution of strong, radiative shock waves. There is ample evidence in observational data from supernova remnants of the afterma th of the passage of radiative shock or blast waves. In the early phases of supernova remnant evolution, the radially expanding shock wave expands nea rly adiabatically since it is traveling at a very high velocity as it begin s to sweep up the surrounding interstellar gas. A Sedov-Taylor blast wave s olution can be applied to this phase when the mass of interstellar gas swep t 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 mas s of the stellar ejecta, the evolution transitions to a radiative phase whe rein 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 wit h a radiative shock as its outer boundary, the pressure-driven snowplow. Un til recently it has not been feasible to devise laboratory experiments wher ein shock waves with initial pressures in excess of several hundred megabar s 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 p hases of evolution. This new experiment was designed to follow the propagat ion 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 irradi ation of the front surface of a layered, solid target with similar to 400 J of 1 mu m laser radiation in a 20 ps pulse focused to a similar to 50 mu m diameter spot, which produces an intensity in excess of 10(18) cm(-2). The se conditions approximate a point explosion, and a blast wave that has an i nitial pressure of cm several hundred megabars and that decays as it travel s approximately radially outward from the interaction region is predicted t o be generated. We have utilized streaked optical pyrometry of the blast fr ont to determine its time of arrival at the rear surface of the target. App lications of a self-similar Taylor-Sedov blast wave solution allows the amo unt of energy deposited to be estimated. By varying the parameters of the l aser pulse that impinges on the target, pressures on the order of 1 Gbar wi th initial temperatures in excess of 1 keV are achievable. At these tempera tures and densities radiative processes are coupled to the hydrodynamic evo lution of the system. Short-pulse lasers produce a unique environment for t he study of coupled radiation hydrodynamics in a laboratory setting.