[en] Experiments are being developed to shock compress metal foils in the solid state to study the material strength under high compression. The x-ray drive has been characterized and hydrodynamics experiments performed to study growth of the Rayleigh-Taylor (RT) instability in Al foils at a peak pressure of about 1.8 Mbar. Pre-imposed modulations with an initial wavelength of lo-50 pm, and amplitude of 0.5 pm show growth. Variation in the growth factors may be a result of shot-shot variation in preheating of the Al sample due to emission from the plasma in the hohlraum target

[en] Simultaneous measurements of shock velocity and optical reflectance at 1064, 808, and 404 nm of a high pressure shock front propagating through liquid deuterium show a continuous increase in reflectance from below 10% and saturating at ∼(40-60)% in the range of shock velocities from 12 to 20 μm/ns (pressure range 17-50 GPa). The high optical reflectance is evidence that the shocked deuterium reaches a conducting state characteristic of a metallic fluid. Above 20 μm/ns shock velocity (50 GPa pressure) reflectance is constant indicating that the transformation is substantially complete. (c) 2000 The American Physical Society

[en] We have developed and tested several optical interferometric diagnostics to measure preheat and shock velocity in high-pressure equation of state experiments on the Nova laser. Theory and practical application of interferometric measurement techniques with illustrative experimental results are presented

[en] A high-intensity laser was used to shock compress liquid deuterium to pressures between 0.22 and 3.4 megabars (Mbar). Shock density, pressure, and temperature were determined using a variety of experimental techniques and diagnostics. This pressure regime spans the transformation of deuterium from an insulating molecular fluid to an atomic metallic fluid. Data reveal a significant increase in compressibility and a temperature inflection near 1 Mbar, both indicative of such a transition. Single-wavelength reflectivity measurements of the shock front demonstrated that deuterium shocked above ∼0.5 Mbar is indeed metallic. (c) 2000 The American Astronomical Society

[en] We present experimental work exploring displacement and velocity interferometry as high spatial and temporal resolution diagnostics for measuring target preheat and the speed, planarity, and steadiness of a shock wave. A chirped pulse reflectometry experiment is also proposed as a frequency domain alternative for shock speed measurements. These techniques fill a need for high-precision diagnostics to derive accurate laboratory-based equation-of-state data at shock wave-driven pressures directly relevant to astrophysical systems. The performance of these optical laser probe techniques may exceed conventional passive techniques such as temporally streaked recording of optical emission upon shock breakout or side-on streaked X-ray radiography. Results from Nova laser and high-intensity ultrashort pulse experiments are presented. (c) 2000 The American Astronomical Society

[en] Pyrometric measurements of single-shock-compressed liquid deuterium reveal that shock front temperatures T increase from 0.47 to 4.4eV as the pressure P increases from 31 to 230GPa. Where deuterium becomes both conducting and highly compressible, 30≤P le 50 GPa , T is lower than most models predict and T<< T_Fermi , proving that deuterium is a degenerate Fermi-liquid metal. At P>50 Gpa , where the optical reflectivity is saturated, there is an increase in the rate that T increases with P

[en] 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 where 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 experimentally verify. However, a new class of lasers, such as the Petawatt laser,Perry 1996 are capable of producing focused intensities greater than 10²⁰ W/cm ampersand sup2; where such relativistic effects can be observed and even dominate the laser-target interaction. 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,Taylor 1950, Sedov 1959 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.Chevalier 1982 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 ldw-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.Blondin et al. 1998 Until recently it has not been feasible to devise laboratory experiments wherein shock waves with initial pressures in excess of several hundred Mbar 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. We report on a new experiment 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 N 400 J of 1 pm laser radiation in a 20 ps pulse focused to a N 50 ,um diameter spot, which produces an intensity in excess of 10¹⁸ W/cm ampersand sup2;. These conditions approximate a point explosion and a blast wave is predicted to be generated with an initial pressure of several hundred megabars which decays as it travels approximately radially outward from the interaction region. 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 which 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

[en] An X-ray drive has been developed to shock compress metal foils in the solid state using an internally shielded hohlraum with a high contrast shaped pulse from the Nova laser. The drive has been characterized, and hydrodynamics experiments designed to study the growth of the Rayleigh-Taylor (R-T) instability in Cu foils at 3 Mbar peak pressures in the plastic flow regime have been started. Preimposed modulations with an initial wavelength of 20-50 μm and amplitudes of 1.0-2.5 μm show growth consistent with simulations. In the Nova experiments, the fluid and solid states are expected to behave similarly for Cu. An analytic stability analysis is used to motivate an experimental design with an Al foil where the effects of material strength of the R-T growth are significantly enhanced. The conditions reached in the metal foils at peak compression are similar to those predicted at the core of Earth. (c) 2000 The American Astronomical Society

[en] Initial laser-driven equation of state (EOS) experiments on liquid deuterium employed x-ray radiography to track the shock and particle speeds in the shock compressed sample. With the high pressures available with laser drivers we found that it is also possible to track the shock front directly with a velocity interferometer system for any reflector (VISAR) because the shock front reflects light across the visible spectrum with reflectance around 50% for shocks stronger than 50 GPa in liquid deuterium. We have observed similar reflectances in other dielectric samples, such as diamond, LiF, and water. The pressure required to produce a reflecting shock varies with each material. This phenomenon allows us to design impedance-matched EOS experiments using velocity interferometry to measure the propagation speed in the transparent shocked materials, and step breakout measurements to determine the speed in the pusher. In a different kind of experiment we have observed double shock compression in liquid deuterium by impacting a shock in liquid deuterium at a LiF anvil placed in the liquid sample. VISAR can be used to track the shock in the deuterium as well as the motion of the deuterium--LiF interface subsequent to impact. This allows us to diagnose double-shock states using the VISAR technique. As a final example VISAR can be used to track shock overtake events such as produced by shaped pulse compression or shock reverberation effects in the accelerating pusher. This capability is directly applicable to shock timing experiments needed to tune the drive pulse for inertial confinement fusion capsules on the National Ignition Facility