David M. Gold

Energy Dependence of Emission Intensity and Temperature in a LIBS Plasma Using Femtosecond Excitation

In this paper, we investigate the effect of laser energy on laser-induced breakdown emission intensity and average temperature in a short-pulse plasma generated by using 140 fs laser excitation. Both line emission and continuum background intensity and plasma temperature decrease very rapidly after excitation compared to the more conventional nanosecond pulse excitation. Both emission intensity and plasma temperature increase with increasing laser energy. However, the intensity increase appears to be mostly related to the amount of material ablated. Also, nongated laser-induced breakdown spectroscopy (LIBS) is demonstrated using a high-pulse (1 kHz) pulse repetition rate.

Accurate measurement of laser-driven shock trajectories with velocity interferometry

We describe a velocity interferometer used to measure the velocity and trajectory of laser driven shocks in liquid deuterium accurately and continuously. This demonstration of velocity interferometry to measure shock velocity and shock trajectory in condensed matter shows strong potential for future studies of laser-driven shocks in transparent media. Accuracy of this technique can be better than 1% in velocity and ±0.2 μm in position during a 10 ns interval.

Shock timing technique for the National Ignition Facility

Among the final shots at the Nova laser [Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] was a series testing the VISAR (velocity interferometry system for any reflector) technique that will be the primary diagnostic for timing the shocks in a NIF (National Ignition Facility) ignition capsule. At Nova, the VISAR technique worked over the range of shock strengths and with the precision required for the NIF shock timing job—shock velocities in liquid D2 from 12 to 65 μm/ns with better than 2% accuracy. VISAR images showed stronger shocks overtaking weaker ones, which is the basis of the plan for setting the pulse shape for the NIF ignition campaign. The technique is so precise that VISAR measurements may also play a role in certifying beam-to-beam and shot-to-shot repeatability of NIF laser pulses.

Equation of state measurements of hydrogen isotopes on Nova

High intensity lasers can be used to perform measurements of materials at extremely high pressures if certain experimental issues can be overcome. We have addressed those issues and used the Nova laser to shock-compress liquid deuterium and obtain measurements of density and pressure on the principal Hugoniot at pressures from 300 kbar to more than 2 Mbar. The data are compared with a number of equation of state models. The data indicate that the effect of molecular dissociation of the deuterium into a monatomic phase may have a significant impact on the equation of state near 1 Mbar.

Equation of State and Material Property Measurements of Hydrogen Isotopes at the High-Pressure, High-Temperature, Insulator-Metal Transition

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 {approx}0.5 Mbar is indeed metallic. (c) 2000 The American Astronomical Society.

Prepulse suppression using a self-induced ultrashort pulse plasma mirror

The plasma mirror is a self-induced, plasma-based optical element which can be inserted into existing experiments to reduce prepulse energy without significant degradation of ultrashort pulse laser light. We have directly observed the nonlinear reflectivity of the plasma mirror as well as the spatial and temporal characteristics of the reflected pulse. The initial measurements indicate that the incident pulse reflects specularly from a high density, highly reflective plasma. The reflected pulse has a smoothed spatial profile and reduced pulsewidth. We outline future work to characterize both the plasma mirror technique of prepulse suppression and its reflected pulse.

Interferometric and Chirped Optical Probe Techniques for High-Pressure Equation-of-State Measurements*

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.

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.

DEVELOPING SOLID-STATE EXPERIMENTS ON THE NOVA LASER

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 {mu}m and amplitudes of 1.0-2.5 {mu}m 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.

High pressure solid state experiments on the nova laser

Abstract An x-ray drive has been developed to shock compress metal foils in the solid state in order to study the material strength under high compression. The drive has been characterized and hydrodynamics experiments designed to study growth of the Rayleigh-Taylor (RT) instability in Cu foils at 3 Mbar peak pressures have been started. Pre-imposed modulations with an initial wavelength of 20–50 μm, and amplitudes of 1.0–2.5 μm show growth consistent with simulations. In this parameter regime, 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 on the RT growth are significantly enhanced. Improved x-ray drive design will allow the material to stay solid under compression throughout the experiment, and dynamic diffraction techniques are being developed to verify the compressed state.