[en] The purpose of this project was to develop a high peak power laser system (100 TW) and begin initial high intensity experiments that exploit its short pulse width (30 fs) and high repetition rate (1 - 10 Hz). Such a laser system presents unique capabilities such as permitting ultrafast time-resolved plasma physics experiments by probing the plasma with the 30 fs laser pulse. The high repetition rate also allows detailed, systematic studies of phenomena, not possible with large, single shot laser systems. During the previous year we have made good progress on the development of the laser. We have demonstrated the production of pulses up to the 5 TW level at 10 Hz and have installed an additional amplifier to take the system to 20 TW. We have pulse compressed the pulses to 30 fs and have developed a number of diagnostics to characterize the laser prepulse. During this year we have also activated a target chamber to begin plasma physics experiments in gas jet targets

[en] An intense ultrafast laser pulse can be very strongly absorbed in a moderate density gas composed of van der Waals bonded clusters. In this paper, the deposition of the energy of intense 30 fs light pulses in a gas of deuterium clusters has been diagnosed using a technique based on analysis of the trajectories of the resulting cylindrically symmetric blast waves. Using the well-known relation between blast wave velocity and energy deposition in gas, the laser energy deposited per unit length as a function of distance in gas jet plume was measured. These measurements were conducted in jets containing either deuterium clusters or simple deuterium molecules

[en] A high brightness photoinjector has been developed at LLNL. This injector combined with the 100 TW FALCON laser and the LLNL 100 MeV S-Band RF linac will enable development of a high brightness, femtosecond-scale, tunable, hard x-ray probe for time-resolved material measurements, based on Thomson scattering. Short pulse x-rays enable time-resolved characterization of shock dynamics, and examination of materials under extremes of pressure and temperature. Examples include Equation of State characterization on high-density materials, Crystal disorganization and re-growth in shocked and heated materials, and measurement of short time scale phase transition phenomena. Single shot evaluation, requiring high peak flux, is important for complex experiments such as probing of laser shocked actinides. A low emittance electron beam synchronized with femtosecond accuracy to an intense laser will revolutionize x-ray dynamics studies of materials. This project will lead development of ultrafast x-ray dynamics research on problems important in physics, chemistry, biology, and materials

[en] An understanding of radiation effects on the evolution of shock waves is of great importance to many problems in astrophysics. Shock waves driven by a laser-heated plasma are attractive for laboratory investigation of these phenomena. Recent studies of intense short-pulse laser interactions with gases of atomic clusters indicate a potential avenue to access this regime of radiative hydrodynamics. We have measured the energy absorption efficiency of high-intensity, picosecond laser pulses in low-density gases composed of large atomic clusters and find that the energy absorption can be very high (>95%), producing a high-temperature plasma filament which consequently produces a strong blast wave. Interferometric characterization of these shock waves indicates that in high-Z gases such as Xe, radiation transport plays an important role in the evolution of the shock wave. (c) 2000 The American Astronomical Society

[en] We have resolved the expansion of intensely irradiated atomic clusters on a femtosecond time scale. These data show evidence for resonant heating, similar to resonance absorption, in spherical cluster plasmas

No abstract available

[en] In this work we present temperature diagnostics of an expanding laser-produced argon plasma. A short-pulse (35fs) laser with an intensity of I = 10¹⁷W/cm² deposits ∼ 100 mJ of energy into argon clusters. This generates a hot plasma filament that develops into a cylindrically expanding shock. We develop spectral diagnostics for the temperatures of the argon plasma in the shock region and the preionized region ahead of the shock. A collisional-radiative model is applied to explore line intensity ratios derived from Ar II - Ar IV spectra that are sensitive to temperatures in a few eV range. The results of hydrodynamic simulations are employed to derive a time dependent radiative transport calculation that generates the theoretical emission spectra from the expanding plasma

[en] We simulate experiments performed with the Falcon laser at Lawrence Livermore National Laboratory to generate strong, cylindrically diverging blast waves of relevance to astrophysics. In particular, we are interested in producing and modeling radiative shocks. We compare numerical simulations with the data and with an analytic approximation to blast-wave propagation with a radiative-loss term included. Our goal is to develop a laboratory setting for studying radiative shocks of relevance to supernova remnants, gamma-ray burst afterglows, and other high-energy astrophysics phenomena. We will show that a good degree of agreement exists between the experimental data and the numerical simulations, demonstrating that it is indeed possible to generate radiative shocks in the laboratory using tabletop femtosecond lasers. In addition, we show how we can determine the energy-loss rate from the blast-wave evolution. This analytic method is independent of the exact mechanism of radiative cooling and is scalable to both the laboratory and astrophysical radiative blast waves. (c) 2000 The American Astronomical Society

[en] The dynamics of electrons ionized from high charge states by lasers with intensity >10²⁰ W/cm² have been studied. At these intensities vxB forces drive the electrons subsequent to ionization in a trajectory nearly parallel to the laser propagation direction. This gives rise to large energy gains as the electron rides in phase with the laser field over a long distance. Monte Carlo simulations illustrate that, unlike in case of ionization in sub- and near-relativistic intensity fields (<10¹⁹ W/cm²), the electron dynamics in the ultrarelativistic case are strongly influenced by the longitudinal electric fields found near the focus of a tightly focused laser

[en] High Mach number shockwaves were launched in laboratory plasmas to simulate supernova shockwave propagation. The experiments were carried out at inertial fusion facilities using large lasers. Spherical shocks were created by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas. Ablated material from the pin would rapidly expand and launch a shock through the surrounding gas. Planar shocks were created by ablating material from one end of a cylindrical shocktube. Laser pulses were typically 1 ns in duration with ablative energies ranging from <1 J to >4 kJ. Shocks were propagated through various plasmas, and observed at spatial scales of up to 5 cm using optical and x-ray cameras. Interferometry techniques were used to deduce densities, and emission spectroscopy data were obtained to infer electron temperatures. Experimental results confirm that spherical shocks are Taylor-Sedov, and that radiative shocks stall sooner than non-radiative shocks. Unexpected results include the birth of a second shock ahead of the original, stalling shock, at the edge of the radiatively preheated region. We have begun experiments to simulate the interaction between shocks and interstellar material (ISM), and the subsequent turbulent mixing. Comparisons between experimental data and numerical simulations of shock evolution, stall, second shock birth, and interstellar material (ISM) interaction will be presented