Showing papers by "T. W. Phillips published in 1998"

Diagnostics for high-density implosions at Nova and the national ignition facility

Abstract: The proposed National Ignition Facility (NIF) is a large (1.8 MJ on target at 0.35 {micro}m) multi-beam laser facility that will be used for Inertial Confinement Fusion (ICF). ICF implosions at this facility will produce core plasma temperatures over 10 keV and densities over 100 g/cm{sup 3}. Properties of these plasmas can be measured by a variety of optical, x-ray, and nuclear diagnostic techniques such as those used at existing facilities like the Nova laser at the Lawrence Livermore National Laboratory (LLNL). Some of these currently used techniques will be directly applicable to NIF; others require significant development. Damage of components close to the target will be a much greater issue at NIF, necessitating the development of distant detector techniques. To penetrate the larger targets, x-ray-based core diagnostics will need to utilize substantially higher energies than are in routine use today. Penetrating nuclear-particle-based diagnostics will be particularly well suited to these implosions, and the higher nuclear yields will allow new techniques to be developed. Some examples of diagnostics used for high-density-implosion experiments at Nova and corresponding development of new techniques for NIF are discussed.

Nuclear experiments with petawatt class lasers

Abstract: A number of experiments using the petawatt laser system at LLNL have now been performed to examine the creation of hot electrons and their use in the fast ignition approach to ICF. By changing the experimental conditions, the electron spectrum can be shifted towards higher energy, even in solid density plasmas. In this regime the laser-target interaction is dominated by the enormous light pressure and relativistic effects. New instruments and techniques are required to diagnose the plasma conditions because many traditional plasma diagnostics are not longer effective. High-field magnetic dipole spectrometers were fielded to measure the electron spectrum emerging from the back of the target and orthogonal to the laser axis. The dispersed electron spectrum is recorded as tracks in a standard emulsion. Electrons were observed at energies extending above 90 MeV, however the bulk of the distribution was in the range of /spl sim/5-15 MeV where the emission was found to be forward directed. High-energy bremsstrahlung X-rays generated by these electrons in the gold target produced photonuclear reactions in both the gold and surrounding copper target-holder producing activation and transmutation to platinum and nickel daughter isotopes. Post-shot g-spectroscopy was used to examine the production of these and other radioactive isotopes via photonuclear reactions. The threshold gamma-ray energy for photoactivation of the gold and copper indicates a large flux of high-energy bremsstrahlung. A gamma spectrum and decay curve are shown. In addition to these data, neutron spectra and other high energy X-ray spectra are also presented. Together, these diagnostics are providing us the first glimpse into this new regime of laser-matter interactions.