Laboratory astroparticle physics

Abstract:

We have measured the production of by 4.7 keV x-rays from low-density Ti-doped aerogel (rho approximate to 3 mg/cc) targets at the OMEGA laser facility (University of Rochester), with the goal of maximizing x-ray output. Forty OMEGA beams (lambda(L) = 0.351 mum) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser fully ionizes the target (n(e)/n(crit) less than or equal to 0.1), and a laser-bleaching wave excites, supersonically, the high-Z emitter ions in the sample. Heating in the target was imaged with gated x-ray framing cameras and an x-ray streak camera. Ti K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and also with a set of filtered aluminum x-ray diodes, both instruments provide absolute measurement of the multi-keV x-ray emission. We find between 40 - 260 J of output with 4.67 less than or equal to by less than or equal to 5.0 keV. Radiation-hydrodynamic calculations predict late time enhancement of the x-ray power due first to axial stagnation of the heating waves, then, ablatively-driven radial compression from the target walls.

Abstract:

We present simplified expressions for the dynamic structure factor, or form factor S(k, omega), which is the quantity describing the inelastic x-ray scattering cross section from a dense plasma or a simple liquid. Our results, based on the random phase approximation (RPA) for the treatment on the charged particle coupling, are compared with analytical expressions for the free electron dynamic structure factor which include effects of strong coupling in both classical and degenerate plasmas. We will show that these modifications introduce minimal corrections to the RPA for typical conditions found in recent non-collective x-ray Thomson scattering experiment on solid density isochorically heated laser plasmas. On the other hand, strong collective scattering may exhibit significant deviations from the RPA. The results shown in this work can be applied to interpreting future x-ray scattering in warm dense plasmas occurring in inertial confinement fusion experiments or for the modelling of solid density matter found in the interior of planets.

Abstract:

A study on the x-ray scattering from solid density plasmas was presented. By applying spectrally resolved multi-keV scattering, the measurements of the microscopic properties of dense matter were demonstrated. The scattering spectra from solid density beryllium demonstrated the inelastic Compton-down shifted feature that is spectrally broadened when heating the solid density plasmas isochorically and homogeneously to temperatures of several times the Fermi energy.

Abstract:

We present the first spectrally resolved x-ray scattering measurements from solid-density plasmas. The scattering spectra show the broadened Compton down-shifted feature allowing us to determine the electron temperature and density with high accuracy. In the low temperature limit, our data indicate that the ionization balance reflects the electrons in the conduction band consistent with calculations that include quantum mechanical corrections to the interaction potential.

Abstract:

An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I<1.5x10(15) W cm(-2), SBS is in a linear regime and is moderated primarily by velocity gradients, while for intensities above this threshold, nonlinear trapping is relevant. Direct evidence of detuning of SBS by a velocity gradient was achieved by directly measuring the frequency of the SBS-driven acoustic wave relative to the local resonant acoustic frequency. The frequency and amplitude of the ion-acoustic wave directly responsible for SBS has been measured as a function of space using a 3omega 200 ps Thomson-scattering probe beam. Furthermore, direct evidence of kinetic effects associated with the SBS process in the nonlinear regime has been investigated through a novel use of Thomson scattering. Specifically, a measured twofold increase in the ion temperature has been linked with ion-acoustic waves that have been driven to large amplitudes by the SBS instability. Ion-acoustic waves were excited to large amplitude with a 2omega 1.2-ns-long interaction beam with intensities up to 7x10(15) W cm(-2). The measured twofold increase in the ion temperature and its correlation with SBS reflectivity measurements provides quantitative evidence of hot ions created by ion trapping in laser plasmas. These detailed and accurate measurements in well-characterized plasma conditions allow a direct test of linear and nonlinear models of the saturation of SBS. (C) 2003 American Institute of Physics.