[en] Thirty wave features, observed in 3.6 and 13 cm-wavelength optical depth profiles of Saturn's rings obtained by Voyager 1 radio occultation, are analyzed individually and comparatively. Many are the signature of spiral density waves and bending waves excited by gravitational resonances with Saturn's satellites. A new technique for locating waveform extrema, which fits a sinusoid to each half cycle of wave data, quantifies the wavelength variation across a feature. Fitting dispersion models to the derived wavelengths provides new estimates of ambient surface mass density σ in each wave region. For fourteen weak density waves in Ring A, modelling of the waveform near resonance with linear density wave theory gives independent estimates of σ, as well as reliable estimates of resonance location. Measurements of wave amplitude damping give an upper bound for ring thickness 2H, where H is the ring scale height. In the wave regions studied, Rings A, B, and C have 30 approx-lt σ approx-lt 70, σ approx-gt 65, and σ ∼ 1 g/cm², respectively. Mass loading estimates from waveform modelling are 20 to 40% larger than dispersion-derived values, suggesting accumulation of mass in the wave regions. The average offset of derived wave location from theoretical resonance is about 1 km. Model waveforms of overlapping waves excited by the satellites Janus and Epimethenus agree well with observed morphologies in the linear region near resonance. In Ring C, dispersion analysis indicates that the most prominent wave feature, previously unidentified, is a one-armed spiral wave

[en] The development of reconstruction algorithms that correct for diffraction effects in radio occultation measurements is described. The reciprocal Fresnel transform relationship between the complex amplitude of the observed coherent signal and the complex microwave transmittance of the rings is derived using the Huygens-Fresnel formulation of the diffraction problem. The effects of the finite data segment width, the uncertainties in the Fresnel scale, systematic phase errors in the kernel of the inverse transform, reference oscillator instabilities, and random noise measurements on the resolution of the reconstructed transmittance are analyzed. Examples of reconstructed opacity profiles for some regions of Saturn's rings derived by applying the reconstruction theory to Voyager 1 at Saturn data are presented. 35 references

[en] We describe x-ray streak camera measurements of wall motion and plasma filling in hohlraum targets heated by the AWE HELEN laser. An x-ray streak camera using a transmission mode photocathode on a thin plastic substrate (1000 Angstrom Parylene-N) was coupled to a 15 degree incidence gold mirror to define a spectral channel response of width 45 eV full width at half-maximum centered around 120 eV. A 20 μm diam pinhole was used to image the hohlraum interior onto the photocathode slit of the streak camera, via the gold reflector. Plasma expansion from the laser hot spots, and the indirectly heated wall, was recorded. The experimental data are compared with simulations using the AWE Lagrangian hydrocode NYM. copyright 1997 American Institute of Physics

[en] The Voyager 2 encounter with the Neptune system included radio science investigations of the masses and densities of Neptune and Triton, the low-order gravitational harmonics of Neptune, the vertical structures of the atmospheres and ionospheres of Neptune and Triton, the composition of the atmosphere of Neptune, and characteristics of ring material. Demanding experimental requirements were met successfully, and study of the large store of collected data has begun. Neptune's atmosphere was probed to a pressure level of about 5 x 10⁵ pascals, and effects of a methane cloud region and probable ammonia absorption below the cloud are evident in the data. Results for the mixing ratios of helium and ammonia are still being investigated; the methane abundance below the clouds is at least 1 percent by volume. Derived temperature-pressure profiles to 1.2 x 10⁵ pascals and 78 kelvins (K) show a lapse rate corresponding to frozen equilibrium of the para- and ortho-hydrogen states. Neptune's ionosphere exhibits an extended topside at a temperature of 950 ± 160 K if H⁺ is the dominant ion, and narrow ionization layers of the type previously seen at the other three giant planets. Triton has a dense ionosphere with a peak electron concentration of 46 x 10⁹ per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 ± 16 K in N₂⁺ is the principal ion. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; however, the accuracy of the measurements is limited by uncertainties in the frequency of the spacecraft reference oscillator. Preliminary values for the surface pressure of 1.6 ± 0.3 pascals and an equivalent isothermal temperature of 48 ± 5 K are suggested, on the assumption that molecular nitrogen dominates the atmosphere

[en] Supersonic fluid flow and the interaction of strong shock waves to produce jets of material are ubiquitous features of inertial confinement fusion (ICF), astrophysics, and other fields of high energy-density science. The availability of large laser systems provides an opportunity to investigate such hydrodynamic systems in the laboratory, and to test their modeling by radiation hydrocodes. We describe experiments to investigate the propagation of a structured shock front within a radiation-driven target assembly, the formation of a supersonic jet of material, and the subsequent interaction of this jet with an ambient medium in which a second, ablatively driven shock wave is propagating. The density distribution within the jet, the Kelvin-Helmholz roll-up at the tip of the jet, and the jet's interaction with the counterpropagating shock are investigated by x-ray backlighting. The experiments were designed and modeled using radiation hydrocodes developed by Los Alamos National Laboratory, AWE, and Lawrence Livermore National Laboratory. The same hydrocodes are being used to model a large number of other ICF and high energy-density physics experiments. Excellent agreement between the different simulations and the experimental data is obtained, but only when the full geometry of the experiment, including both laser-heated hohlraum targets (driving the jet and counter-propagating shock), is included. The experiments were carried out at the University of Rochester's Omega laser [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)]

[en] The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The spatial structure and mass evolution of supersonic jets has previously been investigated in detail. In this paper, the results from the first series of hydrodynamic experiments will be presented in which the mass distribution within the jet was quantified. In these experiments, two of the first four beams of NIF (national ignition facility, Usa) are used to drive a 40 Mbar shock wave into millimeter scale aluminum targets backed by 100 mg/cm³ carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray back-lighter source for diagnosing the structure of the jet. Comparisons between data and simulations using several codes are presented. Particularly the areal mass density distribution measured in the 2-dimensional jet experiment is compared to the results of 5 codes. It appears first, that the data has no regions with an areal mass density greater than 15 mg/cm² whereas each simulation shows regions between 15 and 22 mg/cm² and secondly that the data shows a higher percentage of regions in the 4-7 mg/cm² range. These 2 discrepancies are consistent with mixing of the higher density region jet material into lower density regions. This mixing can be attributed to the small-scale flow features of the high Reynolds number (> 10⁶) jet which are not captured in the simulations

[en] A first set of shock timing, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4 beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics (HEDP). In parallel, a robust set of optical and X-ray spectrometers, interferometer, calorimeters and imagers have been activated. The experiments have been undertaken with laser powers and energies of up to 8 TW and 17 kJ in flattop and shaped 1-9 ns pulses focused with various beam smoothing options. The experiments have demonstrated excellent agreement between measured and predicted laser-target coupling in foils and hohlraums, even when extended to a longer pulse regime unattainable at previous laser facilities, validated the predicted effects of beam smoothing on intense laser beam propagation in long scale-length plasmas and begun to test 3-dimensional codes by extending the study of laser driven hydrodynamic jets to 3-dimensional geometries. (authors)