An experiment is described in which an interface between materials of different density is subjected to an acceleration history consisting of a strong shock followed by a period of deceleration. The resulting flow at this interface, initiated by the deposition of strong laser radiation into the initially well characterized solid materials, is unstable to both the Richtmyer-Meshkov ͑RM͒ and Rayleigh-Taylor ͑RT͒ instabilities. These experiments are of importance in their ability to access a difficult experimental regime characterized by very high energy density ͑high temperature and pressure͒ as well as large Reynolds number and Mach number. Such conditions are of interest, for example, in the study of the RM/RT induced mixing that occurs during the explosion of a core-collapse supernova. Under these experimental conditions, the flow is in the plasma state and given enough time will transition to turbulence. By analysis of the experimental data and a corresponding one-dimensional numerical simulation of the experiment, it is shown that the Reynolds number is sufficiently large (ReϾ10 5 ) to support a turbulent flow. An estimate of three key turbulence length scales ͑the Taylor and Kolmogorov microscales and a viscous diffusion scale͒, however, shows that the temporal duration of the present flow is insufficient to allow for the development of a turbulent inertial subrange. A methodology is described for estimating the time required under these conditions for the development of a fully turbulent flow.

We measured the stopping of energetic protons in an isochorically heated solid-density Be plasma with an electron temperature of ∼32 eV, corresponding to moderately coupled ½ðe 2 =aÞ=ðk B T e þ E F Þ ∼ 0.3 and moderately degenerate ½k B T e =E F ∼ 2 "warm-dense matter" (WDM) conditions. We present the first highaccuracy measurements of charged-particle energy loss through dense plasma, which shows an increased loss relative to cold matter, consistent with a reduced mean ionization potential. The data agree with stopping models based on an ad hoc treatment of free and bound electrons, as well as the average-atom local-density approximation; this work is the first test of these theories in WDM plasma.

Laser induced fluorescence measurements of the parallel and perpendicular ion temperatures in a helicon source indicate the existence of a substantial ion temperature anisotropy, T ⊥ /T > 1. The magnitude of the ion temperature anisotropy depends linearly on the source magnetic field. The parallel ion temperature is independent of magnetic field strength while the perpendicular temperature increases linearly with increasing magnetic field. Bohm-like particle confinement is proposed as an explanation for the linear dependence on magnetic field of the perpendicular ion temperature. In the helicon mode, the ion temperature components are independent of RF driving frequency and power and show a trend towards isotropy at high neutral fill pressures.

Variations in the plasma parameters of a large volume, helicon source as a function of applied rf power ͑0-2 kW͒, driving frequency ͑8-18 MHz͒, magnetic field ͑0-1.4 kG͒ and fill pressure ͑2-10 mTorr͒ have been studied. The transitions between the capacitive, inductive, and resonant helicon mode are consistent with previous experiments. Our data indicate that the transition to the helicon mode occurs at a unique magnetic field, independent of the driving frequency. Based on the helicon wave dispersion relation, from which helicon wavelengths can be calculated, the observed variations in plasma density as a function of driving frequency suggest that the wavelength of the helicon wave is a weak function of driving frequency. Calculation of the electron energies which correspond to the phase velocity of the driving wave ͑i.e., Landau damping͒ suggest that either Landau damping cannot be responsible for the efficient ionization of helicon sources, or that the helicon portion of the discharge does not extend over the entire radius of the apparatus.

More than a decade after the explosion of supernova 1987A, unresolved discrepancies still remain in attempts to numerically simulate the mixing processes initiated by the passage of a very strong shock through the layered structure of the progenitor star. Numerically computed velocities of the radioactive 56 Ni and 56 Co, produced by shock-induced explosive burning within the silicon layer, for example, are still more than 50% too low as compared with the measured velocities. To resolve such discrepancies between observation and simulation, an experimental testbed has been designed on the Omega Laser for the study of hydrodynamic issues of importance to supernovae ͑SNe͒. In this paper, results are presented from a series of scaled laboratory experiments designed to isolate and explore several issues in the hydrodynamics of supernova explosions. The results of the experiments are compared with numerical simulations and are generally found to be in reasonable agreement.