A quasi-spherical z-pinch may directly compress foam or deuterium and tritium in three dimensions as opposed to a cylindrical z-pinch, which compresses an internal load in two dimensions only. Because of compression in three dimensions the quasi-spherical z-pinch is more efficient at doing pdV work on an internal fluid than a cylindrical pinch. Designs of quasi-spherical z-pinch loads for the 28MA 100ns driver ZR, results from zero-dimensional (0D) circuit models of quasi-spherical implosions, and results from 1D hydrodynamic simulations of quasi-spherical implosions heating internal fluids will be presented. Applications of the quasi-spherical z-pinch implosions include a high radiation temperature source for radiation driven experiments, a source of neutrons for treating radioactive waste, and a source of fusion energy for a power generator.

Particle-in-cell simulations aimed at developing methods to control the relativistic electron beam blowup observed in recent laser-plasma experiments are described. By radially layering vacuum gaps and/or dissimilar materials with varying ionizability, a negative radial gradient in plasma density would be formed. This gradient results in confining fields that can, in principle, confine the hot electron column to nearly the laser injection spot size. Fully kinetic ion dynamics are included, to account for heavy particle transport effects across interfaces. Potential applications include radiography, electron beam focusing, and perhaps beam collimation for fast ignition. Experiments are presently being planned to test this concept.

We describe how the powerful technique of spectrally resolved Thomson scattering can be extended to the x-ray regime, for direct measurements of the ionization state, density, temperature, and the microscopic behavior of dense cool plasmas. Such a direct measurement of microscopic parameters of solid density plasmas could eventually be used to properly interpret laboratory measurements of material properties such as thermal and electrical conductivity, EOS and opacity. In addition, xray Thomson scattering will provide new information on the characteristics of hitherto difficult to diagnose Fermi degenerate and strongly coupled plasmas.

Rayleigh–Taylor (RT) instability of cylindrical, imploding plasma liners in a Z-pinch is analyzed. The reduction in total perturbation growth for multicascade systems (multiple imploding shells) is presented. This reduction is effective if the pressure produced by the impacting shell exceeds the magnetic pressure at the time of impact. Analytical and numerical solutions are also obtained for the RT instability of an imploding plasma liner accelerated into undisturbed plasma. The snowplow model is used in which the mass encountered by the imploding magnetic piston is swept into an infinitely thin sheath. A shock front launched ahead of the liner is shown to reduce the growth rate. It is also shown that accumulating the mass increases the growth rate. However, the total perturbation growth can be reduced if the liner accumulates the mass during the implosion compared to a liner with the same mass imploding into vacuum. Finally, it is shown that the final kinetic energy density for a given shell nonuniformity is largest if the final liner mass is accumulated during the implosion.

We report on recent Thomson scattering measurements from two ion species laser-produced plasmas. A flat disk coated with multiple alternate thin layers of gold and beryllium was irradiated with one laser beam of the Nova laser facility and the resulting two ion species plasma was probed with a second laser beam at a distance of 500 μm with time-resolved spectroscopy of the Thomson scattered light. A controlled variation of the Au fraction of the plasma was achieved by changing the relative thicknesses of the individual multilayers of the target. Two ion acoustic waves belonging to the Au and Be species were clearly observed. Besides their usual application to deduce electron temperatures, the relative damping of both waves provides an accurate measurement of the ion temperature of the plasma. In addition, the relative ion densities can be measured with high accuracy from the relative phase velocities of both waves if the ion charge state is known independently or vice versa.

Annular Al-wire Z-pinch implosions on the Saturn accelerator [D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference (Institute of Electrical and Electronics Engineers, New York, 1987), p. 310] that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh–Taylor (RT) instability prior to the first compression seeds an m=0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional (2-D) radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These simulations agree qualitatively with the measured characteristics of the first x-ray pulse and the overall energetics, and they provide a 2-D view into the plasma hydrodynamics of the implosion.

In this paper we report 2D radiation magnetohydrodynamic simulations of a dynamic hohlraum target designed to be driven by the Z accelerator (1) at Sandia National Laboratory, Albuquerque New Mexico. Z generates currents up 20 MA with a rise time of 100ns and peak electrical power of 40 TW. In this design we attempt to reduce the effects of magneto-Rayleigh Taylor (RT) modes by using a distributed initial density profile. Earlier work (2,3) showed that "tailoring" the initial density profile could reduce the sheath acceleration and the number of e-foldings that the RT instability grows during the implosion. As the sheath moves in radially, fresh material is swept up or "snow plowed", providing a back pressure that counters the J x B force. A special profile can be found in which the unstable outer surface of the sheath implodes at constant velocity, reducing the classical growth rate to zero, although residual Richtmeyer-Meshkov type instability (instability of the snow-plow shock front) may be present. In practice, it is hard to create tailored initial density profiles due to the difficulty of machining and otherwise manipulating very low density materials. It becomes easier to manufacture these complex targets as the current, energy and load mass increase with large drivers. Z is the first fast pulse power device with enough energy to consider loads of this type.

We report the first observations of two separate ion-acoustic waves in a two-ion-species plasma with Thomson scattering. A flat disk coated with thin multilayers of gold and beryllium was irradiated with one laser beam, and the resulting two-ion-species plasma was probed with a second laser at a distance of 500 mu m. The phase velocities of the ion-acoustic waves are shown to be a sensitive function of the relative concentrations of the two-ion species. Moreover, an accurate measurement of the ion temperature can be derived from the relative damping of the two-ion-acoustic waves.

Two-dimensional LASNEX [National Technical Information Service Document No. DE 81026329 (Zimmerman, Report No. UCRL-74811, 1973)] calculations are made for a Z pinch on Saturn, the low-impedance, low-inductance electron accelerator at the Sandia National Laboratories [D. D. Bloomquist et al. Proceedings of the Sixth IEEE Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (Institute of Electronics and Electrical Engineers, New York, 1987), p. 310]. The experiment is characterized by a current of 6 MA with a tungsten wire load (4 mg) at 2 mm. Two-dimensional calculations show the evolution of the Rayleigh–Taylor instability to the bubble and spike phase, causing high-density islands to form in the pinch opposite the bubbles. The two-dimensional energy flow causes a ‘‘hot spot’’ to evolve, which is shown to agree in its size and brightness with pinhole camera measurements. This is the first explicit calculation of a hot spot in two dimensions employing the full magnetohydrodynamic equations.

Z-pinch implosions driven by the SATURN device [D. D. Bloomquist et al., Proceedings of the 6th Institute of Electrical and Electronics Engineers (IEEE) Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] at Sandia National Laboratory are modeled with a two-dimensional radiation magnetohydrodynamic (MHD) code, showing strong growth of the magneto-Rayleigh–Taylor (MRT) instability. Modeling of the linear and nonlinear development of MRT modes predicts growth of bubble-spike structures that increase the time span of stagnation and the resulting x-ray pulse width. Radiation is important in the pinch dynamics, keeping the sheath relatively cool during the run-in and releasing most of the stagnation energy. The calculations give x-ray pulse widths and magnitudes in reasonable agreement with experiments, but predict a radiating region that is too dense and radially localized at stagnation. We also consider peaked initial density profiles with constant imploding sheath velocity that should reduce MRT instability and improve performance. Krypton simulations show an output x-ray power ≳80 TW for the peaked profile.

A systematic experimental study of annular aluminum-wire Z-pinches on a 20-TW electrical generator shows that the measured spatial characteristics and emitted x-ray power agree more closely with rad-hydro simulations when large numbers of wires are used. The measured x-ray power increases first slowly and then rapidly with decreasing interwire gap spacing. Simulations suggested that this increase reflects the transition from implosion of individual wire plasmas to one of an azimuthally symmetric plasma shell. In the plasma-shell regime, x-ray powers of 40 TW are achieved.

We have conducted experiments with aluminum, copper, and gold wires arrays to study x‐ray production, z‐pmch stability, and heating mechanisms. Time‐resolved x‐ray pinhole camera data and PCD data clearly show that the maximum in the keV x‐ray production occurs after peak compression during an expansion phase.The data are consistent with a sheath whose width is determined by the number of wires of the initial array (azimuthal uniformity) and the array diameter. The final stagnated size of the pinch is set by that sheath thickness. The stagnation event is characterized by a relatively uniform initial pinch reaching, in some cases, a diameter < 1 mm, followed by a disruption and increased x‐ray production. The source of this energy is apparently not kinetic. The kinetic portion of the energy should have been converted to thermal energy near the time of maximum compression. For aluminum and copper wire arrays, heating from classical Spitzer resistivity is insufficient to explain the observed x‐ray yields. While our present data does not show the actual location of current sheath nor does it give the position of non‐radiating plasmas and, hence, is incomplete, the data when taken as a whole is strongly suggestive of enhanced resistive dissipation and an MHD instability with a helical shape.