Session ZI2: Z Pinches, HED Science, Target Fabrication and Invited Postdeadline

The implosion and stagnation of wire array z-pinches

Experiments at all levels of current drive have demonstrated that the first 60-80 percent of the evolution of a wire array z- pinch is dominated by the gradual ablation of cold, dense wire cores into low density coronal plasma that is projected towards the axis of the array. Implosion of the array only begins when the wire cores start to break up, at which time a piston of current snowploughs up coronal plasma as it accelerates towards the axis. Here we present the detailed measurements of the snowplough process and the dynamics of the array during its stagnation on axis. The stability and width of the snowplough and the compression of the plasma at stagnation are related to X-ray emission, providing data on the mechanisms responsible for X-ray production.\newline Several methods to alter the implosion of an array are explored. The interaction between the outer and inner of a nested array configuration is directly observed for the first time, highlighting how X-ray emission can be shaped. In a new type of array – a ``coiled'' array – the magnetic field topology is altered, resulting in large changes to the ablation dynamics and an implosion that snowploughs less mass to a higher velocity. With a relatively low number of wires, the use of coiled arrays can increase X-ray emission by $\sim$5x over that usually observed at stagnation. Using a radial array configuration, meanwhile, the scale of the stagnating plasma can be reduced without adversely affecting X-ray power. This may enable arrays to couple to far smaller hohlraums, significantly raising the available temperatures for HEDP experiments. \newline \newline This research was sponsored by Sandia National Labs and the NNSA under DOE Cooperative Agreement DE-F03-02NA00057.   

Deuterium Gas-Puff Z-Pinch Implosions on the Z accelerator

The generation of neutrons via current driven sources, including z-pinch driven hohlraums, deuterium gas puffs, deuterium fiber pinches, deuterium liners, and dense plasma foci, has been studied for many years. Experiments with methods other than inertial confinement fusion have produced significant neutron output (up to $\sim $10$^{12})$ from experiments with current drives $<$ 8 MA. In this paper, the results of experiments at the Z Accelerator to study the neutron production and implosion characteristics of a deuterium gas puff will be presented. Two current levels (12MA and 15MA) were fielded to evaluate the scaling of the neutron output; neutron outputs of 1 x 10$^{13}$ and 3 x 10$^{13}$ were measured. The neutron output measured was the first with a load of this type at this current level and has been demonstrated to be repeatable, with side-on time-of-flight measurements showing 2.34 MeV. While the mechanism for the neutrons has not been identified experimentally, this neutron output is 100 times more than previously observed from neutron producing experiments at Z. Comparison of the neutron output with previous experiments at 7 MA shows that the neutron output scales approximately as I$^{4}$. Time-of-flight measurements from multiple directions, as well as the results of activation diagnostics will be presented. The experimental results will be compared with 1D, 2D, and 3D magneto-hydrodynamic (MHD) calculations, which have shown that thermal neutron outputs from Z could be expected to be in the (0.3 to 1.0) x 10$^{14}$ range. Dopant gases were added to track the implosion characteristics of the gas through x-ray yield measurements and spectroscopy. X-ray diagnostics have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2 keV and ion densities of 2 x 10$^{20}$ cm$^{-3}$, in agreement with the MHD calculations. **Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. NRL work was supported by DTRA. **in collaboration with C. Deeney, C. Ruiz, Sandia National Labs, G. Cooper, Univ. of New Mexico, A.L. Velikovich, J. Davis, R.W. Clark, Y.K. Chong, and J.P. Apruzese, Naval Research Lab, J.Franklin, S. Chantrenne, and P.D. LePell, Ktech, J. Chittenden, Imperial College, J.Levine and J. Banister, L-3 Communications.   

Magnetic Rayleigh-Taylor Instability Mitigation and Efficient Radiation Production in Gas Puff Z-Pinch Implosions

For a long time it was believed that tightness and uniformity of Z-pinch plasmas imploded from large radii are inherently low because the adverse effect of the magnetic Rayleigh-Taylor (RT) instability that distorts the imploding plasma column is stronger for a longer acceleration path. None of the wire-array implosions from a diameter exceeding 7 cm were successful; a significant decrease of the argon K-shell radiation yield was observed when a 2.5 cm diameter annular shell load was replaced with a 4 cm diameter one. We report how we solved the problem of imploding z-pinch plasmas from large initial radii, making it possible to efficiently produce x-ray radiation with z pinches driven by longer current pulses than previously thought possible. Our novel load design[1] that mitigates the RT instability and enhances energy coupling to the radiating plasma column consists of a ``pusher,'' outer region plasma that carries the current and couples energy from the driver, a ``stabilizer,'' inner region plasma that stabilizes the implosion and a ``radiator,'' high-density center jet plasma that radiates. It increased the Ar K-shell yield at 3.46 MA in 200-ns implosions from 12-cm initial diameter by a factor of two, to 21 kJ, matching the yields obtained earlier on the same accelerator with 100-ns implosions. Test results of this load on all other major US accelerators will be presented [2]. Using laser shearing images, we illustrate the RT growth, its suppression and stabilization of an imploding plasma in a structured gas puff load that lead to a high compression, high yield z pinch. Similar images obtained for gas puff loads whose design does not ensure stabilization show the evolution of highly unstable z pinches which perform poorly as radiators. This research points the way to improved z-pinch implosions from large initial radii, either in the form of wire arrays or gas puffs. \newline \newline [1] H. Sze \textit{et al}., Phys. Rev. Lett. \textbf{95}, 105001 (2005) \newline [2] J. Levine \textit{et al}., Phys. Plasma (August 2006)   

Developing Depleted Uranium and Gold Hohlraums for the National Ignition Facility

Fusion ignition experiments will begin at the National Ignition Facility (NIF) using the indirect drive configuration. Although the x-ray drive in this configuration is highly symmetric, energy is lost in the conversion process because the x-rays penetrate the hohlraum wall. To mitigate this loss, calculations show that adding depleted uranium to the traditional gold hohlraum increases the efficiency of the laser to x-ray energy conversion by making the wall more opaque to the x-rays [1]. To this end, multi-layered depleted uranium (DU) and gold hohlraums are being fabricated by alternately rotating a hohlraum mold in front of separate DU and Au sputter sources to build up multi-layers to the desired wall thickness. This mold is removed to leave a freestanding hohlraum half. The two halves are used to assemble the complete NIF hohlraum to the design specifications. DU will quickly oxidize in air as well as in the chemicals required for the hohlraum fabrication process, so our greatest experimental challenge is to protect it from damage. Oxidized DU is unacceptable for two reasons: 1) the lattice expands significantly during oxidation, resulting in severe structural damage to the hohlraum, and 2) oxygen increases the ionization heat capacity of the hohlraum wall, effectively canceling the efficiency gains associated with the addition of DU to a gold-only wall. The unique production techniques required to fabricate these hohlraums will be presented, as well as results from Auger electron spectroscopy which show a minimal presence of oxygen within the hohlraum wall.\par \vskip6pt \noindent [1] T.J. Orzechowski, et al., Phys.\ Rev.\ Lett.\ {\bf 77}, 3545 (1996).   

Dynamics and control of the expansion of finite-size plasmas produced in ultraintense laser-matter interactions

The expansion dynamics of nanometer-sized plasmas is a key issue for applications involving the interaction of ultraintense infrared laser pulses with cluster jets [1], or the irradiation of biological samples with ultraintense x-ray pulses, for biomolecular imaging purposes [2]. Typically, these scenarios involve the prompt formation and expansion of dense plasmas, composed of cold ions and hot electrons. Control over the expansion can be achieved by exploiting the role of the electron dynamics: in particular, using suitable sequences of laser pulses, one can tailor the phase-space dynamics of the ions [3]. This provides the ability to generate large-scale shock shells [4], and opens the way towards intracluster fusion reactions within large D or D-T clusters [3]. Such new possibilities urge the need for a deeper comprehension of the expansion process, in regimes far from that of a pure Coulomb explosion. To this end, a novel Lagrangian model is used, which provides a self-consistent, kinetic description of the collisionless expansion of spherical nanoplasmas: simple relationships are deduced for the key expansion features, valid for a wide range of initial conditions [5], and a threshold in the electron energy is identified, beyond which the energy spectrum becomes monotonic and the Coulomb explosion regime is approached. \newline \newline [1] T. Ditmire \textit{et al.}, Nature \textbf{386}, 54 (1997); T. Ditmire \textit{et al.}, Nature \textbf{398}, 489 (1999). \newline [2] R. Neutze \textit{et al.}, Nature \textbf{406}, 752 (2000); H. Wabnitz \textit{et al.}, Nature \textbf{420}, 482 (2002). \newline [3] F. Peano \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 033401 (2005); Phys. Rev. A, \textbf{73} , 053202 (2006). \newline [4] A. E. Kaplan\textit{ et al.}, Phys. Rev. Lett. \textbf{91}, 143401 (2003) \newline [5] F. Peano\textit{ et al.}, Phys. Rev. Lett. \textbf{96}, 175002 (2006).   

Multi-Mbar Measurements of Shock Hugoniots and Melt in Beryllium and Diamond for ICF Capsule Physics

Both beryllium and diamond are being considered as ablator materials in the design of capsules for inertial confinement fusion as part of the National Ignition Campaign. Understanding the shock melting of these materials is key in the ability to design capsules and drive pulse-shaping that minimizes microstructure effects during the implosion phase. Recently, tri-laboratory experimental campaigns utilizing the flyer plate capability (7-34 km/s) at the Sandia Z accelerator have been performed to determine the Hugoniot and the shock melting properties of polycrystalline beryllium and micro- and nano-crystalline diamond. Composite aluminum/copper flyer plates were used to shock load beryllium and diamond samples to pressures ranging from 1 to 5 Mbar and 5 to 14 Mbar, respectively. The impedance mismatch at the aluminum/copper interface in the flyer resulted in a well defined release wave that followed the shock into the sample. Multiple sample thicknesses allowed for the measurement of the release wave velocity, which is sensitive to the phase of the material in the shocked state. Results of these experiments, including conclusions regarding the onset and completion of melt in both materials, will be discussed. The inferred melt properties will also be compared to various models for beryllium and diamond including models based on recent quantum molecular dynamics calculations.