Publications

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ALEGRA is an arbitrary Lagrangian-Eulerian finite element code that emphasizes large distortion and shock propagation in inviscid fluids and solids. This document describes user options for modeling resistive magnetohydrodynamic, thermal conduction, and radiation emission effects.

A dynamic hohlraum is created when an annular z-pinch plasma implodes onto a cylindrical 0.014 g/cc 6-mm-diameter CH{sub 2} foam. The impact launches a radiating shock that propagates toward the axis at {approx}350 {micro}m/ns. The radiation trapped by the tungsten z-pinch plasma forms a {approx}200 eV hohlraum that provides X-rays for indirect drive inertial confinement fusion capsule implosion experiments. We are developing the ability to diagnose the hohlraum interior using emission and absorption spectroscopy of Si atoms added as a tracer to the central portion of the foam. Time- and space-resolved Si spectra are recorded with an elliptical crystal spectrometer viewing the cylindrical hohlraum end-on. A rectangular aperture at the end of the hohlraum restricts the field of view so that the 1D spectrometer resolution corresponds approximately to the hohlraum radial direction. This enables distinguishing between spectra from the unshocked radiation-heated foam and from the shocked foam. Typical spectral lines observed include the Si Ly{alpha} with its He-like satellites and the He-like resonance sequence including He{alpha}, He{beta}, and He{gamma}, along with some of their associated Li-like satellites. Work is in progress to infer the hohlraum conditions using collisional-radiative modeling that accounts for the radiation environment and includes both opacity effects and detailed Stark broadening calculations. These 6-mm-scale radiation-heated plasmas might eventually also prove suitable for testing Stark broadening line profile calculations or for opacity measurements.

ALEGRA is an arbitrary Lagrangian-Eulerian multi-material finite element code used for modeling solid dynamics problems involving large distortion and shock propagation. This document describes the basic user input language and instructions for using the software.

This manual describes the input syntax to the ALEGRA radiation transport package. All input and output variables are defined, as well as all algorithmic controls. This manual describes the radiation input syntax for ALEGRA-HEDP. The ALEGRA manual[2] describes how to run the code and general input syntax. The ALEGRA-HEDP manual[13] describes the input for other physics used in high energy density physics simulations, as well as the opacity models used by this radiation package. An emission model, which is the lowest order radiation transport approximation, is also described in the ALEGRA-HEDP manual. This document is meant to be used with these other manuals.

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Symmetric capsule implosions in the double-ended vacuum hohlraum (DEH) on Z have demonstrated convergence ratios of 14-21 for 2.15-mm plastic ablator capsules absorbing 5-7 kJ of x-rays, based on backlit images of the compressed ablator remaining at peak convergence [1]. Experiments with DD-filled 3.3-mm diameter capsules designed to absorb 14 kJ of x-rays have begun as an integrated test of drive temperature and symmetry, complementary to thin-shell symmetry diagnostic capsules. These capsule implosions are characterized by excellent control of symmetry (< 3% time-integrated), but low hohlraum efficiency (< 2%). Possible methods to increase the capsule absorbed energy in the DEH include mixed-component hohlraums, large diameter foam ablator capsules, transmissive shine shields between the z-pinch and capsule, higher spoke electrode x-ray transmission, a double-sided power feed, and smaller initial radius z-pinch wire arrays. Simulations will explore the potential for each of these modifications to increase the capsule coupling efficiency for near-term experiments on Z and ZR.

We have studied the feasibility of using the 3D fully electromagnetic implicit hybrid particle code LSP (Large Scale Plasma) to study laser plasma interactions with dense, compressed plasmas like those created with Z, and which might be created with the planned ZR. We have determined that with the proper additional physics and numerical algorithms developed during the LDRD period, LSP was transformed into a unique platform for studying such interactions. Its uniqueness stems from its ability to consider realistic compressed densities and low initial target temperatures (if required), an ability that conventional PIC codes do not possess. Through several test cases, validations, and applications to next generation machines described in this report, we have established the suitability of the code to look at fast ignition issues for ZR, as well as other high-density laser plasma interaction problems relevant to the HEDP program at Sandia (e.g. backlighting).

The impact of 3D structure on wire array z-pinch dynamics is a topic of current interest, and has been studied by the controlled seeding of wire perturbations. First, Al wires were etched at Sandia, creating 20% radial perturbations with variable axial wavelength. Observations of magnetic bubble formation in the etched regions during experiments on the MAGPIE accelerator are discussed and compared to 3D MHD modeling. Second, thin NaF coatings of 1 mm axial extent were deposited on Al wires and fielded on the Zebra accelerator. Little or no axial transport of the NaF spectroscopic dopant was observed in spatially resolved K-shell spectra, which places constraints on particle diffusivity in dense z-pinch plasmas. Finally, technology development for seeding perturbations is discussed.

Recent 3D hybrid simulation of a plasma current-carrying column revealed two regimes of sausage and kink instability development. In the first regime, with small Hall parameter, development of instabilities leads to appearance of large-scale axial perturbations and eventually to the bending of the plasma column. In the second regime, with five times larger Hall parameter, small-scale perturbations dominated and no bending of the plasma column was observed. Simulation results are compared to recent experimental data, including laser probing, x-ray spectroscopy and time-gated x-ray imaging during wire array implosions at NTF.

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We report on unique particle-in-cell simulations to understand the relativistic electron beam thermalization and subsequent heating of highly compressed plasmas. The simulations yield heated core parameters in good agreement with the GEKKO-PW experimental measurements, given reasonable assumptions of laser-to-electron coupling efficiency and the distribution function of laser-produced electrons. The classical range of the hot electrons exceeds the mass density-core diameter product {rho}L by a factor of several. Anomalous stopping appears to be present and is created by the growth and saturation of an electromagnetic filamentation mode that generates a strong back-EMF impeding hot electrons on the injection side of the density maxima.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

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ITS is a powerful and user-friendly software package permitting state of the art Monte Carlo solution of linear time-independent couple electron/photon radiation transport problems, with or without the presence of macroscopic electric and magnetic fields of arbitrary spatial dependence. Our goal has been to simultaneously maximize operational simplicity and physical accuracy. Through a set of preprocessor directives, the user selects one of the many ITS codes. The ease with which the makefile system is applied combines with an input scheme based on order-independent descriptive keywords that makes maximum use of defaults and internal error checking to provide experimentalists and theorists alike with a method for the routine but rigorous solution of sophisticated radiation transport problems. Physical rigor is provided by employing accurate cross sections, sampling distributions, and physical models for describing the production and transport of the electron/photon cascade from 1.0 GeV down to 1.0 keV. The availability of source code permits the more sophisticated user to tailor the codes to specific applications and to extend the capabilities of the codes to more complex applications. Version 5.0, the latest version of ITS, contains (1) improvements to the ITS 3.0 continuous-energy codes, (2)multigroup codes with adjoint transport capabilities, and (3) parallel implementations of all ITS codes. Moreover the general user friendliness of the software has been enhanced through increased internal error checking and improved code portability.

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Inertial confinement fusion capsule implosions absorbing up to 35 kJ of x-rays from a {approx}220 eV dynamic hohlraum on the Z accelerator at Sandia National Laboratories have produced thermonuclear D-D neutron yields of (2.6 {+-} 1.3) x 10{sup 10}. Argon spectra confirm a hot fuel with Te {approx} 1 keV and n{sub e} {approx} (1-2) x 10{sup 23} cm{sup -3}. Higher performance implosions will require radiation symmetry control improvements. Capsule implosions in a {approx}70 eV double-Z-pinch-driven secondary hohlraum have been radiographed by 6.7 keV x-rays produced by the Z-beamlet laser (ZBL), demonstrating a drive symmetry of about 3% and control of P{sub 2} radiation asymmetries to {+-}2%. Hemispherical capsule implosions have also been radiographed in Z in preparation for future experiments in fast ignition physics. Z-pinch-driven inertial fusion energy concepts are being developed. The refurbished Z machine (ZR) will begin providing scaling information on capsule and Z-pinch in 2006. The addition of a short pulse capability to ZBL will enable research into fast ignition physics in the combination of ZR and ZBL-petawatt. ZR could provide a test bed to study NIF-relevant double-shell ignition concepts using dynamic hohlraums and advanced symmetry control techniques in the double-pinch hohlraum backlit by ZBL.

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