Publications

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Recent experiments on the refurbished Z-machine were conducted using large diameter stainless steel arrays which produced x-ray powers of 260 TW. Follow-up experiments were then conducted utilizing tungsten wires with approximately the same total mass with the hypothesis that the total x-ray power would increase. On the large diameter tungsten experiments, the x-ray power averaged over 300 TW and the total x-ray energy was greater than 2MJ. Different analysis techniques for inferring the x-ray power will be described in detail.

AASC is designing multiple-shell gas puff loads for Z. Here we assess the influence of the loads initial gas distribution on its K-shell yield performance. Emphasis is placed on designing an optimal central jet initial gas distribution, since it is believed to have a controlling effect on pinch stability, pinch conditions, and radiation physics. We are looking at distributions that optimize total Ar K-shell emission and high energy (>10 KeV) continuum radiation. This investigation is performed with the Mach2 MHD code with non-LTE kinetics and ray trace based radiation transport.

Fast z-pinches provide intense 1-10 keV photon energy radiation sources. Here, we analyze time-, space-, and spectrally-resolved {approx}2 keV K-shell emissions from Al (5% Mg) wire array implosions on Sandia's Z machine pulsed power driver. The stagnating plasma is modeled as three separate radial zones, and collisional-radiative modeling with radiation transport calculations are used to constrain the temperatures and densities in these regions, accounting for K-shell line opacity and Doppler effects. We discuss plasma conditions and dynamics at the onset of stagnation, and compare inferences from the atomic modeling to three-dimensional magneto-hydrodynamic simulations.

Recent copper wire array shots on Z, when spectroscopically analyzed on a spatially-averaged basis, appear to have achieved ion densities near 10{sup 21} cm{sup -3}, electron temperatures of 1.25 keV, and K-shell radiating participation of 70-85% of the load mass. However, pinhole images of the shots reveal considerable structure, including several well-defined intensely radiating 'bright spots', which may be due to enhanced density, temperature, or some combination of the two. We have analyzed these individual spots on selected shots, using line-outs of their spectrum and inferred powers based on their images. We compare the properties of these spots (are they dense, hot, or both?), and examine their effect on inferring the radiating mass.

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Spatially and temporally resolved X-ray emission lines contain information about temperatures, densities, velocities, and the gradients in a plasma. Extracting this information from optically thick lines emitted from complex ions in dynamic, three-dimensional, non-LTE plasmas requires self-consistent accounting for both non-LTE atomic physics and non-local radiative transfer. We present a brief description of a hybrid-structure spectroscopic atomic model coupled to an iterative tabular on-the-spot treatment of radiative transfer that can be applied to plasmas of arbitrary material composition, conditions, and geometries. The effects of Doppler line shifts on the self-consistent radiative transfer within the plasma and the emergent emission and absorption spectra are included in the model. Sample calculations for a two-level atom in a uniform cylindrical plasma are given, showing reasonable agreement with more sophisticated transport models and illustrating the potential complexity - or richness - of radially resolved emission lines from an imploding cylindrical plasma. Also presented is a comparison of modeled L- and K-shell spectra to temporally and radially resolved emission data from a Cu:Ni plasma. Finally, some shortcomings of the model and possible paths for improvement are discussed.

This final report for Project 117863 summarizes progress made toward understanding how X-pinch load designs scale to high currents. The X-pinch load geometry was conceived in 1982 as a method to study the formation and properties of bright x-ray spots in z-pinch plasmas. X-pinch plasmas driven by 0.2 MA currents were found to have source sizes of 1 micron, temperatures >1 keV, lifetimes of 10-100 ps, and densities >0.1 times solid density. These conditions are believed to result from the direct magnetic compression of matter. Physical models that capture the behavior of 0.2 MA X pinches predict more extreme parameters at currents >1 MA. This project developed load designs for up to 6 MA on the SATURN facility and attempted to measure the resulting plasma parameters. Source sizes of 5-8 microns were observed in some cases along with evidence for high temperatures (several keV) and short time durations (<500 ps).

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Large diameter nested wire array z-pinches imploded on the Z-generator at Sandia National Laboratories have been used extensively to generate high intensity K-shell radiation. Large initial radii are required to obtain the high implosion velocities needed to efficiently radiate in the K-shell. This necessitates low wire numbers and large inter-wire gaps which introduce large azimuthal non-uniformities. Furthermore, the development of magneto-Rayleigh-Taylor instabilities during the implosion are known to generate large axial non-uniformity These effects motivate the complete, full circumference 3-dimensional modeling of these systems. Such high velocity implosions also generate large voltages, which increase current losses in the power feed and limit the current delivery to these loads. Accurate representation of the generator coupling is therefore required to reliably represent the energy delivered to, and the power radiated from these sources. We present 3D-resistive MHD calculations of the implosion and stagnation of a variety of large diameter stainless steel wire arrays (hv {approx} 6.7 keV), imploded on the Z-generator both before and after its refurbishment. Use of a tabulated K-shell emission model allows us to compare total and K-shell radiated powers to available experimental measurements. Further comparison to electrical voltage and current measurements allows us to accurately assess the power delivered to these loads. These data allow us to begin to constrain and validate our 3D MHD calculations, providing insight into ways in which these sources may be further optimized.

This paper presents initial designs of multiple-shell gas puff imploding loads for the refurbished Z generator. The nozzle has three independent drivers for three independent plena. The outer and middle plena may be charged to 250psia whilst the central jet can be charged to 1000psia. 8-cm and 12-cm outer diameter nozzles have been built and tested on the bench. The unique valve design provides a very fast opening, hence the amount of stray gas outside the core nozzle flow is minimized. A similar 8-cm nozzle was characterized earlier using a fiber optic interferometer, but at lower pressures and without the central jet. Those data have been scaled to the higher pressures required for refurbished Z and used to estimate performance. The use of three independent plena allows variation of the pressure (hence mass distribution) in the nozzle flow, allowing optimization of implosion stability and the on-axis mass that most contributes to K-shell emission. Varying the outer/middle mass ratios influences the implosion time and should affect the details of the assembly on axis as well as the radiation physics. Varying the central jet pressure will have a minor effect on implosion dynamics, but a strong effect on pinch conditions and radiation physics. Optimum mass distributions for planned initial Ar shots on refurbished Z are described. Additional interferometer data including the central jet and at higher pressures will also be presented.

Large diameter (50-70 mm) wire array z pinches are fielded on the refurbished Z machine to generate 1-10 keV K-shell x-ray radiation. Imploding with velocities approaching 100 cm/{micro}s, these loads create large dL/dt which generates a high voltage, stresses the convolute, and leads to current loss. High velocities are required to reach the few-keV electron temperatures required to strip moderate-atomic-number plasmas to the K shell, thus there is an inherent trade-off between achieving high velocity and stressing the pulsed power driver via the large dL/dt.Here, we present experiments in which the length of stagnated Cu and stainless steel z pinches was varied from 12-24 mm. The motivation in reducing the pinch height is to lower the final inductance and improve coupling to the generator. Shortening a Cu pinch from 20 to 12 mm by angling the anode glide plane reduced the final L and dL/dt, enhancing the feed current by 1.4 MA, nearly doubling the K-shell power per unit length, and increasing the net K-shell yield by 20%. X-ray spectroscopy is employed to assess differences in plasma conditions between the loads. Lengthening the pinch could lead to yield enhancements by increasing the mass participating in the implosion, provided the increased inductance is not overly detrimental to the current coupling. In addition to the experimental results, these scenarios are studied via thin-shell 0D and also magneto-hydrodynamic modeling with a coupled driver circuit model.

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Two-dimensional (r, z) magnetohydrodynamic simulations with nonlocal thermodynamic equilibrium ionization and radiation transport are used to investigate the K-shell radiation output from doubly nested large-diameter (> 60 mm) stainlesssteel arrays fielded on the refurbished Z pulsed-power generator. The effects of the initial density perturbations, wire ablation rate, and current loss near the load on the total power, K-shell power, and K-shell yield are examined. The broad mass distribution produced by wire ablation largely overcomes the deleterious impact on the K-shell power and yield of 2-D instability growth. On the other hand, the possible current losses in the final feed section lead to substantial reductions in K-shell yield. Following a survey of runs, the parameters for the perturbation level, ablation rate, and current loss are chosen to benchmark the simulations against existing 65-mm-diameter radiation data. Themodel is then used to predict the K-shell properties of larger diameter (70 mm) arrays to be imploded on the Z generator. © 2010 IEEE.

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Radial wire arrays provide an alternative x-ray source for Z-pinch driven Inertial Confinement Fusion. These arrays, where wires are positioned radially outwards from a central cathode to a concentric anode, have the potential to drive a more compact ICF hohlraum. A number of experiments were performed on the 7MA Saturn Generator. These experiments studied a number of potential risks in scaling radial wire arrays up from the 1MA level, where they have been shown to provide similar x-ray outputs to larger diameter cylindrical arrays, to the higher current levels required for ICF. Data indicates that at 7MA radial arrays can obtain higher power densities than cylindrical wire arrays, so may be of use for x-ray driven ICF on future facilities. Even at the 7MA level, data using Saturn's short pulse mode indicates that a radial array should be able to drive a compact hohlraum to temperatures {approx}92eV, which may be of interest for opacity experiments. These arrays are also shown to have applications to jet production for laboratory astrophysics. MHD simulations require additional physics to match the observed behavior.

Post-hole convolutes are used in high-power transmission line systems and join several individual transmission lines in parallel, transferring the combined currents to a single transmission line attached to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due, in part, to the formation of magnetic nulls, resulting in current losses. At very high-power operating levels, the formation of electrode plasmas is considered likely which can lead to additional losses. A recent computational analysis of the Sandia Z accelerator suggested that modest plasma desorption rates in the convolute region could explain measured current losses [1]. The recently completed Sandia ZR accelerator has utilized new convolute designs to accommodate changes to the parallel-plate transmission lines on ZR. Detailed particle-in-cell simulations that are fully electromagnetic and relativistic, and include plasma desorption from electrode surfaces in the post-hole convolutes, are carried out to assess the measured current losses on ZR. We find that the plasma desorption rate used to model the Z convolute also applies to three different ZR convolute designs that have been fielded. Based on these findings, the simulation model is being used to develop newer convolute designs with the goal of reducing the current losses, particularly for higher-impedance loads. ©2009 IEEE.

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Highly collimated outflows or jets are produced by a number of astrophysical objects including protostars. The morphology and collimation of these jets is thought to be strongly influenced by the effects of radiative cooling, angular momentum and the interstellar medium surrounding the jet. Astrophysically relevant experiments are performed with conical wire array z-pinches investigating each of these effects. It is possible in each case to enter the appropriate parameter regime, leading the way towards future experiments where these different techniques can be more fully combined. © 2009 American Institute of Physics.

Many physical effects can produce unstable plasma behavior that affect K-shell emission from arrays. Such effects include: asymmetry in the initial density profile, asymmetry in power flow, thermal conduction at the boundaries, and non-uniform wire ablation. Here we consider how asymmetry in the radiation field also contributes to the generation of multidimensional plasma behavior that affects K-shell power and yield. To model this radiation asymmetry, we have incorporated into the MACH2 r-z MHD code a self-consistent calculation of the non-LTE population kinetics based on radiation transport using multi-dimensional ray tracing. Such methodology is necessary for modeling the enhanced radiative cooling that occurs at the anode and cathode ends of the pinch during the run-in phase of the implosion. This enhanced radiative cooling is due to reduced optical depth at these locations producing an asymmetric flow of radiative energy that leads to substantial disruption of large initial diameter (>5 cm) pinches and drives ID into 2D fluid (i.e., Rayleigh-Taylor like) flows. The impact of this 2D behavior on K-shell power and yield is investigated by comparing ID and 2D model results with data obtained from a series of single wire array stainless steel experiments performed on the Z generator. © 2009 American Institute of Physics.

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At the Nevada Terawatt Facility we investigated the generation of a sheared plasma flow using conical wire arrays with an additional wire located on the axis of the pinch. The additional center wire generates axial current carrying plasma that serves as a target for the plasma accelerated from the outer wires, generating a sheared plasma flow which leads to the growth of the Kelvin-Helmholtz instability. These experiments were conducted on Zebra, a 2 TW pulse power device capable of delivering a 1 MA current in 100 ns. This paper will focus on the implosion dynamics that lead to shear flow and the development of the Kelvin Helmholtz instability.

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A series of ten shots were performed on the Saturn generator in short pulse mode in order to study planar and small-diameter cylindrical tungsten wire arrays at {approx}5 MA current levels and 50-60 ns implosion times as candidates for compact z-pinch radiation sources. A new vacuum hohlraum configuration has been proposed in which multiple z pinches are driven in parallel by a pulsed power generator. Each pinch resides in a separate return current cage, serving also as a primary hohlraum. A collection of such radiation sources surround a compact secondary hohlraum, which may potentially provide an attractive Planckian radiation source or house an inertial confinement fusion fuel capsule. Prior to studying this concept experimentally or numerically, advanced compact wire array loads must be developed and their scaling behavior understood. The 2008 Saturn planar array experiments extend the data set presented in Ref. [1], which studied planar arrays at {approx}3 MA, 100 ns in Saturn long pulse mode. Planar wire array power and yield scaling studies now include current levels directly applicable to multi-pinch experiments that could be performed on the 25 MA Z machine. A maximum total x-ray power of 15 TW (250 kJ in the main pulse, 330 kJ total yield) was observed with a 12-mm-wide planar array at 5.3 MA, 52 ns. The full data set indicates power scaling that is sub-quadratic with load current, while total and main pulse yields are closer to quadratic; these trends are similar to observations of compact cylindrical tungsten arrays on Z. We continue the investigation of energy coupling in these short pulse Saturn experiments using zero-dimensional-type implosion modeling and pinhole imaging, indicating 16 cm/?s implosion velocity in a 12-mm-wide array. The same phenomena of significant trailing mass and evidence for resistive heating are observed at 5 MA as at 3 MA. 17 kJ of Al K-shell radiation was obtained in one Al planar array fielded at 5.5 MA, 57 ns and we compare this to cylindrical array results in the context of a K-shell yield scaling model. We have also performed an initial study of compact 3 mm diameter cylindrical wire arrays, which are alternate candidates for a multi-pinch vacuum hohlraum concept. These massive 3.4 and 6 mg/cm loads may have been impacted by opacity, producing a maximum x-ray power of 7 TW at 4.5 MA, 45 ns. Future research directions in compact x-ray sources are discussed.

Bright, intense x-ray sources with extreme plasma parameters (micropinch plasmas) have previously been characterized at 0.1-0.4 MA, but the scaling of such sources at higher current is poorly understood. The x-ray source size and radiation power of 1 MA X pinches were studied as a function of wire material (Al, Ti, Mo, and W) and number (1-, 2-, 8-, 32-, and 64-wire configurations). The smallest bright spots observed were from 32-wire tungsten X pinches, which produced {le} 11-16 {micro}m, {approx}2 J, 1-10 GW sources of 3-5 keV radiation.

Planar wire arrays are studied at 3-6 MA on the Saturn pulsed power generator as potential drivers of compact hohlraums for inertial confinement fusion studies. Comparison with zero-dimensional modeling suggests that there is significant trailing mass. The modeled energy coupled from the generator cannot generally explain the energy in the main x-ray pulse. Preliminary comparison at 1-6 MA indicates sub-quadratic scaling of x-ray power in a manner similar to compact cylindrical wire arrays. Time-resolved pinhole images are used to study the implosion dynamics.

Highly collimated outflows or jets are produced by a number of astrophysical objects including protostars. The morphology and collimation of these jets is thought to be strongly influenced by the effects of radiative cooling, angular momentum and the interstellar medium surrounding the jet. Astrophysically relevant experiments are performed with conical wire array z-pinches investigating each of these effects. It is possible in each case to enter the appropriate parameter regime, leading the way towards future experiments where these different techniques can be more fully combined.

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The first laboratory astrophysics experiments to produce a radiatively cooled plasma jet with dynamically significant angular momentum are discussed. A new configuration of wire array z pinch, the twisted conical wire array, is used to produce convergent plasma flows each rotating about the central axis. Collision of the flows produces a standing shock and jet that each have supersonic azimuthal velocities. By varying the twist angle of the array, the rotation velocity of the system can be controlled, with jet rotation velocities reaching ∼18% of the propagation velocity. © 2008 The American Physical Society.

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