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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Helical plasma striations in liners in the presence of an external axial magnetic field

Awe et al. found on the 20 MA Z machine [Acta Phys. Pol. A 115, 956 (2009)] that applying an externally generated axial magnetic field to an imploding liner leads to a helical pattern in the liner when viewed with soft x-ray radiography ([Phys. Rev. Lett. 111, 235005 (2013)] and [Phys. Plasmas 21, 056303 (2014)]). We show that this phenomenon is also observed in extreme ultraviolet self-emission images of 10 mm long cylindrical metal liners having varying diameters and varying wall thicknesses on a 1 MA, 100–200 ns pulsed power generator. The magnetic field in these experiments is created using either twisted return current wires positioned close to the liner, generating a time-varying B z , or a Helmholtz coil, generating a steady-state B z .

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Experimental characterization and constitutive modeling of the mechanical behavior of molybdenum under electromagnetically applied compression-shear ramp loading

Magnetically applied pressure-shear (MAPS) is a new experimental technique that provides a platform for direct measurement of material strength at extreme pressures. The technique employs an imposed quasi-static magnetic field and a pulsed power generator that produces an intense current on a planar driver panel, which in turn generates high amplitude magnetically induced longitudinal compression and transverse shear waves into a planar sample mounted on the drive panel. In order to apply sufficiently high shear traction to the test sample, a high strength material must be used for the drive panel. Molybdenum is a potential driver material for the MAPS experiment because of its high yield strength and sufficient electrical conductivity. To properly interpret the results and gain useful information from the experiments, it is critical to have a good understanding and a predictive capability of the mechanical response of the driver. In this work, the inelastic behavior of molybdenum under uniaxial compression and biaxial compression-shear ramp loading conditions is experimentally characterized. It is observed that an imposed uniaxial magnetic field ramped to approximately 10 T through a period of approximately 2500 μs and held near the peak for about 250 μs before being tested appears to anneal the molybdenum panel. In order to provide a physical basis for model development, a general theoretical framework that incorporates electromagnetic loading and the coupling between the imposed field and the inelasticity of molybdenum was developed. Based on this framework, a multi-axial continuum model for molybdenum under electromagnetic loading is presented. The model reasonably captures all of the material characteristics displayed by the experimental data obtained from various experimental configurations. Additionally, data generated from shear loading provide invaluable information not only for validating but also for guiding the development of the material model for multiaxial loadings.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A gas-dynamical approach to radiation pressure acceleration

The study of high intensity ion beams driven by high power pulsed lasers is an active field of research. Of particular interest is the radiation pressure acceleration, for which simulations predict narrow band ion energies up to GeV. We derive a laser-piston model by applying techniques for non-relativistic gas-dynamics. The model reveals a laser intensity limit, below which sufficient laser-piston acceleration is impossible. The relation between target thickness and piston velocity as a function of the laser pulse length yields an approximation for the permissible target thickness. We performed one-dimensional Particle-In-Cell simulations to confirm the predictions of the analytical model. These simulations also reveal the importance of electromagnetic energy transport. We find that this energy transport limits the achievable compression and rarefies the plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Effects of a Xe dopant on an Ar gas-puff implosion on Z

Two Ar gas-puff experiments, each using loads consisting of two annuli and a center jet, were recently performed on Sandia National Laboratories' Z machine. These shots had the same load except that one of them (Z2603) employed a Xe dopant of 0.8% by number in the jet, which was not present in the otherwise identical Z2605. The extensive diagnostics deployed in these experiments reveal that the presence of this small fraction of Xe had a significant effect on the emitted K-shell radiation. Use of the Xe dramatically reduced the Ar K-shell yield from 373 ± 9% to 129 ± 9% kJ. However, the total yield increased, from 1.02 ± 17% to 1.14 ± 17% MJ. Also, the K-shell power pulse for Z2603 exhibited two nearly equal peaks separated by ∼5 ns, but that of Z2605 consisted of a more conventional single peak. Analysis of time- and space resolved K-shell spectra using a detailed atomic model for Ar ions indicates that a much greater fraction of the load mass was heated to the K-shell in the non-Xe shot Z2605. Previous average-ion atomic calculations of Xe cooling rates [Post et al., At. Data Nucl. Data Tables 20, 397 (1977)] show that the radiative cooling of the small 0.8% Xe fraction is nonetheless significant compared to that of the Ar and is therefore likely responsible for the differences in the shots.

Apruzese, J. P.↗
DOE OSTI2017

Detection of an anomalous pressure on a magneto-inertial-fusion load current diagnostic

Recent Magnetized Liner Inertial Fusion experiments at the Sandia National Laboratories Z pulsed power facility have featured a PDV (Photonic Doppler Velocimetry) diagnostic in the final power feed section for measuring load current. In this paper, we report on an anomalous pressure that is detected on this PDV diagnostic very early in time during the current ramp. Early time load currents that are greater than both B-dot upstream current measurements and existing Z machine circuit models by at least 1 MA would be necessary to describe the measured early time velocity of the PDV flyer. This leads us to infer that the pressure producing the early time PDV flyer motion cannot be attributed to the magnetic pressure of the load current but rather to an anomalous pressure. Using the MHD code ALEGRA, we are able to compute a time-dependent anomalous pressure function, which when added to the magnetic pressure of the load current, yields simulated flyer velocities that are in excellent agreement with the PDV measurement. As a result, we also provide plausible explanations for what could be the origin of the anomalous pressure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Laser-driven magnetized liner inertial fusion on OMEGA

Magneto-inertial fusion (MIF) combines the compression of fusion fuel, a hallmark of inertial confinement fusion (ICF), with strongly magnetized plasmas that suppress electron heat losses, a hallmark of magnetic fusion. It can reduce the traditional velocity, pressure, and convergence ratio requirements of ICF. The magnetized liner inertial fusion (MagLIF) concept being studied at the Z Pulsed-Power Facility is a key target concept in the U.S. ICF Program. Laser-driven MagLIF is being developed on OMEGA to test the scaling of MagLIF over a range of absorbed energy of the order of 1 kJ on OMEGA to 500 kJ on Z. It is also valuable as a platform for studying the key physics of MIF. An energy-scaled point design has been developed for OMEGA that is roughly 10 × smaller in linear dimensions than Z MagLIF targets. A 0.6-mm-outer-diameter plastic cylinder filled with 2.4 mg/cm3 of D2 is placed in a ∼10-T axial magnetic field, generated by a Magneto-inertial fusion electrical discharge system, the cylinder is compressed by 40 OMEGA beams, and the gas fill is preheated by a single OMEGA beam propagating along the axis. Preheating to >100 eV and axially uniform compression over 0.7 mm have been demonstrated, separately, in a series of preparatory experiments that meet our initial expectations. The preliminary results from the first integrated experiments combining magnetization, compression, and preheat demonstrating a roughly 2 x increase in the neutron yield will be reported here for the first time.

Barnak, D. H. (ORCID:0000000246467517)↗
DOE OSTI2017

Axial magnetic field injection in magnetized liner inertial fusion

MagLIF is a fusion concept using a Z-pinch implosion to reach thermonuclear fusion. In current experiments, the implosion is driven by the Z-machine using 19 MA of electrical current with a rise time of 100 ns. MagLIF requires an initial axial magnetic field of 30 T to reduce heat losses to the liner wall during compression and to confine alpha particles during fusion burn. This field is generated well before the current ramp starts and needs to penetrate the transmission lines of the pulsed-power generator, as well as the liner itself. Consequently, the axial field rise time must exceed hundreds of microseconds. Any coil capable of being submitted to such a field for that length of time is inevitably bulky. The space required to fit the coil near the liner, increases the inductance of the load. In turn, the total current delivered to the load decreases since the voltage is limited by driver design. Yet, the large amount of current provided by the Z-machine can be used to produce the required 30 T field by tilting the return current posts surrounding the liner, eliminating the need for a separate coil. However, the problem now is the field penetration time, across the liner wall. This paper discusses why skin effect arguments do not hold in the presence of resistivity gradients. Numerical simulations show that fields larger than 30 T can diffuse across the liner wall in less than 60 ns, demonstrating that external coils can be replaced by return current posts with optimal helicity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Effects of a chirped bias voltage on ion energy distributions in inductively coupled plasma reactors

The metrics for controlling reactive fluxes to wafers for microelectronics processing are becoming more stringent as feature sizes continue to shrink. Recent strategies for controlling ion energy distributions to the wafer involve using several different frequencies and/or pulsed powers. Although effective, these strategies are often costly or present challenges in impedance matching. With the advent of matching schemes for wide band amplifiers, other strategies to customize ion energy distributions become available. In this study, we discuss results from a computational investigation of biasing substrates using chirped frequencies in high density, electronegative inductively coupled plasmas. Depending on the frequency range and chirp duration, the resulting ion energy distributions exhibit components sampled from the entire frequency range. However, the chirping process also produces transient shifts in the self-generated dc bias due to the reapportionment of displacement and conduction with frequency to balance the current in the system. Finally, the dynamics of the dc bias can also be leveraged towards customizing ion energy distributions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Generation of highly symmetric, cylindrically convergent shockwaves in water

We report on pulsed power driven, exploding copper wire array experiments conducted to generate cylindrical convergent shockwaves in water employing μs risetime currents >550 kA in amplitude and with stored energies of >15 kJ—a substantial increase over previous results. The experiments were carried out on the recently constructed Mega-Ampere-Compression-and-Hydrodynamics facility at Imperial College London in collaboration with colleagues of Technion, Israel. 10 mm diameter arrays consisting of 60 × 130 μm wires were utilized, and the current and voltage diagnostics of the load region suggested that ~8 kJ of energy was deposited in the wires (and the load region close to the wires) during the experiments, resulting in the formation of dense, highly resistive plasmas that rapidly expanded driving the shockwaves in water. Laser-backlit framing images of the shockfront were obtained at radii <0.25 mm for the first time, and there was strong evidence that even at radii <0.1 mm this front remains stable, resulting in a convergence ratio of >50:1. Framing images and streak photographs showed that the velocity of the shockwave reached ~7.5 km s -1 at 0.1 mm from the axis. 2D hydrodynamic simulations that match the experimentally obtained implosion trajectory suggest that pressures >1 Mbar are produced within 10 μm of the axis along with water densities of 3gcm -3 and temperatures of many 1000 s of Kelvin. Under these conditions, Quotidian Equation of State suggests that a strongly coupled plasma with an ionization fraction of ~0.7 would be formed. The results represent a “stepping stone” in the application of the technique to drive different material samples into high pressure, warm dense matter regimes with compact, university scale generators, and provide support in scaling the technique to multi-mega ampere currents.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Note: Infrared laser diagnostics for deuterium gas puff Z pinches

Deuterium gas puff Z pinches have been used for generation of strong neutron fluxes on the MA class pulse power machines. Due to the low electron density of deuterium Z-pinch plasma, regular laser diagnostics in the visible range cannot be used for observation and study of the pinch. Laser probing at the wavelength of 1064 nm was used for visualization of deuterium plasma. Finally, infrared schlieren and interferometry diagnostics showed the deuterium gas puff plasma dynamics, instabilities, and allowed for the reconstruction of the profile of the plasma density.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Applied axial magnetic field effects on laboratory plasma jets: Density hollowing, field compression, and azimuthal rotation

We experimentally measure the effects of an applied axial magnetic field (B z ) on laboratory plasma jets and compare experimental results with numerical simulations using an extended magnetohydrodynamics code. A 1 MA peak current, 100 ns rise time pulse power machine is used to generate the plasma jet. On application of the axial field, we observe on-axis density hollowing and a conical formation of the jet using interferometry, compression of the applied B z using magnetic B-dot probes, and azimuthal rotation of the jet using Thomson scattering. Experimentally, we find densities ≤ 5×10 17 cm -3 on-axis relative to jet densities of ≥ 3×10 18 cm -3 . For aluminum jets, 6.5 ± 0.5 mm above the foil, we find on-axis compression of the applied 1.0 ± 0.1 T B z to a total 2.4 ± 0.3 T, while simulations predict a peak compression to a total 3.4 T at the same location. On the aluminum jet boundary, we find ion azimuthal rotation velocities of 15-20 km/s, while simulations predict 14 km/s at the density peak. We discuss possible sources of discrepancy between the experiments and simulations, including: surface plasma on B-dot probes, optical fiber spatial resolution, simulation density floors, and 2D vs. 3D simulation effects. Lastly, this quantitative comparison between experiments and numerical simulations helps elucidate the underlying physics that determine the plasma dynamics of magnetized plasma jets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Spatially resolved single crystal x-ray spectropolarimetry of wire array z-pinch plasmas

This journal article describes the design, development, and implementation of a single-crystal x-ray spectropolarimeter. The article also reports polarization sensitive spectra for He-like Al x-ray lines emitted from hot dense plasma formed from wire-array loads driven by the z-pinch pulsed power machine at the University of Nevada, Reno Nevada Terawatt Facility.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Observation of impact of eddy current on laser targets in a strong fast rising magnetic field

Pulsed power generators are capable of producing strong magnetic fields. Foil laser targets for high intensity interactions with matter were characterized in a strong magnetic field produced by different types of loads. Longitudinal magnetic fields generated by coil loads were measured by Faraday rotation of the polarization plane of laser beams in a small glass sample at two wavelengths. Magnetic fields of 0.8–1.5 MG were measured in different coil loads at 1 MA current. For the first time, the impact of eddy current generated by the fast rising magnetic field with dB/dt= 10 9 T/s was observed in Al laser targets using shadowgraphy and interferometry. Material of the Al 10 μm foil target expanded from both sides at 0.1–0.2 mm. This effect was not observed in dielectric targets. The impact of eddy current on targets is important for designing experiments for laser plasma interactions in strong magnetic fields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

The electro-thermal stability of tantalum relative to aluminum and titanium in cylindrical liner ablation experiments at 550 kA

Presented are the results from the liner ablation experiments conducted at 550 kA on the Michigan Accelerator for Inductive Z-Pinch Experiments. These experiments were performed to evaluate a hypothesis that the electrothermal instability (ETI) is responsible for the seeding of magnetohydrodynamic instabilities and that the cumulative growth of ETI is primarily dependent on the material-specific ratio of critical temperature to melting temperature. This ratio is lower in refractory metals (e.g., tantalum) than in non-refractory metals (e.g., aluminum or titanium). The experimental observations presented herein reveal that the plasma-vacuum interface is remarkably stable in tantalum liner ablations. This stability is particularly evident when contrasted with the observations from aluminum and titanium experiments. These results are important to various programs in pulsed-power-driven plasma physics that depend on liner implosion stability. Furthermore, examples include the magnetized liner inertial fusion (MagLIF) program and the cylindrical dynamic material properties program at Sandia National Laboratories, where liner experiments are conducted on the 27-MA Z facility.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Design and testing of a magnetically driven implosion peak current diagnostic

We report that a critical component of the magnetically driven implosion experiments at Sandia National Laboratories is the delivery of high-current, 10s of MA, from the Z pulsed power facility to a target. In order to assess the performance of the experiment, it is necessary to measure the current delivered to the target. Recent Magnetized Liner Inertial Fusion (MagLIF) experiments have included velocimetry diagnostics, such as PDV (Photonic Doppler Velocimetry) or Velocity Interferometer System for Any Reflector, in the final power feed section in order to infer the load current as a function of time. However, due to the nonlinear volumetrically distributed magnetic force within a velocimetry flyer, a complete time-dependent load current unfold is typically a time-intensive process and the uncertainties in the unfold can be difficult to assess. In this paper, we discuss how a PDV diagnostic can be simplified to obtain a peak current by sufficiently increasing the thickness of the flyer. This effectively keeps the magnetic force localized to the flyer surface, resulting in fast and highly accurate measurements of the peak load current. Additionally, we show the results of experimental peak load current measurements from the PDV diagnostic in recent MagLIF experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Switching Purcell effect with nonlinear epsilon-near-zero media

An optical topological transition is defined as the change in the photonic iso-frequency surface around epsilon-near-zero (ENZ) frequencies which can considerably change the spontaneous emission of a quantum emitter placed near a metamaterial slab. In this work, we show that due to the strong Kerr nonlinearity at ENZ frequencies, a high-power pulse can induce a sudden transition in the topology of the iso-frequency dispersion curve, leading to a significant change in the transmission of propagating as well as evanescent waves through the metamaterial slab. This evanescent wave switch effect allows for the control of spontaneous emission through modulation of the Purcell effect. We determine a theory of the enhanced nonlinear response of ENZ media to s and p polarized inputs and show that this nonlinear effect is stronger for p polarization and is almost independent of the incident angle. We perform finite-difference time-domain simulations to demonstrate the transient response of the metamaterial slab to an ultrafast pulse and fast switching of the Purcell effect at the sub-picosecond scale. The Purcell factor changes at ENZ by almost a factor of three which is an order of magnitude stronger than that away from ENZ. We additionally demonstrate that due to the inhomogeneous spatial field distribution inside the multilayer metal-dielectric super-lattice, a unique spatial topological transition metamaterial can be achieved by the control pulse induced nonlinearity. This work can lead to ultra-fast control of quantum phenomena in ENZ metamaterials.

42 ENGINEERING↗
DOE OSTI2018

Enhancing performance of magnetized liner inertial fusion at the Z facility

Here, the Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17–18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10–1000 MJ) should be possible with pulsed power machines producing larger drive currents (45–60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Synchrotron based X-ray radiography of convergent shock waves driven by underwater electrical explosion of a cylindrical wire array

We present x-ray radiography images showing the propagation of shock waves generated by electrical explosion of a cylindrical arrangement of wires in water driven by pulsed power. In previous experiments [S. N. Bland et al., Phys. Plasmas 24, 082702 (2017)] the merger of shock waves from adjacent wires has produced a highly symmetrical, cylindrical shock wave converging on the axis, where it is expected to produce a high density, strongly coupled plasma ideal for warm dense matter research. However, diagnostic limitations have meant that much of the dynamics of the system have been inferred from the position of the front of the cylindrical shock and timing/spectra of light emitted from the axis. Here we present synchrotron-based radiography of such experiments – providing direct quantitative measurements on the formation of the convergent shock wave, the increased density of water on axis caused by its arrival, and its ‘bounce’ after arrival on axis. The obtained images are compared with twodimensional hydrodynamic simulations which reproduce the observed dynamics with a satisfactory agreement in density values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗