W. A. Stygar

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (322)877.63 Total impact

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    ABSTRACT: A novel algorithm for the simulation of cathode plasmas in particle-in-cell codes is described and applied to investigate cathode plasma evolution in magnetically insulated transmission lines (MITLs). The MITL electron sheath is modeled by a fully kinetic electron species. Electron and ion macroparticles, both modeled as fluid species, form a dense plasma which is initially localized at the cathode surface. Energetic plasma electron particles can be converted to kinetic electrons to resupply the electron flux at the plasma edge (the “effective” cathode). Using this model, we compare results for the time evolution of the cathode plasma and MITL electron flow with a simplified (isothermal) diffusion model. Simulations in 1D show a slow diffusive expansion of the plasma from the cathode surface. But in multiple dimensions, the plasma can expand much more rapidly due to anomalous diffusion caused by an instability due to the strong coupling of a transverse magnetic mode in the electron sheath with the expanding resistive plasma layer.
    Physics of Plasmas 03/2015; 22(3):032101. DOI:10.1063/1.4913805 · 2.25 Impact Factor
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    ABSTRACT: Presented are voltage measurements taken near the load region on the Z pulsed-power accelerator using an inductive voltage monitor (IVM). Specifically, the IVM was connected to, and thus monitored the voltage at, the bottom level of the accelerator's vacuum double post-hole convolute. Additional voltage and current measurements were taken at the accelerator's vacuum-insulator stack (at a radius of 1.6 m) by using standard D -dot and B -dot probes, respectively. During postprocessing, the measurements taken at the stack were translated to the location of the IVM measurements by using a lossless propagation model of the Z accelerator's magnetically insulated transmission lines (MITLs) and a lumped inductor model of the vacuum post-hole convolute. Across a wide variety of experiments conducted on the Z accelerator, the voltage histories obtained from the IVM and the lossless propagation technique agree well in overall shape and magnitude. However, large-amplitude, high-frequency oscillations are more pronounced in the IVM records. It is unclear whether these larger oscillations represent true voltage oscillations at the convolute or if they are due to noise pickup and/or transit-time effects and other resonant modes in the IVM. Results using a transit-time-correction technique and Fourier analysis support the latter. Regardless of which interpretation is correct, both true voltage oscillations and the excitement of resonant modes could be the result of transient electrical breakdowns in the post-hole convolute, though more information is required to determine definitively if such breakdowns occurred. Despite the larger oscillations in the IVM records, the general agreement found between the lossless propagation results and the results of the IVM shows that large voltages are transmitted efficiently through the MITLs on Z . These results are complementary to previous studies [R. D. McBride et al., Phys. Rev. ST Accel. Beams 13, 120401 (2010)] that showed efficient transmission of large currents through the MITLs on Z . Taken together, the two studies demonstrate the overall efficient delivery of very large electrical powers through the MITLs on Z .
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    ABSTRACT: Magnetizing the fuel in inertial confinement fusion relaxes ignition requirements by reducing thermal conductivity and changing the physics of burn product confinement. Diagnosing the level of fuel magnetization during burn is critical to understanding target performance in magneto-inertial fusion (MIF) implosions. In pure deuterium fusion plasma, 1.01 MeV tritons are emitted during deuterium-deuterium fusion and can undergo secondary deuterium-tritium reactions before exiting the fuel. Increasing the fuel magnetization elongates the path lengths through the fuel of some of the tritons, enhancing their probability of reaction. Based on this feature, a method to diagnose fuel magnetization using the ratio of overall deuterium-tritium to deuterium-deuterium neutron yields is developed. Analysis of anisotropies in the secondary neutron energy spectra further constrain the measurement. Secondary reactions also are shown to provide an upper bound for the volumetric fuel-pusher mix in MIF. The analysis is applied to recent MIF experiments [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] on the Z Pulsed Power Facility, indicating that significant magnetic confinement of charged burn products was achieved and suggesting a relatively low-mix environment. Both of these are essential features of future ignition-scale MIF designs.
    Physical Review Letters 10/2014; 113(15):155004. · 7.73 Impact Factor
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    ABSTRACT: This Letter presents results from the first fully integrated experiments testing the magnetized liner inertial fusion concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010).
    Physical Review Letters 10/2014; 113(15):155003. · 7.73 Impact Factor
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    ABSTRACT: Recent experiments at the Sandia National Laboratories Z Facility have, for the first time, studied the implosion dynamics of magnetized liner inertial fusion (MagLIF) style liners that were pre-imposed with a uniform axial magnetic field. As reported [T. J. Awe et al., Phys. Rev. Lett. 111, 235005 (2013)] when premagnetized with a 7 or 10 T axial field, these liners developed 3D-helix-like hydrodynamic instabilities; such instabilities starkly contrast with the azimuthally correlated magneto-Rayleigh-Taylor (MRT) instabilities that have been consistently observed in many earlier non-premagnetized experiments. The helical structure persisted throughout the implosion, even though the azimuthal drive field greatly exceeded the expected axial field at the liner's outer wall for all but the earliest stages of the experiment. Whether this modified instability structure has practical importance for magneto-inertial fusion concepts depends primarily on whether the modified instability structure is more stable than standard azimuthally correlated MRT instabilities. In this manuscript, we discuss the evolution of the helix-like instability observed on premagnetized liners. While a first principles explanation of this observation remains elusive, recent 3D simulations suggest that if a small amplitude helical perturbation can be seeded on the liner's outer surface, no further influence from the axial field is required for the instability to grow.
    Physics of Plasmas 05/2014; 21(5):056303. DOI:10.1063/1.4872331 · 2.25 Impact Factor
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    ABSTRACT: Novel experimental data are reported that reveal helical instability formation on imploding z-pinch liners that are premagnetized with an axial field. Such instabilities differ dramatically from the mostly azimuthally symmetric instabilities that form on unmagnetized liners. The helical structure persists at nearly constant pitch as the liner implodes. This is surprising since, at the liner surface, the azimuthal drive field presumably dwarfs the axial field for all but the earliest stages of the experiment. These fundamentally 3D results provide a unique and challenging test for 3D-magnetohydrodynamics simulations.
    Physical Review Letters 12/2013; 111(23):235005. DOI:10.1103/PhysRevLett.111.235005 · 7.73 Impact Factor
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    ABSTRACT: Quasiequilibrium power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied at $50\text{ }\text{ }\mathrm{TW}\char21{}200\text{ }\text{ }\mathrm{TW}$ using three-dimensional particle-in-cell simulations. The key physical dimensions in the model are based on the ZR accelerator [D. H. McDaniel, et al., Proceedings of 5th International Conference on Dense Z-Pinches, edited by J. Davis (AIP, New York, 2002), p. 23]. The voltages assumed for this study result in electron emission from all cathode surfaces. Electrons emitted from the MITL cathodes upstream of the convolute cause a portion of the MITL current to be carried by an electron sheath. Under the simplifying assumptions made by the simulations, it is found that the transition from the two MITLs to the convolute results in the loss of most of the sheath current to anode structures. The loss is quantified as a function of radius and correlated with Poynting vector stream lines which would be followed by individual electrons. For a fixed MITL-convolute geometry, the current loss, defined to be the difference between the total (i.e. anode) current in the system upstream of the convolute and the current delivered to the load, increases with both operating voltage and load impedance. It is also found that in the absence of ion emission, the convolute is efficient when the load impedance is much less than the impedance of the two parallel MITLs. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases. Ion emission from anode surfaces in the convolute is found to increase the current loss by a factor of 2\char21{}3. When SCL ion emission is allowed from anode surfaces in the MITLs upstream of the convolute, substantially higher current losses are obtained. Note that the results reported here are valid given the spatial resolution used for the simulations.
    Physical Review Special Topics - Accelerators and Beams 12/2013; 16(12). DOI:10.1103/PhysRevSTAB.16.120401 · 1.52 Impact Factor
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    ABSTRACT: Recent experiments at the 20 MA Z Accelerator have demonstrated, for the first time, implosion velocities up to 110-130 cm/μs in imploding stainless steel wire arrays. These velocities, the largest inferred in a magnetically driven implosion, lead to ion densities of 2 × 1020 cm-3 with electron temperatures of ~5 keV. These plasma conditions have resulted in significant increases in the K-shell radiated output of 5-10 keV photons, radiating powers of >30 TW and yields >80 kJ, making it the brightest laboratory x-ray source in this spectral region. These values represent a doubling of the peak power and a 30% increase in the yield relative to previous studies. The experiments also included wire arrays with slower implosions, which were observed to have lower temperatures and reduced K-shell output. These colder pinches, however, radiated 260 TW in the soft x-ray region, making them one of the brightest soft x-ray sources available.
    Physics of Plasmas 10/2013; 20(10):3116-. DOI:10.1063/1.4823711 · 2.25 Impact Factor
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    ABSTRACT: form only given. Large pulsed power drivers often utilize multiple self-magnetically insulated transmission lines (MITL) in parallel to reduce inductance. The MITL currents must be recombined into a single anode-cathode gap, often through a post-hole convolute. Efficient post-hole convolute operation is necessary to maximize the current delivered to the load.The Z machine utilizes 4 parallel MITLs and a double posthole convolute. The current at several radial locations in the MITLs is inferred from B-dot monitor measurements. The MITL current downstream of the convolute can be several Mega-amperes less than the sum of the currents flowing in the MITLs upstream of the convolute. The convolute current is defined as the difference in the current upstream and downstream of the convolute. The convolute impedance is defined as the voltage across the convolute divided by the convolute current. A systematic study of the convolute current and convolute impedance for several Z experiments has been conducted. Despite considerable differences in the amplitude and shape of the driving current pulse, similar convolute impedance behavior is observed for many of these experiments. Impedance variations for nominally identical experiments are significant, but follow similar trends.
    2013 IEEE 40th International Conference on Plasma Sciences (ICOPS); 06/2013
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    ABSTRACT: Multiple experimental campaigns have been executed to study the implosions of initially solid beryllium (Be) liners (tubes) on the Z pulsed-power accelerator. The implosions were driven by current pulses that rose from 0 to 20 MA in either 100 or 200 ns (200 ns for pulse shaping experiments). These studies were conducted in support of the recently proposed Magnetized Liner Inertial Fusion concept [Slutz et al., Phys. Plasmas 17, 056303 (2010)], as well as for exploring novel equation-of-state measurement techniques. The experiments used thick-walled liners that had an aspect ratio (initial outer radius divided by initial wall thickness) of either 3.2, 4, or 6. From these studies, we present three new primary results. First, we present radiographic images of imploding Be liners, where each liner contained a thin aluminum sleeve for enhancing the contrast and visibility of the liner's inner surface in the images. These images allow us to assess the stability of the liner's inner surface more accurately and more directly than was previously possible. Second, we present radiographic images taken early in the implosion (prior to any motion of the liner's inner surface) of a shockwave propagating radially inward through the liner wall. Radial mass density profiles from these shock compression experiments are contrasted with profiles from experiments where the Z accelerator's pulse shaping capabilities were used to achieve shockless (“quasi-isentropic”) liner compression. Third, we present “micro-” measurements of azimuthal magnetic field penetration into the initially vacuum-filled interior of a shocked liner. Our measurements and simulations reveal that the penetration commences shortly after the shockwave breaks out from the liner's inner surface. The field then accelerates this low-density “precursor” plasma to the axis of symmetry.
    Physics of Plasmas 05/2013; 20(5). DOI:10.1063/1.4803079 · 2.25 Impact Factor
  • Physics of Plasmas 05/2013; · 2.25 Impact Factor
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    ABSTRACT: Spark gap switches are integral components in pulsed power applications ranging from pulsed microwaves and flash x-ray sources to, more recently, the linear transformer drivers (LTDs) powering next-generation petawatt-class pulsed power accelerators at Sandia National Laboratories (SNL) in Albuquerque, New Mexico. An LTD system uses a triggered switch to conduct current from two capacitors to form a magnetic field which accelerates particles down magnetically insulated transmission lines (MITLs). Existing spark gap switches suffer from limited lifetimes 1 and levels of jitter and inductance in excess of what will be required for these new accelerators2. Kinetech, under a contract from SNL, has designed, developed, and demonstrated a switch that exceeds all of the established requirements3 by firing for 57,200 shots with an inductance of 35 nH and a one-sigma switch jitter of 1.2 ns variation over a 1,000 shot sampling after 1k, 25k, and 50k shots. This paper will detail the design, development, and demonstration of this switch by Kinetech and SNL.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Electron power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied using 3D particle-in-cell simulations. The simulation uses parameters based on the Z accelerator MITL-convolute at Sandia National Laboratories and is designed for high computational efficiency. At sufficiently high voltages, electron emission upstream of the convolute results in a portion of the current carried by the transmission lines to flow in an electron sheath along the cathode surfaces. The simulations show that at 50-200 TW, the transition from the individual MITLs to the convolute results in a portion of the MITL sheath current being lost to both anode and cathode structures. The losses are identified as a function of radius and correlated with Poynting vector stream lines which can be followed by individual electrons. The difference between the current in the system upstream of the convolute and current delivered to the load (defined as the loss current) increases with both operating voltage and load impedance. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases in both steady-state and time-dependent simulation models. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. For the case of a 0.3 Ω load, the loss current fraction increases by ~7 times between the electron only and plasma simulations. Collectively, these simulation results are being used to help formulate design criteria for high-power MITL-convolute systems.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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    ABSTRACT: A system of 36 laser-triggered gas switches (LTGS) serves as the last set of command-triggered switches in the 80-TW refurbished Z accelerator at Sandia National Laboratories (SNL). The system is instrumental in the overall performance of Z and allows for flexibility in pulse shaping for various experimental campaigns. It is desirable to push the operating envelope of the switches to higher voltages and currents to allow for a higher peak power to be delivered to the load while at the same time reducing jitter and pre-fire rate for increased precision and reliability. We have accomplished this with a new LTGS (which we refer to as the C1.1 switch) while keeping the overall switch size consistent with the physical space available. Like previous gas switches [1,2], the C1.1 LTGS consists of laser-triggered and cascade electrode sections. However, the C1.1's cascade section is cantilevered and is not supported mechanically near the trigger section. An insulating rod located within the cascade electrodes (which supports the cascade section mechanically) is scalloped to reduce the likelihood of electrical tracking. Material choice for the center support rod was important due to both the mechanical and electrical requirements placed on this component. Mechanical shock testing of the new switch was performed on a shaker table available at SNL prior to installation on Z. All electrical testing of the improved LTGS was performed on the Z machine. To date, we have accumulated over 120 shots on C1.1 switches without a pre-fire. Runtime statistics are determined after each shot and show that the C1.1 switches are very tolerant to voltage and pressure variations, exhibiting median runtimes of ~37-41 ns with a jitter of
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Sandia's Z-Facility is used to conduct high energy density science experiments. Large pulsed power drivers, such as Z, are designed to deliver a large current with a short risetime to a magnetically-driven load. This often requires the use of multiple self-magnetically insulated transmission lines (MITL) in parallel to reduce inductance. The MITL currents must be recombined into a single anode-cathode gap at the load, often through a post-hole convolute. Efficient post-hole convolute operation is necessary to maximize the current delivered to the load. The Z machine utilizes four parallel MITLs and a double post-hole convolute. The current at several radial locations in the MITLs is inferred from B-dot monitor measurements. The MITL current downstream of the convolute can be several Mega-amperes less than the sum of the currents flowing in the MITLs upstream of the convolute. A systematic study of the convolute shunt current and convolute impedance for several types of Z experiments has been conducted. Convolute behavior is highly dependent on convolute voltage, which is a strong function of load type. Variations for nominally identical experiments are measurable, but small by comparison.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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    ABSTRACT: form only given. Wire array z-pinches on the Z generator are bright sources of radiation from 200eV to 9keV. Typically, wire materials are varied to provide emission in a specific spectral band, and the array setup is varied in order to provide appropriate energy per ion to heat the stagnated plasma to an electron temperature conducive to efficient emission within that band. Here we discuss a series of experiments where the wire material is varied (Al, Stainless Steel, Cu, W), however the array setup is fixed at 65mm diameter and 2.5mg mass. We discuss similarities and differences in the implosion dynamics and the differences in the plasma conditions achieved at stagnation. We will explore the changes in total, thermal K-shell and non-thermal K-shell emission between the different materials.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: The transmission line circuit model of the Z machine is used extensively to aid in the design and analysis of experiments conducted on Z. The circuit model consists of both 1-D and 2-D networks of transmission lines modeling Z's 36 pulselines, vacuum insulator stack, MITLs, vacuum convolute, and load [1].
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: form only given. The modern high current, high voltage pulsed accelerators utilize vacuum-post-hole convolutes to add the current of a number of parallel self Magnetic Insulated Transmission Lines (MITL) to a single one located very close to the centrally located load.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: As the last command-triggered switch in the Refurbished Z accelerator at Sandia National Laboratories (SNL), the laser-triggered gas switch (LTGS) system is instrumental in the overall performance of Z and allows for flexibility in pulse shaping for various experimental campaigns. It is desirable to push the operating envelope of the switch to higher voltages and currents to allow for a higher peak power to be delivered to the load while at the same time reducing jitter and pre-fire rate for increased precision and reliability. We have accomplished this in a version of the LTGS that we call the C1.1 with the constraint of keeping the overall switch size consistent with physical space available. The C1.1 LTGS consists of laser-triggered and cascade portions which has been reported on previously.[1] However, the C1.1 eliminates the trigger plate and supports the cascade section in a cantilevered fashion. Improvements to this iteration of the LTGS were mainly mechanical in nature. Other minor electrical improvements were made to reduce regions of electric field enhancement and to reduce the likelihood of tracking by adding scalloping to the center support rod. Materials choice for the center support rod was important due to both the mechanical and electrical requirements placed on this component. Mechanical shock testing of the improved switch was performed on a shaker table available at SNL prior to installation on Z and showed that the improvements resulted in less displacement of the cantilevered end and less rotation of components in the cascade section. All electrical testing of the improved LTGS was performed on the Z machine. To date, we have accumulated 377 shots on C1.1 switches without a pre-fire. Runtime statistics are determined after each shot and show that the C1.1 switches are very tolerant to voltage and pressure variations exhibiting median runtimes of 43.9 ns with a jitter (1-σ) of 5.4 ns for all switch closures. The modifications made to and- the performance results of the improved LTGS system are detailed in this manuscript.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013

Publication Stats

3k Citations
877.63 Total Impact Points

Institutions

  • 1985–2014
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2010–2013
    • Institute of High Current Electronics
      Tomsk, Tomsk, Russia
  • 2007
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States
  • 1999
    • Defense Threat Reduction Agency
      Fort Belvoir, Virginia, United States
  • 1997
    • Los Alamos National Laboratory
      • Plasma Physics Group
      Лос-Аламос, California, United States