W. A. Stygar

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (314)882.63 Total impact

  • [Show abstract] [Hide abstract]
    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. · 7.73 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-. · 2.38 Impact Factor
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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). · 2.38 Impact Factor
  • Physics of Plasmas 05/2013; · 2.38 Impact Factor
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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: 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. 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. For a fixed MITL-convolute geometry, 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 operating modes. The impact of cathode plasma formation on the loss current is also considered for the time-dependent simulation results. Collectively, these simulation results are being used to help formulate design criteria for high-power convolute-MITL systems.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 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: 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: 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: 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: Three-dimensional fully electromagnetic (EM) models are being used to optimize the design of a petawatt-class pulsepower accelerator.1,2 In this design, a cylindrical array of linear-transformer-driver (LTD) modules3 feed power into radial-transmission-line impedance transformers followed by a vacuum line and convolute section which delivers the combined current to the load.1 In the limit of small ratio of pulse width to one-way wave transit time, we have previously demonstrated that transport efficiency is maximized when the impedance profile is exponential but deviates for increasing ratio.4 We build on that result here with the construction of a virtual accelerator that includes a 3D EM model of a realistic next-generation design for the accelerator's impedance transformer, and circuit models for the LTD drive circuits, vacuum magnetically insulated transmission lines, and load. Variable timing for each LTD module will enable pulse shaping.
    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
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    ABSTRACT: 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. The reason of course of using several parallel MITL's to transfer the current pulse from large, ~1.5 m, radii to the 1-2 cm load is to reduce the transfer inductance. For example, the vacuum chamber of the 24-26-MA Z machine has a 1.45-m radius vacuum section containing four parallel conical MITLs merging into one 6cm radial disc MITL adjacent to the centrally located load via a double post-hole convolute array located at 7.62 cm from the axis. Although special care has been taken to reduce the electrical stresses on the cathode hole surfaces in order to avoid electron emission, substantial current losses, 4-6 MA, are observed most probably due to plasma formation and the unavoidable magnetic nulls. In the proposed experiments we will study the behavior of only one convolute using the MYKONOS-V driver. MYKONOS-V is a Linear Transformer Driver (LTD) voltage adder composed of 5 nominally 1-MA cavities connected in series. The voltage adder radial A-K cavity is deionized water insulated. The experimental set-up is designed in such a way to reach conditions on the convolute very similar to those existing on Z. Most importantly, in contrast to Z, it provides full view of the convolute for optical and spectroscopic imaging and gives the flexibility and freedom to study various options in an effort to reduce the convolute losses without affecting the day-to-day Z experiments. This is going to be a dedicated convolute study experiment. The hardware design, numerical simulations and proposed diagnostics will be presented and discussed.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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    ABSTRACT: The implosions of initially solid beryllium liners (tubes) have been imaged with penetrating radiography through to stagnation. These novel radiographic data reveal a high degree of azimuthal correlation in the evolving magneto-Rayleigh-Taylor structure at times just prior to (and during) stagnation, providing stringent constraints on the simulation tools used by the broader high energy density physics and inertial confinement fusion communities. To emphasize this point, comparisons to 2D and 3D radiation magnetohydrodynamics simulations are also presented. Both agreement and substantial disagreement have been found, depending on how the liner's initial outer surface finish was modeled. The various models tested, and the physical implications of these models are discussed. These comparisons exemplify the importance of the experimental data obtained.
    Physical Review Letters 09/2012; 109(13):135004. · 7.73 Impact Factor
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    Physics of Plasmas 07/2012; 19(7). · 2.38 Impact Factor
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    ABSTRACT: Current pulse shaping techniques, originally developed for planar dynamic material experiments on the Z-machine [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], are adapted to the design of controlled cylindrical liner implosions. By driving these targets with a current pulse shape that prevents shock formation inside the liner, shock heating is avoided along with the corresponding decrease in electrical conductivity ahead of the magnetic diffusion wave penetrating the liner. This results in an imploding liner with a significant amount of its mass in the solid phase and at multi-megabar pressures. Pressures in the solid region of a shaped pulse driven beryllium liner fielded on the Z-machine are inferred to 5.5 Mbar, while simulations suggest implosion velocities greater than 50kms-1. These solid liner experiments are diagnosed with multi-frame monochromatic x-ray backlighting which is used to infer the material density and pressure. This work has led to a new platform on the Z-machine that can be used to perform off-Hugoniot measurements at higher pressures than are accessible through magnetically driven planar geometries.
    Physics of Plasmas 04/2012; 19(5). · 2.38 Impact Factor
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    ABSTRACT: The linear transformer driver (LTD) technological approach can result in relatively compact devices that can deliver fast, high current, and high-voltage pulses straight out of the LTD cavity without any complicated pulse forming and pulse compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. The usual LTD architecture [A. A. Kim, M. G. Mazarakis, V. A. Sinebryukhov, B. M. Kovalchuk, V. A. Vizir, S. N Volkov, F. Bayol, A. N. Bastrikov, V. G. Durakov, S. V. Frolov, V. M. Alexeenko, D. H. McDaniel, W. E. Fowler, K. LeCheen, C. Olson, W. A. Stygar, K. W. Struve, J. Porter, and R. M. Gilgenbach, Phys. Rev. ST Accel. Beams 12, 050402 (2009); M. G. Mazarakis, W. E. Fowler, A. A. Kim, V. A. Sinebryukhov, S. T. Rogowski, R. A. Sharpe, D. H. McDaniel, C. L. Olson, J. L. Porter, K. W. Struve, W. A. Stygar, and J. R. Woodworth, Phys. Rev. ST Accel. Beams 12, 050401 (2009)] provides sine shaped output pulses that may not be well suited for some applications like z-pinch drivers, flash radiography, high power microwaves, etc. A more suitable power pulse would have a flat or trapezoidal (rising or falling) top. In this paper, we present the design and first test results of an LTD cavity that generates such a type of output pulse by including within its circular array a number of third harmonic bricks in addition to the main bricks. A voltage adder made out of a square pulse cavity linear array will produce the same shape output pulses provided that the timing of each cavity is synchronized with the propagation of the electromagnetic pulse.
    Physical Review Special Topics - Accelerators and Beams 04/2012; 15(4). · 1.57 Impact Factor

Publication Stats

2k Citations
882.63 Total Impact Points

Institutions

  • 1990–2013
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2007
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States
    • University of Michigan
      • Department of Nuclear Engineering and Radiological Sciences
      Ann Arbor, MI, United States
  • 2001
    • Swarthmore College
      Swarthmore, Pennsylvania, United States
  • 1999
    • Defense Threat Reduction Agency
      Fort Belvoir, Virginia, United States