D. E. Bliss

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (63)195.6 Total impact

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    ABSTRACT: Since October 2007 Sandia National Laboratories has operated the refurbished Z machine at an improved load current of 26 MA yielding 400 TW of x-ray power. The current pulse shape to the load is controlled by 36 independently timed laser triggered gas switches. As part of the refurbishment effort, a fiber coupled laser spark detector system has been installed which is able to detect the laser generated plasma in situ inside the trigger section of the high voltage switch. In this paper we describe how this detection system can be used to characterize the discharge dynamics of these 5.9 MV, 820 kA switches.
    Review of Modern Physics 07/2012; · 44.98 Impact Factor
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    ABSTRACT: A 6.1-MV, 0.79-MA laser-triggered gas switch (LTGS) is used to synchronize the 36 modules of the Z machine at Sandia National Laboratories. Each module includes one switch, which serves as the last command-fired switch of the module, and hence is used to determine the time at which each module electrically closes relative to the other modules. The switch is ˜81-cm in length, ˜45-cm in diameter, and is immersed in mineral oil. The outer switch envelope consists of six corrugated monomer-cast acrylic insulators and five contoured stainless-steel rings. The trigger electrodes are fabricated from copper-infused tungsten. The switch is pressurized with several atmospheres of sulfur hexafluoride (SF6), which is turbulently purged within 2 seconds after every shot. Each switch is powered from a 6-MV, 0.78-MJ Marx generator which pulse charges a 24-nF intermediate-store water capacitor in 1.4-mus. Closure of the switch allows power to flow into pulse-forming transmission lines. The power pulse is subsequently compressed by water switches, which results in a total accelerator output power in excess of 70-TW. A previous version of the LTGS performed exceptionally at a 5.4-MV, 0.7-MA level on an engineering test module used for switch development. It exhibited a 1-sigma jitter of ˜5ns, a prefire and flashover rate less than 0.1%, and a lifetime in excess of 150 shots. When installed on the Z accelerator, however, the switch exhibited a prefire probability of ˜3%, a flashover probability of ˜7%, and a 15-ns jitter. The difference in performance is attributed to several factors such as higher total charge transfer, exposure to more debris, and more stressful dynamic mechanical loading upon machine discharge. Under these conditions, the replacement lifetime was less than ten shots. Since refurbishment of Z in October 2007, there have been three LTGS design iterations to improve the performance at 6.1-MV. The most recent design exhibits a prefire rate of less than 0.1%, a flashover rate of ˜0.2%, a single switch jitter of ˜6-ns, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.
    Review of Modern Physics 01/2010; 13(3). · 44.98 Impact Factor
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    ABSTRACT: The refurbished Z pulsed power driver has been operational since October of 2007 delivering a peak current of ˜26 MA to the load. A critical component of the refurbished Z accelerator was the Laser Triggered Gas Switch (LTGS) with a maximum proven operating point of 6.5 MV, 820 kA and an overall 1-σ timing jitter of ˜6 ns. We have identified a feature in the V-dot monitor on the Pulse Forming Line (PFL) downstream of the LTGS which is indicative of the closure of the trigger section of the switch. The PFL "squiggle" feature allows us to independently measure the runtime of the cascade and trigger sections and identify problems associated with the laser triggering of the switch, such as poor alignment or degrading transmission of the focusing lens. The squiggle also helps characterize the effect of changes in operating conditions and switch design. For the most recent design version of the LTGS, the trigger and cascade section runtimes with ±1-σ jitter are 0.8 ±1.3 ns and 46 ± 5.3 ns respectively. The trigger and cascade section runtimes are not correlated suggesting that the trigger and cascade sections operate independently of each other.
    01/2009;
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    ABSTRACT: A 6.1-MV, 790-kA laser triggered gas switch (LTGS) is utilized to synchronize the 36-modules of the Z-machine at Sandia National Laboratories. The switch is ~81-cm in length, 45-cm in diameter, and is immersed in transformer oil. The switch is pulse-charged from a 780-kJ, 6-MV Marx generator in 1.4-?s. Closure of the switch allows energy stored in a 24-nF intermediate-store water capacitor to flow into subsequent pulse-forming stage. The entire system (36-modules) generates a ~70-TW at the present system operating level, but after near term improvements are complete, this level will increase to ~100-TW. The initial design of the LTGS exhibited exceptional performance at a 5.4-MV, 700-kA level (1-? jitter of ~5 ns, prefire and flashover rate less than 0.1%, lifetime in excess of 150 shots) on an engineering test-module utilized for component development. When initially implementing the same design on the 36-module Z-machine, the switch exhibited a prefire probability of ~3%, a flashover probability of ~7%, and a jitter increase of a factor of 3. The difference in performance is attributed to several factors such as higher Coulomb transfer, exposure to a more debris prone environment, and more stressful dynamic mechanical loading when the machine discharges, compared to that of the engineering test module. Under these conditions the replacement lifetime was less than 10 shots, which is unacceptably frequent. Since refurbishment of Z in October 2007, there have been three major LTGS design iterations to improve the performance at a 6.1-MV operating point. The most recent design iteration includes modifications to the switch geometry and electrode and insulating materials that reduces the random prefire rate to less than 0.1%, reduces the flashover rate to -0.2%, reduces single switch jitter to ~6-ns, while increasing the average switch life to greater than 50 shots. Modifications and performance improvements are detailed in this manuscript.
    01/2009;
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    ABSTRACT: The effect of short-circuit across the final anode-cathode gap of powerful pulsed current generators could hamper efficient power delivery to the Z-pinch plasma. To study this effect, a novel EUV diagnostics of plasmas created in the final section of the transmission line (the anode-cathode gap near the main load) of the Z-Machine high-current generator (Sandia National Laboratories, United States) was developed. The work included developing spectroscopic instruments, theoretical and experimental studies of EUV spectra of iron ions in well-diagnosed laser-produced plasmas, and a comparison of these spectra with those of plasmas created in the final anode-cathode gap of the transmission line. The EUV spectra of highly charged Fe ions in the spectral range λ ∼ 20–800 Å were investigated. In experiments performed at Sandia National Laboratories, spectra of FeXIII-FeXVII ions were observed. A comparison of the measured and calculated spectra shows that the electron plasma temperature in the anode-cathode gap is T e ∼ 200 eV.
    Plasma Physics Reports 10/2008; 34(11):944-954. · 0.66 Impact Factor
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    ABSTRACT: Laser triggered, megavolt, megampere gas switches are frequently utilized to synchronize multiple pulsed power driver modules for inertial-confinement fusion, isentropic compression, and radiation physics experiments. The device developed to synchronize the 36 modules of the refurbished Z accelerator at Sandia National Laboratories is a 5.4 MV, 700 kA, sulfur-hexafluoride (SF6) filled, laser triggered gas switch. At this operating level, switch jitter is 5 ns, the prefire rate is less than 0.1%, the average optic lifetime is greater than 200 shots, and the flashover rate is less than 1%. Over 1000 shots on a single-module test facility were conducted while iterating several potential design improvements, including utilizing low-erosion electrode material, varying SF6 pressure, and modifying internal switch geometry all while keeping the basic switch architecture and footprint constant. Results of this development effort are presented herein.
    Review of Modern Physics 01/2008; 11(6). · 44.98 Impact Factor
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    ABSTRACT: We study the physics of water switching at 4 to 5 MV as part of Sandia's Z-Refurbishment program. We present time histories of the light emitted by two sets of water switches and correlate the light emission to the accelerator power pulse. Framing camera images demonstrate how the initial "bushy" streamers collapse to narrow current channels at late time. Spectroscopy of the water switches indicates that the early-time streamers are already hot-dense blackbodies with temperatures between 7000 K and 9000 K. Late time hydrogen emission lines allow estimates of electron densities at that time ranging from 0.2 to 2 times 10<sup>18</sup>/cm<sup>3</sup>.
    IEEE Transactions on Plasma Science 01/2008; · 0.87 Impact Factor
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    ABSTRACT: Summary form only given. The Z pulsed power driver at Sandia National Laboratories is in the process of being refurbished as part of the ZR project to improve reliability and increase the energy delivered to the load. The new ZR gas switches currently close at a peak voltage of 6.2 MV to generate a projected 26 MA load current as compared to 4.6 MV and ~20 MA for the old switches on Z. The Laser Trigger System (LTS) has been redesigned to meet requirements that it trigger this higher voltage switch with comparable optic lifetime (>100 shots) and jitter (1sigma < 5ns) to the old Z switches. We will discuss critical design features of the LTS and show performance results from the Z20 test bed which was used to study the operation of a single ZR module.
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
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    ABSTRACT: The effect of a short circuit across the final anode-cathode (A-K) gap of the powerful Z-Accelerator could hamper effective power delivery to z-pinch plasmas. The objective of this work is to develop an extreme ultraviolet (EUV) diagnostic technique for diagnosis of the low-temperature plasmas created in the final transmission line (A-K gap near the load) of the Z-Accelerator at the Sandia National Laboratories (SNL). The purpose of this effort is to help in understanding and mitigating this potentially serious problem. This work includes developing EUV grazing incidence spectrometers, investigation of the EUV spectra of highly charged ions in well diagnosed laser-produced plasmas, and the comparison of these laser plasma spectra with the spectra of plasmas created in the inner transmission line. Spectra of highly-charged iron (Fe) ions were investigated using EUV spectroscopy methods in a spectral range of 2 to 80 nm. Experiments at SNL have shown that the most stripped ion observed in the spectra is FeXVII. Comparison of the experimental spectra of FeIII through FeXVII ions with theoretical calculations gives an electron temperature T e of ∼ 200 eV.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: Summary form only given. We present experimental observations of the closure of the power feed gap on the Z machine during the implosion of a wire array load. The cathode surface and wire array edge were imaged by time-resolved laser shadowgraphy and X-ray backlighting. During the run in phase of the wire array which lasts through maximum current, ~20 MA, the radial and axial power-flow surfaces of the cathode expanded < frac14 mm. In contrast, after peak X-ray emission, the radial power-flow surface expanded at a velocity of 4.8 times 10<sup>4</sup> m/s as observed by laser shadowgraphy. Assuming both anode and cathode power-flow surfaces expand with similar velocities, the extrapolated time to close a 4 mm gap is ~40 ns. Previous gap closure experiments indicated closure times less than this. Therefore a low density, low gradient plasma must be responsible for shorting the gap at earlier times. The high density plasma serves as a moving surface and source of ions.
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
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    ABSTRACT: The Z machine at Sandia National Laboratories is presently undergoing an upgrade, called Z-Refurbishment (ZR) [1], that is aimed at improving capacity, precision, and capability to essentially all of its pulsed power components, including its thirty six laser-triggered gas switches (LTGS). Voltage and current requirements for the ZR LTGS have increased 25% from the onset of the ZR program, with no allowable increase to the physical footprint (or inductance) for the device. Initial design studies indicated that a total machine peak current of 26 MA could be achieved with the each LTGS operating at 5 MV and 600 kA. Increases in the final design inductance in the transition from vertical water transmission lines to horizontal magnetically insulated transmission lines, higher inductance in vacuum from changes in the load position for improved diagnostic access, and conservatism in the vacuum power flow requirements caused the LTGS operational goal to become 5.4 MV and 750 kA for a total machine peak current of 23 MA in 100 ns to a 10 mm radius, 10 mm long wire array. A comprehensive research program was initiated in August 2005 to improve the performance of the ZR gas switch at the 5.4 MV level, and results of that effort to date are presented herein.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: The Laser Triggered Switch Program at Sandia National Laboratories is an intensive development study to understand and optimize the laser triggered gas switch (LTGS) for the Z-Refurbishment (ZR) project. The laser triggered gas switch is the final command-triggered switch in the machine. Reliability and performance of the switch is crucial.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: The Z pulsed power driver [1] at Sandia National Laboratories is used to develop high energy density z-pinch x-ray sources for inertial confinement fusion research and radiation effects testing, and to drive megabar pressures in material samples for equation of state studies. The entire pulsed power system is in the process of being replaced, improving reliability and increasing the energy delivered to the load.
    Pulsed Power Conference, 2007 16th IEEE International; 07/2007
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    ABSTRACT: Summary form only given. The Z machine at Sandia National Laboratories is presently undergoing an upgrade, called Z-Refurbishment (ZR), that is aimed at improving capacity, precision, and capability to nearly all of its pulsed power components, including its thirty six laser-triggered gas switches (LTGS). Voltage and current requirements for the ZR LTGS have increased 25% from the onset of the ZR program, with no allowable increase to the physical footprint (or inductance) for the device. Initial design studies indicated that a total machine peak current of 26 MA could be achieved with each LTGS operating at 5 MV and 600 kA. Increases in the final design inductance in the transition from vertical water transmission lines to horizontal magnetically insulated transmission lines, higher inductance in vacuum from changes in the load position for improved diagnostic access, and conservatism in the vacuum power How requirements caused the LTGS operations goals to become 6.25 MV and 750 kA for a total machine peak current of 26 MA. Tests of the proposed switch design at 5 MV with a resistive load demonstrated jitter (<4 ns), voltage precision (range 0.05 -1.5 percent), and lifetime (mean 220 shots), which were all within the initial ZR design goals. The prefire rate was 1.2% which was substantially higher than desired for ZR. Subsequent tests on a single ZR engineering module as the operating voltage increased resulted in random housing flashovers, difficulties in consistently triggering the switch and even higher prefire rate. This paper summarizes LTGS design changes and new cleaning/assembly protocols that were developed to meet evolving ZR goals. Performance of LTGS at 5.2 MV and 6.1 MV on the engineering module are discussed. The data are utilized to project performance on ZR.
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
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    ABSTRACT: Summary form only given. The Laser Triggered Switch Program at Sandia National Laboratories is an intensive development study to optimize and improve the laser triggered gas switch (LTGS) for the Z-Refurbishment (ZR) project. The laser triggered gas switch is the final command-triggered switch in the machine, and reliability and performance of the switch is crucial. A modified LTGS trigger section with optical viewing windows perpendicular to laser propagation is used to analyze a laser induced plasma spark in SF6 gas in order to quantity parameters such as spark length and plasma temperature. The laser spark is created through a focusing lens by the fourth-harmonic (266nm) of a 5ns FWHM pulsed Nd: YAG laser with 30mJ maximum energy output. Several diagnostic methods are used to analyze the laser spark. Visible spark length measurements are made using a lens system mounted to a CCD camera at gas pressures ranging from sub-atmosphere to 4 atmospheres. Differing f-number lenses are compared to determine optimal visible spark length for a given gas pressure. Spark length is used as an indicator of the ability of a switch to trigger at a given gas pressure and charge voltage. Typically, the visible spark length must be at least 30% of the electrode gap spacing to produce acceptable switch run-time and jitter. Experiments have shown that for switch operating pressures near 4 atmospheres, a dramatic increase in spark length (~12mm to 24mm) is noted between f/9.8 and f/14.8 lenses (500mm and 750mm respectively) while the increase in spark length slows markedly (~24mm to 30.5mm) with an increase in lens focal length to f/19.7 (lm).
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
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    ABSTRACT: The mass of the outer and inner wire array used to drive the baseline dynamic hohlraum (DH) with pedestal target [ Sanford et al., Phys. Plasmas 13, 012701 (2006) ] is reversed in order to determine if the nested wire array is operating in a hydrodynamic, or transparent-like mode [ J. Davis et al., Appl. Phys. Lett. 70, 170 (1997) ], when the outer array arrives at the radius of the inner array. In contrast to the baseline, mass reversal allows the modes to be distinguished by the difference in the timing of characteristic features of the x-ray radiation pulses in the two modes. For the reversed-mass DH, all parameters such as wire number, array radii, and target remained the same, except the diameters of the individual wires were adjusted to reverse the array masses. Measurements show unambiguously that the reversed-mass DH operates in a transparent-like mode, the outer array passing through the inner array with limited collisional interaction. Numerical simulations in the r-θ plane suggest that the underlying physics of the outer array collision with the inner between the two DHs (baseline and reversed-mass), remains similar, implying that the baseline also operates with transparency. Inflection in the rate of change of the current is measured 4–7 ns after the radiation signal and is associated with the outer-inner array interaction, indicating that the rear portion of the resulting plasma shell of the outer array carries the current prior to the collision. Numerical simulations together with analytic theory describe probable dynamics of the current switching from the outer to inner array.
    Physics of Plasmas 05/2007; 14(5):052703-052703-17. · 2.38 Impact Factor
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    ABSTRACT: Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.
    Physical Review Letters 03/2007; 98(6):065003. · 7.73 Impact Factor
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    ABSTRACT: In October 2005, an intensive three-year Laser Triggered Gas Switch (LTGS) development program was initiated to investigate and solve observed performance and reliability issues with the LTGS for ZR. The approach taken has been one of mission-focused research: to revisit and reassess the design, to establish a fundamental understanding of LTGS operation and failure modes, and to test evolving operational hypotheses. This effort is aimed toward deploying an initial switch for ZR in 2007, on supporting rolling upgrades to ZR as the technology can be developed, and to prepare with scientific understanding for the even higher voltage switches anticipated needed for future high-yield accelerators. The ZR LTGS was identified as a potential area of concern quite early, but since initial assessments performed on a simplified Switch Test Bed (STB) at 5 MV showed 300-shot lifetimes on multiple switch builds, this component was judged acceptable. When the Z{sub 20} engineering module was brought online in October 2003 frequent flashovers of the plastic switch envelope were observed at the increased stresses required to compensate for the programmatically increased ZR load inductance. As of October 2006, there have been 1423 Z{sub 20} shots assessing a variety of LTGS designs. Numerous incremental and fundamental switch design modifications have been investigated. As we continue to investigate the LTGS, the basic science of plastic surface tracking, laser triggering, cascade breakdown, and optics degradation remain high-priority mission-focused research topics. Significant progress has been made and, while the switch does not yet achieve design requirements, we are on the path to develop successively better switches for rolling upgrade improvements to ZR. This report summarizes the work performed in FY 2006 by the large team. A high-level summary is followed by detailed individual topical reports.
    02/2007;
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    ABSTRACT: Summary form only given. The ZR gas switch is a multistage gas switch designed to hold off a voltage of ~6 MV and then transmit a 600-kA current pulse to the water section of ZR. The first stage of the switch, which holds off approximately 900 kV, is laser triggered by the focused beam from a frequency-quadrupled ND:YAG laser operating at 266 nm. The breakdown of the laser-triggered section causes a wave of breakdowns in the remaining 22 gaps in the switch, starting near the laser-triggered section and cascading to the other end of the switch. The gaps in this "cascade" section normally break down in multiple arc channels, which lowers the overall inductance of the switch. We are observing the details of normal and abnormal switch operation with fast framing and streak cameras, observing the time-history of the light emitted by the switch with fast photo-detectors and taking time-integrated photographs of the switch. The diagnostics indicate that the laser-triggered gap normally begins conducting current 10-to-15 ns after the laser arrives in the gap, with the entire switch breaking down 45-to 55-ns after the laser beam arrives in the switch. Details of the experiments and the operation of different switches are presented.
    IEEE International Conference on Plasma Science 01/2007;
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    ABSTRACT: Over the last several years, rapid progress has been made evaluating the double-z-pinch indirect-drive, inertial confinement fusion (ICF) high-yield target concept (Hammer et al 1999 Phys. Plasmas 6 2129). We have demonstrated efficient coupling of radiation from two wire-array-driven primary hohlraums to a secondary hohlraum that is large enough to drive a high yield ICF capsule. The secondary hohlraum is irradiated from two sides by z-pinches to produce low odd-mode radiation asymmetry. This double-pinch source is driven from a single electrical power feed (Cuneo et al 2002 Phys. Rev. Lett. 88 215004) on the 20 MA Z accelerator. The double z-pinch has imploded ICF capsules with even-mode radiation symmetry of 3.1 ± 1.4% and to high capsule radial convergence ratios of 14–21 (Bennett et al 2002 Phys. Rev. Lett. 89 245002; Bennett et al 2003 Phys. Plasmas 10 3717; Vesey et al 2003 Phys. Plasmas 10 1854). Advances in wire-array physics at 20 MA are improving our understanding of z-pinch power scaling with increasing drive current. Techniques for shaping the z-pinch radiation pulse necessary for low adiabat capsule compression have also been demonstrated.
    Plasma Physics and Controlled Fusion 01/2006; 48(2):R1. · 2.37 Impact Factor