Publications (36)149.11 Total impact
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ABSTRACT: Imploding wire arrays on the 20 MA Z generator have recently provided some of the most powerful and energetic laboratory sources of multikeV photons, including ∼375 kJ of Al Kshell emission (hν ∼ 1–2 keV), ∼80 kJ of stainless steel Kshell emission (hν ∼ 5–9 keV) and a kJlevel of Mo Kshell emission (hν ∼ 17 keV). While the global implosion dynamics of these different wire arrays are very similar, the physical process that dominates the emission from these xray sources fall into three broad categories. Al wire arrays produce a column of plasma with densities up to ∼3 × 1021 ions/cm3, where opacity inhibits the escape of Kshell photons. Significant structure from instabilities can reduce the density and increase the surface area, therefore increase the Kshell emission. In contrast, stainless steel wire arrays operate in a regime where achieving a high pinch temperature (achieved by thermalizing a high implosion kinetic energy) is critical and, while opacity is present, it has less impact on the pinch emissivity. At higher photon energies, line emission associated with inner shell ionization due to energetic electrons becomes important. 
Conference Paper: Z machine circuit model development
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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 1D and 2D networks of transmission lines modeling Z's 36 pulselines, vacuum insulator stack, MITLs, vacuum convolute, and load [1]. 
Article: Circuit Model for Driving ThreeDimensional Resistive MHD Wire Array $Z$ Pinch Calculations
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ABSTRACT: Compact tungsten wire array Z pinches imploded on the Z generator at Sandia National Laboratories have proven to be a powerful reproducible Xray source. Wire arrays have also been used in dynamic hohlraum radiation flow experiments and as an intense Kshell source, while the generator has been used extensively for isentropic compression experiments. A problem shared by all these applications is current loss, preventing the ~20MA drive current from being reliably coupled to the load. This potentially degrades performance, while uncertainties in how this loss is described limit our predictive capability. We present details of a transmission line equivalent circuit model of the Z generator for use in driving 3D resistive MHD simulations of wire array loads. We describe how power delivery to these loads is affected by multiple current losses and demonstrate how these may be calculated or reconstructed from available electrical data for inclusion in the circuit model. We then demonstrate how the circuit model and MHD load calculation may be combined to infer an additional current loss that has not been directly diagnosed for wire arrays.  [Show abstract] [Hide abstract]
ABSTRACT: A Bdot monitor that measures the current 6 cm from the axis of dynamic loads fielded on 107A multiterawatt pulsedpower accelerators has been developed. The monitor improves upon the multimegampere loadcurrent gauge described in Phys. Rev. ST Accel. Beams 11, 100401 (2008)PRABFM1098440210.1103/PhysRevSTAB.11.100401. The design of the improved monitor was developed using threedimensional particleincell simulations that model vacuum electron flow in the transmission line near the monitor. The simulations include important geometric features of the Bdot probe and model the deposition of electron energy within the probe. The simulations show that the improved design reduces by as much as a factor of 5 the electron energy deposition to the interior of the monitor. Data taken on accelerator shots demonstrate that the improved monitor works as well as the original monitor on shots with lowimpedance loads, and delivers superior performance on higherimpedanceload shots. 
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ABSTRACT: Since the completion of the ZR upgrade of the Z accelerator at the Sandia National Laboratories in the fall of 2007, many shots have been taken on the accelerator, and there has been much opportunity to compare circuitcode predictions of the performance of the machine with actual measurements. We therefore show comparisons of measurements, and describe a fullcircuit, 36line Bertha circuit model of the machine. The model has been used for both shortpulse and longpulse (tailored pulse) modes of operation. We also present the asbuilt circuit parameters of the machine and indicate how these were derived. We discuss enhancements to the circuit model that include 2D effects in the water lines, but show that these have little effect on the fidelity of the simulations. Finally, we discuss how further improvements can be made to handle azimuthal coupling of the multiple lines at the vacuum insulator stack.  [Show abstract] [Hide abstract]
ABSTRACT: We have conducted dielectricbreakdown tests on water subject to a single unipolar pulse. The peak voltages used for the tests range from 5.8 to 6.8 MV; the effective pulse widths range from 0.60 to 1.1 μs; and the effective areas tested range from 1.8×105 to 3.6×106 cm2. The tests were conducted on waterinsulated coaxial capacitors. The two electrodes of each capacitor have outer and inner radii of 99 and 56 cm, respectively. Results of the tests are consistent with predictions of the waterdielectricbreakdown relation developed in [ Phys. Rev. ST Accel. Beams 9 070401 (2006)].  [Show abstract] [Hide abstract]
ABSTRACT: We have developed a semianalytic expression for the total energy loss to a vacuum transmissionline electrode operated at high lineal current densities. (We define the lineal current density jℓ≡B/μ0 to be the current per unit electrode width, where B is the magnetic field at the electrode surface and μ0 is the permeability of free space.) The expression accounts for energy loss due to Ohmic heating, magnetic diffusion, j×B work, and the increase in the transmission line’s vacuum inductance due to motion of the vacuumelectrode boundary. The sum of these four terms constitutes the Poynting fluence at the original location of the boundary. The expression assumes that (i) the current distribution in the electrode can be approximated as onedimensional and planar; (ii) the current I(t)=0 for t<0, and I(t)∝t for t≥0; (iii) jℓ≤10 MA/cm; and (iv) the currentpulse width is between 50 and 300 ns. Under these conditions we find that, to first order, the total energy lost per unit electrodesurface area is given by Wt(t)=αtβBγ(t)+ζtκBλ(t), where B(t) is the nominal magnetic field at the surface. The quantities α, β, γ, ζ, κ, and λ are material constants that are determined by normalizing the expression for Wt(t) to the results of 1D magnetohydrodynamic MACH2 simulations. For stainlesssteel electrodes operated at current densities between 0.5 and 10 MA/cm, we find that α=3.36×105, β=1/2, γ=2, ζ=4.47×104, κ=5/4, and λ=4 (in SI units). An effective timedependent resistance, appropriate for circuit simulations of pulsedpower accelerators, is derived from Wt(t). Resistancemodel predictions are compared to energyloss measurements made with stainlesssteel electrodes operated at peak lineal current densities as high as 12 MA/cm (and peak currents as high as 23 MA). The predictions are consistent with the measurements, to within experimental uncertainties. We also find that a previously used electrodeenergyloss model overpredicts the measurements by as much as an order of magnitude.  [Show abstract] [Hide abstract]
ABSTRACT: We have developed a system of differentialoutput monitors that diagnose current and voltage in the vacuum section of a 20MA 3MV pulsedpower accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuuminsulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The innerMITL monitors are located 6 cm from the axis of the load. Each of the stack and outerMITL current monitors comprises two separate Bdot sensors, each of which consists of four 3mmdiameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the Bdot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Commonmodenoise rejection is achieved by combining these signals in a 50Omega balun. The signal cables that connect the Bdot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50Omega cable transmits the output signal of each balun to a doublewall screen room, where the signals are attenuated, digitized (0.5ns/sample), numerically compensated for cable losses, and numerically integrated. By contrast, each innerMITL current monitor contains only a single Bdot sensor. These monitors are fielded in oppositepolarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for commonmodenoise rejection after digitization. All the current monitors are calibrated on a 76cmdiameter axisymmetric radial transmission line that is driven by a 10kA current pulse. The reference current is measured by a currentviewing resistor (CVR). The stack voltage monitors are also differentialoutput gauges, consisting of one 1.8cmdiameter Ddot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MV on dedicated accelerator shots with a shortcircuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outerMITL Bdot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA to a lowimpedance zpinch load, the peak lineal current densities at the stack, outerMITL, and innerMITL monitor locations are 0.5, 1, and 58MA/m, respectively. On such shots the peak currents measured at these three locations agree to within 1%. 
Conference Paper: CircuitCode Modeling of the Refurbished Z Accelerator: Comparison of Measurements with Predictions
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ABSTRACT: With the successful completion of its refurbishment the Z machine at Sandia is now routinely operating with currents over 26 MA into various loads. Now that the machine is operating we can measure current and voltage at various locations throughout the machine and compare with circuit code predictions. These measurements have led to improvements in the model that provide a more accurate predictive capability. In this paper we describe the fullmachine circuit model of Z, and indicate how machine parameters are derived. Many were determined with commerciallyavailable field calculation software, but parameters for switches and other nonlinear elements were determined empirically. We show comparisons of circuit code predictions with machine performance. Finally, we show where improvements to the model can yet be made. 
Conference Paper: 1D and 2D circuitcode modeling of the refurbished Z accelerator
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ABSTRACT: Now that the refurbishment of the Z machine at Sandia is complete we are routinely operating with currents over 26 MA into various loads. With routine operation we are able to measure current and voltage throughout the machine and compare with circuit code predictions. These measurements have motivated improvements to the original model to include twodimensional (2D) and threedimensional (3D) subcircuits. In this paper we describe a fullmachine (36 module), onedimensional (1D) circuit model of Z using the Bertha code developed at the Naval Research Laboratory. We also describe a model that uses 2D submodules. In addition, we discuss plans for implementing a 3D circuit model to allow investigating azimuthal effects that can be important for tailoring the rise time, shape, and length of the Z output pulse. Pulse tailoring on Z is achieved by staggering the timing of the various pulsedforming modules, and by operating individual modules in either long or shortpulse modes. Since the individual lines combine at a large diameter and are separated azimuthally, transittime effects can be important for determining transient field enhancements at the vacuum insulator and in the vacuum transmission lines, as well as pulse shape. Finally, we compare the 1D and 2D models with data, and provide predictions for both short and long pulse modes for various load configurations.  [Show abstract] [Hide abstract]
ABSTRACT: Now that the refurbishment of the Z machine at Sandia is complete we are routinely operating with currents over 26 MA into various loads. With routine operation we are able to measure current and voltage throughout the machine and compare with circuit code predictions. These measurements have motivated improvements to the original model to include twodimensional (2D) and threedimensional (3D) subcircuits. In this paper we describe a fullmachine (36 module), onedimensional (1D) circuit model of Z using the Bertha code developed at the Naval Research Laboratory. We also describe a model that uses 2D submodules. In addition, we discuss plans for implementing a 3D circuit model to allow investigating azimuthal effects that can be important for tailoring the rise time, shape, and length of the Z output pulse. Pulse tailoring on Z is achieved by staggering the timing of the various pulsedforming modules, and by operating individual modules in either long or shortpulse modes. Since the individual lines combine at a large diameter and are separated azimuthally, transittime effects can be important for determining transient field enhancements at the vacuum insulator and in the vacuum transmission lines, as well as pulse shape. Finally, we compare the 1D and 2D models with data, and provide predictions for both short and long pulse modes for various load configurations. 
Conference Paper: An Overview of Pulse Compression and Power Flow in the Upgraded Z Pulsed Power Driver
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ABSTRACT: The Z pulsed power driver [1] at Sandia National Laboratories is used to develop high energy density zpinch xray 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. The upgraded pulsed power system ultimately will deliver more than nine megajoules of forward going wave energy in the first one hundred nanoseconds of its pulse. The system is comprised of thirtysix nominally identical modules, each producing a 3.3terawatt pulse in 6 Omega waterinsulated transmission lines. The peak forwardgoing voltage is about 5 MV. The pulse rise time is similar to 75 ns; the full width at half maximum is similar to 190 us. The thirtysix modules are combined in parallel and drive twenty to twentyfive MA into the single load. Figure 1 shows a cross section of the upgraded Z driver. The outer tank diameter is thirtythree meters.  [Show abstract] [Hide abstract]
ABSTRACT: We have developed an accelerator architecture that can serve as the basis of the design of petawattclass zpinch drivers. The architecture has been applied to the design of two zpinch accelerators, each of which can be contained within a 104mdiameter cylindrical tank. One accelerator is driven by slow (∼1 μs) Marx generators, which are a mature technology but which necessitate significant pulse compression to achieve the short pulses (≪1 μs) required to drive z pinches. The other is powered by linear transformer drivers (LTDs), which are less mature but produce much shorter pulses than conventional Marxes. Consequently, an LTDdriven accelerator promises to be (at a given pinch current and implosion time) more efficient and reliable. The Marxdriven accelerator produces a peak electrical power of 500 TW and includes the following components: (i) 300 Marx generators that comprise a total of 1.8×104 capacitors, store 98 MJ, and erect to 5 MV; (ii) 600 waterdielectric triplate intermediatestore transmission lines, which also serve as pulseforming lines; (iii) 600 5MV lasertriggered gas switches; (iv) three monolithic radialtransmissionline impedance transformers, with triplate geometries and exponential impedance profiles; (v) a 6level 5.5mdiameter 15MV vacuum insulator stack; (vi) six magnetically insulated vacuum transmission lines (MITLs); and (vii) a tripleposthole vacuum convolute that adds the output currents of the six MITLs, and delivers the combined current to a zpinch load. The accelerator delivers an effective peak current of 52 MA to a 10mmlength z pinch that implodes in 95 ns, and 57 MA to a pinch that implodes in 120 ns. The LTDdriven accelerator includes monolithic radial transformers and a MITL system similar to those described above, but does not include intermediatestore transmission lines, multimegavolt gas switches, or a laser trigger system. Instead, this accelerator is driven by 210 LTD modules that include a total of 1×106 capacitors and 5×105 200kV electrically triggered gas switches. The LTD accelerator stores 182 MJ and produces a peak electrical power of 1000 TW. The accelerator delivers an effective peak current of 68 MA to a pinch that implodes in 95 ns, and 75 MA to a pinch that implodes in 120 ns. Conceptually straightforward upgrades to these designs would deliver even higher pinch currents and faster implosions.  [Show abstract] [Hide abstract]
ABSTRACT: We have developed a diagnostic system that measures the spectrally integrated (i.e. the total) energy and power radiated by a pulsed blackbody xray source. The totalenergyandpower (TEP) diagnostic system is optimized for blackbody temperatures between 50 and 350 eV. The system can view apertured sources that radiate energies and powers as high as 2 MJ and 200 TW, respectively, and has been successfully tested at 0.84 MJ and 73 TW on the Z pulsedpower accelerator. The TEP system consists of two pinhole arrays, two silicondiode detectors, and two thinfilm nickel bolometers. Each of the two pinhole arrays is paired with a single silicon diode. Each array consists of a 38×38 square array of 10μmdiameter pinholes in a 50μmthick tantalum plate. The arrays achromatically attenuate the xray flux by a factor of ∼1800. The use of such arrays for the attenuation of soft x rays was first proposed by Turner and coworkers. The attenuated flux from each array illuminates its associated diode; the diode's output current is recorded by a dataacquisition system with 0.6ns time resolution. The arrays and diodes are located 19 and 24 m from the source, respectively. Because the diodes are designed to have an approximately flat spectral sensitivity, the output current from each diode is proportional to the xray power. The nickel bolometers are fielded at a slightly different angle from the arraydiode combinations, and view (without pinhole attenuation) the same xray source. The bolometers measure the total xray energy radiated by the source andon every shotprovide an in situ calibration of the arraydiode combinations. Two arraydiode pairs and two bolometers are fielded to reduce random uncertainties. An analytic model (which accounts for pinholediffraction effects) of the sensitivity of an arraydiode combination is presented. 
Article: Analytic model of a magnetically insulated transmission line with collisional flow electrons
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ABSTRACT: We have developed a relativisticfluid model of the flowelectron plasma in a steadystate onedimensional magnetically insulated transmission line (MITL). The model assumes that the electrons are collisional and, as a result, drift toward the anode. The model predicts that in the limit of fully developed collisional flow, the relation between the voltage Va, anode current Ia, cathode current Ik, and geometric impedance Z0 of a 1D planar MITL can be expressed as Va=IaZ0h(χ), where h(χ)≡[(χ+1)/4(χ1)]1/2ln χ+(χ21)1/2 /2χ(χ1) and χ Ia/Ik. The relation is valid when Va 1 MV. In the minimally insulated limit, the anode current Ia,min =1.78Va/Z0, the electronflow current If,min =1.25Va/Z0, and the flow impedance Zf,min =0.588Z0. {The electronflow current If IaIk. Following Mendel and Rosenthal, we define the flow impedance Zf as Va/(Ia2Ik2)1/2.} In the wellinsulated limit (i.e., when Ia Ia,min ), the electronflow current If=9Va2/8IaZ02 and the flow impedance Zf=2Z0/3. Similar results are obtained for a 1D collisional MITL with coaxial cylindrical electrodes, when the inner conductor is at a negative potential with respect to the outer, and Z0 40 Ω. We compare the predictions of the collisional model to those of several MITL models that assume the flow electrons are collisionless. We find that at given values of Va and Z0, collisions can significantly increase both Ia,min and If,min above the values predicted by the collisionless models, and decrease Zf,min. When Ia Ia,min, we find that, at given values of Va, Z0, and Ia, collisions can significantly increase If and decrease Zf. Since the steadystate collisional model is valid only when the drift of electrons toward the anode has had sufficient time to establish fully developed collisional flow, and collisionless models assume there is no net electron drift toward the anode, we expect these two types of models to provide theoretical bounds on Ia, If, and Zf. 
Article: Waterdielectricbreakdown relation for the design of largearea multimegavolt pulsedpower systems
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ABSTRACT: We have developed an empirical electricalbreakdown relation that can be used to design largearea waterinsulated pulsedpower systems. Such systems often form an integral part of multiterawatt pulsedpower accelerators, and may be incorporated in future petawattclass machines. We find that complete dielectric failure is likely to occur in water between a significantly fieldenhanced anode and a lessenhanced cathode when Eptaueff0.330±0.026=0.135±0.009. In this expression Ep≡Vp/d is the peak value in time of the spatially averaged electric field between the anode and cathode (in MV/cm), Vp is the peak voltage across the electrodes, d is the distance between the anode and cathode, and taueff is the temporal width (in mus) of the voltage pulse at 63% of peak. This relation is based on 25 measurements for which 1  [Show abstract] [Hide abstract]
ABSTRACT: We have developed wirearray z pinch scaling relations for plasmaphysics and inertialconfinementfusion (ICF) experiments. The relations can be applied to the design of z pinch accelerators for highfusionyield (approximately 0.4 GJ/shot) and inertialfusionenergy (approximately 3 GJ/shot) research. We find that (delta(a)/delta(RT)) proportional (m/l)1/4 (Rgamma)(1/2), where delta(a) is the implodingsheath thickness of a wireablationdominated pinch, delta(RT) is the sheath thickness of a RayleighTaylordominated pinch, m is the total wirearray mass, l is the axial length of the array, R is the initial array radius, and gamma is a dimensionless functional of the shape of the current pulse that drives the pinch implosion. When the product Rgamma is held constant the sheath thickness is, at sufficiently large values of m/l, determined primarily by wire ablation. For an ablationdominated pinch, we estimate that the peak radiated xray power P(r) proportional (I/tau(i))(3/2)Rlphigamma, where I is the peak pinch current, tau(i) is the pinch implosion time, and phi is a dimensionless functional of the currentpulse shape. This scaling relation is consistent with experiment when 13 MA < or = I < or = 20 MA, 93 ns < or = tau(i) < or = 169 ns, 10 mm < or = R < or = 20 mm, 10 mm < or = l < or = 20 mm, and 2.0 mg/cm < or = m/l < or = 7.3 mg/cm. Assuming an ablationdominated pinch and that Rlphigamma is held constant, we find that the xraypower efficiency eta(x) congruent to P(r)/P(a) of a coupled pinchaccelerator system is proportional to (tau(i)P(r)(7/9 ))(1), where P(a) is the peak accelerator power. The pinch current and accelerator power required to achieve a given value of P(r) are proportional to tau(i), and the requisite accelerator energy E(a) is proportional to tau2(i). These results suggest that the performance of an ablationdominated pinch, and the efficiency of a coupled pinchaccelerator system, can be improved substantially by decreasing the implosion time tau(i). For an accelerator coupled to a doublepinchdriven hohlraum that drives the implosion of an ICF fuel capsule, we find that the accelerator power and energy required to achieve highyield fusion scale as tau(i)0.36 and tau(i)1.36, respectively. Thus the accelerator requirements decrease as the implosion time is decreased. However, the xraypower and thermonuclearyield efficiencies of such a coupled system increase with tau(i). We also find that increasing the anodecathode gap of the pinch from 2 to 4 mm increases the requisite values of P(a) and E(a) by as much as a factor of 2.  [Show abstract] [Hide abstract]
ABSTRACT: The Z driver at Sandia National Laboratories delivers one to two megajoules of electromagnetic energy inside its ~10 cm radius final feed in 100 ns. The high current (~20 MA) at small diameter produces magnetic pressures well above yield strengths for metals. The metal conductors stay in place due to inertia long enough to deliver current to the load. Within milliseconds however, fragments of metal escape the load region at high velocity. Much of the hardware and diagnostics inside the vacuum chamber is protected from this debris by blast shields with small view ports, and fastclosing valves. The watervacuum insulator requires different protection because the transmission line debris shield should not significantly raise the inductance or perturb the self magnetically insulated electron flow. This report shows calculations and results from a design intended to protect the insulator assembly. 
Conference Paper: Mechanical Design of the ZR Magnetically Insulated Transmission Lines
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ABSTRACT: The ZR upgrade to the Z accelerator at Sandia National Laboratories will replace much of the power flow components. The single most massive of these components is the 1.6 meter diameter, fourfeed MITL system. This 14.5 Ton assembly transports electromagnetic energy from the vacuum interface to the load. The MITLs must cany 30 MA total with 99% reliability. Optimum performance requires careful control of the transmission line gaps. The MITLs must support the load while maintaining strict gap tolerance under their own weight and the weight of diagnostic assemblies. Dynamically they must be robust enough to withstand accelerations and impacts similar if not greater than those on Z today. Stress and deflection calculations will be shown for the selfdeflection, diagnostic loading, and dynamic motion present in the system. The MITLs are built in multiple annular sections to reduce cost and allow replacement of damaged pieces. Reliable power delivery demands that the multipiece MITLs be joined with currentcarrying contacts that do not introduce plasma or neutrals into the power flow gap. The design of the ZR MITL system and the design of the various current contacts will be shown. The current contacts were designed to maintain a continuous static pressure on soft copper gaskets to achieve intimate conductor contact. Testing on Z will be done in order to validate FEA pressure calculations as well as the current contact concept itself.
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1k  Citations  
149.11  Total Impact Points  
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19982014

Sandia National Laboratories
 Advanced Materials Laboratory
Albuquerque, New Mexico, United States
