D. D. Hinshelwood

University of New Mexico, Albuquerque, NM, United States

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Publications (193)88.68 Total impact

  • Physical Review Special Topics - Accelerators and Beams 01/2014; 17:060401. · 1.57 Impact Factor
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    ABSTRACT: Mercury, a 2-TW inductive voltage adder located at the Naval Research Laboratory in Washington, DC, had previously been converted from negative to positive polarity output by rotating each of the cells[1]. Positive polarity was needed to field an ion-beam diode.[2] However, rotating the cells takes about 2 to 3 weeks and is very labor intensive. So, when we next needed to operate in positive polarity, we considered two options to quickly turnaround Mercury to positive polarity; reverse charging the Marx and fielding a vacuum convolute. Charging the Marx with the opposite polarity is the simplest way to operate Mercury in positive polarity. However, because the breakdown and flashover strengths of the components are lowered when the polarity is reversed in this way, it is required to limit the Marx charge to 2/3 of its normal charging voltage to prevent failures. This limits the maximum output voltage, with the existing center conductor, to about 4.5 MV. An alternative approach we considered was to operate in negative polarity but to field a vacuum convolute inside our large diameter load chamber. Simulations and field calculations showed that this approach could work and the maximum output would be about 6 MV. We chose to operate Mercury in positive polarity by reverse charging the Marx and successfully operated an ion diode at a lower power level. To improve the pulse shape, we modified the self-breaking PFL water switch hardware. After this and a few other changes, Mercury can now be switched to a reliable and repeatable positive polarity mode in days instead of weeks, although at a lower than maximum power level. Details and results will be presented.
    Pulsed Power Conference (PPC), 2013 19th IEEE; 01/2013
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    ABSTRACT: form only given. An intense bremsstrahlung x-ray pulse is generated by the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder.1 A study of the diode configuration was undertaken to optimize the forward-directed radiation. To this end, the diode AK gap was varied between 23 and 43 cm and an ID-reducing insert in the vacuum chamber wall was added to adjust the incidence angle and the electron charge at the tantalum anode converter.2. Anode current monitors measure the portion of load-region current reaching the converter. Arrays of CaF2 TLDs, x-ray pin diodes and an x-ray pinhole camera are used to measure the x-ray dose distributions. Anode current data are presented which show that electron losses to the insert and to the outer-conductor wall increase with AK gap, in agreement with pinhole camera measurements. Pin diode signals are analyzed to determine beam dynamics during the pulse. TLD data are presented which show that, as the AK gap increases, the angular dose distribution narrows and that the on-axis dose increases, until the 43-cm AK gap configuration. Here, axial dose decreases as electron losses to the insert and vacuum chamber wall override any further benefit due to smaller incidence angle for electrons striking the anode. Bremsstrahlung produced by electrons at large radius impacting the walls is removed from the x-ray beam by a thick steel collimator. Though wall losses with a small, 23-cm gap are low, LSP/ITS simulations predict large electron impact angles, which would reduce the on-axis dose downstream of the diode. Research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal3.
    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: There is ongoing interest in the use of an intense bremsstrahlung pulse to induce photofission in fissionable material.1, 2 To optimize the radiation for inducing fissions in the forward direction, electrons should approach normal incidence and their charge on the anode converter should be maximum. Here, these are optimized on the 8-MeV, 200-kA, 50-ns Mercury inductive voltage adder3 by varying the diode AK gap between 23 and 43 cm and adding an ID-reducing insert in the outer-conductor wall.2 Current monitors at the anode determine the load-region current fraction reaching the converter. TLDs and an x-ray pinhole camera measure the angular and radial x-ray dose distributions4, and 3 He detectors measure fissions induced in a 30×30×2.5-cm3 depleted uranium (DU) plate. Measurements are in broad agreement with LSP/ITS simulations of the electron dynamics and radiation transport. Current traces show that electron losses to the insert and outer-conductor wall increase with AK gap, confirming LSP predictions and pinhole camera measurements. With increasing AK gap, measured angular dose distributions narrow, on-axis dose and DU fission-neutrons increase, indicating that electron angles are approaching the converter normal. With the 43-cm gap, axial dose and fission neutrons decrease. For 40-cm gaps or larger, LSP electron impact angles on the converter are within 5 degrees of normal. For 8-MeV electrons, x-ray divergence does not narrow significantly for smaller electron angles. However, electron losses to the insert and wall continue to increase for larger gaps, explaining reduced performance at 43 cm. Bremsstrahlung produced by these electrons at large radius is collimated out of the x-ray beam reaching the DU. If the current lost to the insert and walls could reach the converter, LSP/ITS predicts a fission yield 1.75-times higher than that achieved with a 40-cm gap. Though wall losses wi- h a 23-cm gap are small, the predicted 20-deg electron impact angles produce only about 1/3 of the on-axis fissions of the 40-cm gap. Therefore, research has begun to modify such smaller-gap diodes to magnetically steer electrons to converter impact angles closer to the normal.5
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: For many applications, control and manipulation of the electron orbits in a high-current electron beam is desirable. This is especially true when a weakly-self-pinched, multi-MV electron-beam is used to make bremsstrahlung radiation. In this case, the radiation pattern is highly peaked along the direction that the electron beam makes when it strikes the x-ray target. Therefore, to maximize the number of photons in the forward direction, it is desirable that the electrons strike the x-ray target as close to normal with as little spread in the beam angles as possible. In this paper, a method for controlling the macroscopic angle of a high-power electron beam using a post-diode magnetic-field structure is presented. The idea is to extract the electron beam into a vacuum cavity through a thin, low-mass foil where a portion of the return-current flows through a central post. The amount of current that flows through the central post and therefore the amount of beam straightening is controlled by inductively splitting the return current so that a portion of it returns through the central post and a portion returns outside the beam. By adjusting the balance between these two currents one can alter the electron orbits and achieve a wide range of angles that the electron beam makes with the target without the need for plasma or an external pulser.1 Particle-in-cell simulations have been performed to determine the parameters required to straighten an 8-MV, 200-kA, 23-cm-diameter hollow electron beam with an inward 20° macroscopic (average) angle so that it approaches the x-ray target at normal incidence. The simulations show an increase in the forward photon spectrum by up to a factor of 3. Experiments with similar beam parameters using the Mercury Inductive-Voltage Adder at the Naval Research Laboratory have shown an increase of a factor of two in the forward dose using this technique and are in good qualitative agreement with the simulations. Additional s- mulations and experiments are planned to optimize the forward dose and will be reported on during this talk.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: form only given. The self-magnetic-pinch (SMP) electron beam diode is being developed for 4 to 10 M V, 30 to 50 ohms, 50 ns, flash radiography of explosively driven objects by AWE, SNL, and NRL. The goal is a reproducible
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Proton-beam-generation experiments have been conducted on the NRL Mercury pulsed-power generator operating in positive polarity with a lithium metal target embedded in the cathode. The accelerating voltage was limited to below 2.7 MV in order to limit the energy of neutrons produced in the 7Li(p,n)7Be reaction (Q = 1.88 MeV) to below 1 MeV. Analyses based on published results1 and calculations presented here are used to predict the angular distribution of neutron yield and spectrum for each shot. Predicted neutron yields are compared to Rh-counter and Al-activation measurements. The results of these comparisons are quite encouraging, showing better than factor-of-2 agreement between the two sets of measurements and the analysis over the voltage range of the shot series. In order to achieve this level of agreement, a series of MCNPX computations has been carried out to determine the spectral contribution of neutrons reflected from the Mercury test-cell environment2 and the associated changes in detector calibrations. The agreement between measurements and modeling provides a check on the voltage calculated using a positive-polarity ion-diode model. For operation at 2.5-2.6 MV, on-axis neutron yields from the p-Li reaction are in the 1011 neutrons/steradian range.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: Flash x-radiography usually involves the creation of a pulse of intense bremsstrahlung whose duration is short compared to the motion of the object being radiographed. In many applications it is also desirable to have a very bright source that produces high-spatial-resolution images. For these applications a figure-of-merit (FOM) that provides a measure of the source brightness can be defined as, FOM = D/R2, where D is the dose in CaF2 at a distance of 1 m from the source and R (usually referred to as the radiographic spot size) is a number that characterizes the spatial extent of the radiation source. One method of creating a high FOM source uses a self-magnetic-pinch (SMP) electron-beam diode. [1] In relevant experiments on RITS-6 [2] and Mercury [3] inductive voltage adders IVA's, magnetically-insulated electron-flow in the adder can be a significant fraction of the current available to drive the SMP. It is believed that the magnetically-insulated flow produces an undesirably large radiographic spot if it is allowed to strike the x-ray target. Therefore, the approach adopted on RITS-6 and Mercury has been to dump this electron flow prior to the x-ray target. This usually involves the use of very large hardware and a loss of up to 50% of the available current. In this talk, we will present results from 3D LSP [4] particle-in-cell simulations of the coupling of magnetically-insulated electron-flow to an SMP diode. These simulations show that asymmetries in the electrical power-flow can cause the radiographic spot to wander. These simulations further show that nearly all the electrical current can be coupled to the SMP diode. However, in this situation the radiographic spot size is larger than cases without electron-flow in the SMP diode. In this paper we use LSP to explore methods of stabilizing the position and reducing the size of the radiographic spot.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: An ongoing programme looking at the active detection of special nuclear material is being undertaken by the Atomic Weapons Establishment in collaboration with the Naval Research Laboratory (NRL). As part of this programme, proton beam generation experiments were conducted at the NRL Mercury pulsed power accelerator using a lithium metal cathode to generate neutrons through the 7Li(p,n)7Be reaction. A modification to GEANT4 to include the 7Li(p,n)7Be reaction for the purposes of modeling the doubly differential neutron yield from these experiments has been implemented. The Large Scale Plasma (LSP) Particle-in-Cell (PiC) code was used to obtain the proton kinematics and flux at the lithium conversion target; an algorithm was subsequently written to transform this data to a complete GEANT4 source description. The modification to the GEANT4 simulation toolkit is presented and the output compared to experimental data with good agreement. The work presented shows that the new GEANT4 process alongside the existing packages can be used to model the 7Li(p,n)7Be reaction with reasonable accuracy given an input proton phase space from a suitable simulation code such as LSP.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: An ongoing program me looking at the active detection of special nuclear material (SNM) is being undertaken by the Atomic Weapons Establishment (A WE) in collaboration with the Naval Research Laboratory (NRL). As part of this programme, pulsed-power driven neutron experiments were conducted at the NRL Mercury accelerator. Mercury was used in a positive polarity mode to produce and accelerate protons into lithium metal foils, generating neutrons via the 7Li(p,n)7Be reaction. 13 shots were carried out at varying machine voltages and over 30 separate neutron and gamma-ray diagnostics were fielded to characterise the angular distribution and energy spectrum of the neutrons generated. Machine performance, neutron, and gamma-ray data are presented and discussed. Neutron yields of up to 1011 neutrons/steradian were recorded, with yields at 60° off axis being approximately 50% of the on axis yield. Previously published analysis [1] of data has been used to validate GEANT4 modelling of the experiments (2). Machine performance data has been used in conjunction with modelled neutron spectra to predict the performance of the Mercury 7Li(p,n)7Be source as a system for detecting SNM.
    Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2012 IEEE; 01/2012
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    ABSTRACT: form only given. The plasma opening switch (POS) is a pulsed-power switching technology that has been developed for inductive-energy-storage applications.[1] During the conduction phase of the POS, the discharge current initially flows in a plasma that is injected between the anode and cathode prior to the arrival of the main pulse. Lorentz forces act on the current-carrying plasma to distort the plasma distribution eventually leading to a disruption of the current channel on a fast timescale, producing a rapid transfer of magnetic energy to the load. The fast opening of the POS is crucial to the overall operation of the switch because it allows for a sharpening of the pulse rise-time and inductive voltage multiplication.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: form only given. A series of gas-cell experiments were performed at NRL to be able to validate dry air chemistry/collision models available in Particle-in-Cell (PIC) codes for cavity System Generated Electromagnetic Pulse (SGEMP). The experimental setup is a co-axial power feed which transitions to a disk-shaped cathode. The electron beam then passes through a thin anode foil and then a pressure foil, which separates the vacuum diode from the gas cell; scattering and propagation in the foils is energy and angle dependent. The peak electron energy entering the gas cell is about 90 keV and the pulse-duration is about 100ns. In addition to voltage and current measurements at various locations, a feature of these experiments is laser interferometry, which measures the line-integrated electron density at various locations in the gas cell for a gas pressures ranging from 50 mTorr to 300 Torr. This paper will analyze the numerical error as well as aleatory and epistemic uncertainty. PIC and Monte Carlo electron transport algorithms are inherently stochastic in nature; therefore, an extension to Richardson extrapolation is developed to obtain an error bound on the simulations. To account for aleatory and epistemic uncertainty we use second order probability, an inner aleatory loop sampling the approximate distribution of variation in parameters in the experiment with an outer epistemic loop varying the parameters of the distribution and model form. By including both aleatory and epistemic uncertainty and numerical error, the simulation can be compared with the NRL gas cell experiment.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: A pinch-reflex ion diode is fielded on the pulsed-power machine Mercury (R. J. Allen, et al., 15th IEEE Intl. Pulsed Power Conf., Monterey, CA, 2005, p. 339), which has an inductive voltage adder (IVA) architecture and a magnetically insulated transmission line (MITL). Mercury is operated in positive polarity resulting in layered MITL flow as emitted electrons are born at a different potential in each of the adder cavities. The usual method for estimating the voltage by measuring the bound current in the cathode and anode of the MITL is not accurate with layered flow, and the interaction of the MITL flow with a pinched-beam ion diode load has not been studied previously. Other methods for determining the diode voltage are applied, ion diode performance is experimentally characterized and evaluated, and circuit and particle-in-cell (PIC) simulations are performed. Results indicate that the ion diode couples efficiently to the machine operating at a diode voltage of about 3.5 MV and a total current of about 325 kA, with an ion current of about 70 kA of which about 60 kA is proton current. It is also found that the layered flow impedance of the MITL is about half the vacuum impedance.
    Physics of Plasmas 05/2011; 18(5):053106-053106-16. · 2.38 Impact Factor
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    ABSTRACT: A single burst of radiation created by 100-ns pulsed power can be used to induce photofission in fissionable material. In NRL experiments, the 8-MV Mercury bremsstrahlung spectrum irradiates a depleted-uranium plate several meters from the diode, and the resulting delayed-fission neutrons are measured. In this work, the LSP particle-in-cell code is used to model electron flow in the vacuum feed and diode, and the ITS Monte-Carlo codes are used to extract the x-ray spectrum from the diode and transport it to the DU plate. The delayed-fission-neutron yield is determined from the known dependence of the fission cross-section on photon energy. In this calculational chain, the only free parameter is the fraction of ions flowing in the diode, which affects the orbits of electrons impacting the bremsstrahlung converter. The ion fraction therefore determines the radial and angular distributions of x-rays emitted by the converter, which in turn, determine the DU neutron yield, the radial x-ray distribution emitted by the anode, and the angular distribution of x-ray dose. These three experimental measurements are compared to the modeling to determine the ion fraction that gives the best agreement between the measurements and computations: about 0.4% for the experiments discussed here.
    01/2011;
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    ABSTRACT: Mykonos is a linear transformer driver (LTD) pulsed power accelerator currently undergoing testing at Sandia National Laboratories. Mykonos-2, the initial configuration, includes two 1-MA, 200-kV LTD cavities driving a water-filled transmission line terminated by a resistive load. Transmission line and 3D electromagnetic (EM) simulation models of high-current LTD cavities have been developed [D.V. Rose et al. Phys. Rev. ST Accel. Beams 13, 90401 (2010)]. These models have been used to develop an equivalent two-cavity transmission line model of Mykonos-2 using the BERTHA transmission line code. The model explicitly includes 40 bricks per cavity and detailed representations of the water-filled transmission line and resistive load. (A brick consists of two capacitors and a switch connected in series.) This model is compared to 3D EM simulations of the entire accelerator including detailed representations of the individual capacitors and switches in each cavity. Good agreement is obtained between the two simulation models and both models are in good agreement with preliminary data from Mykonos-2.
    01/2011;
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    ABSTRACT: form only given. A reflex triode consists of two grounded cathodes on either side of a positive-high-voltage, tantalum anode. Emitted electrons interact with the tantalum to produce bremsstrahlung and line radiation. By making the tantalum much thinner than the electron range, the self absorption of the x-rays by the tantalum can be reduced. The reflex triode configuration makes the electrons pass through the tantalum many times, improving the coupling of the electrons to the tantalum. Recent experiments on the Saturn generator at Sandia National Laboratories, Albuquerque, NM, used a triode configuration with cylindrical cathodes on either side of a flat, annular anode. For typical beam parameters (250 kV, 1.2 MA per side) and triode dimensions (19 cm radius cathodes, 3 mm anode-cathode gaps) the beam does not pinch rapidly toward the axis. As a result, the x-ray source is partially shadowed by the cathode located between the tantalum and the observation plane outside the vacuum chamber. The external x-ray signal starts about 15 ns later than expected from the current and voltage waveforms. At the time of maximum dose rate, it appears that only 60% or less of the total current produces x-rays from the part of the anode inside the inner diameter of the cathode. This beam behavior also occurs in particle-in-cell simulations of the Saturn experiments. A new triode configuration has promise for eliminating the delay and shadowing of the x-rays by the cathodes. The tantalum anode is in the form of a short cylinder, extending through the gap between two flat annular cathodes that lie in the same plane, one cathode inside the anode cylinder and one outside. This triode was tested on the Gamble II generator at the Naval Research Laboratory, Washington, DC, at the same voltage and anode-cathode gap as the Saturn triode, but at half the current and half the radius. The magnetic and electric fields should therefore be the same and scale directly to Saturn. The initial- results indicate the x-ray delay is eliminated and the measured dose matches the expected value based on measured current and voltage waveforms. The near field dose distribution is also more uniform and higher than the corresponding distribution for the triode with a flat anode. These results will be presented along with future possibilities to utilize this new triode in series or parallel configurations to increase the x-ray output from Saturn by using more of its current at the same voltage and outer diameter.
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: form only given. A high-impedance, small spot size diode for flash radiography has been successfully tested on a 2.3-MV generator. The generator is an L-3 Communications, Pulse Sciences Division, Pulserad 43703B, the largest in a series of generators originally called Febetron. These generators are normally used with long-life, larger diameter anodes in a sealed glass envelope. Previous work with the 1-MV Pulserad 43710A generator has demonstrated that the sealed tube can be replaced with a stacked ring insulator and the large anode replaced with a small, 1-mm diameter anode in order to achieve a much smaller spot size for single shot use. Recently, the cathode design for this retrofitted diode has been improved for optimal performance. With only small changes, this vacuum stack and diode have been retrofitted to the 43703B 2.3-MV generator. Because of the higher voltage, extra rings were added to the insulator stack, and the outer conductor hardware was rearranged to provide a smooth surface. This new diode hardware has been successfully tested at the full, 35-kV charge voltage and preliminary results show the spot size reduced by about a factor of 5 to about 1 mm with only a modest decrease in dose. If a figure of merit for the radiography source is defined as the dose divided by the spot size squared, this retrofitted diode improves the figure of merit by more than a factor of 10. Future work will be to optimize the hardware design for field use and to optimize the performance of the diode with the aid of computer models.
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: A series of gas-cell experiments were performed at NRL to be able to validate the dry air (as well as pure N2) chemistry/collision models available in Particle-in-Cell (PIC) codes. These experiments were constructed and performed with many diagnostics enabling comprehensive comparison between simulation and experiment. This multi-physics/stage problem (i.e. the pulse power circuit, cathode, foil, gas region and collection/anode) has diagnostics that can validate the simulation before and after each component. Because of the experimental access at each individual stage, this experiment is ideal for investigating the many models incorporated in various PIC codes. These measurements include the nonlinear coupling between each stage affecting the results both upstream and downstream of each measurement location. Therefore, these measurements are a stringent test of the models. In addition to numerical error, there is uncertainty of the parameters (e.g. the multi-group cross section used in the foil and the collision cross-sections used in the gas cell) in each component of the experiment. This study will examine the error propagated from one stage to the next; by accounting for these uncertainties we will be able to definitively validate the models. The experimental set up is a co-axial power feed which transitions to a disk-shaped cathode. The electron beam then passes through a thin anode foil which separates the vacuum diode from the gas cell; scattering and propagation in the foil is energy and angle dependent as calculated from ITS and SCEPTRE. The energy of the electrons going into the gas peaks at about 100 keV and the beam last for about 200ns. In addition to voltage and current measurements at various locations, the novel feature of these experiments is laser interferometry which measures the line-integrated electron density at various locations in the gas cell for a gas pressures ranging from 50 mTorr to 300 Torr.
    Plasma Science (ICOPS), 2011 Abstracts IEEE International Conference on; 01/2011
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    ABSTRACT: This paper describes the use of intense, pulsed bremsstrahlung for inducing photofission in fissionable material. 8-MV bremsstrahlung triggers photofission in depleted uranium (DU). Particle-in-cell and Monte-Carlo codes model the source, gamma and neutron production and transport, and detector response. A variety of detectors have been fielded successfully to measure fission in DU from delayed-neutron, delayed-gamma, and prompt-neutron signals. This work represents the first clean measurement of prompt photofission neutrons in an intense, pulsed-power environment.
    Pulsed Power Conference (PPC), 2011 IEEE; 01/2011