Search

Radioactivity induced in material irradiated by accelerated deuterons has been investigated in experiments conducted using a unique configuration of dense plasma focus (DPF) driven by the Hawk pulsed-power generator at the Naval Research Laboratory. The high inductance (607 nH) and associated high voltage (640 kV) and fast rise time (1.2 µs) of Hawk are unusual for a DPF driver, as is the initialization of the DPF using local mass injection rather than a conventional neutral gas fill. The local mass injection involves injecting transient neutral gas and plasma into a vacuum chamber at prescribed locations just prior to application of the main current pulse that drives the DPF. Evidence of ions accelerated to energies over an order of magnitude higher than the applied generator voltage has been observed in ion multi-pinhole images obtained with absorbers of varying thickness and in neutron time-of-flight detector signals. In this work an aluminum plate was positioned for irradiation by accelerated deuterons escaping the plasma during the DPF pulse. Following irradiation, a NaI(Tl) detector was used to record the energy spectrum and decay rate of the radioactivity induced in the plate. At the same time, an x-ray/γ-ray sensitive image plate placed in contact with the irradiated plate was used to record the spatial distribution of that radioactivity. The measurements showed γ-ray spectra and half-lives consistent with induced 27 Al(d,p) 28 Al and 12 C(d,n) 13 N reactions in the aluminum and from carbon impurities in the chamber. The induced radioactivity was concentrated at the center of the plate, which was located on the axis of the machine during irradiation.

The plasma chemistry generated by an electron beam emitted from a Febetron pulsed-power generator operating in different background gases is being investigated spectroscopically. The generator located at the Naval Research Laboratory produces a peak voltage of 80 kV, peak current of 4 kA with a pulse width of 100 ns. Once formed, the electron beam passes through a thin anode followed by a thin pressure barrier into a volume filled with different background gases at pressures ranging from 1000 to 10 mtorr. The volume is filled with both molecular (nitrogen and air) and atomic gases (Ar) to study the behavior of the generated plasma in these different conditions. The present work focuses on analysis of the emission spectrum from the electron beam generated plasma; of particular interest are spectral lines from the near ultraviolet to the visible (300 nm to 600 nm). The temporal and spatial variation of excited species are studied to understand the characteristics of the plasma.

A high-resolution spectroscopic characterization is conducted on the vibrational and rotational spectral emissions of an N2 plasma produced by an electron beam. The electron beam is initiated by the Naval Research Laboratory’s Febetron pulsed-power generator capable of 80 kV and 4 kA peak voltage and current pulses with a 100 ns pulse time width. The beam is directed into an N2 pressurized chamber, ranging in pressures from 10mTorr to 10 Torr, where the electron beam excites the N2 gas into both molecular and atomic emissions. A spectroscopic analysis is conducted on these emissions from the near-UV to visible (300-650 nm) by means of a sub-angstrom resolution 1.3 m focal length Czerny-Turner spectrometer. Multiple simultaneous spectral measurements are taken per pulse enabling measurements of spatial variations of the plasma spectra through a 7-fiber input array. Imaging of the spectra is conducted by a 2D intensified gated CCD over small time increments to analyze the time-resolved evolution of the emissions.

Z-pinch experiments with hybrid deuterium gas-puff showed a possibility of efficient production of neutrons and multi-MeV ions. In the experiments on the GIT-12 generator, the total neutron yields are typically above 10 12 at a current of 3 MA. Since the intense pulsed neutron sources are desired for numerous applications as nuclear astrophysics 1 , subcritical experiments 2 , flash neutron radiography 3 , etc., neutron emission from the hybrid Z-pinches deserves to be thoroughly characterized. For this purpose, a comprehensive system of neutron diagnostic is exploited on the GIT-12 generator. Using scintillation detectors, nuclear activation methods, and bubble detectors, the fundamental features of neutron emission, e.g. energy spectrum, angular anisotropy, and number of non-DD reactions are evaluated. The analogous experiments on the HAWK generator allow comparison and scaling of the neutron emission at two different values of the Z-pinch current.

Naval Research Laboratory is starting to study the connection between plasma instabilities and fast ion acceleration for neutron production on a dense plasma focus (DPF). The experiments will be performed on the HAWK generator (665 kA) that has a fast rise time, $1.2~\mu \text{s}$ , and a high inductance, 720 nH. There is a conjecture that the neutron yield on a DPF may be enhanced on a high-impedance generator, since the assembly on the axis will not significantly reduce the current drive. Furthermore, to prevent the upstream breakdown during the implosion, there are plans to inject the gas load as plasma far from the insulator stack. As an introductory study, we performed MHD simulations using the Athena code. Athena is a Eulerian 3-D, unsplit finite volume MHD code that includes shock capturing with Riemann solvers and resistive diffusion for the magnetic field components. The simulations are coupled to an equivalent circuit model for HAWK. We report the dynamics and implosion time as a function of the initial injected plasma distribution and study the implications of nonideal effects. We also calculated a stochastic particle acceleration resulting from the motional electric field.

Several experiments have shown that the anode initiates breakdown of vacuum gaps in cases where the anode and cathode field enhancements are approximately equal [1]. The previous understanding was that only explosive emission at the cathode initiates breakdown between metal electrodes in vacuum with nanosecond pulses. Past experiments have shown clear evidence of particles from the anode initiating breakdown when the cathode was not specially roughened or treated to increase field enhancement. However, the type of particles coming from the anode was not known. Now, we have measurements from a specially designed anode-cathode structure that has allowed us to capture these particles. CR-39 film was used as a particle detector. A Thomson parabola and also a Kimfol filter allowed us to determine the particle species.

The 670 kA Hawk pulsed power generator at NRL has been configured as a fast (1.2 µs rise time), high inductance (607 nH) driver for a dynamic dense plasma focus (DPF) load in order to create a testbed for the study of charged particle acceleration mechanisms in imploding plasmas. The current pulse is initiated in a deuterium plasma injected radially by three Marshall guns. This plasma is accelerated through a coaxial region, and pinches onto neutral deuterium injected axially by a gas-puff valve. Promising experimental results along with recent improvements to the Marshall gun design have prompted efforts to better characterize the density profiles produced by the guns. A multichannel heterodyne interferometer focused into a ribbon beam is used to provide time-resolved measurements of the Marshall gun plasma density, allowing parameters to be tuned and mass distributions to be established for a wide range of initial conditions. These interferometric measurements are used in establishing a standard operating condition for the experiment in a high neutron yield regime.

Acceleration of ions to multi-MeV energies is investigated in various plasma devices to better understand processes in astrophysical plasmas and to develop efficient accelerators for a variety of applications. This paper reports the production of proton, deuteron, and electron beams in a z-pinch—a cylindrically symmetric plasma column that is compressed by its own magnetic field. For this work, the GIT-12 pulsed-power generator was used to drive a novel configuration of z-pinch that dramatically enhanced ion acceleration associated with disruption of the current by instabilities in the compressed plasma. During the disruption of 3 MA current, hydrogen ions were accelerated up to at least 50 MeV, which is almost a hundred-times the ion energy provided by the generator driving voltage of 0.6 MV. Under optimal conditions, the total numbers of hydrogen ions with energies above 20 and 50 MeV were 4 × 1013 and 1011, respectively. Accelerated deuterons produced one 20 ns (full width at half maximum) pulse of fast neutrons via D(d, n)3He and other nuclear reactions. A maximum neutron output of (1.0 ± 0.2) × 1012 neutrons/sr was observed downstream, i.e., in the anode to cathode direction. In this direction, the maximum neutron energy reached 58 ± 7 MeV. Both ion and neutron beams in our experiment reached an end-point energy of about 60 MeV, which is the highest value observed in pulsed-power devices. A localized peak voltage of ≳60 MV was driven by the inductive energy that was stored around the plasma column and that was extracted during a sub-nanosecond current drop. Considering the natural occurrence of current-carrying columns in laboratory and space plasmas, the current interruption observed in z-pinches could represent a more general physical process that contributes to the efficient conversion of magnetic energy into the energy of particle beams in various plasmas.

Z-pinch experiments with a hybrid configuration of a deuterium gas puff have been carried out on the HAWK (NRL, Washington, DC) and GIT-12 (IHCE, Tomsk) pulsed power generators at 0.7 MA and 3 MA currents, respectively. On GIT-12, neutron yields reached an average value of 2 × 1012 neutrons, and deuterons were accelerated up to an energy of 30 MeV. This was 50 times the ion energy provided by the generator driving voltage of 0.6 MV and the highest energy observed in z-pinches and dense plasma foci. To confirm these unique results independently on another device, we performed several experimental campaigns on the HAWK generator. Comparison of the experiments on GIT-12 and HAWK helped us to understand which parameters are essential for optimized neutron production. Since the HAWK generator is of a similar pulsed power architecture as GIT-12, the experiments on GIT-12 and HAWK are important for the study of how charged-particle acceleration scales with the current.

Recent experiments on the Mercury pulsed power accelerator were performed with a large-area bremsstrahlung diode operated at 5 MV. Extensive measurements of the x-ray dose distribution were made using TLDs, both in the near field and far field of the x-ray beam. In order to understand the operation of the diode and the properties of the x-ray beam, numerical simulations of the diode and beam were performed. Particle-in-cell simulations of the diode were driven using the measured currents from the experiment in order to simulate the electron beam produced by the accelerator. These beam electrons were then used as a source in Monte Carlo simulations of the bremsstrahlung converter to compute the generated x-ray beam. The photons from this simulation were then transported to the near- and far-field TLD locations and doses were computed. The dose predictions were compared to measurements, showing reasonably good agreement.

The Naval Research Laboratory's Hawk pulsed power generator is being used to drive a dense plasma focus (DPF) in order to study the phenomenon of charged particle acceleration in imploding plasmas. The fast rise time, 1.2 μS, and high inductance, 607 nH, of the 220 kJ, 0.65 MA peak current Hawk Marx are unusual compared to typical DPF driver parameters. In addition to the unusual electrical parameters, local plasma or gas injection is used to initialize the DPF, rather than a more traditional static fill gas. A suite of diagnostics is in development to benchmark models of the DPF and potential acceleration mechanisms. Initial measurements of the neutron yield and other plasma and accelerated particle characteristics will be discussed and compared with modeling results, in order to evaluate coupling of the high-inductance driver to the DPF.

The 650 kA Hawk pulsed power generator at NRL has been configured as a fast (1.2 μs), high in ductance (607 nH) driver for a dense plasma focus load. The current pulse is initiated in a radially injected deuterium plasma. This plasma is accelerated through a coaxial region, and pinches onto neutral deuterium injected axially by a gas-puff valve. A ribbon beam interferometer is used to diagnose the plasma in the coaxial region at several radii, providing time resolved electron density measurements through the initiation and axial run-down phases of the implosion. We present interferometry results alongside imaging, spectroscopic, electrical, and radiation measurements that track the evolution of the imploding current sheath.

Strong evidence for the emission of ions from anode surfaces as being a mechanism for the initiation of cathode plasma formation and electrical breakdown of vacuum gaps has been previously presented.1 It was shown, for example, that treatment of the anode surface can dramatically affect the turn on of the cathode. It is believed that field emission of ions from the anode is the responsible mechanism. Here we report on a more direct measurement of these ions using a Thomson parabola as a mass spectrometer using CR-39 film as an ion detector. The results of these measurements give us the species of ion responsible for the observed effects and the sensitivity of this emission to anode preparation.

We compare MHD simulation results to experimental measurements taken from a dense plasma focus driven by the HAWK pulsed-power generator (0.65 MA peak current, $1.2\mu \mathrm{s}$ rise time). USim is a 3D capable, fluid plasma modeling framework that simulates the dynamics of charged fluids using the two temperature MHD equations, among others. The DPF device is modeled using an unstructured mesh, and initial conditions are applied to account for plasma injected radially inward by 3 Marshall guns, as well as additional mass injected by an on-axis gas puff valve. The non-axisymmetric nature of the Marshall guns means that full 3D simulations are required. The initial plasma and gas density profiles were varied to determine their effect on the pinch parameters. Simulated current, voltage, inductance and neutron yield are compared with experimental results. We also calculate simulated end-on optical imaging using multiple opacity models and compare with experiment.

We report on a collaborative test campaign conducted at the Naval Research Laboratory's Mercury pulsed power facility in December of 2012. The experiment sought to use Mercury in the Intense Pulsed Active Detection (IPAD) [1] mode to interrogate a fissionable material target (depleted uranium, DU) and benchmark the effects of shielding the target with either a low-Z (2% borated high-density polyethylene, BPE) or high-Z (steel) material. A large suite of instrumentation, including 3He, BF3, NaI(Tl), and liquid scintillation detectors were used to measure the delayed γ and neutron signatures from the DU. The test campaign consisted of a series of single IPAD pulses, i.e., “shots,” employing incremental shielding configurations of BPE (up to 50 g/cm2) and steel (up to 150 g/cm2) encapsulating the DU target. We show the results from each detector array, for varying amounts of shielding, in terms of the signal-to-noise vs. time.

Active interrogation is a method used to enhance the likelihood of detection of shielded special nuclear material (SNM); an external source of radiation is used to interrogate a target and to stimulate fission within any SNM present. Radiation produced by the fission process can be detected and used to infer the presence of the SNM. The Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have carried out a joint experimental study into the use of single pulse, high-intensity sources of bremsstrahlung x-rays and ${\bf D}({\gammab}, {\bf n}){\bf H}$ photoneutrons in an active interrogation system. The source was operated in both x-ray-only and mixed x-ray/photoneutron modes, and was used to irradiate a depleted uranium (DU) target which was enclosed by up to $150 \, {\bf g}{\cdot}{\bf cm}^{ - 2}$ of steel shielding. Resulting radiation signatures were measured by a suite of over 80 detectors and the data used to characterise detectable fission signatures as a function of the areal mass of the shielding. This paper describes the work carried out and discusses data collected with $^{3}{\bf He}$ proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( $ >\!\!\! {3\sigmab }$ ) of the DU target through a minimum of $ 113 \, {\bf g}{\cdot}{\bf cm}^{ - 2}$ of steel, dropping to $85 \, {\bf g}{\cdot}{\bf cm}^- {2}$ when using a mixed x-ray/photoneutron source. The $^{3}{\bf He}$ proportional counters demonstrate detection ( $ >\!\! { 3\sigmab }$ ) of the DU target through the maximum ${149}. 7 \, {\bf g}{\cdot}{\bf cm}^{ - {2}}$ steel shielding deployed for both photon and mixed x-ray/photoneutron sources.

Plasma density gradients are known to drive magnetic shocks in electron-magnetohydrodynamics (EMHD). Previous slab modeling has been extended to cylindrical modeling of radially imploding shocks. The main new effect of the cylindrical geometry is found to be a radial dependence in the speed of shock propagation. This is shown here analytically and in numerical simulations. Ion acceleration by the magnetic shock is shown to possibly become substantial, especially in the peaked structures that develop in the shock because of electron inertia., Comment: Accepted for publication in Physics of Plasmas

Well diagnosed measurements of electrical breakdown between metal electrodes in vacuum have been made using a 1-MV, 50-ns pulse-generator-based test stand [1]. Results with bare, as-machined electrodes showed that cathode surface finish and material have little to no effect on the breakdown electric field. However, changing the AK gap did affect the breakdown field level, suggesting that the anode is involved in cathode turn on. Further tests showed that applying a carbon coating to the anode can dramatically decrease the breakdown field [2]. Here, we report on other anode and cathode surface treatments and how they can enhance or suppress breakdown. Polishing of the anode can prevent breakdown at certain field strengths and decrease the breakdown current at higher fields. Marring a small spot on a polished anode is shown to initiate cathode turn on at a location opposite that spot. The effects of other surface treatments such as metallic and dielectric coatings on anode and cathode surfaces will also be discussed.

NRL is embarking on a program to study the connection between plasma instabilities and fast ion acceleration for neutron production on a Dense Plasma Focus (DPF). The experiments will be performed on the HAWK generator (665 kA, 1.5 micro-sec), which has a high inductance (619 nH). There is a conjecture that the neutron yield on a DPF may be enhanced on a high impedance generator since the assembly on axis will not significantly reduce the current drive. Furthermore, to prevent upstream breakdown during the implosion there are plans to inject the gas load as a plasma far from the insulator stack. As a propaedeutic study, we performed MHD simulations using the Athena code. Athena is an Eulerian 3D, unsplit finite volume MHD code that includes shock capturing with Riemann solvers and resistive diffusion for the magnetic field components. The simulations are coupled to an equivalent circuit model for HAWK. This initial report will examine the dynamics and implosion time as a function of the initial injected plasma distribution and study the implications of non-ideal effects.

There is an interest in 10 keV to 100 keV radiation sources for some types of radiation-interaction studies. The approach presented here uses non-thermal radiation from single-wire radiators (SWRs). An SWR is a high-Z exploding wire that produces many small, bright radiation sources along its length during a fast (< 100 ns) high-current (∼ MA) discharge. In previous experiments,2 tungsten wires produced spectra with 60 J above 10 keV with an effective endpoint of about 100 keV. An array of many, non-interacting SWRs is proposed to generate a higher yield in this energy range.

Direct irradiation of materials by electron beams (e-beams) has been used to study material response.2,3 The desire to utilize high-power (∼ TW) generators to achieve higher specific energy deposition over larger areas has led to several approaches. One approach utilizes a monolithic e-beam diode with an external magnetic field (B field). The external B field allows the diode to operate in the bipolar, space-charge-limited regime without the current being magnetically limited to a lower value. The field also is used to guide the e-beam through the gas-filled region between the vacuum diode and the object to be irradiated. An alternate approach, discussed in this presentation, utilizes multiple diodes electrically in parallel, with each diode running below the critical current to obtain a high current. The e-beams are then scattered in foils and combined in the gas-transport region to achieve the desired irradiation uniformity and area. We report on experiments that have been performed on the Gamble II generator at NRL (∼ 1 MV, ∼ 800 kA, ∼ 60 ns) designed to study this second approach. Diagnostics include diode voltage and current, a net-current monitor, interferometry, spectroscopy, an axial array of Ta-strip x-ray witness plates, and a segmented calorimeter. Experiments have been performed with a single ring diode and two nested ring diodes. Where possible, the measurements are compared with results from ITS and from the recently-developed ABC model4 for the interaction of the e-beam with the gas. Results show that the beam is very nearly charge and current neutralized as it propagates ∼ 20 cm in 1-Torrr N2 gas. Beam scattering from both the anode and a Ti scattering foil in the gas results in a relatively uniform radial beam profile.

Measurements of the breakdown field of vacuum gaps have been made with a 1-MV, 50-ns test stand [1]. The first set of experiments was performed with planar, bare metal electrodes [2]. This geometry had an enhanced anode edge that affected measurements. Here, we report on a new set of measurements using anodes without field enhancement. Diagnostics include current and voltage probes, x-ray detector, x-ray pinhole camera and a fast framing camera. The results of these measurements give new insight into the cause of anode interactions that reduce the breakdown field with large gaps.

Intense pulsed active detection (IPAD) is a promising technique for detecting fissile material to prevent the proliferation of special nuclear materials. With IPAD, fissions are induced in a brief, intense radiation burst and the resulting gamma ray or neutron signals are acquired during a short period of elevated signal-to-noise ratio. The 8 MV, 200 kA Mercury pulsed-power generator at the Naval Research Laboratory coupled to a high-power vacuum diode produces an intense 30 ns bremsstrahlung beam to study this approach. The work presented here reports on Mercury experiments designed to maximize the photofission yield in a depleted-uranium (DU) object in the bremsstrahlung far field by varying the anode-cathode (AK) diode gap spacing and by adding an inner-diameter-reducing insert in the outer conductor wall. An extensive suite of diagnostics was fielded to measure the bremsstrahlung beam and DU fission yield as functions of diode geometry. Delayed fission neutrons from the DU proved to be a valuable diagnostic for measuring bremsstrahlung photons above 5 MeV. The measurements are in broad agreement with particle-in-cell and Monte Carlo simulations of electron dynamics and radiation transport. These show that with increasing AK gap, electron losses to the insert and outer conductor wall increase and that the electron angles impacting the bremsstrahlung converter approach normal incidence. The diode conditions for maximum fission yield occur when the gap is large enough to produce electron angles close to normal, yet small enough to limit electron losses.

Magnetic field penetration in current-carrying plasmas is being studied in a plasma opening switch geometry. Several Marshall guns1 are used to inject single or multi-species plasmas between coaxial conductors connected to the output of NRL's Hawk pulsed-power generator. Following injection of the plasma, the generator is used to apply an electrical pulse with a peak current of 700 kA, a peak voltage of 640 kV, and a rise time of 1.2 µs. Initially the plasma acts as a short, conducting all of the current. Over time, the resulting magnetic field interacts with the plasma through a combination of magnetohydrodynamic (MHD) plasma translation and field penetration that is not well understood2–4. Eventually a quasi-neutral gap forms in the plasma5,6, allowing electrical power to flow downstream. The quality of this switching is affected by the manner in which the gap is formed. This process is monitored using magnetic probes and a ribbon-beam interferometer running parallel to the axis of the accelerator and spanning the inter-electrode plasma region. Particle-in-cell (PIC) modeling shows that the relative importance of MHD translation and field penetration in the gap formation process is dependent upon the radial density gradient and composition of the plasma7. These parameters of the initial injected plasma are adjusted experimentally using the Marshall guns for light (hydrogen), heavy (argon), and mixed light and heavy components. The experimentally-observed behavior of the resulting opening switch plasmas in the presence of the interacting magnetic field is compared with results from PIC modeling.

We are studying species separation and magnetic field penetration in current-carrying plasmas in a coaxial plasma opening switch geometry1. PIC modeling has shown that these processes are dependent upon the radial density gradient and multi-species composition of the plasma2. Historically, flashboards, cable guns3, or an inverse pinch source4 have been used to inject plasma into the switch region before the main power pulse is applied. In this case, the plasma is injected using Marshall guns5, allowing greater control of the initial density gradient and composition than with other injection methods. A ribbon beam interferometer, in a Mach-Zehnder configuration, passes axially through the 4-cm-long, 2-cm-wide switch region to measure the radial dependence of the line-integrated electron density as a function of time. Measurements are taken both for the guns alone and during short circuit plasma opening switch shots on NRL's Hawk pulsed-power generator (640 kV, 700 kA, 1.2 µs rise time) to determine the initial density gradient and its evolution as the plasma interacts with the driving magnetic field. Experimental data are compared with previous opening switch experiments using Marshall guns and current particle-in-cell simulations.

Understanding the interaction of a strong magnetic field with a plasma is one of the fundamental problems in plasma physics. In this talk we report on a new systematic study using two-dimensional particle-in-cell simulations designed to explore the interplay between magnetic pushing1 (MHD time scales) and magnetic field penetration due to the Hall effect2. A two-ion-species plasma that consists of a heavy-ion species and a light-ion species is simulated. In a single-species plasma where the ions are infinitely massive and ∇n×B > 0, the magnetic field penetrates into the plasma at the Hall speed. In a two-species plasma, the simulations show simultaneous pushing of the light ions and magnetic field penetration through the heavy ions, when the Hall speed is between the Alfven speeds of the heavy and light ion species3–4 (i.e. V AH Hall AL ). The simultaneous pushing of the light ion species and penetration of the magnetic field through the heavy species leads to a separation of the two ion species. If the mass of the light ions is increased, a transition to magnetic penetration of both species is observed when V AH AL Hall . In the opposite limit where the Hall speed is small compared to the Alfven speeds of both ion species, magnetic pushing of both species is observed.

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.

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 with 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

Summary form only given. Pulsed bremsstrahlung from the Hermes-III generator produces photofission for research in Intense Pulsed Active Detection (IPAD)2. Hermes-III is operated in three modes producing bremsstrahlung with different endpoint energies: 8, 12, and 16 MeV, all with 30 ns x-ray pulse width. The electron-beam current increases with output voltage, from 300 kA at 8 MV to 600 kA at 16 MV. Previous experiments demonstrated detection of delayed neutrons from fissions induced in a depleted uranium (DU) plate. The work reported here focuses on detecting prompt fission neutrons, which are about 100 times more numerous than delayed neutrons. Neutrons are detected using 3He tubes surrounded by two different thicknesses of polyethylene (PE) to moderate the neutrons. A thin layer of cadmium absorbs neutrons with energy less than 1 eV. The “low-energy” detector has 2.5 cm PE and its response peaks at 6 keV. The “high-energy” detector has 12 cm PE and its response peaks at 1.6 MeV. MCNPX calculations for Hermes-III experiments at 8 MV indicate the high-energy detector is capable of detecting prompt fission neutrons (from DU) while effectively excluding photoneutrons produced in the environment or from a lead target. The detector electronics produce pulses that are counted as a function of time after a Hermes-III shot. Scattered x-rays make the detector signals useless for the first 10 microseconds, the time range when prompt neutrons first reach the detectors. But the thick PE disperses the prompt neutrons so they are recorded in the 3He gas at later times. Prompt neutrons are detected between 20 and 500 microseconds, in agreement with MCNPX predictions. When Hermes is operated in the 8-MV mode, the photoneutron signal from the background (no target) or a lead target is zero for times less than 500 microseconds. In the 12-MV mode, photoneutron signals are not negligible, but the presence of fission neutrons can be detected by comparing pulse counts from the two detectors, essentially identifying higher energy neutrons than expected from nonfissionable materials. At 16 MV, photoneutron energies are comparable to the prompt fission neutron energies and detection of fission neutrons by this method is more difficult.

Summary 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.

An ongoing programme investigating 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). The programme is funded through the UK Home Office, Ministry of Defence and Cabinet Office and the Naval Research Laboratory supported primarily through the US Defence Threat Reduction Agency with support also from the Office of Naval Research and the Defence Nuclear Detection Office. The process by which the UK are applying active detection techniques to border protection and a review of the current challenges and opportunities for this technology as assessed by the authors is provided. As part of this programme, the NRL Mercury IVA was operated in positive polarity mode to produce photons characteristic of the 19P(p,αγ)160 reaction, at energies of 6.13, 6.92 and 7.12 MeV. Protons produced by Mercury interact with a thick Teflon (PTFE) target to produce characteristic gamma radiation. These in turn were used to induce photofission in a depleted uranium (DU) sample. Eighteen experiments were fielded in September 2011, in which thirty-five detectors were fielded, including 3He tubes, NaI detectors, liquid scintillators and high purity germanium detectors, capable of detecting both gamma radiation and neutrons. The results from a selection of those detectors are discussed here. A variety of high-Z (lead) and hydrogenous (borated polyethylene) and hydrogenous shielding configurations was employed and positive detection was made up to the maximum shielding tested, 8.Sg/cm2. Effects of secondary reactions in the photon production are visible in the results and some employed reduction techniques are discussed. Monte Carlo modelling has been employed for a subset of the 3

An ongoing programme investigating 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, the NRL Mercury IVA was operated in negative polarity mode to produce an 8MeV endpoint bremsstrahlung spectrum, which in turn was used to induce photofission in a depleted uranium (DU) sample. Twenty-six experiments were fielded in March 2011 in which twenty-seven detectors were fielded, including 3He tubes, NaI detectors, liquid scintillators and high purity germanium detectors, capable of detecting both gamma radiation and neutrons. The results from a selection of those detectors are discussed here. A variety of high-Z (lead) and hydrogenous (borated polyethylene) shielding configurations was employed and positive detection was made up to the maximum shielding tested, 75g/cm2. A detailed source has been modelled using MCNPX and MCNP6 to predict the number of (n,p) detector events within four of the 3He tubes fielded. The modelling is consistent with the experiment to within a factor of two, when integrating between 0.1 and 20s.

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.

Summary form only given. We investigate the use of pulsed bremsstrahlung as a standoff active interrogation tool for the production of neutron and gamma signatures in fissionable materials.1 Photons are generated by an electron beam striking a Ta converter on the 16 MV Hermes III facility at Sandia National Laboratories, and propagate to a target location 19 m from the electron beam diode, at which various materials are placed. X-rays produce (γ,f) and (γ,n) reactions in fissionable material, while nonfissionable materials such as Pb will result in competing (γ,n) reactions. The intense prompt 30 ns x-ray burst from the diode produces γ-rays and neutrons from materials in the environment, all of which must be discriminated by a successful detector set.

Summary form only given. 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.

Summary form only given. 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.

Summary form only given. The HERMES-III generator1 is used for research in photofission and induced background. This work follows experiments using the 8-MV, 200 kA, 40 ns bremsstrahlung pulse from the Mercury generator to induce photofission in a variety of targets.2 HERMES is operated in two modes: half-machine at 8 MV and 300 kA, similar to Mercury, and full-machine at 16 MV and 600 kA. The objective of the research is to understand radiation background in the environment as the bremsstrahlung endpoint is increased.

In this paper, the MCNPX code is used to optimize the moderator materials and thicknesses for a neutron detector based on a 6Li-loaded glass scintillator. It is shown that the response of the 6Li detector to a delayed-neutron-fission spectrum is comparable to a 3He-based gas proportional counter that is configured with a similar active area and moderator materials. For a delayed-neutron spectrum, a polyethylene thickness between 4 cm and 5 cm optimizes the number of 6Li(n,α)3H reactions in the glass. Furthermore, when the polyethylene is surrounded by additional thickness of 2-cm of Flexi-boron the response of the detector to neutrons with energies of 1–100 eV (where the emission of delayed fission neutrons is small) is reduced by a factor of two or more while reducing the response at the peak of the delayed spectrum by only 20–30%. The addition of this thick Flexi-boron layer can be an effective way to reduce the unwanted signals from cosmic-ray neutrons and other background sources. A good neutron detector should also be able to distinguish between high-energy gamma-rays and neutrons. In a pulsed-power environment, the bremsstrahlung from the generator can also temporarily blind the detector. Lead shielding can be used to reduce these effects so that the detector can recover fast enough to measure delayed fission neutrons. We have used MCNPX to address the effects of lead shielding on the response of the 6Li detector to delayed fission neutrons. It is found that the addition of lead has little effect on the optimum polyethylene thickness. The addition of 5-cm-thick lead (10-cm) reduces the response to the delayed neutron spectrum by about 30% (50%) while reducing the effects of 1 MeV gamma-rays by a factor of 30 (1000).

A series of experiments has been performed on the Mercury generator (8 MV, 200 kA, 50 ns) to investigate the use of a single, intense radiation pulse to induce photo fission.1 In these studies, a plate of depleted uranium (DU) is irradiated by the 8 MV-endpoint bremsstrahlung pulse produced by Mercury. Within this x-ray pulse, photons whose energies lie above the threshold of 5.26 MeV induce fission in the DU, producing about 2.7 neutrons per fission. The neutrons consist of two populations: prompt neutrons, which escape the DU within a few hundred ns of the bremsstrahlung pulse, and delayed neutrons, which continue to appear for minutes after the pulse. The results reported here pertain only to the delayed neutron population. These neutrons are unambiguously detected using an array of He-3 detectors deployed 1.0–4.5 m from the DU. However, the measured neutrons do not follow an inverse square law as a function of detector-to-DU distance. The farthest detector measured more than twice the counts, relative to the closest detector, that would be expected for an inverse-square distribution. Containment and scattering of the fission neutrons by the walls, floor, and ceiling of the Mercury chamber, and various nearby objects such as electrical boxes, are the likeliest explanation for the measured spatial distribution. Modeling of the experiment by the Monte Carlo transport code MCNPX 2 supports this hypothesis. Results showing the influence of various features of the Mercury chamber, as well as their composition, are presented.

In an ionized atom the Kα and Kβ fluorescence lines have different, and usually slightly higher, energies than the same lines in a cold, un-ionized atom. The increase in fluorescence line energy Δ(hv) that pulsed power discharges can produce in high atomic number atoms is usually much smaller than the line energies hv themselves: typical numbers are hv = 60 keV and Δ(hv) = 10–20 eV, while the resolution obtained so far with a transmission crystal spectrometer developed for the purpose is around 100 eV. Hence any change in the energy of the fluorescence x-rays, from tungsten weakly ionized in the tungsten of the Plasma-Filled Rod Pinch (PFRP), could not be observed.

A 640 kV Marx bank and plasma opening switch (POS) produce an intense pulse of MeV protons that stop in a fluorine-bearing target. The 19F(p,αγ)16O reaction produces 6- to 7-MeV characteristic γ-rays that can induce fission reactions in fissionable materials, such as depleted uranium (DU). Some fission reactions are also induced by neutrons generated by proton and/or deuteron reactions in the target, indicating that this system could be used to produce a mixed source of γ-rays and neutrons. Experimental results are presented, and future options are described to improve this kind of mixed source.

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.

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.

In intense, pulsed active detection, a single, intense pulse of radiation is used to induce photofission in fissionable material, increasing its detectability. The Mercury pulsed-power generator was converted to positive polarity (+3.7 MV, 325 kA, 50-ns FWHM) to drive an intense, pulsed radiation source based on the FIGARO active detection concept. The probing radiation source consisted of an ion-beam diode and a thick PTFE (Teflon) converter where 6–7 MeV γ-rays were produced via the 19F(p,αγ)16O reaction. A suite of radiation detectors was used both to detect the presence of irradiated fissionable material and to characterize the probing radiation source. Four types of detectors were used for the source characterization. Thermoluminescent dosimeters were used to measure the angular distribution of the dose associated with x-rays and γ-rays from the ion-beam diode. Plastic scintillator-photodiode detectors were used to characterize the time dependence of this dose. A plastic scintillator-photomultiplier detector was used to monitor the γ-ray intensity of the probing radiation source and to monitor changes in the production of background neutrons by the diode and PTFE converter. A set of rhodium foil activation counters was used to measure the absolute yield of these background neutrons. Two types of detectors with comparable sensitivities were used to measure delayed neutrons resulting from photofission: 3He proportional counters and a 6Li-loaded-glass-scintillator detector. The neutron detection rate from each detector following the probing radiation pulse was over 100 times higher with depleted uranium present than with lead.

For pulsed power machines with an inductive voltage adder (IVA) architecture and a magnetically insulated transmission line (MITL), positive polarity operation results in layered MITL flow as emitted electrons are born at different potentials in each of the adder cells.1 Because of this layered flow, the usual prediction of the voltage using the Mendel formula is not accurate in positive polarity. In an attempt to directly measure the voltage across an ion diode on Mercury in positive polarity2,3, a number of diagnostics have been fielded, including a vacuum wire voltmeter, stacked radiachromic films, nuclear activation, and a 7Li(p, n) neutron-yield technique. Additionally, the ion current produced by the diode is measured using magnetic probes and the proton current is estimated by the same 7Li(p, n) neutron-yield technique. An ion diode model is developed to predict the diode voltage and ion current based on the measured total current. This model is based on previous pinched beam ion diode work4,5 and is only applicable when the diode presents an under-matched load impedance compared with the self-limited impedance of the MITL so that the flow current becomes part of the diode current. The self-limited impedance in positive polarity layered flow on Mercury is also investigated. PIC and circuit simulations are used to analyze the problem as well. The experimental results will be discussed and compared with the theoretical predictions.

The detection of fissile material by passive and active techniques is an area of intense interest. Many active approaches detect the products of fission induced by photon and/or neutron irradiation.1 One such approach has focused on using intense, pulsed, beam-target interactions to produce an irradiation source of 6.13-, 6.92-, and 7.13-MeV characteristic gamma rays from the 19F(p, αγ)16O reaction.2,3 Initially, work at NRL utilized the Gamble-II, 3-Ω, water-line generator with a pinch-reflex ion diode that accelerates ions to 2 MeV at ~ 0.5-TW peak ion power.3 This pulsed source also simultaneously produces neutrons from: (1) a collective ion-acceleration mechanism associated with the diode and (2) the reactions resulting from the isotopic abundance of deuterons in the ion beam, e.g., 12C(d, n)13N and 19F(d, n)20Ne. In this presentation, we review progress toward characterizing a similar mixed source using the Mercury inductive voltage adder operated in positive polarity producing ~ 4-MeV4 ion energy and ~ 0.3-TW peak ion power. An array of diagnostics is used to determine the number of gamma rays and neutrons produced by the source, including a shielded scintillator-photomultiplier for the gamma-ray time history and a rhodium-activation counter to measure the total number of neutrons produced. In addition, the gamma and neutron source strengths are inferred from delayed neutrons that result from fissions in depleted uranium and that are measured using 3He and 6Li neutron detectors.

Argon gas puff experiments using the long pulse mode of Saturn (230-ns rise time) have promise to increase the coupled energy and simplify operations because the voltage is reduced in vacuum and the forward-going energy is higher for the same Marx charge. The issue addressed in this work is to determine if the 12-cm-diameter triple nozzle used in Saturn long-pulse-mode experiments to date1 provides maximum K-shell yield, or if a different-radius nozzle provides additional radiation. Long-pulse implosions are modeled by starting with measured density distributions from the existing 12-cm-diameter nozzle2, and then varying the outer radius in an implosion-energy-conserving self-similar manner to predict the gas-puff diameter that results in the maximum K-shell yield. The snowplow-implosions and multi-zone radiation transport3 models used in the analysis are benchmarked against detailed measurements from the 12-cm-diameter experiments. These calculations indicate that the maximum K-shell emission is produced with very nearly the existing nozzle radius.