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Photons of the appropriate energy can induce photofission in fissile material. We are investigating the applicability of this mechanism, using photons from bremsstrahlung, for long-range ( 100 m) detection of fissile material using high-power ( 1 TW), pulsed technology so called intense, pulsed active detection. In this report, we describe the results of laboratory, proof-of-principle experiments, supported by computations and analyses, in which a single pulse of 8-MeV endpoint bremsstrahlung from the Naval Research Laboratory Mercury pulsed-power generator (200-kA peak current, 50-ns pulse width) induces photofission in depleted uranium. Fission products are measured using He-3 proportional counters, and plastic (BC408), sodium-iodide (NaI:Tl), and bismuth-germinate-oxide scintillators. Prompt neutrons, delayed neutrons, and delayed gamma-rays from the induced photofission are measured unambiguously. Delayed neutron and gamma-ray measurements are also carried out with various thicknesses of low- and high-atomic-number materials attenuating the bremsstrahlung and fission signatures. Results of simulations and analyses carried out in support of the experiments are in agreement with the measurements. In addition, we demonstrate that relatively simple variations of the diode geometry can increase the efficiency of inducing fissions three-fold over the original geometry. These geometry changes cause the electron beam to be more normally incident onto the tantalum converter and thereby increase the forward-directed bremsstrahlung intensity without changing the driving, electrical power pulse. The results of this work and future directions for this research are presented in this report., The original document contains color images. Sponsored in part by Office of Naval Research. Prepared in collaboration with Naval Research Laboratory, Space Sciences Division, Washington, DC, Engility, Inc., Chantilly, VA, and Idaho State University, Idaho Falls, ID.

The output voltage of the NRL Mercury generator has been successfully increased to 8 MV. Mercury was originally designed for 6 MV, 300 kA operation in negative polarity [1]. However, new experiments required a bremsstrahlung x-ray source with a higher endpoint voltage [2],[3]. The threshold energy for photofission in targets of interest is about 5.5 MeV and, given the roughly Gaussian shape of the Mercury output pulse, very few x rays above threshold could be generated by Mercury with a 6-MV peak output voltage. Also, the cross section for fission rises sharply above the threshold energy. It was therefore decided to increase the output voltage of Mercury to the maximum possible without imposing undo cost or risk. A new center conductor was designed for Mercury that increased the effective output impedance of the MITL from about 23 to 40 , thereby increasing the maximum output voltage to 8 MV. The new center conductor has carefully designed tapers (steps down in diameter) at each stage of the adder so that the adder cell voltages are equally balanced at the design limit of 1.3 MV. One limiting factor is the breakdown probability of the insulator stack in the adder cells. However, simulations have shown that the new center conductor only increases the breakdown probability a small amount. The 40 output impedance was selected because it allowed us to reuse several pieces of the existing center conductor. Aluminum was used for most of the new parts. These steps allowed us to save time and keep material costs to a minimum. PIC and circuit simulations were performed to validate the design. Mercury has now been successfully operated for over 200 shots at 8 MV., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color images.

Intense pulsed active detection (IPAD, [1]; also see presentations at this conference by B.V. Weber, et al., D.P Murphy et al., S.B. Swanekamp et al. and J.C. Zier, et al.) has been proposed as a means of detecting contraband fissile material from a distance. In this approach, an intense bremsstrahlung pulse is used to induce photo fission, the products of which are detected. In this work, we report on an initial effort to evaluate the applicability of various pulsed-power architectures to this approach. The electron energy is 12 MeV with an effective electron charge at 12 MeV of 3 mC to 5 mC delivered in 100 ms, in either a single pulse or a burst. The eventual goals for the accelerator are compactness, especially short length; relatively light weight; transportability; and ease of setup and operation in the field. We first consider designs that could be constructed in a few years with minimum development. We compare induction voltage adders (IVA), ferrite-core linear induction accelerators (LIA), and linear transformer drivers (LTD). Conceptual point-designs are developed for each approach using essentially demonstrated technology, though the LTD assumes scaling and repackaging. The IVA design was considered for further design development [2]. We then sought approaches that are not demonstrated technology but have promise for achieving substantially less weight and volume. We considered recirculating LIAs, auto-accelerators, air-core LIAs, dielectric wall accelerators, and vacuum inductive stores with plasma opening switches. A partial pre-conceptual design of a 100 kA, single-pulse air-core LIA suggested that this might be a promising advanced candidate, and it is described here. An evaluation of the other advanced concepts will be published elsewhere [3]., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color images. Supported in part by DTRA.

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., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013, The original document contains color images.

A new circuit model for the Sandia Saturn generator was constructed to aid analysis of measurements from recent experiments with reflex triode loads. This configuration is easier to model than others because there are no convolutes in the vacuum region. Also, the low impedance of the reflex triode makes the amount of vacuum flow current insignificant, which also simplifies analysis. Saturn has 36 identical pulse generator modules that each drive a water transmission line output. These transmission line outputs are bussed together in water before driving the vacuum insulator stack. Measured voltage waveforms from all 36 of the water transmission lines (TL) are used to drive the simulation. The 94-ns two-way electrical length between the location of the TL voltage probe and the end of the TL makes the measured TL voltages a high fidelity input for the simulation. The load currents from the circuit model are compared to measured currents for short circuit and real load shots. A physics-based circuit model for the reflex triode was used to aid in analysis of the reflex triode behavior., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013. The original document contains color images.

We present optical emission-line spectra for outlying H II regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer disk of NGC2915 to be enriched to a metallicity of 0.4 Z. An analysis of the metal yields shows that the outer disk of NGC2915 is overabundant for its gas fraction, while the central star-forming core is similarly underabundant for its gas fraction. Star formation rates derived from very deep 14 ks GALEX FUV exposures indicate that the low level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider three plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.

Contains reports on four research projects., Lincoln Laboratory Purchase Order No. 748, National Aeronautics and Space Administration (Contract NSR-22-009-120), National Aeronautics and Space Administration (Grant NsG-419)

Contains reports on five research projects., National Aeronautics and Space Administration (Grant NsG-419), National Aeronautics and Space Administration (Contract NSR-22-009-120), Lincoln Laboratory Purchase Order No. 748

After 616 shots in a negative polarity configuration, Mercury, a 6-MV and 300-kA inductive voltage adder (IVA), has been converted to positive polarity in order to extract ion beams. Conversion to positive polarity was achieved by rotating all six of the adder cells by 180 . In principle, we could have chosen to instead insert the center conductor from the other end of the adder to change polarity, but rotating the cells minimized the time required to make the transition. Although most of the same pieces were used, the center conductor had to be reconfigured in order to align the transition pieces with the cell feed gaps. Because the electron flow was anticipated to be very different in positive polarity, a result of emission from surfaces of different potential, a simple blade diode was fielded for the initial shots to gain a better understanding of operation in positive polarity. The blade diode consisted of the same cathode used as a dummy load in the first negative polarity shots on Mercury, but with a different carbon anode that just covered the end of the center conductor. After a few short circuit and initializing shots, a series of shots were taken where only the blade diode AK gap was varied in order to characterize self-limited and load-limited operation and to compare measurements with theory and simulation., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color images.

The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Here we report observations of the kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to determine the rotation curve of the halo gas. We have obtained a Fabry-P\'erot data cube in Halpha to measure the kinematics of the halo gas with angular resolution much higher than obtained from HI 21 cm observations. The data cube was obtained with the TAURUS II spectrograph at the WHT on La Palma. The velocity information of the diffuse ionized gas extracted from the data cube is compared to model distributions to constrain the distribution of the gas and in particular the halo rotation curve. The best fit model has a central attenuation tau_H-alpha=6, a dust scale length of 8.1 kpc, an ionized gas scale length of 5.0 kpc. Above the plane the rotation curve lags with a vertical gradient of -18.8 km/s/kpc. We find that the scale length of the H-alpha must be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation curve above the plane rises less steeply than in the plane. This is all in agreement with the velocities measured in the HI., Comment: A&A, in press. 13 pages, 19 figures

The opacity of a spiral disk due to dust absorption influences every measurement we make of it in the UV and optical. Two separate techniques directly measure the total absorption by dust in the disk: calibrated distant galaxy counts and overlapping galaxy pairs. The main results from both so far are a semi-transparent disk with more opaque arms, and a relation between surface brightness and disk opacity. In the Spitzer era, SED models of spiral disks add a new perspective on the role of dust in spiral disks. Combined with the overall opacity from galaxy counts, they yield a typical optical depth of the dusty ISM clouds: 0.4 that implies a size of ~60 pc. Work on galaxy counts is currently ongoing on the ACS fields of M51, M101 and M81. Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift pairs. Evolution in disk opacity could influence distance measurements (SN1a, Tully-Fisher relation). Here, we present first results from spectroscopically selected occulting pairs in the SDSS. The redshift range for this sample is limited, but does offer a first insight into disk opacity evolution as well as a reference for higher redshift measurements., Comment: 4 pages, 3 figures, to appear in the proceedings of "The Formation and Evolution of Galaxy Disks", Rome 2007, organized by the Vatican Observatory, editors J. G. Funes, S.J. and E. M. Corsini

The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in H$\alpha$ emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in H$\alpha$ emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. From the 24 $\mu$m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., $\sim 30%$. The additional asymmetry in H$\alpha$ is modeled as additional symmetric dust attenuation which extends up to $\sim$ 40\arcsec (1.9 kpc) above the plane of the galaxy with a mid-plane value of $\tau$=0.8 and a scale height of 0.5 kpc, Comment: A&A in press. 5 pages, 3 figures

The Naval Research Laboratory (NRL) has initiated a program focusing on railgun bore materials science. The objective of the program is to study the conditions in a high power railgun barrel during launch. An 11-MJ prime energy, 6-m long, 5-cm bore diameter railgun is being built for testing. This laboratory launcher will be used to study sliding contacts under the extreme current, temperature, and pressure conditions found in a high power railgun. Real time diagnostics coupled with extensive post-shot materials analysis will be used to understand railgun operation. Results will be compared with other high power railguns., Presented at the International Symposium on Electromagnetic Launch Technology (EML) (13th) held 22-25 May 2006 in Potsdam, Germany.

Context At least a fraction of the atomic hydrogen in spiral galaxies is suspected to be the result of molecular hydrogen which has been dissociated by radiation from massive stars. Aims In this paper, we extend our earlier set of data from a small region of the Western spiral arm of M81 with CO observations in order to study the interplay between the radiation field and the molecular and atomic hydrogen. Methods We report CO(1-0) observations with the Nobeyama 45 m dish and the Owens Valley interferometer array of selected regions in the Western spiral arm of M81. Results From our Nobeyama data, we detect CO(1-0) emission at several locations, coinciding spatially with HI features near a far-UV source. The levels and widths of the detected CO profiles are consistent with the CO(1-0) emission that can be expected from several large photo-dissociation regions with typical sizes of some 50x150 pc located within our telescope beam. We do not detect emission at other pointings, even though several of those are near far-UV sources and accompanied by bright HI. This non-detection is likely a consequence of the marginal area filling factor of photo-dissociation regions in our observations. We detect no emission in our Owens Valley data, consistent with the low intensity of the CO emission detected in that field by the Nobeyama dish. Conclusions We explain the lack of CO(1-0) emission at positions farther from far-UV sources as a consequence of insufficient heating and excitation of the molecular gas at these positions, rather than as an absence of molecular hydrogen., Comment: 8 pages latex, 4 figures, Accepted for publication as a Research Note in Astronomy & Astrophysics

We have found the peculiar galaxy NGC922 to be a new drop-through ring galaxy using multi-wavelength (UV-radio) imaging and spectroscopic observations. Its `C'-shaped morphology and tidal plume indicate a recent strong interaction with its companion which was identified with these observations. Using numerical simulations we demonstrate that the main properties of the system can be generated by a high-speed off-axis drop-through collision of a small galaxy with a larger disk system, thus making NGC922 one of the nearest known collisional ring galaxies. While these systems are rare in the local Universe, recent deep HST images suggest they were more common in the early Universe., Comment: 5 pages, accepted for publication in MNRAS Letters

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a spacecraft-borne nulling interferometer for high-resolution astronomy and the direct detection of exoplanets and assay of their environments and atmospheres. FKSI is a high angular resolution system operating in the near to mid-infrared spectral region and is a scientific and technological pathfinder to the Darwin and Terrestrial Planet Finder (TPF) missions. The instrument is configured with an optical system consisting, depending on configuration, of two 0.5 - 1.0 m telescopes on a 12.5 - 20 m boom feeding a symmetric, dual Mach- Zehnder beam combiner. We report on progress on our nulling testbed including the design of an optical pathlength null-tracking control system and development of a testing regime for hollow-core fiber waveguides proposed for use in wavefront cleanup. We also report results of integrated simulation studies of the planet detection performance of FKSI and results from an in-depth control system and residual optical pathlength jitter analysis.

Mercury, Figure 1, is a magnetically-insulated inductive voltage adder that was acquired, assembled, and made operational by the Pulsed Power Physics Branch at the Naval Research Laboratory in Washington, DC1,2. Mercury is designed to produce a 50-ns pulse of 6-MV peak voltage, and 360-kA peak current when operated at full power. This is accomplished using four, SF6 filled, laser-triggered switches (LTSs) to transfer energy from four intermediate-storage capacitors to 12 pulse forming lines (PFLs). By discharging the PFLs in a parallel/series configuration via self-break water output switches (OSs) into six induction cells, the output pulse is realized. To achieve optimal power flow, OS closure times should be staggered according to the delay time between adjacent induction cavities3. Consequently, both LTS and OS jitter need to be kept to a minimum. During different stages of assembly, the LTSs and the OSs were tested using dummy loads3. This document will review the results of these tests., See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color images.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.