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Spectral measurements of the second harmonic of the SASE FEL radiation at APS.

Description: We present the z-dependent spectral measurement results for the second harmonic of the SASE FEL radiation before and after saturation. The measurements were performed at the Advanced Photon Source FEL with the fundamental wavelength of 530 nm. The spectral properties of the second harmonic are compared with those of the fundamental and with the theoretical expectations.
Date: October 3, 2002
Creator: Sajaev, V. & Huang, Z.
Partner: UNT Libraries Government Documents Department

Pressure and fluid saturation prediction in a multicomponent reservoir, using combined seismic and electromagnetic imaging

Description: This paper presents a method for combining seismic and electromagnetic measurements to predict changes in water saturation, pressure, and CO{sub 2} gas/oil ratio in a reservoir undergoing CO{sub 2} flood. Crosswell seismic and electromagnetic data sets taken before and during CO{sub 2} flooding of an oil reservoir are inverted to produce crosswell images of the change in compressional velocity, shear velocity, and electrical conductivity during a CO{sub 2} injection pilot study. A rock properties model is developed using measured log porosity, fluid saturations, pressure, temperature, bulk density, sonic velocity, and electrical conductivity. The parameters of the rock properties model are found by an L1-norm simplex minimization of predicted and observed differences in compressional velocity and density. A separate minimization, using Archie's law, provides parameters for modeling the relations between water saturation, porosity, and the electrical conductivity. The rock-properties model is used to generate relationships between changes in geophysical parameters and changes in reservoir parameters. Electrical conductivity changes are directly mapped to changes in water saturation; estimated changes in water saturation are used along with the observed changes in shear wave velocity to predict changes in reservoir pressure. The estimation of the spatial extent and amount of CO{sub 2} relies on first removing the effects of the water saturation and pressure changes from the observed compressional velocity changes, producing a residual compressional velocity change. This velocity change is then interpreted in terms of increases in the CO{sub 2}/oil ratio. Resulting images of the CO{sub 2}/oil ratio show CO{sub 2}-rich zones that are well correlated to the location of injection perforations, with the size of these zones also correlating to the amount of injected CO{sub 2}. The images produced by this process are better correlated to the location and amount of injected CO{sub 2} than are any of the individual images of ...
Date: 2002-10~
Creator: Hoversten, G. M.; Gritto, Roland; Washbourne, John & Daley, Tom
Partner: UNT Libraries Government Documents Department

Organic Dye Effects on DNAPL Entry Pressure in Water Saturated Porous Media

Description: One of three diazo dyes with the same fundamental structure have been used in most studies of DNAPL behavior in porous media to stain the NAPL: Sudan III, Sudan IV, or Oil-Red-O. The dyes are generally implicitly assumed to not influence DNAPL behavior. That assumption was tested using simple entry pressure experiments.
Date: October 2, 2001
Creator: Iversen, G.M.
Partner: UNT Libraries Government Documents Department

Analysis of Breakthrough Profiles Based on Gamma Ray Emission Along Loaded Packed Bed Columns: Comparative Evaluation of Ionsiv IE-911 and Chabazite Zeolite for the Removal of Radiostrontium and Cesium from Groundwater

Description: A gamma counting system has been assembled that can profile the breakthrough fronts of gamma-emitting radioisotopes longitudinally and axially along a loaded column. This profiling technique has been particularly useful in columns studies such as those performed with IONSP IE-911, a crystalline silicotitanate (CST) manufactured by UOP, in which unusually long operating times are required to observe cesium breakthrough in column effluent. The length of the mass transfer zone and extent of column saturation can be detected early in a column study by viewing the relative emission of gamma emitters along I the length of the column. In this study, gamma scans were used to analyze loaded CST and zeolite columns used in the treatment of process wastewater simulant and actual groundwater. Results indicate good run-to-run reproductibility in acquiring the scans. The longitudinal gamma scans for both {sup 90}Sr and {sup 137}Cs conformed with breakthrough results reported on the basis of column effluent activity. Although not obvious from data obtained by monitoring effluent activity, the gamma scans indicated that both cesium and strontium in the saturated zone of the CST column are slowly displaced by the higher levels of groundwater cations and are then resorbed further down the column. This displacement phenomenon identified by gamma scans was verified using data from a zeolite column, in which both the gamma scan and column effluent data exhibited radionuclide displacement by groundwater cations. The gamma emission intensities from the CST column runs are used to quantitate and compare the distribution coefficient and loading capacity of {sup 137}Cs on CST versus zeolite.
Date: October 18, 1999
Creator: Bostick, D.T.; DePaoli, S.M. & Lucero, A.J.
Partner: UNT Libraries Government Documents Department

LAUNCHER PERFORMANCE AND THERMAL CAPABILITY OF THE DIII-D ECH SYSTEM

Description: OAK-B135 The temperatures of components of DIII-D ECH launchers were observed during 2003 tokamak operation. The injected power was typically 500-700 kW and the pulse length was typically 2s. Plasma shots were performed at intervals of about 17 min from 9 a.m. to 5 p.m. The temperatures of a movable mirror, a fixed mirror and a launcher reached an equilibrium after about six hours of repetitive pulsing. The saturation temperature depends to some extent on the plasma stored energy. However, even in high {beta} plasma, the temperatures plateaued at acceptable values.
Date: October 1, 2003
Creator: KAJIWARA,K; LOHR,J; GORELOV,I.A; GREEN,M.T; PONCE,D; CALLIS,R.W et al.
Partner: UNT Libraries Government Documents Department

Modeling seepage into heated waste emplacement tunnels in unsaturated fractured rock

Description: Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that-for the first several hundred years after emplacement-will be heated to above-boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above-boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. In this paper, we describe numerical analyses of the coupled thermal-hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock-drift interface-independent of the thermal conditions-a second barrier exists to downward percolation at above-boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual-permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions.
Date: October 1, 2003
Creator: Birkholzer, Jens T.; Mukhopihadhyay, Sumit & Tsang, Yvonne W.
Partner: UNT Libraries Government Documents Department

Optimization of the parameters of a storage ring for a high power XUV free electron laser

Description: In this paper we describe the operation of an XUV high gain FEL operating within a bypass of an electron storage ring, and discuss the implications on storage ring optimization imposed by FEL requirements. It transpires that, in the parameter regime of interest, collective effects within the beam play an important role. For example, intrabeam scattering dilutes the transverse emittance of the beam and the microwave instability increases the momentum spread. Both phenomena reduce the effectiveness of the FEL. A computer code, ZAP, has been written which, for a given lattic design, takes all such effects into consideration and produces a figure of merit for FEL operation for that machine. We show the results of ZAP for several storage ring designs, all optimized for FEL operation, and present a design example of a facility capable of producing coherent radiation at 400 A with tens of megawatts of peak power.
Date: October 1, 1985
Creator: Jackson, A.; Bisognano, J.; Chattopadhyay, S.; Cornacchia, M.; Garren, A.; Halbach, K. et al.
Partner: UNT Libraries Government Documents Department

Origin of the patchy emission pattern at the ZERT CO2 Release Test

Description: A numerical experiment was carried out to test whether the patchy CO{sub 2} emission patterns observed at the ZERT release facility are caused by the presence of packers that divide the horizontal injection well into six CO2-injection zones. A three-dimensional model of the horizontal well and cobble-soil system was developed and simulations using TOUGH2/EOS7CA were carried out. Simulation results show patchy emissions for the seven-packer (six-injection-zone) configuration of the field test. Numerical experiments were then conducted for the cases of 24 packers (23 injection zones) and an effectively infinite number of packers. The time to surface breakthrough and the number of patches increased as the number of packers increased suggesting that packers and associated along pipe flow are the origin of the patchy emissions. In addition, it was observed that early breakthrough occurs at locations where the horizontal well pipe is shallow and installed mostly in soil rather than the deeper cobble. In the cases where the pipe is installed at shallow depths and directly in the soil, higher pipe gas saturations occur than where the pipe is installed slightly deeper in the cobble. It is believed this is an effect mostly relevant to the model rather than the field system and arises through the influence of capillarity, permeability, and pipe elevation of the soil compared to the cobble adjacent to the pipe.
Date: October 15, 2009
Creator: Lewicki, J.L.; Pan, L.; Dobeck, L.; Spangler, L. & Oldenburg, C.M.
Partner: UNT Libraries Government Documents Department

Microscopic return point memory in Co/Pd multilayer films

Description: We report soft x-ray speckle metrology measurements of microscopic return point and complementary point memory in Co/Pd magnetic films having perpendicular anisotropy. We observe that the domains assemble into a common labyrinth phase with a period that varies by nearly a factor of two between initial reversal and fields near saturation. Unlike previous studies of similar systems, the ability of the film to reproduce its domain structure after magnetic cycling through saturation varies from loop to loop, from position to position on the sample, and with the part of the speckle pattern used in the metrology measurements. We report the distribution of memory as a function of field and discuss these results in terms of the reversal process.
Date: October 1, 2009
Creator: Seu, K.A.; Su, R.; Roy, S.; Parks, D.; Shipton, E.; Fullerton, E.E. et al.
Partner: UNT Libraries Government Documents Department

Oxidative cleavage of erucic acid for the synthesis of brassylic acid

Description: The main focus of this work is to synthesize Brassylic Acid (BA) using oxidative cleavage of Erucic Acid (EA). Crambe (Crambe abyssinica) is an industrial oilseed grown in North Dakota. Crambe has potential as an industrial fatty acid feedstock as a source of Erucic acid (EA). It has approximately 50-60 % of EA, a C{sub 22} monounsaturated fatty acid. Oxidative cleavage of unsaturated fatty acids derived from oilseeds produces long chain (9, 11, and 13 carbon atoms) dibasic and monobasic acids. These acids are known commercial feedstocks for the preparation of nylons, polyesters, waxes, surfactants, and perfumes. Other sources of EA are Rapeseed seed oil which 50-60 % of EA. Rapeseed is grown outside USA. The oxidative cleavage of EA was done using a high throughput parallel pressure reactor system. Kinetics of the reaction shows that BA yields reach a saturation at 12 hours. H{sub 2}WO{sub 4} was found to be the best catalyst for the oxidative cleavage of EA. High yields of BA were obtained at 80 C with bubbling of O{sub 2} or 10 bar of O{sub 2} for 12 hours.
Date: October 29, 2010
Creator: Nasrullah, Mohammed J.; Thapliyal, Pooja; Pfarr, Erica N.; Dusek, Nicholas S.; Schiele, Kristofer L. & Bahr, James A.
Partner: UNT Libraries Government Documents Department

Test-to-Failure of Crystalline Silicon Modules: Preprint

Description: Accelerated lifetime testing of five crystalline silicon module designs was carried out according to the Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol. This protocol compares the reliability of various module constructions on a quantitative basis. The modules under test are subdivided into three accelerated lifetime testing paths: 85..deg..C/85% relative humidity with system bias, thermal cycling between ?40..deg..C and 85..deg..C, and a path that alternates between damp heat and thermal cycling. The most severe stressor is damp heat with system bias applied to simulate the voltages that modules experience when connected in an array. Positive 600 V applied to the active layer with respect to the grounded module frame accelerates corrosion of the silver grid fingers and degrades the silicon nitride antireflective coating on the cells. Dark I-V curve fitting indicates increased series resistance and saturation current around the maximum power point; however, an improvement in junction recombination characteristics is obtained. Shunt paths and cell-metallization interface failures are seen developing in the silicon cells as determined by electroluminescence, thermal imaging, and I-V curves in the case of negative 600 V bias applied to the active layer. Ability to withstand electrolytic corrosion, moisture ingress, and ion drift under system voltage bias are differentiated.
Date: October 1, 2010
Creator: Hacke, P.; Terwilliger, K.; Glick, S.; Trudell, D.; Bosco, N.; Johnston, S. et al.
Partner: UNT Libraries Government Documents Department

Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solution

Description: A surface reaction kinetic model is developed for predicting Ca isotope fractionation and metal/Ca ratios of calcite as a function of rate of precipitation from aqueous solution. The model is based on the requirements for dynamic equilibrium; i.e. proximity to equilibrium conditions is determined by the ratio of the net precipitation rate (R{sub p}) to the gross forward precipitation rate (R{sub f}), for conditions where ionic transport to the growing crystal surface is not rate-limiting. The value of R{sub p} has been experimentally measured under varying conditions, but the magnitude of R{sub f} is not generally known, and may depend on several factors. It is posited that, for systems with no trace constituents that alter the surface chemistry, R{sub f} can be estimated from the bulk far-from-equilibrium dissolution rate of calcite (R{sub b} or k{sub b}), since at equilibrium R{sub f} = R{sub b}, and R{sub p} = 0. Hence it can be inferred that R{sub f} {approx} R{sub p} + R{sub b}. The dissolution rate of pure calcite is measureable and is known to be a function of temperature and pH. At given temperature and pH, equilibrium precipitation is approached when R{sub p} (= R{sub f} - R{sub b}) << R{sub b}. For precipitation rates high enough that R{sub p} >> R{sub b}, both isotopic and trace element partitioning are controlled by the kinetics of ion attachment to the mineral surface, which tend to favor more rapid incorporation of the light isotopes of Ca and discriminate weakly between trace metals and Ca. With varying precipitation rate, a transition region between equilibrium and kinetic control occurs near R{sub p} {approx} R{sub b} for Ca isotopic fractionation. According to this model, Ca isotopic data can be used to estimate R{sub f} for calcite precipitation. Mechanistic models for calcite precipitation indicate that ...
Date: October 15, 2010
Creator: DePaolo, D.
Partner: UNT Libraries Government Documents Department

Multiscale thermohydrologic model: addressing variability and uncertainty at Yucca Mountain

Description: Performance assessment and design evaluation require a modeling tool that simultaneously accounts for processes occurring at a scale of a few tens of centimeters around individual waste packages and emplacement drifts, and also on behavior at the scale of the mountain. Many processes and features must be considered, including non-isothermal, multiphase-flow in rock of variable saturation and thermal radiation in open cavities. Also, given the nature of the fractured rock at Yucca Mountain, a dual-permeability approach is needed to represent permeability. A monolithic numerical model with all these features requires too large a computational cost to be an effective simulation tool, one that is used to examine sensitivity to key model assumptions and parameters. We have developed a multi-scale modeling approach that effectively simulates 3D discrete-heat-source, mountain-scale thermohydrologic behavior at Yucca Mountain and captures the natural variability of the site consistent with what we know from site characterization and waste-package-to-waste-package variability in heat output. We describe this approach and present results examining the role of infiltration flux, the most important natural-system parameter with respect to how thermohydrologic behavior influences the performance of the repository.
Date: October 1, 2000
Creator: Buscheck, T; Rosenberg, N D; Gansemer, J D & Sun, Y
Partner: UNT Libraries Government Documents Department

Exceptional Electron Transport Properties of In-rich InGaN

Description: Recent years have seen an explosion of interest in the narrow band gap end of the InGaN alloy system, particularly in InN. The existence of surface electron accumulation and a tendency for n-type conductivity have been well-established and are explained by an extremely large electron affinity and the location of the Fermi level stabilization energy (E{sub FS}) high in the conduction band [1]. These characteristics pose significant challenges to the integration of In-rich InGaN into devices and demonstrate the need for a better understanding of the relationship between native defects and electronic transport in the alloy system. It has been previously shown that high-energy particle irradiation can predictably control the electronic properties of In-rich InGaN [1]. With increasing irradiation dose, the electron concentration (n) increases and the electron mobility ({mu}) decreases until the Fermi level reaches E{sub FS}, which is the saturation point. The value of n at saturation decreases with decreasing In fraction, due to the raising of the conduction band edge with respect to E{sub FS}.
Date: October 22, 2006
Creator: Jones, R.E.; van Genuchten, H.C.M.; Yu, K.M.; Walukiewicz, W.; Li, S.X.; Liliental-Weber, Z. et al.
Partner: UNT Libraries Government Documents Department

RESISTIVITY MODELING FOR ARBITRARILY SHAPED THREE-DIMENSIONAL STRUCTURES

Description: A numerical technique has been developed to solve the three-dimensional potential distribution about a point source of current located in or on the surface of a half-space containing an arbitrary three-dimensional conductivity distribution. Self-adjoint difference equations are obtained for Poisson's equation using finite-difference approximations in conjunction with an elemental volume discretization of the lower half-space. Potential distribution at all points in the set defining the subsurface are simultaneously solved for multiple point sources of current. Accurate and stable solutions are obtained using full, banded, Cholesky decomposition of the capacitance matrix as well as the recently developed Incomplete Cholesky-Conjugate Gradient Iterative method. A comparison of the two- and three-dimensional simple block-shaped models, for the collinear dipole-dipole array, indicates substantially lower anomaly indices for inhomogeneities of finite strike-extent. In general, the strike-extents of inhomogeneities have to be approximately 10 times the dipole lengths before the response becomes two-dimensional. The saturation effect with increasing conductivity contrasts appears sooner for the three-dimensional conductive inhomogeneities than for corresponding models with infinite strike lengths. A downhole-to-surface configuration of electrodes produces diagnostic total field apparent resistivity maps for three-dimensional buried inhomogeneities. Experiments with various lateral and depth locations of the current pole indicate that mise a la masse surveys give the largest anomaly if a current pole is located asymmetrically and preferably near the top-surface of the buried conductor.
Date: October 1, 1977
Creator: Dey, Abhijit & Morrison, H. Frank
Partner: UNT Libraries Government Documents Department

Effects of Ion-Ion Collisions and Inhomogeneity in Two-Dimensional Kinetic Ion Simulations of Stimulated Brillouin Backscattering

Description: Two-dimensional simulations with the BZOHAR [B.I. Cohen, B.F. Lasinski, A.B. Langdon, and E.A. Williams, Phys. Plasmas 4, 956 (1997)] hybrid code (kinetic particle ions and Boltzmann fluid electrons) have been used to investigate the saturation of stimulated Brillouin backscatter (SBBS) instability including the effects of ion-ion collisions and inhomogeneity. Ion-ion collisions tend to increase ion-wave dissipation, which decreases the gain exponent for stimulated Brillouin backscattering; and the peak Brillouin backscatter reflectivities tend to decrease with increasing collisionality in the simulations. Two types of Langevin-operator, ion-ion collision models were implemented in the simulations. In both models used the collisions are functions of the local ion temperature and density, but the collisions have no velocity dependence in the first model. In the second model, the collisions are also functions of the energy of the ion that is being scattered so as to represent a Fokker-Planck collision operator. Collisions decorrelate the ions from the acoustic waves in SBS, which disrupts ion trapping in the acoustic wave. Nevertheless, ion trapping leading to a hot ion tail and two-dimensional physics that allows the SBS ion waves to nonlinearly scatter remain robust saturation mechanisms for SBBS in a high-gain limit over a range of ion collisionality. SBS backscatter in the presence of a spatially nonuniform plasma flow is also investigated. Simulations show that depending on the sign of the spatial gradient of the flow relative to the backscatter, ion trapping effects that produce a nonlinear frequency shift can enhance (auto-resonance) or decrease (anti-auto-resonance) reflectivities in agreement with theoretical arguments.
Date: October 17, 2005
Creator: Cohen, B I; Divol, L; Langdon, A B & Williams, E A
Partner: UNT Libraries Government Documents Department

A Unified Model of Secondary Electron Cascades in Diamond

Description: In this paper we present a detailed and unified theoretical treatment of secondary electron cascades that follow the absorption of an X-ray photon. A Monte Carlo model has been constructed that treats in detail the evolution of electron cascades induced by photoelectrons and by Auger electrons following inner shell ionizations. Detailed calculations are presented for cascades initiated by electron energies between 0.1-10 keV. The present paper expands our earlier work by extending the primary energy range, by improving the treatment of secondary electrons, especially at low electron energies, by including ionization by holes, and by taking into account their coupling to the crystal lattice. The calculations describe the three-dimensional evolution of the electron cloud, and monitor the equivalent instantaneous temperature of the free-electron gas as the system cools. The dissipation of the impact energy proceeds predominantly through the production of secondary electrons whose energies are comparable to the binding energies of the valence (40-50 eV) and of the core electrons (300 eV). The electron cloud generated by a 10 keV electron is strongly anisotropic in the early phases of the cascade (t {le} 1 fs). At later times, the sample is dominated by low energy electrons, and these are scattered more isotropically by atoms in the sample. Our results for the total late time number of secondary electrons agree with available experimental data, and show that the emission of secondary electrons approaches saturation within about 100 fs, following the primary impact.
Date: October 13, 2004
Creator: Ziaja, B; London, R A & Hajdu, J
Partner: UNT Libraries Government Documents Department

Advances in Performance of Microchannel Plate Detectors for HEDP Diagnostics

Description: In recent years, a team from NSTec and SNL has built a unique capability to develop microchannel plate (MCP)?based framing x-ray cameras for HEDP diagnostics. At the SNL Z facility, multistrip MCP detectors to record up to eight channels are employed in 2-D, sub-nanosecond time-resolved imaging and time- and space-resolved spectroscopy diagnostics. Progressively more stringent technical temporal resolution and response uniformity requirements have necessitated a systematic design approach based on iterative modeling of the MCP using inputs from electrical circuit characterization. An inherently large exponential dependence in MCP gain, V{sup 11.5}, has mandated a firm understanding of the applied voltage pulse shape propagating across the strip. We pioneered direct measurements of the propagating waveform using a Picoprobe{reg_sign} and developed a Monte Carlo code to simulate MCP response to compare against test measurements. This scheme is shown in Figure 1. The simulation detailed a physical model of the cascade and amplification process of the MCP that includes energy conservation for the secondary electrons, the effects of elastic scattering of low-energy electrons from the channel wall, and gain saturation mechanisms from wall charging and space charge. Our model can simulate MCP response for both static and pulsed voltage waveforms. Using this design approach, we began to characterize the newly developed second-generation detector (H-CA-65) by using a Manson x-ray source to evaluate the following DC characteristics: MCP sensitivity as a function of bias voltage, flat-field uniformity and spatial resolution, and variation of spatial resolution and sensitivity as a function of phosphor bias voltage. Dynamic performance and temporal response were obtained by using an NSTec short-pulse laser to measure optical gate profiles, saturation, and dynamic range. These data were processed and combined to obtain the gain variation and gate profiles for any position along an MCP strip. Typical position-sensitive gate profiles of the detector ...
Date: October 1, 2009
Creator: Ming Wu, Craig Kruschwitz, Ken Moy, Greg Rochau
Partner: UNT Libraries Government Documents Department

Oxidation and Volatilization of TZM Alloy in Air

Description: The excellent high temperature strength and thermal conductivity of molybdenum-base alloys provide attractive features for components in advanced magnetic and inertial fusion devices. Refractory metal base alloys react readily with oxygen and other gases, and molybdenum alloys are susceptible to losses from highly volatile molybdenum trioxide (MoOsub3) species. Transport of radioactivity by the volatilization, migration, and re-deposition of MoO3 during a potential accident involving a loss of vacuum or inert environment represents a safety issue. We have experimentally measured the oxidation, volatilization and re-deposition of molybdenum from TZM in flowing air between 400 and 800°C. Calculations using chemical thermodynamic data for vapor pressures over pure MoOsub3 and a vaporization mass transfer model correlate well with experimental data between 600 and 800°C. Partial saturation of (MoOsub3) gas species account for influences of flow rate at 700°C. Some anomalies in oxidation rate below 650°C, suggesting that other phases, e.g., MoOsub2 or other non-stoichiometric oxides may influence oxidation and volatilization processes under some limited conditions.
Date: October 1, 1999
Creator: Smolik, Galen Richard; Petti, David Andrew & Schuetz, Stanley Thomas
Partner: UNT Libraries Government Documents Department

Modeling Interactions of Surface-Subsurface Flow Using a Free-Surface Overland Flow Boundary Condition in a Parallel Flow Simulator

Description: Models incorporating interactions between surface and subsurface flow are commonly based on the conductance concept that presumes a distinct interface at the land surface, separating the surface from the subsurface domain. In these models the subsurface and surface domains are linked via an exchange flux that depends upon the magnitude and direction of the hydraulic gradient across the interface and a proportionality constant (a measure of the hydraulic connectivity). Because experimental evidence of such a distinct interface is often lacking in the field, a more general coupled modeling approach would be preferable. We present a more general approach that incorporates a two-dimensional overland flow simulator into the parallel three-dimensional variably saturated subsurface flow code ParFlow developed at LLNL. This overland flow simulator takes the form of an upper, free-surface boundary condition and is, thus, fully integrated without relying on the conductance concept. Another advantage of this approach is the efficient parallelism of ParFlow, which is exploited by the overland flow simulator. Several verification and simulation examples are presented that focus on the two main processes of runoff production: excess infiltration and saturation. The usefulness of our approach is demonstrated in an application of the model to an urban watershed. The influence of heterogeneity of the shallow subsurface on overland flow and transport is also examined. The results show the uncertainty in flow and transport predictions due to heterogeneity. This is important in determining, for example, total maximum daily loads of surface water systems.
Date: October 25, 2005
Creator: Kollet, S J & Maxwell, R M
Partner: UNT Libraries Government Documents Department

LCLS Undulator Commissioning, Alignment, and Performance

Description: The LCLS x-ray FEL has recently achieved its 1.5-Angstrom lasing and saturation goals upon first trial. This was achieved as a result of a thorough pre-beam checkout, both traditional and beam-based component alignment techniques, and high electron beam brightness. The x-ray FEL process demands very tight tolerances on the straightness of the electron beam trajectory (<5 {micro}m) through the LCLS undulator system. Tight, but less stringent tolerances of {approx}100 {micro}m rms were met for the transverse placement of the individual undulator segments with respect to the beam axis. The tolerances for electron beam straightness can only be met through a beam-based alignment (BBA) method, which is implemented using large electron energy variations and sub-micron resolution cavity beam position monitors (BPM), with precise conventional alignment used to set the starting conditions. Precision-fiducialization of components mounted on remotely adjustable girders, and special beam-finder wires (BFW) at each girder have been used to meet these challenging alignment tolerances. Longer-term girder movement due to ground motion and temperature changes are being monitored, continuously, by a unique stretched wire and hydrostatic level Alignment Diagnostics System (ADS).
Date: October 30, 2009
Creator: Nuhn, Heinz-Dieter
Partner: UNT Libraries Government Documents Department

Monte Carlo Simulations of High-speed, Time-gated MCP-based X-ray Detectors: Saturation Effects in DC and Pulsed Modes and Detector Dynamic Range

Description: We present here results of continued efforts to understand the performance of microchannel plate (MCP)–based, high-speed, gated, x-ray detectors. This work involves the continued improvement of a Monte Carlo simulation code to describe MCP performance coupled with experimental efforts to better characterize such detectors. Our goal is a quantitative description of MCP saturation behavior in both static and pulsed modes. We have developed a new model of charge buildup on the walls of the MCP channels and measured its effect on MCP gain. The results are compared to experimental data obtained with a short-pulse, high-intensity ultraviolet laser; these results clearly demonstrate MCP saturation behavior in both DC and pulsed modes. The simulations compare favorably to the experimental results. The dynamic range of the detectors in pulsed operation is of particular interest when fielding an MCP–based camera. By adjusting the laser flux we study the linear range of the camera. These results, too, are compared to our simulations.
Date: October 31, 2008
Creator: Craig Kruschwitz, Ming Wu, Ken Moy, Greg Rochau
Partner: UNT Libraries Government Documents Department

Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

Description: The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10{sup 7}/sec to 10{sup 10}/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.
Date: October 7, 2003
Creator: Seppala, E T; Belak, J & Rudd, R E
Partner: UNT Libraries Government Documents Department

A Picosecond 14.7 nm X-Ray Laser for Probing Matter Undergoing Rapid Changes

Description: With laser-driven tabletop x-ray lasers now operating in the efficient saturation regime, the source characteristics of high photon flux, high monochromaticity, picosecond pulse duration, and coherence are well-matched to many applications involving the probing of matter undergoing rapid changes. We give an overview of recent experiments at the Lawrence Livermore National Laboratory (LLNL) Compact Multipulse Terawatt (COMET) laser using the picosecond 14.7 nm x-ray laser as a compact, ultrafast probe for surface analysis and for interferometry of laser-produced plasmas. The plasma density measurements for known laser conditions allow us to reliably and precisely benchmark hydrodynamics codes. In the former case, the x-ray laser ejects photo-electrons, from the valence band or shallow core-levels of the material, and are measured in a time-of-flight analyzer. Therefore, the electronic structure can be studied directly to determine the physical properties of materials undergoing rapid phase changes.
Date: October 7, 2002
Creator: Dunn, J; Smith, R F; Nilsen, J; Nelson, A J; Van Buuren, T W; Moon, S J et al.
Partner: UNT Libraries Government Documents Department