Publications

Optical filters based on resonant gratings have spectral characteristics that are lithographically defined. Nanoimprint lithography is a relatively new method for producing large area gratings with sub-micron features. Computational modeling using rigorous coupled-wave analysis allows gratings to be designed to yield sharp reflectance maxima and minima. Combining these gratings with microfluidic channels and micromechanical actuators produced using micro electromechanical systems (MEMS) technology forms the basis for producing tunable filters and other wavelength selective elements. These devices achieve tunable optical characteristics by varying the index of refraction on the surface of the grating. Coating the grating surface with water creates a 33% change in the resonant wavelength whereas bringing a grating into contact with a quartz surface shifts the resonant wavelength from 558 nm to 879 nm, a fractional change of 58%. The reflectivity at a single wavelength can be varied by approximately a factor of three. Future applications of these devices may include tunable filters or optical modulators. © 2008 Society of Photo-Optical Instrumentation Engineers.

We describe a method of performing trilinear analysis on large data sets using a modification of the PARAFAC-ALS algorithm. Our method iteratively decomposes the data matrix into a core matrix and three loading matrices based on the Tuckerl model. The algorithm is particularly useful for data sets that are too large to upload into a computer's main memory. While the performance advantage in utilizing our algorithm is dependent on the number of data elements and dimensions of the data array, we have seen a significant performance improvement over operating PARAFAC-ALS on the full data set. In one case of data comprising hyperspectral images from a confocal microscope, our method of analysis was approximately 60 times faster than operating on the full data set, while obtaining essentially equivalent results. Published in 2008 by John Wiley & Sons, Ltd.

Chemical solution deposition has been used to fabricate continuous ultrathin lead lanthanum zirconate titanate (PLZT) films as thin as 20 nm. Further, multilayer capacitor structures with as many as 10 dielectric layers have been fabricated from these ultrathin PLZT films by alternating spin-coated dielectric layers with sputtered platinum electrodes. Integrating a photolithographically defined wet etch step to the fabrication process enabled the production of functional multilayer stacks with capacitance values exceeding 600 nF. Such ultrathin multilayer capacitors offer tremendous advantages for further miniaturization of integrated passive components.

The author will describe two-photon-resonant LIF detection of CO, O, and H. Application of these techniques in flames frequently suffers from significant photolytic interferences caused by the intense UV excitation pulses required to produce measurable signal. When compared to nanosecond excitation, the use of short pulse (picosecond) excitation can significantly reduce the effect of the photolytic interference. Results of recent atomic oxygen imaging experiments using picosecond- and nanosecond-duration laser pulses will be presented, and potential improvements to CO and H imaging will be discussed.

A Dish Stirling parabolic concentrator typically consists of a number of mirror facets that must be aligned to focus the concentrated sunlight on the engine receiver. An alignment strategy must be developed to deliver the energy uniformly to the receiver while maximizing system performance. Several criteria must be met in order to maximize the performance and lifetime of the system. The peak flux should be minimized at the receiver to extend life. This is accomplished by locally optimizing the mirror aimpoints, minimizing overlap of facet images. The energy delivered to each cylinder of a multi-cylinder engine should be balanced to maximize the power production capability of the engine. This is accomplished through globally optimizing the mirror aimpoints. Depending on dish geometry, both of these constraints will be met by moving the aimpoints of certain facets away from a single point at the center of the aperture. However, this often results in a larger aperture or more flux spillage. The larger aperture results in greater thermal and reflective losses from the receiver cavity. This paper proposes and demonstrates a novel approach to optimizing the alignment strategy while obeying these constraints. The method uses an approach similar to molecular dynamics to globally and locally distribute the power on the receiver, while imposing movement constraints at the aperture to limit the focal plane spot size. The method can also impose additional geometric constraints at the receiver plane to accommodate un-cooled surfaces. The method is explored and demonstrated on the Stirling Energy Systems 25kW dish Stirling system at Sandia National Laboratories. The approach provides a receiver flux distribution and power balance equal to the strategy developed by McDonnell Douglas in the early 1980's, but with an aperture size equal to that of the single aimpoint strategy. This should result in about a 1kW increase in power generated at rated conditions, with no additional cost, due to reduced thermal losses from the receiver. The method can be extended to other point-focus concentrating solar technologies. On a tower, the heliostat aiming strategy could be dynamically updated to accommodate flux profile needs, sun position, or maintenance in the field. Copyright © 2007 by US Government.

Within this paper, we provide a solution to the static frame validation challenge problem (see this issue) in a manner that is consistent with the guidelines provided by the Validation Challenge Workshop tasking document. The static frame problem is constructed such that variability in material properties is known to be the only source of uncertainty in the system description, but there is ignorance on the type of model that best describes this variability. Hence both types of uncertainty, aleatoric and epistemic, are present and must be addressed. Our approach is to consider a collection of competing probabilistic models for the material properties, and calibrate these models to the information provided; models of different levels of complexity and numerical efficiency are included in the analysis. A Bayesian formulation is used to select the optimal model from the collection, which is then used for the regulatory assessment. Lastly, bayesian credible intervals are used to provide a measure of confidence to our regulatory assessment.

This paper recalls the events leading up to the author's 1973 discovery of Deep Level Transient Spectroscopy (DLTS). It discusses the status of junction capacitance techniques in the late 1960s and points out why the typical capacitance instrumentation of that era would not have lead the author to the DLTS discovery. This discovery is discussed in the context of the novel NMR-inspired instrumentation used by the author to study fast capacitance transients of the ZnO center in GaP LEDs. Finally, the author makes some general comments about the innovation process. © 2007 Elsevier B.V. All rights reserved.

This work explores the feasibility of mechanistically modeling the transient behavior of defects and carriers in bipolar Si devices exposed to pulses of MeV neutrons. Our approach entails a detailed, finite-element treatment of the diffusion, field-drift, and reactions of well-established primal defects and reacted states, taking into account the localization of displacement damage within secondary cascades. The modeling captures a variety of the properties of pulse-neutron-irradiated transistors observed from electrical measurements and deep-level transient spectroscopy, using parameter values consistent with independently available information. © 2007 Elsevier B.V. All rights reserved.

A standard approach to cross-language information retrieval (CLIR) uses Latent Semantic Analysis (LSA) in conjunction with a multilingual parallel aligned corpus. This approach has been shown to be successful in identifying similar documents across languages - or more precisely, retrieving the most similar document in one language to a query in another language. However, the approach has severe drawbacks when applied to a related task, that of clustering documents 'language independently', so that documents about similar topics end up closest to one another in the semantic space regardless of their language. The problem is that documents are generally more similar to other documents in the same language than they are to documents in a different language, but on the same topic. As a result, when using multilingual LSA, documents will in practice cluster by language, not by topic. We propose a novel application of PARAFAC2 (which is a variant of PARAFAC, a multi-way generalization of the singular value decomposition [SVD]) to overcome this problem. Instead of forming a single multilingual term-by-document matrix which, under LSA, is subjected to SVD, we form an irregular three-way array, each slice of which is a separate term-by-document matrix for a single language in the parallel corpus. The goal is to compute an SVD for each language such that V (the matrix of right singular vectors) is the same across all languages. Effectively, PARAFAC2 imposes the constraint, not present in standard LSA, that the 'concepts' in all documents in the parallel corpus are the same regardless of language. Intuitively, this constraint makes sense, since the whole purpose of using a parallel corpus is that exactly the same concepts are expressed in the translations. We tested this approach by comparing the performance of PARAFAC2 with standard LSA in solving a particular CLIR problem. From our results, we conclude that PARAFAC2 offers a very promising alternative to LSA not only for multilingual document clustering, but also for solving other problems in crosslanguage information retrieval. © 2007 ACM.

The rotational dynamics of chemically similar systems based on freely jointed and freely rotating chains are studied. The second Legendre polynomial of vectors along chain backbones is used to investigate the rotational dynamics at different length scales. In a previous study, it was demonstrated that the additional bond-angle constraint in the freely rotating case noticeably perturbs the character of the translational relaxation away from that of the freely jointed system. Here, it is shown that differences are also apparent in the two systems' rotational dynamics. The relaxation of the end-to-end vector is found to display a long time, single-exponential tail and a stretched exponential region at intermediate times. The stretching exponents Β are found to be 0.75±0.02 for the freely jointed case and 0.68±0.02 for the freely rotating case. For both system types, time-packing-fraction superposition is seen to hold on the end-to-end length scale. In addition, for both systems, the rotational relaxation times are shown to be proportional to the translational relaxation times, demonstrating that the Debye-Stokes-Einstein law holds. The second Legendre polynomial of the bond vector is used to probe relaxation behavior at short length scales. For the freely rotating case, the end-to-end relaxation times scale differently than the bond relaxation times, implying that the behavior is non-Stokes-Einstein, and that time-packing-fraction superposition does not hold across length scales for this system. For the freely jointed case, end-to-endrelaxation times do scale with bond relaxation times, and both Stokes-Einstein and time-packing-fraction-across-length-scales superposition are obeyed. © 2007 American Institute of Physics.

We measure frictional properties of liquid-expanded and liquid-condensed phases of lipid Langmuir-Blodgett monolayers by interfacial force microscopy. We find that over a reasonably broad surface-density range, the friction shear strength of the lipid monolayer film is proportional to the surface area (42-74 Â2/ molecule) occupied by each molecule. The increase in frictional force (i.e., friction shear strength with molecular area can be attributed to the increased conformational freedom and the resulting increase in the number of available modes for energy dissipation. © 2007 American Chemical Society.

In this work, we present two methods for solving overdetermined systems of the Time of Arrival (TOA) geolocation equations that achieve the minimum possible variance in all cases, not just when the satellites are at large equal radii. One of these techniques gives two solutions, and the other gives four solutions.

A model for the simulation of the electron energy distribution in nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET) devices, using a kinetic simulation technique, is implemented. The convective scheme (CS), a method of characteristics, is an accurate method of solving the Boltzmann transport equation, a nonlinear integrodifferential equation, for the distribution of electrons in a MOSFET device. The method is used to find probabilities for use in an iterative scheme which iterates to find collision rates in cells. The CS is also a novel approach to 2D semiconductor device simulation. The CS has been extended to handle boundary conditions in 2D as well as to calculation of polygon overlap for polygons of more than three sides. Electron energy distributions in the channel of a MOSFET are presented. © 2007 Elsevier Inc. All rights reserved.

The immersed- Bz diode is being developed as a high-brightness, flash x-ray radiography source at Sandia National Laboratories. This diode is a foil-less electron-beam diode with a long, thin, needlelike cathode that is inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces forward-directed bremsstrahlung. In addition, electron beam heating of the anode produces surface plasmas allowing ion emission. Two different operating regimes for this diode have been identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous operating regime characterized by a dramatic impedance collapse where the total diode current greatly exceeds the bipolar limit. Data from a comprehensive series of experiments fielded at 4 and 5 MV, where the diode operates in the nominal or stable impedance regime, with beam currents ranging from 20-40 kA on target are presented. In this mode, both the measured diode current and experimental radiation production are consistent with physics based models including two-dimensional particle-in-cell simulations. The analysis indicates that intermediate mass ions (e.g., 12-18 amu) control the nominal impedance evolution rather than expected lighter mass ions such as hydrogen. © 2007 American Institute of Physics.

As part of the US DOE Nuclear Hydrogen Initiative, Sandia National Laboratories is designing and constructing a process for the conversion of sulfuric acid to produce sulfur dioxide. This process is part of the thermochemical Sulfur-Iodine (S-I) cycle that produces hydrogen from water. The Sandia process will be integrated with other sections of the S-I cycle in the near future to complete a demonstration-scale S-I process. In the Sandia process, sulfuric acid is concentrated by vacuum distillation and then catalytically decomposed at high temperature (850°C) to produce sulfur dioxide, oxygen and water. Major problems in the process, corrosion, and failure of high-temperature connections of process equipment, have been eliminated through the development of an integrated acid decomposer constructed of silicon carbide. The unit integrates acid boiling, superheating and decomposition into a single unit operation and provides for exceptional heat recuperation. The design of acid decomposition process, the new acid decomposer, other process units, and materials of construction for the process are described and discussed.

A preliminary study was conducted which considered capturing carbon dioxide from fossil-fired power plants and combining it with nuclear hydrogen in order to produce alternative liquid fuels for transportation. Among the alternative liquid hydrocarbons which can be used as fuel in internal combustion engines, the two that are most promising are methanol and ethanol. We choose these two because they are relatively simple compounds and can be used with only minor changes to the fuel systems of most automobiles today. In fact, there are some vehicles today which can operate with any combination of conventional gasoline, ethanol, or methanol. We estimated the quantity of carbon dioxide that would be emitted by fossil-fired power plants in the future. We then use this information to determine how much ethanol or methanol can be created if enough hydrogen is made available. Using the quantity of hydrogen required and the thermodynamics of the reactions involved, we estimate the nuclear power that would be needed to produce the liquid fuel. This amount of liquid fuel is then used to estimate the effect of such a program on conventional gasoline usage, need for foreign oil, and decrease in CO 2 emissions.

Study of the Triggered Plasma Opening Switch (TPOS) characteristics is in progress via an ion current collection diagnostic (ICCD), in addition to offline apparatus. This initial ion current collection diagnostic has been designed, fabricated, and tested on the TPOS in order to explore the opening profile of the main switch. The initial ion current collection device utilizes five collectors which are positioned perpendicularly to the main switch stage in order to collect radially traveling ions. It has been shown through analytical prowess that this specific geometry can be treated as a planar case of the Child-Langmuir law with only a 6% deviation from the cylindrical case. Additionally, magnetostatic simulations with self consistent space charge emitting surfaces of the main switch using the Trak code are under way. It is hoped that the simulations will provide evidence in support of both the analytical derivations and experimental data. Finally, an improved design of the ICCD (containing 12 collectors in the axial direction) is presently being implemented. © 2005 IEEE.

The BROOM system was developed to collect, manage and analyze information from bioterrorist attacks on strategic buildings. GIS features help decision-makers and analysts rapidly assess the current status of contaminated facilities and develop optimized cleanup strategies. BROOM consists of networked server, desktop and PDA components. PDAs are deployed to collect samples of suspected bioagents, such as anthrax. Novel geostatistical methods are used to generate contaminant maps and define optimum locations for subsequent sampling. Efficiency and accuracy gains witnessed in field tests show that GIS technology can play a vital role in visualizing, managing and analyzing data from bioterrorism incidents. © 2007 The Haworth Press, Inc. All rights reserved.

The ZR accelerator is a refurbishment of Sandia National Laboratories Z accelerator [1]. The ZR accelerator components were designed using electrostatic and circuit modeling tools. Transient electromagnetic modeling has played a complementary role in the analysis of ZR components [2]. In this paper we describe a 3D transient electromagnetic analysis of the ZR water convolute and stack using edge-based finite element techniques. © 2005 IEEE.

The Z driver at Sandia National Laboratories delivers one to two megajoules of electromagnetic energy inside its ∼10 cm radius final feed in 100 ns. The high current (∼20 MA) at small diameter produces magnetic pressures well above yield strengths for metals. The metal conductors stay in place due to inertia long enough to deliver current to the load. Within milliseconds however, fragments of metal escape the load region at high velocity. Much of the hardware and diagnostics inside the vacuum chamber is protected from this debris by blast shields with small view ports, and fast-closing valves. The water-vacuum insulator requires different protection because the transmission line debris shield should not significantly raise the inductance or perturb the self-magnetically insulated electron flow. This report shows calculations and results from a design intended to protect the insulator assembly. © 2005 IEEE.

The Triggered Plasma Opening Switch (TPOS) at SNL is a unique device that exploits the high conductivity and low mass properties of plasma. The TPOS's objective is to take the initial ∼0.8MA (∼250ns rise time) storage inductor current and deliver ∼0.5MA at ∼2.4MV (∼10ns rise time) to a load of ∼5-10Ω. Configuration advantages include low current jitter and resistive voltage drop, power gain, and minimization of trigger input power as the result of using two stages in series. This two-stage design is novel and is the first to demonstrate operation of magnetically triggered stages. Study of TPOS characteristics is in progress via an offline interferometer diagnostic; specifically, a laser interferometer will be used to make density measurements of the source plasma. It is thought that the gross plasma source density is ∼1014 cm-3, but details of the spatial structure and temporal evolution have not previously been studied. In order to better understand switch operation, these details are essential. Presently two interferometer systems are planned for testing: a temporary 1 μm system for initial plasma characterization, and a 10.6 mu;m laser system for routine use. We will start with a single chord measurement then upgrade to a multi-chord system. Future plans involve varying plasma source parameters, such as magnetic field strength and plasma fill time, in order to understand the density dependence on these parameters. Improved knowledge of the plasma source density behavior should allow for improved switch operation. © 2005 IEEE.

Although there is much written in regards to voltage breakdown of polymeric insulators under AC and DC conditions, much less is written involving Rexolite® (1422) [1], non-uniform field geometries, and impulse conditions. Yet, in order to design optimized pulsed power systems with some desired degree of reliability, understanding the behavior of this type of insulating system is needed. Specifically, Sandia National Laboratory's ZR project, which will use anode plugs in the vacuum stack (thus increasing the electrical stress in the Rexolite insulators), needs to be able to estimate the reliability of these vacuum stack insulators [2]. In an effort to estimate the insulator's lifetime small scale testing is in progress. Nine samples have been tested so far and at least ten more will be tested. Results from the current testing suggest that the Rexolite "ages" from pulse to pulse, that there is some volume dependence on breakdown strength, and that the electrode-vacuum-insulator interface has an affect on the insulator lifetime. ©2005 IEEE.

The Refurbished Z machine, ZR, has 3 distinct missions. The first is to increase the capability for the user community by providing higher peak currents. The second is to increase the precision by increasing pulse repeatability and pulse shape flexibility. The third is to increase the capacity by providing operational turnaround time consistent with conducting a shot per shin. The pursuit of the third mission relies heavily on the reliability of the components and well defined maintenance cycles. To test the performance of the ZR design, a system assessment module has been tested and found to meet the capability mission. The system assessment module will be used to test the reliability of the components comprising the ZR pulsed power modules. To assess the program goal of 1 failure in 50 ZR shots we plan to perform 7200 shots of the system assessment module, Z20. At a typical shot rate, this task would take approximately three years. To minimize the impact on the facility while obtaining the required reliability data, the system will be configured to fire and reset autonomously with the ultimate goal of unmanned operation. A systems approach was developed using National Instruments LabVIEW ® software and Field Point I/O hardware. All communications between subsystems are provided via ethernet using fiber optic media converters. The major subsystems for operating the module which will be described are the gas pressure and purge, high voltage power supplies, oil diverter operation, triggering, precision monitoring of Marx system, personnel access control and remote operation of the laser trigger system. © 2005 IEEE.

During the last few years Sandia is actively pursuing the development of new accelerators based on the novel technology of Linear Transformer Driver (LTD). This effort is done in close collaboration with the High Current Electronic Institute (HCEI) in Tomsk, Russia, where the LTD idea was first conceived and developed. LTD based drivers are currently considered for many applications including future very high current Z-pinch drivers like ZX and IFE (Inertial Fusion Energy), medium current drivers with adjustable pulse length for ICE (Isentropic Compression Experiments), and finally relatively lower current accelerators for radiography and x-pinch. Currently we have in operation the following devices: One 500-kA, 100-kV LTD cavity, a 1-MVvoltage adder composed of seven smaller LTD cavities for radiography, and one 1-MA, 100-kV cavity. The first two are in Sandia while the latter one is still in Tomsk. In addition a number of stackable 1-MA cavities are under construction to be utilized as building blocks for a 1-MA, 1-MV voltage adder module. This module will serve as a prototype for longer, higher voltage modules, a number of which, connected in parallel, could become the driver of an IFE fusion reactor or a high current Z-pinch driver (ZX). The IFE requirements are more demanding since the driver must operate in rep-rated mode with a frequency of 0.1 Hz. In this paper we mainly concentrate on the higher current LTDs: We briefly outline the principles of operation and architecture and present a first cut design of an IFE, LTD z-pinch driver. © 2005 IEEE.

The ZR refurbishment project [1] at Sandia National Laboratories (SNL) required a set of diverter switches to protect the Marx generators and intermediate storage (IS) capacitors from Marx pre-fire and/or laser triggered output switch (LTS) no-fire. Thirty-six such diverters, one for each Marx-IS set, will need to operate reliably over the full range of Marx charge voltages and LTS anticipated closure times. Operating voltage is up to 6 MV. A self-closing oil switch diverter was selected and design work began in late 2002. The first diverter (Phase I or just P1) was delivered in the summer of 2003 and tested on SNL's Z20 test-bed. Based on test results, operational experience and overall project budgetary concerns, it was decided to re-design the diverter, resulting a simpler, less costly switch. This new self-closing oil switch (Phase II or P2) was fielded at SNL on the Z20 test-bed in late 2004. Both designs include adjustable electrodes to control the closure time. Also incorporated is a mechanical clamp that minimizes or shorts the oil gap until Marx charge is complete. Both diverters feature liquid resistors sized to safely absorb the energy stored in the Marx or IS. This paper describes the design and test results from these diverters. © 2005 IEEE.