David K. Zerkle

Absolute copper atom density determination in laser-ablated copper plasmas using hook spectroscopy

Hook spectroscopy has been implemented to measure the absolute density of atomic species in a laser‐ablated plasma plume. We use the hook method to determine the density of copper atoms produced by ablation of a copper target in 25 Torr of helium at power densities of ∼1 GW/cm2. The measured copper densities when ablating into 25 Torr of helium backing gas range from 1.5×1015 cm−3 at a delay of 200 μs to 4.4×1013 cm−3 at 5 ms delay. This dramatic decrease in density is due to condensation of the metal vapor to form fine particulate.

Coupled phase transformation, chemical decomposition, and deformation in plastic-bonded explosive: Models

A continuum thermomechanochemical model of the behavior of a plastic-bonded explosive PBX 9501 formulation consisting of the energetic crystal octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine HMX embedded in a polymeric binder is developed. Our main focus is on the study of the ↔ phase transformations PTs in crystalline HMX under a complex pressure-temperature path. To reproduce the pressure-temperature path, in particular during heating of PBX inside of a rigid cylinder, the ↔ PTs in HMX are coupled to chemical decomposition of the HMX and binder leading to gas formation, gas leaking from the cylinder, elastic, thermal, and transformational straining as well as straining due to mass loss. A fully physically based thermodynamic and kinetic model of the ↔ PT in HMX crystal is developed. It is based on a suggested nucleation mechanism via melt mediated nanocluster transformation and the recently revealed growth mechanism via internal stress-induced virtual melting. During the nucleation, nanosize clusters of the phase dissolve in a molten binder and transform diffusionally into phase clusters. During the interface propagation, internal stresses induced by transformation strain cause the melting of the stressed phase much below 120 K the melting temperature and its immediate resolidification into the unstressed phase. These mechanisms explain numerous puzzles of HMX polymorphism and result in overall transformation kinetics that is in good agreement with experiments. Simple phenomenological equations for kinetics of chemical decomposition of the HMX and the binder are in good correspondence with experiments as well. A continuum deformation model is developed in two steps. The geometrically linear small strain theory is used to prove that the internal stresses and macroscopic shear stresses are negligible. Then a large strain theory is developed under hydrostatic loading. The developed continuum thermomechanochemical model is applied in the accompanying paper V. I. Levitas, B. F. Henson, L. B. Smilowitz, D. K. Zerkle, and B. W. Asay, J. Appl. Phys. submitted to modeling the heating of PBX inside of a rigid cylinder.

Time-resolved PLIF imaging of Cu in a laser-ablated copper plasma plume

Planar laser-induced fluorescence (PLIF) has been used to determine the relative number density of ground state copper atoms in laser-ablated plasma plumes. An ablation laser power flux of /spl sim/1.5 GW/cm/sup 2/ is applied to a solid copper target in a background gas, producing a plasma plume suitable for studying homogeneous copper vapor condensation. Density is measured at postablation time delays ranging from 5 /spl mu/s to 10 ms with 1-100 torr of either argon or helium as the background gas. Planar laser-induced fluorescence images are used to spatially resolve the relative density within the plume, The decrease in density is due to the homogeneous condensation of copper vapor to form particulate.

Molten Composition B Viscosity at Elevated Temperature

A shear-thinning viscosity model is developed for molten Composition B at elevated temperature from analysis of falling ball viscometer data. Results are reported with the system held at 85, 110, and 135°C. Balls of densities of 2.7, 8.0, and 15.6 g/cm3 are dropped to generate a range of strain rates in the material. Analysis of video recordings gives the speed at which the balls fall. Computer simulation of the viscometer is used to determine parameters for a non-Newtonian model calibrated to measured speeds. For the first time, viscosity is shown to be a function of temperature and strain rate–dependent maximum RDX (cyclotrimethylenetrinitramine) particle volume fraction.

SAFE Testing Nuclear Rockets Economically

Several studies over the past few decades have recognized the need for advanced propulsion to explore the solar system. As early as the 1960s, Werner Von Braun and others recognized the need for a nuclear rocket for sending humans to Mars. The great distances, the intense radiation levels, and the physiological response to zero‐gravity all supported the concept of using a nuclear rocket to decrease mission time. These same needs have been recognized in later studies, especially in the Space Exploration Initiative in 1989. One of the key questions that has arisen in later studies, however, is the ability to test a nuclear rocket engine in the current societal environment. Unlike the Rover/NERVA programs in the 1960s, the rocket exhaust can no longer be vented to the open atmosphere. As a consequence, previous studies have examined the feasibility of building a large‐scale version of the Nuclear Furnace Scrubber that was demonstrated in 1971. We have investigated an alternative that would deposit the rocket exhaust along with any entrained fission products directly into the ground. The Subsurface Active Filtering of Exhaust, or SAFE, concept would allow variable sized engines to be tested for long times at a modest expense. A system overview, results of preliminary calculations, and cost estimates of proof of concept demonstrations are presented. The results indicate that a nuclear rocket could be tested at the Nevada Test Site for under

Modeling of On-Line Catalyst Addition Effects in a Short Contact Time Reactor /

20 M.

Non-local thermodynamic equilibrium in laser sustained plasmas

Recently developed short-contact-time reactors (SCTR), consisting of porous alumina monoliths coated with platinum, have been shown to produce ethylene from rich ethane/oxygen(hydrogen) mixtures with yields and selectivities comparable to conventional steam cracking, using a reactor of much smaller size. Although the overall mechanism is clearly autothermal and catalytic, the details, in particular the relative contributions of heterogeneous and homogeneous chemistry, are a matter of considerable debate. Recent experiments show that reactor performance can be further enhanced by dripping a dilute platinum solution onto the SCTR front face during reaction, resulting in catalyst deposition within only a short (several millimeter) zone of the reactor. The authors have undertaken a computational study of this system, using two-dimensional computational fluid dynamics simulations with full heat and mass transport and detailed heterogeneous and homogeneous kinetic mechanisms. The results indicate that front-face catalyst loading enhances reactor performance by limiting the opportunity for heterogeneous ethane reactions that produce methane. As a result, ethylene selectivity increases and CH{sub 4} selectivity decreases. The results strongly support a mechanism recently proposed by the authors, in which rapid, heterogeneous oxidation of adsorbed hydrogen consumes most of the oxygen. The resulting heat is then released to the gas phase, causing homogeneous pyrolysis of ethane to occur in an environment containing much less oxygen. This mechanism explains not only the effects of on-line catalyst addition, but also the increase in ethylene selectivity observed upon addition of hydrogen to the reactant mixture.

Integrated rheology model: Explosive Composition B-3

An argon laser sustained plasma (LSP) at atmospheric pressure has been studied spectroscopically, and the existence of a nonlocal thermodynamic equilibrium state has been determined. The spectroscopic data consist of argon neutral and ion line emissions used to spatially resolve electronic energy level population densities in each plasma species. A hydrogen seed is added to the argon flow for the purpose of determining the electron number density by Stark broadening analysis of the Balmer series alpha line. Electron and heavy particle kinetic temperatures are calculated through the use of an appropriate nonequilibrium model. The dominant nonequilibrium effect in this plasma is kinetic nonequilibrium where the electron kinetic temperature can be more than twice the heavy particle kinetic temperature in high laser power flux regions. Typical electron and heavy particle kinetic temperatures are 14,000 K and 8000 K, respectively. Electron number density ranges from 6 x 10 exp 16/cu cm to 2.1 x 10 exp 17/cu cm. 31 refs.

Relating confidence to measured information uncertainty in qualitative reasoning

ABSTRACT Composition B-3 (Comp B-3) is a high explosive formulation composed of 60/40wt% RDX (1,3,5-trinitroperhydro-1,3,5-triazine) /TNT (2,4,6 trinitrotoluene). Above approximately 78°C this formulation partially melts to form a multiphase system with solid RDX particles in a molten TNT matrix. This multiphase system presents a number of phenomena that influence its apparent viscosity. In an earlier study explosive Composition B-3 (Comp B-3, 60/40wt% RDX/TNT) was examined for evidence of yield stress using a non-isothermal falling ball viscometer and a yield stress model was proposed. An integrated viscosity model suitable for use in computational fluid dynamics (CFD) simulations is developed to capture the transition from a heterogeneous solid to a Bingham viscoplastic fluid. This viscosity model is used to simulate the motion of imbedded spheres falling through molten Comp B-3. Comparison of the simulations to physical tests show agreement between the positions predicted by the model and the measured locations of the spheres as a function of temperature between 90C and 165C.

Thermal Decomposition Models for High Explosive Compositions

Qualitative reasoning makes use of qualitative assessments provided by subject matter experts to model factors such as security risk. Confidence in a result is important and useful when comparing competing results. Quantifying the confidence in an evidential reasoning result must be consistent and based on the available information. A novel method is proposed to relate confidence to the available information uncertainty in the result using fuzzy sets. Information uncertainty can be quantified through measures of non-specificity and conflict. Fuzzy values for confidence are established from information uncertainty values that lie between the measured minimum and maximum information uncertainty values.