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High flux isotope reactor redesigned beryllium reflector thermal stress calculations

Description: The Beryllium reflector of the High Flux Isotope Reactor is currently redesigned for upgrading the capability of the reactor. The original design criteria are adopted in the redesign analysis. Both nuclear heating and thermal stress calculations are revised. The results show that more margin of safety have been achieved and the updated design assures more precise design estimates for the reflector thermal stress conditions. 1 ref., 2 figs., 2 tabs.
Date: June 1, 1996
Creator: Chang, S.J.
Partner: UNT Libraries Government Documents Department

Local field and quantum effects for current perpendicular to planes in multilayers

Description: The calculation of giant-magnetoresistance and in general, of electron transport for multilayers in the case of current perpendicular to the planes (CPP) requires both the two-point conductivity and the solution to the local field problem. In this paper we present a solution to the local field problem at an interface using two approaches. In the first approach we find the semiclassical solution for the local field when there is a band mismatch between two sides of an interface, and examine the deviation of the total resistance from the result of ``self-averaging``, in the lowest order of the value of the potential step. In the second approach, we solve for the quantum correction to the local field through a numerical iterative scheme. The oscillations due to the quantum correction are surprisingly large, but their correction to the total resistance is remarkably small. Our results imply that the ``self-averaging`` of the resistance, which is usually assumed in analysis of CPP, is only approximate. 8 refs., 2 figs., 2 tabs.
Date: December 31, 1996
Creator: Zhang, X.G. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Comparison of elastic-viscous-plastic and viscous-plastic dynamics models using a high resolution Arctic sea ice model

Description: A nonlinear viscous-plastic (VP) rheology proposed by Hibler (1979) has been demonstrated to be the most suitable of the rheologies commonly used for modeling sea ice dynamics. However, the presence of a huge range of effective viscosities hinders numerical implementations of this model, particularly on high resolution grids or when the ice model is coupled to an ocean or atmosphere model. Hunke and Dukowicz (1997) have modified the VP model by including elastic waves as a numerical regularization in the case of zero strain rate. This modification (EVP) allows an efficient, fully explicit discretization that adapts well to parallel architectures. The authors present a comparison of EVP and VP dynamics model results from two 5-year simulations of Arctic sea ice, obtained with a high resolution sea ice model. The purpose of the comparison is to determine how differently the two dynamics models behave, and to decide whether the elastic-viscous-plastic model is preferable for high resolution climate simulations, considering its high efficiency in parallel computation. Results from the first year of this experiment (1990) are discussed in detail in Hunke and Zhang (1997).
Date: December 31, 1997
Creator: Hunke, E.C. & Zhang, Y.
Partner: UNT Libraries Government Documents Department

Comparison of beam transport simulations to measurements at the Los Alamos Proton Storage Ring

Description: The ability to model and simulate beam behavior in the Proton Storage Ring (PSR) of the Los Alamos Neutron Science Center (LANSCE) is an important diagnostic and predictive tool. This paper gives the results of an effort to model the ring apertures and lattice and use beam simulation programs to track the beam. The results are then compared to measured activation levels from beam loss in the ring. The success of the method determines its usefulness in evaluating the effects of planned upgrades to the Proton Storage Ring.
Date: October 1, 1997
Creator: Wilkinson, C.; Neri, F.; Fitzgerald, D.H.; Blind, B.; Macek, R.; Plum, M. et al.
Partner: UNT Libraries Government Documents Department

Critical exponents for the 3D Ising model

Description: The authors present a status report on the ongoing analysis of the 3D Ising model with nearest-neighbor interactions using the Monte Carlo Renormalization Group (MCRG) and finite size scaling (FSS) methods on 64{sup 3}, 128{sup 3}, and 256{sup 3} simple cubic lattices. Their MCRG estimates are K{sup c}{sub nn} = 0.221655(1)(1) and {nu} = 0.625(1). The FSS results for K{sup c} are consistent with those from MCRG but the value of {nu} is not. Their best estimate {eta} = 0.025(6) covers the spread in the MCRG and FSS values. A surprise of their calculation is the estimate {omega} {approx} 0.7 for the correction-to-scaling exponent. The authors also present results for the renormalized coupling g{sub R} along the MCRG flow and argue that the data supports the validity of hyperscaling for the 3D Ising model.
Date: March 1, 1996
Creator: Gupta, R. & Tamayo, P.
Partner: UNT Libraries Government Documents Department

Treatment of patient-dependent beam modifiers in photon treatments by the Monte Carlo dose calculation code PEREGRINE

Description: The goal of the PEREGRINE Monte Carlo Dose Calculation Project is to deliver a Monte Carlo package that is both accurate and sufficiently fast for routine clinical use. One of the operational requirements for photon-treatment plans is a fast, accurate method of describing the photon phase-space distribution at the surface of the patient. The open-field case is computationally the most tractable; we know, a priori, for a given machine and energy, the locations and compositions of the relevant accelerator components (i.e., target, primary collimator, flattening filter, and monitor chamber). Therefore, we can precalculate and store the expected photon distributions. For any open-field treatment plan, we then evaluate these existing photon phase-space distributions at the patient`s surface, and pass the obtained photons to the dose calculation routines within PEREGRINE. We neglect any effect of the intervening air column, including attenuation of the photons and production of contaminant electrons. In principle, for treatment plans requiring jaws, blocks, and wedges, we could precalculate and store photon phase-space distributions for various combinations of field sizes and wedges. This has the disadvantage that we would have to anticipate those combinations and that subsequently PEREGRINE would not be able to treat other plans. Therefore, PEREGRINE tracks photons through the patient-dependent beam modifiers. The geometric and physics methods used to do this are described here. 4 refs., 8 figs.
Date: March 1, 1997
Creator: Schach von Wittenau, A.E.; Cox, L.J.; Bergstrom, P.M. Jr.; Hornstein, S.M.; Mohan, R.; Libby, B. et al.
Partner: UNT Libraries Government Documents Department

Surface characterization of optics for EUV lithography

Description: The surface topography of optics fabricated for Extreme Ultraviolet Lithography has been measured using a combination of phase-measuring interferometry and atomic force microscopy. Power Spectral Densities were computed over spatial frequencies extending from 2.0{times}10{sup {minus}8} nm{sup {minus}1} to 7.7{times}10{sup {minus}2} nm{sup {minus}1}. Roughness values for frequencies greater than 1.0{times}10{sup {minus}6} were 0.64 nm rms for a spherical optic and 0.95 nm rms for an aspheric optic. These values are significantly larger than 0.088 nm rms, which as obtained using a spherical optic representative of current limits in surface polishing technology. 9 refs., 4 figs.
Date: May 28, 1996
Creator: Gaines, D.P.; Sweeney, D.W.; DeLong, K.W.; Vernon, S.P.; Baker, S.L.; Tichenor, D.A. et al.
Partner: UNT Libraries Government Documents Department

Stability and halo formation in axisymmetric intense beams

Description: Beam stability and halo formation in high-intensity axisymmetric 2D beams in a uniform focusing channel are analyzed using particle-in-cell simulations. The tune depression-mismatch space is explored for the uniform (KV) distribution of the particle transverse-phase-space density, as well as for more realistic ones (in particular, the water-bag distribution), to determine the stability limits and halo parameters. The numerical results show an agreement with predictions of the analytical model for halo formation.
Date: October 1, 1997
Creator: Gluckstern, R.L. & Kurennoy, S.S.
Partner: UNT Libraries Government Documents Department

Dislocation emission from a three-dimensional crack -- A large-scale molecular dynamics study

Description: A series of massively parallel molecular dynamics simulations with up to 35 million atoms is performed to investigate dislocation emission from a three-dimensional crack. The authors observe dislocation loops emitted from the crack front--the first time this has been seen in computer simulations. The sequence of dislocation emission in the process of crack blunting process strongly depends on the crystallographic orientation of the crack front and differs strikingly from anything previously conjectured. This finding is essential to establish a precise dislocation emission criterion (i.e., intrinsic ductility criterion). They also find that boundary conditions and interatomic force laws have a significant effect on jogging or blunting dislocation emission modes.
Date: December 31, 1996
Creator: Zhou, S.J.; Beazley, D.M.; Lomdahl, P.S.; Voter, A.F. & Holian, B.L.
Partner: UNT Libraries Government Documents Department

Electrical conductivity of compressed argon

Description: The authors report calculations of the electrical conductivity of solid argon as a function of compression within the density functional local density approximation formulation for a norm-conserving pseudopotential using both electron-phonon coupling and molecular dynamics techniques.
Date: October 1, 1997
Creator: Bauer, R.; Windl, W.; Collins, L.; Kress, J. & Kwon, I.
Partner: UNT Libraries Government Documents Department

Discrete ordinates transport methods for problems with highly forward-peaked scattering

Description: The author examines the solutions of the discrete ordinates (S{sub N}) method for problems with highly forward-peaked scattering kernels. He derives conditions necessary to obtain reasonable solutions in a certain forward-peaked limit, the Fokker-Planck (FP) limit. He also analyzes the acceleration of the iterative solution of such problems and offer improvements to it. He extends the analytic Fokker-Planck limit analysis to the S{sub N} equations. This analysis shows that in this asymptotic limit the S{sub N} solution satisfies a pseudospectral discretization of the FP equation, provided that the scattering term is handled in a certain way (which he describes) and that the analytic transport solution satisfies an analytic FP equation. Similar analyses of various spatially discretized S{sub N} equations reveal that they too produce solutions that satisfy discrete FP equations, given the same provisions. Numerical results agree with these theoretical predictions. He defines a multidimensional angular multigrid (ANMG) method to accelerate the iterative solution of highly forward-peaked problems. The analyses show that a straightforward application of this scheme is subject to high-frequency instabilities. However, by applying a diffusive filter to the ANMG corrections he is able to stabilize this method. Fourier analyses of model problems show that the resulting method is effective at accelerating the convergence rate when the scattering is forward-peaked. The numerical results demonstrate that these analyses are good predictors of the actual performance of the ANMG method.
Date: April 1, 1998
Creator: Pautz, S.D.
Partner: UNT Libraries Government Documents Department

Preliminary results of GODIVA-IV prompt burst modeling

Description: The dynamic computer model developed to simulate GODIVA-IV prompt bursts adequately predicts the magnitude of power bursts. Also, it demonstrates the characteristic features of prompt bursts in metal assemblies, such as the change in shape of power pulses and the ringing of fuel surfaces at the onset of inertial effects. The model will be used to test more sophisticated reactivity feedback coefficients and neutronic-hydrodynamic coupling schemes. It will also be used for a more detailed analysis of inertial effects.
Date: June 1, 1996
Creator: Kimpland, R.
Partner: UNT Libraries Government Documents Department

First-principles simulations and Z-contrast imaging of impurities at <001> tilt grain boundaries in MgO

Description: First-principles density-functional calculations were used to study the effects of CA impurities on the {Sigma} = 5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
Date: December 1, 1997
Creator: Yan, Y.; Chisholm, M.F.; Pennycook, S.J.; Duscher, G.; Maiti, A. & Pantelides, S.T.
Partner: UNT Libraries Government Documents Department

Cooperative chemical rebonding in the segregation of impurities in silicon grain boundaries

Description: With ab initio calculations the authors show that the experimentally observed large segregation energies of As at Si grain boundaries can be explained by the formation of isolated dimers or ordered chains of dimers of threefold-coordinated As along the cores of grain boundary dislocations. They also find the intriguing possibility that As segregation may drive structural transformation of certain grain boundaries. Recently, they have obtained the first atomic-resolution STEM images of As in a Si grain boundary, consistent with the formation of As dimers. Segregation energy of As dimers was found to be significantly higher in isolated dislocation cores, where larger site-variation in strain than in grain boundaries lead to further lowering of the electronic levels of As deep into the bandgap.
Date: December 1996
Creator: Maiti, A.; Chisholm, M. F.; Pennycook, S. J. & Pantelides, S. T.
Partner: UNT Libraries Government Documents Department

The perturbative resummed series for top production

Description: Our calculation of the total cross section for inclusive production of t{bar t} pairs in hadron collisions is summarized. Principal ingredient of this calculation is resummation of the universal leading-logarithm effect of gluon radiation to all orders in the quantum chromodynamics coupling strength, restricted to the region of phase space that is manifestly perturbative. We present predictions of the physical cross section as a function of top quark mass in proton-antiproton reactions at center-of-mass energies of 1.8 and 2.0 TeV. 7 refs., 3 figs., 2 tabs.
Date: May 2, 1996
Creator: Berger, E.L. & Contopanagos, H.
Partner: UNT Libraries Government Documents Department

Application of soft x-ray laser interferometry to study large-scale-length, high-density plasmas

Description: We have employed a Mach-Zehnder interferometer, using a Ne-like Y x- ray laser at 155 {Angstrom} as the probe source, to study large-scale- length, high-density colliding plasmas and exploding foils. The measured density profile of counter-streaming high-density colliding plasmas falls in between the calculated profiles using collisionless and fluid approximations with the radiation hydrodynamic code LASNEX. We have also performed simultaneous measured the local gain and electron density of Y x-ray laser amplifier. Measured gains in the amplifier were found to be between 10 and 20 cm{sup {minus}1}, similar to predictions and indicating that refraction is the major cause of signal loss in long line focus lasers. Images showed that high gain was produced in spots with dimensions of {approximately} 10 {mu}m, which we believe is caused by intensity variations in the optical drive laser. Measured density variations were smooth on the 10-{mu}m scale so that temperature variations were likely the cause of the localized gain regions. We are now using the interferometry technique as a mechanism to validate and benchmark our numerical codes used for the design and analysis of high-energy-density physics experiments. 11 refs., 6 figs.
Date: May 1, 1996
Creator: Wan, A.S.; Barbee, T.W., Jr. & Cauble, R.
Partner: UNT Libraries Government Documents Department

A collisional-radiative average atom model for hot plasmas

Description: A collisional-radiative `average atom` (AA) model is presented for the calculation of opacities of hot plasmas not in the condition of local thermodynamic equilibrium (LTE). The electron impact and radiative rate constants are calculated using the dipole oscillator strengths of the average atom. A key element of the model is the photon escape probability which at present is calculated for a semi infinite slab. The Fermi statistics renders the rate equation for the AA level occupancies nonlinear, which requires iterations until the steady state. AA level occupancies are found. Detailed electronic configurations are built into the model after the self-consistent non-LTE AA state is found. The model shows a continuous transition from the non-LTE to the LTE state depending on the optical thickness of the plasma. 22 refs., 13 figs., 1 tab.
Date: October 17, 1996
Creator: Rozsnyai, B. F.
Partner: UNT Libraries Government Documents Department

Coupling impedance of a long slot and an array of slots in a circular vacuum chamber

Description: We find the real part of the longitudinal impedance for both a small hole and a long slot in a beam vacuum chamber with a circular cross section. The slot can be arbitrarily long; the only requirement on the dimensions of the slots is that its width be much smaller than c/w. Regular array of N slots periodically distributed along the pipe is also considered.
Date: June 1, 1995
Creator: Stupakov, G.V.
Partner: UNT Libraries Government Documents Department

Cellular automata modeling of weld solidification structure

Description: The authors explore the use of cellular automata in modeling arc-welding processes. A brief discussion of cellular automata and their previous use in micro-scale solidification simulations is presented. Macro-scale thermal calculations for arc-welding at a thin plate are shown to give good quantitative and qualitative results. Combining the two calculations in a single cellular array provides a realistic simulation of grain growth in a welding process. Results of simulating solidification in a moving melt pool in a poly-crystalline alloy sheet are presented.
Date: December 31, 1993
Creator: Dress, W.B.; Zacharia, T. & Radhakrishnan, B.
Partner: UNT Libraries Government Documents Department

Measurement of direct photons in 200{center_dot}A GeV {sup 32}S + Au collisions

Description: Two years ago at the 10th Winter Workshop on Nuclear Dynamics, we reported preliminary results from our measurements of photons in reactions of 200{center_dot}A GeV {sup 32}S ions with Au nuclei. The results had been presented earlier at Quark Matter `93. For central collisions we reported a small, but statistically significant, excess of photons over those that can be accounted for by the two-photon decay branches of neutral pions and eta mesons and by the small photon contributions from other radiative decays. This paper is, in a sense, a sequel to the 1994 presentation. Here we present the final WA80 single `direct` photon results, and we discuss their implications for various theoretical calculations. 13 refs., 2 figs., 1 tab.
Date: April 1, 1996
Creator: Plasil, F. & Collaboration, The WA80
Partner: UNT Libraries Government Documents Department

Results from a double Vlasov model for negative ion extraction from volume sources

Description: A new negative ion source-extraction model has been formulated and implemented which explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that: (1) for high-beam currents, the beam current is limited by a transverse space-charge limit, not an emission limit; (2) there is a saddle point with a concomitant potential barrier preventing most volume produced negative ions from being extracted (the combination of 1 and 2 indicates that in some interesting cases there is the opportunity to increase extraction currents above values presently observed); (3) introduction of cesium may cause an actual increase in the transverse space-charge limit by accumulation of positive ions of cesium in the presheath.
Date: July 1, 1998
Creator: Olsen, D.K.; Raridon, R.J. & Whealton, J.H.
Partner: UNT Libraries Government Documents Department

Impedance of the PEP-II DIP screen

Description: The vacuum chamber of a storage ring normally consists of periodically spaced pumping slots. The longitudinal impedance of slots are analyzed in this paper. It is found that although the broad-band impedance is tolerable, the narrow-band impedance, as a consequence of the periodicity of the slots, may exceed the stability limit given by natural damping with no feedback system on. Based on this analysis, the PEP-II distributed-ion-pump (DIP) screen uses long grooves with hidden holes cut halfway to reduce both the broad-band and narrow-band impedances.
Date: September 1, 1995
Creator: Ng, C.K. & Weiland, T.
Partner: UNT Libraries Government Documents Department

Study of microwave instability

Description: The microwave instability is usually described by linearizing Vlasov equation in the angle-action variables I, {phi} and assuming that the interaction of azimuthal harmonics {rho}{sub n}(I) of the distribution function {rho} is weak. The argument implied here is that the Hamiltonian flow smears out particles over invariant tori characterized by the action variables, and the remaining azimuthal dependence of the distribution function is small. Indeed, such an approach successfully describes bunch spectrum and the threshold of the microwave instability. However, recently there have been interesting observations of bunch centroid and bunch shape oscillations above instability threshold at LEP and the damping ring at SLAC. There are also indicates that the oscillations sometimes occur in localized region in the longitudinal coordinate instead of affecting the entire longitudinal distribution as one expects by an action-angle analysis. In this paper the authors describe an alternative approach to the problem of bunch stability using decomposition of the Fokker-Plank equation in the system of nonlinear equations for the moments of the distribution function. In particular, this approach allows them to avoid the conventional action-angle decomposition. The physical quantities they are interested in, the moments, are expressed in the Cartesian z {minus} {delta} phase space. To close the infinite hierarchy of moments equations, the authors assume that higher order correlations are small. Although both the action-angle and the Cartesian languages must be equivalent before truncation, they may have different speed of convergence depending on the problem being studied. It is hoped that Cartesian expansion approach would converge faster for the cases corresponding to those observed recently above threshold. The recent experimental observations made them interested in it again. This note is a progress report of their work.
Date: December 1, 1996
Creator: Heifets, S. & Chao, A.
Partner: UNT Libraries Government Documents Department

Specification of multipole tolerances for the APS quadrupole magnet

Description: This note will address a proposed method for specifying the multipole tolerance for the design and production of APS quadrupole magnets. The tolerances for the multipole components for the quadrupole magnets will be set to that level which reduces the dynamic aperture by about 10--15% from the ideal machine dynamic aperture (as specified in CDR-87). This level may appear rather stringent, especially compared to the 50--60% reduction resulting from quad placement errors. However, when all tolerances are taken together, the residual dynamic aperture would be prohibitively small and commissioning would be difficult if these tolerances were at twice this level. The dynamic aperture was determined using the numerical tracking program RACETRACK.
Date: August 1, 1988
Creator: Kramer, S. L.
Partner: UNT Libraries Government Documents Department