Electronic structure of clusters modeling silica

Self-consistent-field-Xα-scattered wave calculations on clusters Si₂O₇^6? and H₆Si₂O₇ modeling silica have been performed. Incorporation of Si 3d orbitals produces significant changes in the overall valence structure. In addition to σ Si — O bonds, there exists a bonding π character due to the participation of O 2p and Si 3d. Hydrogen terminators do not seem to correct edge effects for these π states.     

Nuclear data for space radiation

Human space flight requires protecting astronauts from the harmful effects of space radiation. The availability of measured nuclear cross-section data needed for these studies is reviewed in the present paper. The energy range of interest for radiation protection is approximately 100 MeV/n–10 GeV/n. The majority of data are for projectile fragmentation partial and total cross-sections, including both charge changing and isotopic cross-sections. The cross-section data are organized into categories which include charge changing, elemental, isotopic for total, single and double differential with respect to momentum, energy and angle. Gaps in the data relevant to space radiation protection are discussed and recommendations for future experiments are made.     

Plasma-density-gradient injection of low absolute-momentum-spread electron bunches

Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76+/-0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies.     

Effects of tank tracking on range grasses

The Department of Defense (DoD) must constantly balance its military mission and its commitment to stewardship on large tracks of federal land. These military training lands are some of the most intensely used land in the United States, and training requires that vegetation, primarily grasses, be as resilient as possible. The objectives of this study were to evaluate (i) plant establishment and (ii) the effect of light to heavy tank traffic that ranged from zero to four passes on range grasses at the Yakima Training Center, Yakima, Washington. Characteristics measured include the number of 10-cm gaps without plant materials in seeded rows of the target (sown) species, the biomass of the target species, and the percentages of bluegrass colonization, bare ground, and cheatgrass encroachment in tracked and untracked areas. Both stand establishment and the ability to produce rhizomes are associated with the ability to recover after training. The wildryes did not establish well and subsequently had more 10-cm gaps and lower yields than the other introduced species tested. Among the natives, Snake River wheatgrass and western wheatgrass exhibited the most resilience across the different tracking intensities. The largest decline in percentage target species was observed between the two- and four-pass treatments. After five years, cheatgrass was not able to encroach on Snake River wheatgrass, Siberian wheatgrass, and bluebunch wheatgrass plots. Tracking generally reduced the stand of the target species and increased the amount of bare ground.     

Model calculations of the particle spectrum of the galactic cosmic ray (GCR) environment: Assessment with ACE/CRIS and MARIE measurements

For a given galactic cosmic ray (GCR) environment, understanding the distribution of the particle flux (protons, alpha particles, and heavy ions) in deep-space and on the surface of planetary systems such as Mars are essential for the risk assessment of future human exploration missions. In our model calculations, we make use of the NASA's HZETRN (High Z and Energy Transport) code along with the nuclear fragmentation cross sections that are described by the quantum multiple scattering (QMSFRG) model with the time-dependant variation of the GCR environment derived making use of the solar modulation potential, phi. Data from the cosmic ray isotope spectrometer (CRIS) instrument onboard the advanced composition explorer (ACE) has been available from 1998. Data from the CRIS instrument is being analyzed to understand the short term variations in the particle spectrum during the current solar cycle phase. In this report, particle spectrum for December 2002 of the CRIS instrument was compared with the model calculations. Current assessment of the model calculations show very good agreement of the particle flux (15% for low Z and 5% for high Z) data from Boron (Z=5) through Nickel (Z=28) nuclei for the energy bins reported by CRIS. Similarly, the model calculated dose–rate calculations are compared with the proton flux measured dose–rate from the Martian Radiation Environment Experiment (MARIE) currently onboard the 2001 Mars Odyssey spacecraft in Martian orbit for the month of August 2003 that is representative of a quiet-time GCR data at Mars. Current model calculations are well within 10% of the measured observations of MARIE. The predictive capabilities of the quiet-time GCR particle flux model calculations are promising for biological and shielding studies and for the radiation risk assessment for future human explorations.     

Application of Dey–Mittra conformal boundary algorithm to 3D electromagnetic modeling

The Dey–Mittra conformal boundary conditions have been implemented for the finite-difference time-domain (FDTD) electromagnetic solver of the VORPAL plasma simulation framework and studied in the context of three-dimensional, large-scale computations. The maximum stable time step when using these boundary conditions can be arbitrarily small, due to the presence of small fractional cells inside the vacuum region. Use of the Gershgorin Circle theorem allows the determination of a rigorous criterion for exclusion of small cells in order to have numerical stability for particular values of the ratio f_DM≡Δt/Δt_CFL of the time step to the Courant–Friedrichs–Lewy value for the infinite system. Application to a spherical cavity shows that these boundary conditions allow computation of frequencies with second-order error for sufficiently small f_DM. However, for sufficiently fine resolution, dependent on f_DM, the error becomes first order, just like the error for stair-step boundary conditions. Nevertheless, provided one does use a sufficiently small value of f_DM, one can obtain third-order accuracy through Richardson extrapolation. Computations for the TESLA superconducting RF cavity design compare favorably with experimental measurements.     

Mendelsohn designs admitting the affine group

All Mendelsohn designs containing a Frobenius group with cyclic complement of orderv – 1 as a subgroup of the automorphism are found. Furthermore, the automorphism group of each of the designs is constructed. These designs generalize Mendelsohn's construction of Mendelsohn designs containing a certain doubly transitive automorphism group.The research on this paper was partially supported by North Texas State Faculty Research Grant #35524.     

Correlations of periodic,area-preserving maps

A simple analytical decay law for correlation functions of periodic, area-preserving maps is obtained. This law is compared with numerical experiments on the standard map. The agreement between experiment and theory is good when islands are absent, but poor when islands are present. When islands are present, the correlations have a long, slowly decaying tail.     

Characterizing Polyoxovanadate-Alkoxide Clusters Using Vanadium K-Edge X-Ray Absorption Spectroscopy

A number of technologies would benefit from developing inorganic compounds and materials with specific electronic and magnetic exchange properties. Unfortunately, designing compounds with these properties is difficult because metal⋅⋅⋅metal coupling schemes are hard to predict and control. Fully characterizing communication between metals in existing compounds that exhibit interesting properties could provide valuable insight and advance those predictive capabilities. One such class of molecules are the series of Lindqvist iron-functionalized and hexavanadium polyoxovanadate-alkoxide clusters, which we characterized here using V K-edge X-ray absorption spectroscopy. Substantial changes in the pre-edge peak intensities were observed that tracked with the V 3d-electron count. The data also suggested substantial delocalization between the vanadium cations. Meanwhile, the Fe^III cations were electronically isolated from the polyoxovanadate core.