Development of Improved Rhenium Coatings for Fluorine Engine Thrust Chambers

Coating trials were undertaken to evaluate the application of rhenium to carbon-carbon composite sheet by plasma spraying. Optimum spray parameters and coating thickness were identified for production of coatings free from continuous defects and with adequate adherence to the substrate. A tungsten underlayer was not beneficial and possibly detracted from coating integrity. Stress calculations indicated that the proposed operating cycle of the rocket engine would not cause spalling of the rhenium coating. Calculations indicated that permeation of gases through the coating would not be significant during the expected life of the thrust chamber. The feasibility of applying rhenium coatings by laser melting was also studied. Poor wetting of the composite surface by the liquid rhenium precluded production of uniform coatings. Borate/carborate fluxes did not improve wetting characteristics

Green's Dyadic Approach of the Self-Stress on a Dielectric-Diamagnetic Cylinder with Non-Uniform Speed of Light

We present a Green's dyadic formulation to calculate the Casimir energy for a dielectric-diamagnetic cylinder with the speed of light differing on the inside and outside. Although the result is in general divergent, special cases are meaningful. It is pointed out how the self-stress on a purely dielectric cylinder vanishes through second order in the deviation of the permittivity from its vacuum value, in agreement with the result calculated from the sum of van der Waals forces.Comment: 8 pages, submitted to proceedings of QFEXT0

Tidally Triggered Star Formation in Close Pairs of Galaxies: Major and Minor Interactions

We study star formation in a sample of 345 galaxies in 167 pairs and compact groups drawn from the original CfA2 Redshift Survey and from a follow-up search for companions. We construct our sample with attention to including pairs with luminosity contrast |\Delta m_R| >= 2. These 57 galaxies with |\Delta m_R| >= 2 provide a set of nearby representative cases of minor interactions, a central feature of the hierarchical galaxy formation model. Here we report the redshifts and positions of the 345 galaxies in our sample, and of 136 galaxies in apparent pairs that are superpositions. In the pairs sample as a whole, there are strong correlations between the equivalent width of the H\alpha emission line and the projected spatial and the line-of-sight velocity separation of the pair. For pairs of small luminosity contrast, |\Delta m_R| < 2, the member galaxies show a correlation between the equivalent width of H\alpha and the projected spatial separation of the pair. However, for pairs with large luminosity contrast, |\Delta m_R| >= 2, we detect no correlation between the equivalent width of H\alpha and the projected spatial separation. The relative luminosity of the companion galaxy is more important in a gravitational tidal interaction than the intrinsic luminosity of the galaxy. Central star formation across the entire pairs sample depends strongly on the luminosity ratio, |\Delta m_R|, a reasonable proxy for the mass ratio of the pair; pairs composed of similarly luminous galaxies produce the strongest bursts of star formation. Pairs with |\Delta m_R| >= 2 rarely have EW(H\alpha) >~ 70 Ang.Comment: Minor revisions following journal proof

Surface Divergences and Boundary Energies in the Casimir Effect

Although Casimir, or quantum vacuum, forces between distinct bodies, or self-stresses of individual bodies, have been calculated by a variety of different methods since 1948, they have always been plagued by divergences. Some of these divergences are associated with the volume, and so may be more or less unambiguously removed, while other divergences are associated with the surface. The interpretation of these has been quite controversial. Particularly mysterious is the contradiction between finite total self-energies and surface divergences in the local energy density. In this paper we clarify the role of surface divergences.Comment: 8 pages, 1 figure, submitted to proceedings of QFEXT0

Casimir energy of a dilute dielectric ball in the mode summation method

In the $(\epsilon_1-\epsilon_2)^2$--approximation the Casimir energy of a dilute dielectric ball is derived using a simple and clear method of the mode summation. The addition theorem for the Bessel functions enables one to present in a closed form the sum over the angular momentum before the integration over the imaginary frequencies. The linear in $(\epsilon_1-\epsilon_2)$ contribution into the vacuum energy is removed by an appropriate subtraction. The role of the contact terms used in other approaches to this problem is elucidated.Comment: 14 pages, REVTeX, no figures, no tables; presentation is made better, new references are adde

Kinematic Effects of Tidal Interaction on Galaxy Rotation Curves

We use self-consistent N-body models, in conjunction with models of test particles moving in galaxy potentials, to explore the initial effects of interactions on the rotation curves of spiral galaxies. Using nearly self-consistent disk/bulge/halo galaxy models (Kuijken & Dubinski 1995), we simulate the first pass of galaxies on nearly parabolic orbits; we vary orbit inclinations, galaxy halo masses and impact parameters. For each simulation, we mimic observed rotation curves of the model galaxies. Transient interaction-induced features of the curves include distinctly rising or falling profiles at large radii and pronounced bumps in the central regions. Remarkably similar features occur in our statistical sample of optical emission-line rotation curves of spiral galaxies in tight pairs and n-tuples.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter

Dynamical Casimir Effect and Quantum Cosmology

We apply the background field method and the effective action formalism to describe the four-dimensional dynamical Casimir effect. Our picture corresponds to the consideration of quantum cosmology for an expanding FRW universe (the boundary conditions act as a moving mirror) filled by a quantum massless GUT which is conformally invariant. We consider cases in which the static Casimir energy is repulsive and attractive. Inserting the simplest possible inertial term, we find, in the adiabatic (and semiclassical) approximation, the dynamical evolution of the scale factor and the dynamical Casimir stress analytically and numerically (for SU(2) super Yang-Mills theory). Alternative kinetic energy terms are explored in the Appendix.Comment: 14 pages, REVTeX 4, 6 eps figure

Casimir Energies and Pressures for δ\delta-function Potentials

The Casimir energies and pressures for a massless scalar field associated with $\delta$-function potentials in 1+1 and 3+1 dimensions are calculated. For parallel plane surfaces, the results are finite, coincide with the pressures associated with Dirichlet planes in the limit of strong coupling, and for weak coupling do not possess a power-series expansion in 1+1 dimension. The relation between Casimir energies and Casimir pressures is clarified,and the former are shown to involve surface terms. The Casimir energy for a $\delta$-function spherical shell in 3+1 dimensions has an expression that reduces to the familiar result for a Dirichlet shell in the strong-coupling limit. However, the Casimir energy for finite coupling possesses a logarithmic divergence first appearing in third order in the weak-coupling expansion, which seems unremovable. The corresponding energies and pressures for a derivative of a $\delta$-function potential for the same spherical geometry generalizes the TM contributions of electrodynamics. Cancellation of divergences can occur between the TE ($\delta$-function) and TM (derivative of $\delta$-function) Casimir energies. These results clarify recent discussions in the literature.Comment: 16 pages, 1 eps figure, uses REVTeX

Mode-by-mode summation for the zero point electromagnetic energy of an infinite cylinder

Using the mode-by-mode summation technique the zero point energy of the electromagnetic field is calculated for the boundary conditions given on the surface of an infinite solid cylinder. It is assumed that the dielectric and magnetic characteristics of the material which makes up the cylinder $(\epsilon_1, \mu_1)$ and of that which makes up the surroundings $(\epsilon_2, \mu_2)$ obey the relation $\epsilon_1\mu_1= \epsilon_2\mu_2$. With this assumption all the divergences cancel. The divergences are regulated by making use of zeta function techniques. Numerical calculations are carried out for a dilute dielectric cylinder and for a perfectly conducting cylindrical shell. The Casimir energy in the first case vanishes, and in the second is in complete agreement with that obtained by DeRaad and Milton who employed a Green's function technique with an ultraviolet regulator.Comment: REVTeX, 16 pages, no figures and tables; transcription error in previous version corrected, giving a zero Casimir energy for a tenuous cylinde

The heat kernel coefficients for the dielectric cylinder

We calculate the \hkks for the \elm field in the background of a dielectric cylinder with non equal speeds of light inside and outside. The coefficient $a_{2}$ whose vanishing makes the vacuum energy of a massless field unique, turns out to be zero in dilute order, i.e., in order (\ep-1)^{2}, and nonzero beyond. As a consequence, the vanishing of the vacuum energy in the presence of a dielectric cylinder found by Casimir-Polder summation must take place irrespectively of the methods by which it might be calculated.Comment: 14 pages, 1 figur