13,358 research outputs found
Sputtering Holes with Ion Beamlets
Ion beamlets of predetermined configurations are formed by shaped apertures in the screen grid of an ion thruster having a double grid accelerator system. A plate is placed downstream from the screen grid holes and attached to the accelerator grid. When the ion thruster is operated holes having the configuration of the beamlets formed by the screen grid are sputtered through the plate at the accelerator grid
Potential biomedical applications of ion beam technology
Electron bombardment ion thrusters used as ion sources have demonstrated a unique capability to vary the surface morphology of surgical implant materials. The microscopically rough surface texture produced by ion beam sputtering of these materials may result in improvements in the biological response and/or performance of implanted devices. Control of surface roughness may result in improved attachment of the implant to soft tissue, hard tissue, bone cement, or components deposited from blood. Potential biomedical applications of ion beam texturing discussed include: vascular prostheses, artificial heart pump diaphragms, pacemaker fixation, percutaneous connectors, orthopedic pros-thesis fixtion, and dental implants
Nonsingular Black Hole Evaporation and ``Stable'' Remnants
We examine the evaporation of two--dimensional black holes, the classical
space--times of which are extended geometries, like for example the
two--dimensional section of the extremal Reissner--Nordstrom black hole. We
find that the evaporation in two particular models proceeds to a stable
end--point. This should represent the generic behavior of a certain class of
two--dimensional dilaton--gravity models. There are two distinct regimes
depending on whether the back--reaction is weak or strong in a certain sense.
When the back--reaction is weak, evaporation proceeds via an adiabatic
evolution, whereas for strong back--reaction, the decay proceeds in a somewhat
surprising manner. Although information loss is inevitable in these models at
the semi--classical level, it is rather benign, in that the information is
stored in another asymptotic region.Comment: 23 pages, 6 figures, harvmac and epsf, RU-93-12, PUPT-1399,
NSF-ITP-93-5
A Pyramid Scheme for Particle Physics
We introduce a new model, the Pyramid Scheme, of direct mediation of SUSY
breaking, which is compatible with the idea of Cosmological SUSY Breaking
(CSB). It uses the trinification scheme of grand unification and avoids
problems with Landau poles in standard model gauge couplings. It also avoids
problems, which have recently come to light, associated with rapid stellar
cooling due to emission of the pseudo Nambu-Goldstone Boson (PNGB) of
spontaneously broken hidden sector baryon number. With a certain pattern of
R-symmetry breaking masses, a pattern more or less required by CSB, the Pyramid
Scheme leads to a dark matter candidate that decays predominantly into leptons,
with cross sections compatible with a variety of recent observations. The dark
matter particle is not a thermal WIMP but a particle with new strong
interactions, produced in the late decay of some other scalar, perhaps the
superpartner of the QCD axion, with a reheat temperature in the TeV range. This
is compatible with a variety of scenarios for baryogenesis, including some
novel ones which exploit specific features of the Pyramid Scheme.Comment: JHEP Latex, 32 pages, 1 figur
Model validation for a noninvasive arterial stenosis detection problem
Copyright @ 2013 American Institute of Mathematical SciencesA current thrust in medical research is the development of a non-invasive method for detection, localization, and characterization of an arterial stenosis (a blockage or partial blockage in an artery). A method has been proposed to detect shear waves in the chest cavity which have been generated by disturbances in the blood flow resulting from a stenosis. In order to develop this methodology further, we use both one-dimensional pressure and shear wave experimental data from novel acoustic phantoms to validate corresponding viscoelastic mathematical models, which were developed in a concept paper [8] and refined herein. We estimate model parameters which give a good fit (in a sense to be precisely defined) to the experimental data, and use asymptotic error theory to provide confidence intervals for parameter estimates. Finally, since a robust error model is necessary for accurate parameter estimates and confidence analysis, we include a comparison of absolute and relative models for measurement error.The National Institute of Allergy and Infectious Diseases, the Air Force Office of Scientific Research, the Deopartment of Education and the Engineering and Physical Sciences Research Council (EPSRC)
Status of a five-centimeter-diameter ion thruster technology program
Prototype auxiliary propulsion subsystem with isolated single tank propellant feed system and 5-cm-diameter ion thruste
Quantum Moduli Spaces of String Theories
Generically, string models with supersymmetry are not expected to have
moduli beyond perturbation theory; stringy non-perturbative effects as well as
low energy field-theoretic phenomena such as gluino condensation will lift any
flat directions. In this note, we describe models where some subspace of the
moduli space survives non-perturbatively. Discrete symmetries forbid any
inherently stringy effects, and dynamical considerations control the
field-theoretic effects. The surviving subspace is a space of high symmetry;
the system is attracted to this subspace by a potential which we compute.
Models of this type may be useful for considerations of duality and raise
troubling cosmological questions about string theory. Our considerations also
suggest a mechanism for fixing the expectation value of the dilaton.Comment: 26 pages; uses harvmac. Footnote re fixing dilaton adde
Natural PQ symmetry in the 3-3-1 model with a minimal scalar sector
In the framework of a 3-3-1 model with a minimal scalar sector we make a
detailed study concerning the implementation of the PQ symmetry in order to
solve the strong CP problem. For the original version of the model, with only
two scalar triplets, we show that the entire Lagrangian is invariant under a
PQ-like symmetry but no axion is produced since an U(1) subgroup remains
unbroken. Although in this case the strong CP problem can still be solved, the
solution is largely disfavored since three quark states are left massless to
all orders in perturbation theory. The addition of a third scalar triplet
removes the massless quark states but the resulting axion is visible. In order
to become realistic the model must be extended to account for massive quarks
and invisible axion. We show that the addition of a scalar singlet together
with a Z_N discrete gauge symmetry can successfully accomplish these tasks and
protect the axion field against quantum gravitational effects. To make sure
that the protecting discrete gauge symmetry is anomaly free we use a discrete
version of the Green-Schwarz mechanism.Comment: 18 pages, 1 figure, 3 table
Lymphotoxins and cytomegalovirus cooperatively induce interferon-beta, establishing host-virus détente
Tumor necrosis factor (TNF)-related cytokines regulate cell death and survival and provide strong selective pressures for viruses, such as cytomegalovirus (CMV), to evolve counterstrategies in order to persist in immune-competent hosts. Signaling by the lymphotoxin (LT)-β receptor or TNF receptor-1, but not Fas or TRAIL receptors, inhibits the cytopathicity and replication of human CMV by a nonapoptotic, reversible process that requires nuclear factor κB (NF-κB)-dependent induction of interferon-β (IFN-β). Efficient induction of IFN-β requires virus infection and LT signaling, demonstrating the need for both host and viral factors in the curtailment of viral replication without cellular elimination. LTα-deficient mice and LTβR-Fc transgenic mice were profoundly susceptible to murine CMV infection. Together, these results reveal an essential and conserved role for LTs in establishing host defense to CMV
Nonsingular Lagrangians for Two Dimensional Black Holes
We introduce a large class of modifications of the standard lagrangian for
two dimensional dilaton gravity, whose general solutions are nonsingular black
holes. A subclass of these lagrangians have extremal solutions which are
nonsingular analogues of the extremal Reissner-Nordstrom spacetime. It is
possible that quantum deformations of these extremal solutions are the endpoint
of Hawking evaporation when the models are coupled to matter, and that the
resulting evolution may be studied entirely within the framework of the
semiclassical approximation. Numerical work to verify this conjecture is in
progress. We point out however that the solutions with non-negative mass always
contain Cauchy horizons, and may be sensitive to small perturbations.Comment: 27 pages, three figures, RU-92-61. (Replaced version contains some
corrections to incorrect equations. The zero temperature extremal geometry
(the conjectured end-point of the Hawking evaporation) is not as stated in
the previous version, but rather is a nonsingular analogue of the zero
temperature Reissner-Nordstrom space-time.
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