5,918 research outputs found
Industrial GaInP/GaAs Power HBT MMIC Process
UMS has developed an industrial power HBT process especially dedicated to power MMICs in the 10GHz frequency range. The process has been qualified and meets the very demanding specifications required for X-Band high power amplifiers. Aside from the obvious RF performances, this includes the demonstration of the necessary stability and reproducibility of the process, associated with state-of-art reliability. It is important to note that the later has been achieved without affecting the high frequency capability of the devices, and demonstrated directly on high power transistors. Thanks to its intrinsic qualities this process can naturally also be used for other applications, like low phase noise voltage controlled oscillators, and power amplifiers at lower frequencies (for mobile phones for instance)
Generally covariant theories: the Noether obstruction for realizing certain space-time diffeomorphisms in phase space
Relying on known results of the Noether theory of symmetries extended to
constrained systems, it is shown that there exists an obstruction that prevents
certain tangent-space diffeomorphisms to be projectable to phase-space, for
generally covariant theories. This main result throws new light on the old fact
that the algebra of gauge generators in the phase space of General Relativity,
or other generally covariant theories, only closes as a soft algebra and not a
a Lie algebra.
The deep relationship between these two issues is clarified. In particular,
we see that the second one may be understood as a side effect of the procedure
to solve the first. It is explicitly shown how the adoption of specific
metric-dependent diffeomorphisms, as a way to achieve projectability, causes
the algebra of gauge generators (constraints) in phase space not to be a Lie
algebra --with structure constants-- but a soft algebra --with structure {\it
functions}.Comment: 22 pages, version to be published in Classical & Quantum Gravit
A singular structure: Monopost made in composites
p. 3002-3012This work presents the design process, the analysis, and the performance of a cylindrical hollow monopost, made entirely in composites. It is about a translucent structure of height 40m, external diameter 1.60m and the average wall-thickness 11mm. The material is a polymer made up of vinylster resin and reinforced by glass fiber (GFRP). The manufactured processing used is filament winding. Moreover, due to geographical emplacement where it is situated, the structure has to support wind velocity value above 180Km/h and its elastic modulus of the material does not exceed 25GPa. Eventually, we was able to achieve an optimum solution and strentgh structure, considering and developing differents types of approaches and analysis, such as linear, non-linear and buckling.Rovira, JA.; Martin, P.; Pons, D.; Almerich Chulia, AI. (2009). A singular structure: Monopost made in composites. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/670
Evolutionary Laws, Initial Conditions, and Gauge Fixing in Constrained Systems
We describe in detail how to eliminate nonphysical degrees of freedom in the
Lagrangian and Hamiltonian formulations of a constrained system. Two important
and distinct steps in our method are the fixing of ambiguities in the dynamics
and the determination of inequivalent initial data. The Lagrangian discussion
is novel, and a proof is given that the final number of degrees of freedom in
the two formulations agrees. We give applications to reparameterization
invariant theories, where we prove that one of the constraints must be
explicitly time dependent. We illustrate our procedure with the examples of
trajectories in spacetime and with spatially homogeneous cosmological models.
Finally, we comment briefly on Dirac's extended Hamiltonian technique.Comment: 23 pages; plain TeX. To appear: Classical & Quantum Gravit
Carbon burning in intermediate mass primordial stars
The evolution of a zero metallicity 9 M_s star is computed, analyzed and
compared with that of a solar metallicity star of identical ZAMS mass. Our
computations range from the main sequence until the formation of a massive
oxygen-neon white dwarf. Special attention has been payed to carbon burning in
conditions of partial degeneracy as well as to the subsequent thermally pulsing
Super-AGB phase. The latter develops in a fashion very similar to that of a
solar metallicity 9 M_s star, as a consequence of the significant enrichment in
metals of the stellar envelope that ensues due to the so-called third dredge-up
episode. The abundances in mass of the main isotopes in the final ONe core
resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28
and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly
composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4}
M_sComment: 11 pages, 11 figures, accepted for publication in A&
Explosion of white dwarfs harboring hybrid CONe cores
Recently, it has been found that off-centre carbon burning in a subset of
intermediate-mass stars does not propagate all the way to the center, resulting
in a class of hybrid CONe cores. Here, we consider the possibility that stars
hosting these hybrid CONe cores might belong to a close binary system and,
eventually, become white dwarfs accreting from a non-degenerate companion at
rates leading to a supernova explosion. We have computed the hydrodynamical
phase of the explosion of Chandrasekhar-mass white dwarfs harboring hybrid
cores, assuming that the explosion starts at the center, either as a detonation
(as may be expected in some degenerate merging scenarios) or as a deflagration
(that afterwards transitions into a delayed detonation). We assume these hybrid
cores are made of a central CO volume, of mass M(CO), surrounded by an ONe
shell. We show that, in case of a pure detonation, a medium-sized CO-rich
region, M(CO)<0.4 Msun, results in the ejection of a small fraction of the
mantle while leaving a massive bound remnant. Part of this remnant is made of
the products of the detonation, Fe-group nuclei, but they are buried in its
inner regions, unless convection is activated during the ensuing cooling and
shrinking phase of the remnant. In contrast, and somehow paradoxically, delayed
detonations do not leave remnants but for the minimum M(CO) we have explored,
M(CO)=0.2 Msun, and even in this case the remnant is as small as 0.13 Msun. The
ejecta produced by these delayed detonations are characterized by slightly
smaller masses of 56Ni and substantially smaller kinetic energies than obtained
for a delayed detonation of a 'normal' CO white dwarf. The optical emission
expected from these explosions would hardly match the observational properties
of typical Type Ia supernovae, although they make interesting candidates for
the subluminous class of SN2002cx-like or SNIax.Comment: Accepted for Astronomy and Astrophysics, 11 pages, 4 figure
Database Tuning and its Role in Information Technology Education
Course curriculum in database management systems encompasses many topics, from data modeling to implementation and testing. These topics establish a strong foundation for Information Technology students to analyze, design, and implement a database system. Beyond these basic skills, Information Technology students must be equipped with the necessary capabilities to address database performance issues, such as when end-user expectations are not met. The topic of database tuning includes various techniques associated with enhancing database performance, and is important in providing a well-rounded database curriculum, but this skill is not generally covered in most database management systems courses. Our study focuses on testing the researcher’s assumption that database tuning is not part of current Information Technology curriculums. In addition, its significance and importance in the curriculums is researched. We surveyed academics and professionals to quantify the importance of database tuning and establish an understanding of its role within the education of students taking database related courses
Optimization of the e-e- option for the ILC
The e-e- running mode is one of the interesting physics options at the
International Linear Collider (ILC). The luminosity for e-e- collisions is
reduced by the beam-beam effects. The resulting beamstrahlung energy loss and
beam-beam deflection angles as function of the vertical transverse offset are
different compared to the e+e- collisions. In this paper, the dependence of
these observables with the offset for different beam sizes has been analyzed to
optimize performances for the e-e- mode, taking into account the requirements
of the beam-beam deflection based intra-train feedback system. A first study of
the implications for the final focus and extraction line optics is also
presented for the cases of the 20 mrad and 2 mrad ILC base line crossing angle
geometries
Extensive population synthesis of isolated neutron stars with field decay
We perform population synthesis studies of different types of neutron stars
taking into account the magnetic field decay. For the first time, we confront
our results with observations using {\it simultaneously} the Log N -- Log S
distribution for nearby isolated neutron stars, the Log N -- Log L distribution
for magnetars, and the distribution of radio pulsars in the --
diagram. We find that our theoretical model is consistent with all sets of data
if the initial magnetic field distribution function follows a log-normal law
with and . The
typical scenario includes about 10% of neutron stars born as magnetars,
significant magnetic field decay during the first million years of a NS life.
Evolutionary links between different subclasses may exist, although robust
conclusions are not yet possible.
We apply the obtained field distribution and the model of decay to study
long-term evolution of neuton stars till the stage of accretion from the
interstellar medium. It is shown that though the subsonic propeller stage can
be relatively long, initially highly magnetized neutron stars ( G) reach the accretion regime within the Galactic lifetime if their
kick velocities are not too large. The fact that in previous studies made 10
years ago, such objects were not considered results in a slight increase of the
Accretor fraction in comparison with earlier conclusions. Most of the neutron
stars similar to the Magnificent seven are expected to become accreting from
the interstellar medium after few billion years of their evolution. They are
the main predecestors of accreting isolated neutron stars.Comment: 4 pages, conference "Astrophysics of Neutron Stars - 2010" in honor
of M. Ali Alpar, Izmir, Turke
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