42,119 research outputs found
Large-Signal Simulation of 94 GHz Pulsed Silicon DDR IMPATTs Including the Temperature Transient Effect
In this paper large-signal modeling and simulation has been carried to study the frequency chirping due to temperature transients and the large-signal power and efficiency of pulsed silicon Double-Drift Region (DDR) Impact Avalanche Transit Time (IMPATT) device operating at 94 GHz. A large-signal simulation method based on non-sinusoidal voltage excitation incorporating the transient thermal effect has been developed by the authors. Results show that the device is capable of delivering a peak pulsed power output of 17.5 W with 12.8% efficiency when the voltage modulation is 60%. The maximum junction temperature rise is 350.2 K for a peak pulsed bias current of 6.79 A with 100 ns pulsewidth and 0.5 percent duty cycle; whereas the chirp bandwidth is 8.3 GHz
Hamiltonian embedding of the massive Yang-Mills theory and the generalized St\"uckelberg formalism
Using the general notions of Batalin, Fradkin, Fradkina and Tyutin to convert
second class systems into first class ones, we present a gauge invariant
formulation of the massive Yang-Mills theory by embedding it in an extended
phase space. The infinite set of correction terms necessary for obtaining the
involutive constraints and Hamiltonian is explicitly computed and expressed in
a closed form. It is also shown that the extra fields introduced in the
correction terms are exactly identified with the auxiliary scalars used in the
generalized St\"uckelberg formalism for converting a gauge noninvariant
Lagrangian into a gauge invariant form.Comment: 31 pages, Latex, very minor changes, a concluding paragraph inserted,
version to appear in Nucl. Phys.
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Free vibration of functionally graded beams and frameworks using the dynamic stiffness method
The free vibration analysis of functionally graded beams (FGBs) and frameworks containing FGBs is carried out by applying the dynamic stiffness method and deriving the elements of the dynamic stiffness matrix in explicit algebraic form. The usually adopted rule that the material properties of the FGB vary continuously through the thickness according to a power law forms the fundamental basis of the governing differential equations of motion in free vibration. The differential equations are solved in closed analytical form when the free vibratory motion is harmonic. The dynamic stiffness matrix is then formulated by relating the amplitudes of forces to those of the displacements at the two ends of the beam. Next, the explicit algebraic expressions for the dynamic stiffness elements are derived with the help of symbolic computation. Finally the Wittrick-Williams algorithm is applied as solution technique to solve the free vibration problems of FGBs with uniform cross-section, stepped FGBs and frameworks consisting of FGBs. Some numerical results are validated against published results, but in the absence of published results for frameworks containing FGBs, consistency checks on the reliability of results are performed. The paper closes with discussion of results and conclusions
Why are some galaxy disks extremely thin?
Some low surface brightness galaxies are known to have extremely thin stellar
disks with the vertical to planar axes ratio 0.1 or less, often referred to as
superthin disks. Although their existence is known for over three decades, the
physical origin for the thin distribution is not understood. We model the
stellar thickness for a two-component (gravitationally coupled stars and gas)
disk embedded in a dark matter halo, for a superthin galaxy UGC 7321 which has
a dense, compact halo, and compare with a typical dwarf galaxy HoII which has a
non-compact halo. We show that while the presence of gas does constrain the
disk thickness, it is the compact dark matter halo which plays the decisive
role in determining the superthin disk distribution in low-mass disks. Thus the
compact dark matter halo significantly affects the disk structure and this
could be important for the early evolution of galaxies.Comment: Submitted to MNRA
State of the art review : language testing and assessment (part two).
In Part 1 of this two-part review article (Alderson & Banerjee, 2001), we first addressed issues of washback, ethics, politics and standards. After a discussion of trends in testing on a national level and in testing for specific purposes, we surveyed developments in computer-based testing and then finally examined self-assessment, alternative assessment and the assessment of young learners. In this second part, we begin by discussing recent theories of construct validity and the theories of language use that help define the constructs that we wish to measure through language tests. The main sections of the second part concentrate on summarising recent research into the constructs themselves, in turn addressing reading, listening, grammatical and lexical abilities, speaking and writing. Finally we discuss a number of outstanding issues in the field
Non-Abelian Proca model based on the improved BFT formalism
We present the newly improved Batalin-Fradkin-Tyutin (BFT) Hamiltonian
formalism and the generalization to the Lagrangian formulation, which provide
the much more simple and transparent insight to the usual BFT method, with
application to the non-Abelian Proca model which has been an difficult problem
in the usual BFT method. The infinite terms of the effectively first class
constraints can be made to be the regular power series forms by ingenious
choice of and -matrices. In this new
method, the first class Hamiltonian, which also needs infinite correction terms
is obtained simply by replacing the original variables in the original
Hamiltonian with the BFT physical variables. Remarkably all the infinite
correction terms can be expressed in the compact exponential form. We also show
that in our model the Poisson brackets of the BFT physical variables in the
extended phase space are the same structure as the Dirac brackets of the
original phase space variables. With the help of both our newly developed
Lagrangian formulation and Hamilton's equations of motion, we obtain the
desired classical Lagrangian corresponding to the first class Hamiltonian which
can be reduced to the generalized St\"uckelberg Lagrangian which is non-trivial
conjecture in our infinitely many terms involved in Hamiltonian and Lagrangian.Comment: Notable improvements in Sec. I
Nuclear Matter Studies with Density-dependent Meson-Nucleon Coupling Constants
Due to the internal structure of the nucleon, we should expect, in general,
that the effective meson nucleon parameters may change in nuclear medium. We
study such changes by using a chiral confining model of the nucleon. We use
density-dependent masses for all mesons except the pion. Within a
Dirac-Brueckner analysis, based on the relativistic covariant structure of the
NN amplitude, we show that the effect of such a density dependence in the NN
interaction on the saturation properties of nuclear matter, while not large, is
quite significant. Due to the density dependence of the , as
predicted by the chiral confining model, we find, in particular, a looping
behavior of the binding energy at saturation as a function of the saturation
density. A simple model is described, which exhibits looping and which is shown
to be mainly caused by the presence of a peak in the density dependence of the
medium modified coupling constant at low density.
The effect of density dependence of the coupling constants and the meson
masses tends to improve the results for and density of nuclear matter at
saturation. From the present study we see that the relationship between binding
energy and saturation density may not be as universal as found in
nonrelativistic studies and that more model dependence is exhibited once medium
modifications of the basic nuclear interactions are considered.Comment: Acknowledgements have been modified. 34 pages, revtex, uuencoded
gz-compressed tar fil
Self dual models and mass generation in planar field theory
We analyse in three space-time dimensions, the connection between abelian
self dual vector doublets and their counterparts containing both an explicit
mass and a topological mass. Their correspondence is established in the
lagrangian formalism using an operator approach as well as a path integral
approach. A canonical hamiltonian analysis is presented, which also shows the
equivalence with the lagrangian formalism. The implications of our results for
bosonisation in three dimensions are discussed.Comment: 15 pages,Revtex, No figures; several changes; revised version to
appear in Physical Review
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