5,832 research outputs found
Random-matrix approach to the statistical compound nuclear reaction at low energies using the Monte-Carlo technique
Using a random-matrix approach and Monte-Carlo simulations, we generate
scattering matrices and cross sections for compound-nucleus reactions. In the
absence of direct reactions we compare the average cross sections with the
analytic solution given by the Gaussian Orthogonal Ensemble (GOE) triple
integral, and with predictions of statistical approaches such as the ones due
to Moldauer, to Hofmann, Richert, Tepel, and Weidenm\"{u}ller, and to Kawai,
Kerman, and McVoy. We find perfect agreement with the GOE triple integral and
display the limits of validity of the latter approaches. We establish a
criterion for the width of the energy-averaging interval such that the relative
difference between the ensemble-averaged and the energy-averaged scattering
matrices lies below a given bound. Direct reactions are simulated in terms of
an energy-independent background matrix. In that case, cross sections averaged
over the ensemble of Monte-Carlo simulations fully agree with results from the
Engelbrecht-Weidenm\"{u}ller transformation. The limits of other approximate
approaches are displayed
Optimization of a Langmuir-Taylor detector for lithium
This paper describes the construction and optimization of a Langmuir-Taylor
detector for lithium, using a rhenium ribbon. The absolute detection
probability of this very sensitive detector is measured and the dependence of
this probability with oxygen pressure and surface temperature is studied.
Sources of background signal and their minimization are also discussed in
details. And a comparison between our data concerning the response time of the
detector and literature values is given. A theoretical analysis has been made:
this analysis supports the validity of the Saha-Langmuir law to relate the
ionization probability to the work function. Finally, the rapid variations of
the work function with oxygen pressure and temperature are explained by a
chemical equilibrium model.Comment: 11 pages, 7 figures, to appear in Rev. Sci. Instru
Reduction of Second-Order Network Systems with Structure Preservation
This paper proposes a general framework for structure-preserving model
reduction of a secondorder network system based on graph clustering. In this
approach, vertex dynamics are captured by the transfer functions from inputs to
individual states, and the dissimilarities of vertices are quantified by the
H2-norms of the transfer function discrepancies. A greedy hierarchical
clustering algorithm is proposed to place those vertices with similar dynamics
into same clusters. Then, the reduced-order model is generated by the
Petrov-Galerkin method, where the projection is formed by the characteristic
matrix of the resulting network clustering. It is shown that the simplified
system preserves an interconnection structure, i.e., it can be again
interpreted as a second-order system evolving over a reduced graph.
Furthermore, this paper generalizes the definition of network controllability
Gramian to second-order network systems. Based on it, we develop an efficient
method to compute H2-norms and derive the approximation error between the
full-order and reduced-order models. Finally, the approach is illustrated by
the example of a small-world network
Krasovskii's Passivity
In this paper we introduce a new notion of passivity which we call
Krasovskii's passivity and provide a sufficient condition for a system to be
Krasovskii's passive. Based on this condition, we investigate classes of
port-Hamiltonian and gradient systems which are Krasovskii's passive. Moreover,
we provide a new interconnection based control technique based on Krasovskii's
passivity. Our proposed control technique can be used even in the case when it
is not clear how to construct the standard passivity based controller, which is
demonstrated by examples of a Boost converter and a parallel RLC circuit
Microstructure and superconducting properties of hot isostatically pressed MgB2
Bulk samples of MgB2 have been formed by hot isostatic pressing (HIPping) of
commercial powder at 100MPa and 950=B0C. The resulting material is 100% dense
with a sharp superconducting transition at 37.5K. Microstructural studies have
indicated the presence of small amounts of second phases within the material,
namely MgO and B rich compositions, probably MgB4. Magnetisation measurements
performed at 20K have revealed values of Jc=1.3 x 106A/cm2 at zero field, and
9.3 x 105A/cm2 at 1T. Magneto optical (MO) studies have shown direct evidence
for the superconducting homogeneity and strong intergranular current flow in
the material.Comment: 3 pages, 6 figures, text updated, new references included and
discussed. Submitted to Superconductor Science and Technolog
Heterodyne interferometer with unequal path lengths
Laser interferometry is an extensively used diagnostic for plasma
experiments. Existing plasma interferometers are designed on the presumption
that the scene and reference beam path lengths have to be equal, a requirement
that is costly in both the number of optical components and the alignment
complexity. It is shown here that having equal path lengths is not necessary -
instead what is required is that the path length difference be an even multiple
of the laser cavity length. This assertion has been verified in a heterodyne
laser interferometer that measures typical line-average densities of with an error of .Comment: 15 pages, 9 figures, to be published in Rev. Sci. Instrum. 77 (2006
Reconstructing Loads in Nanoplates from Dynamic Data
It was recently proved that the knowledge of the transverse displacement of a nanoplate in an open subset of its mid-plane, measured for any interval of time, allows for the unique determination of the spatial components (Formula presented.) of the transverse load (Formula presented.), where (Formula presented.) and (Formula presented.) is a known set of linearly independent functions of the time variable. The nanoplate mechanical model is built within the strain gradient linear elasticity theory, according to the KirchhoffâLove kinematic assumptions. In this paper, we derive a reconstruction algorithm for the above inverse source problem, and we implement a numerical procedure based on a finite element spatial discretization to approximate the loads (Formula presented.). The computations are developed for a uniform rectangular nanoplate clamped at the boundary. The sensitivity of the results with respect to the main parameters that influence the identification is analyzed in detail. The adoption of a regularization scheme based on the singular value decomposition turns out to be decisive for the accuracy and stability of the reconstruction
On Isospectral Composite Beams
We consider a composite system consisting of two identical straight elastic beams under longitudinal vibration connected by an elastic interface capable of counteracting the relative vibration of the two beams with its shearing stiffness. We construct examples of isospectral composite beams, i.e., countable one-parameter families of beams having different shearing stiffness but exactly the same eigenvalues under a given set of boundary conditions. The construction is explicit and is based on the reduction to a one-dimensional SturmâLiouville eigenvalue problem and the application of a Darbouxâs lemma
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