207 research outputs found
Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal
We report on the observation of a strong perturbation of the electron gas
induced by 20 MeV/u U ions and 13 MeV/u Pb ions channeled in
silicon crystals. This collective response (wake effect) in-duces a shift of
the continuum energy level by more than 100 eV, which is observed by means of
Radiative Electron Capture into the K and L-shells of the projectiles. We also
observe an increase of the REC probability by 20-50% relative to the
probability in a non-perturbed electron gas. The energy shift is in agreement
with calculations using the linear response theory, whereas the local electron
density enhancement is much smaller than predicted by the same model. This
shows that, for the small values of the adiabaticity parameter achieved in our
experiments, the density fluctuations are not strongly localized at the
vicinity of the heavy ions
A New Preconditioning Approachfor an Interior PointâProximal Method of Multipliers for Linear and Convex Quadratic Programming
In this paper, we address the efficient numerical solution of linear and
quadratic programming problems, often of large scale. With this aim, we devise
an infeasible interior point method, blended with the proximal method of
multipliers, which in turn results in a primal-dual regularized interior point
method. Application of this method gives rise to a sequence of increasingly
ill-conditioned linear systems which cannot always be solved by factorization
methods, due to memory and CPU time restrictions. We propose a novel
preconditioning strategy which is based on a suitable sparsification of the
normal equations matrix in the linear case, and also constitutes the foundation
of a block-diagonal preconditioner to accelerate MINRES for linear systems
arising from the solution of general quadratic programming problems. Numerical
results for a range of test problems demonstrate the robustness of the proposed
preconditioning strategy, together with its ability to solve linear systems of
very large dimension
Refined saddle-point preconditioners for discretized Stokes problems
This paper is concerned with the implementation of efficient solution algorithms for elliptic problems with constraints. We establish theory which shows that including a simple scaling within well-established block diagonal preconditioners for Stokes problems can result in significantly faster convergence when applying the preconditioned MINRES method. The codes used in the numerical studies are available online
Estimation of photovoltaic potential for electricity self-sufficiency: A study case of military facilities in northwest Spain
Renewable energies, including photovoltaic energy, are attracting widespread
international attention, in reaction to worsening environmental problems and the
diminishing long-term sustainability of fossil fuel energies. In this work, the potential
benefits of installing photovoltaic panels on several buildings at the Spanish
Naval Military School (Escuela Naval Militar, ENM) of Mar ın are considered. The
two salient advantages are significant economic savings from the production and
the sale of electricity to the Spanish Electricity Network and achieving selfsufficiency
in electricity requirements. Consequently, the main objective of this
work is to estimate the energy potential of photovoltaic installations on the roofs of
the ENM buildings. This is the first time that a project of this nature and size is presented
to the Spanish Navy. To that end, a three-dimensional geographic analysis
of the buildings is performed using three freeware software: Trimble SketchUp,
Skelion, and Photovoltaic Geographical Information System. An economic study is
also conducted to determine the feasibility of the installations, by estimating the
Net Present Value of the photovoltaic installation and the Internal Rate of Return
associated with the project. Subsequently, a sensitivity analysis that considers the
most important parameters for the calculation of the amortization period is reported.
The results show that the installation could fulfill the ENM electrical demands and
could, in addition, generate significant economic benefits. The conclusions end with
a recommendation to consider the merits of the proposed solution.Regional
Government of Castilla y Le on (Ref. BU034U16), under European Regional Development Fund,
and the Spanish Ministry of Economy, Industry and Competitiveness under the IĂŸD ĂŸ i state
programme Challenges for the Society (Ref. ENE-2014-54601-R). One of the authors, David
Gonz alez Pe~na, thanks Junta de Castilla-Le on for economic support (PIRTU Program, ORDEN
EDU/301/2015
An inexact dual logarithmic barrier method for solving sparse semidefinite programs
A dual logarithmic barrier method for solving large, sparse semidefinite programs is proposed in this paper. The method avoids any explicit use of the primal variable X and therefore is well-suited to problems with a sparse dual matrix S. It relies on inexact Newton steps in dual space which are computed by the conjugate gradient method applied to the Schur complement of the reduced KKT system. The method may take advantage of low-rank representations of matrices Ai to perform implicit matrix-vector products with the Schur complement matrix and to compute only specific parts of this matrix. This allows the construction of the partial Cholesky factorization of the Schur complement matrix which serves as a good preconditioner for it and permits the method to be run in a matrix-free scheme. Convergence properties of the method are studied and a polynomial complexity result is extended to the case when inexact Newton steps are employed. A Matlab-based implementation is developed and preliminary computational results of applying the method to maximum cut and matrix completion problems are reported
Ion slowing down and charge exchange at small impact parameters selected by channeling: superdensity effects
CASInternational audienceIn two experiments performed with 20-30 MeV/u highly charged heavy ions (Pb56+, U91+) channeled through thin silicon crystals, we observed the original features of superdensity, associated to the glancing collisions with atomic rows undergone by part of the incident projectiles. In particular the very high collision rate yields a quite specific charge exchange regime, that leads to a higher ionization probability than in random conditions. X-ray measurements show that electrons captured in outershells are prevented from being stabilized, which enhances the lifetime of the projectile innershell vacancies. The charge state distributions and the energy loss spectra are compared to Monte-Carlo simulations. These simulations confirm, extend and illustrate the qualitative analysis of the experimental results
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