1,721,503 research outputs found
Energy distribution of fragments from H2 and H2+ dissociation by electron impact for the use in numerical models applied to negative ion sources
Full text linkIn negative hydrogen ion sources, the kinetic energy of the atoms is directly related to the negative ion yield at the caesiated converter, with a larger contribution from hot atoms. The energy distribution of hydrogen atoms is related to the formation process: either the kinetic energy release, resulting from dissociation of the hydrogen molecules or molecular ions, or the proton neutralization either in the volume or during reflection at walls. The interpretation of recent experimental measurements related to the translational energy distribution of atoms or positive ions could profit from accurate inclusion of the initial energy distribution in numerical models. In this work, we focus on the calculation of the kinetic energy release for the various dissociation channels due to electron impact on H2 and H+2, in the Franck -Condon and delta approximation. Since in negative ion sources non -equilibrium vibrational distributions of H2 are found, the energy distribution of fragments is calculated for all vibrational levels. The inverse cumulative distribution functions related to the main dissociation processes are given, as well as the cumulative distributions for all dissociation channels by electron impact, for simple implementation in Monte Carlo numerical simulations. Finally, the application of the method to few cases of interest for negative ion sources is discussed
Symmetry in a free boundary problem for degenerate parabolic equations on unbounded domains
No full textWell-posedness results for phase field systems with memory effects in the order parameter dynamics
No full textWe study two models arising in phase transition dynamics. Thestate of the system is described by the pair (θ,χ), where θ is the (relative) temperature and χ is the order parameter or phase field. The main difference between the two models relies on whether global constraints on χ are imposed or not: accordingly, the resulting models will be called conserved or nonconserved. Memory effects influencing both the heat flux and the dynamics of χ have been considered in a number of recent papers. Here we assume the Fourier law for the heat flux in the energy balance equation, while we consider memory effects in the order parameter dynamics. We analyze the well-posedness of corresponding Cauchy-Neumann problems for both conserved and nonconserved models. Various results are derived according to properties of the memory kernel involved
La Spalla instabile: quadri clinici ed anatomopatologici
No full textLa Spalla instabile: quadri clinici ed anatomopatologici
Phase field systems with memory effects in the order parameter dynamics: convergence to standard phase field systems
No full textWe shall deal with two models arising in phase transition dynamics. The state of the system is described by the pair (θ,χ), where θ is the (relative) temperature and χ is the order parameter or phase field. The main difference between the two models relies on whether global constraints on χ are imposed or not: the resulting models will be called conserved or nonconserved, accordingly. Memory effects influencing both the heat flux and the dynamics of χ have been considered in a number of recent papers. Here we assume the Fourier law forthe heat flux in the energy balance equation, while we considermemory effects in the order parameter dynamics. We show thatsolutions to the phase field problems with memory converge to the solution to the standard phase field model, when the memorykernels suitably converge to the Dirac mass. This is done for boththe conserved and the nonconserved cases. Some error estimates are also obtained
Symmetry in exterior boundary value problems for quasilinear elliptic equations via blow-up and a priori estimates.
No full textScreening while controlling an externality
No full textWe propose a tractable framework to introduce externalities in a screening model. Agents
differ in both payoff-type and influence-type (ranking how beneficial their actions are
for others). Applications range from pricing network goods to regulating industries that
create externalities. Inefficiencies arise only if the payoff-type is unobservable. When both
dimensions are unobserved, the optimal allocation satisfies lexicographic monotonicity:
increasing along the payoff-type to satisfy incentive compatibility, but tilted towards
influential agents to move the externality in the socially desirable direction. In particular,
the allocation depends on a private characteristic that is payoff-irrelevant for the agent.
We characterize the solution through a two-step ironing procedure that addresses the
non-monotonicity in virtual values arising from the countervailing impact of payoff- and
influence-type. Rents from influence can emerge but only indirectly, i.e. when the observed
level of influence is used as a signal of the unobserved payoff-type
"Evaluation of structural reliability of historic iron bridges",
Full text linkHistorical bridges, even quite recent ones, are precious historical records worthy of study
from the point of view of industrial archaeology. Indeed, because of both the high level of
scientific and technological knowledge and the variety of structural concepts and details they
contain, bridges do offer an important synthesis of the prevailing trends in the civil
engineering of their period.
In Italy, there is growing interest in the historical analysis of theories and techniques of
construction, with particular attention paid to putting intuitions and theories of construction
science into practice. Early metal bridges, mainly built in the second half of the nineteenth
century and in the first decades of the twentieth century, are of special interest because of
their close links with the world of science and construction technology in a period of rapid
innovation and theoretical development and consolidation.
In addition, a large number of metal ancient bridges are still used for roads and railways. The
problem of their preservation and rehabilitation is very felt and starts from the need of
measuring their structural reliability. For this purpose an expert system to be used for the
automatic evaluation of the state of conservation of metal bridges has been developed and
here presented.
For proceeding with systematic data gathering, it has been necessary to choose a powerful and
versatile method for cataloguing the data. Such a method had to be able to facilitate both a
general and a more detailed examination of all the structural deficiencies of the elements of
iron bridges and offer a system of interconnected data which can be interrelated to give an
overall evaluation of the structure.
This automatic method allows then for an inventory of metal bridges with the aim of
encouraging projects of rehabilitation for the conservation of historically interesting bridges
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