Non-perturbative features of driven scattering systems

We investigate the scattering properties of one-dimensional, periodically and non-periodically forced oscillators. The pattern of singularities of the scattering function, in the periodic case, shows a characteristic hierarchical structure where the number Nc of zeros of the solutions plays the role of an order parameter marking the level of the observed self-similar structure. The behavior is understood both in terms of the return map and of the intersections pattern of the invariant manifolds of the outermost fixed points. In the non-periodic case the scattering function does not provide a complete development of the hierarchical structure. The singularities pattern of the outgoing energy as a function of the driver amplitude is connected to the arrangement of gaps in the fundamental regions. The survival probability distribution of temporarily bound orbits is shown to decay asymptotically as a power law. The "stickiness" of regular regions of phase space, given by KAM surfaces and remnant of KAM curves, is responsible for this observation

Progetto architettonico di una cittadella per lo sport a Cisanello-Pisa, comprensiva di palestra polivalente con progetto ed ottimizzazione dei parametri illuminotecnici

Progetto architettonico ed illuminotecnic

Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonances regimes

The generation of harmonics by atoms interacting with two circularly polarized and frequency related laser fields is addressed through ab initio numerical simulations. A detailed charaterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far-off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range.Comment: Submitted to Appl. Phys. B, 4 page

Two-surface wave decay: improved analytical theory and effects on electron acceleration

Two-surface wave decay (TSWD), i.e. the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma which treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities.Comment: 4 figures, submitted to Appl.Phys.B (special issue from HFSW X conference, Biarritz, France, Oct 12-15 2003); slightly revised tex

Transplantation of Pancreatic Islet in an Immunoisolation Device

Background: Type-1 diabetes is caused by autoimmune destruction of insulin-producing B-cells in the pancreas. Today, transplantation of whole pancreas or islets is the only treatment that can restore endogenous insulin production. Within the past 20 years, pancreatic islet transplantation has become a clinical reality and an option in diabetes treatment. However, large use of this therapy is limited by shortage in organ donations and use of immunosuppressive drugs. Immunoisolation potentially allows elimination of immunosuppressive drugs and use of xenogenic tissue solving the problem of a shortage in organ donation. Aims: The present work was designed to ameliorate isolation of islets from different species and encapsulation of islets in immunoisolation devices (hollow fibres and microcapsules). Moreover, a mathematical model for immunoisolation membrane selection was developed. Results and Conclusions: The application of the automated method allows significant increase in the number of islets obtained. These islets were then used for in vitro or in vivo studies of immunoisolation devices, tested in different protocols. Experimental results indicate that islet transplantation using alginate gel microcapsule in the peritoneal cavity successfully control glycaemia in diabetic animals. However, it is not possible to translate in human application for the implantation site. An alternative strategy was developed based on membrane device to be used in the subcutaneous space. Despite permeability properties have been proved to be adequate, the in vivo functional response of transplanted islet in hollow fibre device was only temporary, probably for development of fibrotic tissue around the device. Future work: The use of theoretical model allows evaluation and selection of new membrane materials for islet immunoisolation. Efficacy of new devices would be determined by performing allo- or xeno- transplantations into diabetic recipients, exploring new sites of implantation that minimize surgical procedures and reduce implant failure

A Framework to compare text annotators and its applications

Text in human languages have a low logic structure and are inherently ambiguous. For this reason, the typical approach of Information Retrieval to text documents has been based on the Bag-of-words model, in which documents are analyzed only by the occurrence of terms, discarding any possible structure. But a recently developing line of research is devoted to adding structure to unstructured text, by recognizing the topics contained in a text and annotate them. Topic annotators are systems that have the purpose of linking a natural language document to the topics that are relevant for describing the content of the document. This systems can be applied to many classic problems of Information Retrieval: the categorization of a document can be based on its topics; the clustering of a set of documents can be done using their topics to find similarities; for a search engine, it would be easier to find relevant pages if there was a way to know the topics that the query expresses and search for them in the cached web pages. In this thesis, we present a formal framework that describe the problems related to topic retrieval, the algorithms that solve those problems, and the way they can be benchmarked

Expansion of a finite size plasma in vacuum

The expansion dynamics of a finite size plasma is examined from an analytical perspective. Results regarding the charge distribution as well as the electrostatic potential are presented. The acceleration of the ions and the associated cooling of the electrons that takes place during the plasma expansion is described. An extensive analysis of the transition between the semi infinite and the finite size plasma behaviour is carried out. Finally, a test of the analytical results, performed through numerical simulations, is presented.Comment: 4 pages with 5 figure

Two-Surface Wave Decay

Using an analytical model we discuss the parametric excitation of pairs of electron surface waves (ESW) in the interaction of an ultrashort, intense laser pulse with an overdense plasma which has a step-like density profile. The ESWs can be excited either by the electric or by the magnetic part of the Lorentz force exerted by the laser and, correspondingly, have frequencies around $\omega/2$ or $\omega$, where $\omega$ is the laser frequency.Comment: 4 EPS figures, Revte

"Single-cycle" ionization effects in laser-matter interaction

We investigate numerically effects related to ``single-cycle'' ionization of dense matter by an ultra-short laser pulse. The strongly non-adiabatic response of electrons leads to generation of a megagauss steady magnetic field in laser-solid interaction. By using two-beam interference, it is possible to create periodic density structures able to trap light and to generate relativistic ionization frontsComment: 12 pages, 6 figures, to be published in Laser and Particle Beam

Electric field dynamics and ion acceleration in the self-channeling of a superintense laser pulse

The dynamics of electric field generation and radial acceleration of ions by a laser pulse of relativistic intensity propagating in an underdense plasma has been investigated using an one-dimensional electrostatic, ponderomotive model developed to interpret experimental measurements of electric fields [S. Kar et al, New J. Phys. *9*, 402 (2007)]. Ions are spatially focused at the edge of the charge-displacement channel, leading to hydrodynamical breaking, which in turns causes the heating of electrons and an "echo" effect in the electric field. The onset of complete electron depletion in the central region of the channel leads to a smooth transition to a "Coulomb explosion" regime and a saturation of ion acceleration.Comment: 9 pages, 7 figures, final revised version, to appear on Plasma Phys. Contr. Fus., special issue on "Laser and Plasma Accelerators", scheduled for February, 200