FreDist: Automatic construction of distributional thesauri for French

International audienceIn this article we present FreDist, a freely available software package for the automatic construction of distributional thesauri from text corpora, as well as an evaluation of various distributional similarity metrics for French. Following from the work of Lin (1998) and Curran (2004), we use a large corpus of journalistic text and implement different choices for the type of lexical context relation, the weight function, and the measure function needed to build a distributional thesaurus. Using the EuroWordNet and \wolf wordnet resources for French as gold-standard references for our evaluation, we obtain the novel result that combining bigram and syntactic dependency context relations results in higher quality distributional thesauri. In addition, we hope that our software package and a joint release of our best thesauri for French will be useful to the NLP community

Sensitivity Analysis of the DARHT-II 2.5MV/2kA Diode

This report summarizes the study of the tolerance limits on the assembly of the cathode and the Pierce electrode for the DARHT-II diode (2.5 MV, 2 kA case), performed through a series of computer simulations using the PIC code WARP [1]. We have considered sources of beam quality degradation like the errors in axial and transverse positioning, and the size of the radial gap between the cathode and the Pierce electrode (shroud). The figure of merit was chosen to be the RMS beam (edge) emittance at a distance of 1 meter from the cathode, as defined by {var_epsilon}{sub x} = 4 {beta}{gamma} {radical}(<x{sup 2}><x{prime}{sup 2}>-<xx{prime}>{sup 2}) {center_dot}. The analysis shows that to position the cathode at the correct axial and transverse location is more important than the size of the radial gap

Generation, transport and focusing of high-brightness heavy ion beams

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.Includes bibliographical references (p. 195-201).The Neutralized Transport Experiment (NTX) has been built at the Heavy Ion Fusion Virtual National Laboratory. NTX is the first successful integrated beam system experiment that explores various physical phenomena, and determines the final spot size of a high intensity ion beam on a scaled version of a Heavy Ion Fusion driver. The final spot size is determined by the conditions of the beam produced in the injector, the beam dynamics in the focusing lattice, and the plasma neutralization dynamics in the final transport. A high brightness ion source using an aperturing technique delivers 25 mA of single charged potassium ion beam at 300 keV and a normalized edge emittance of 0.05 r-mm-mr. The ion beam is injected into a large bore magnetic quadrupole lattice, which produces a 20 mm radius beam converging at 20 mr. The converging ion beam is further injected into a plasma neutralization drift section where it is compressed ballistically down to a 1 mm spot size.(cont.) NTX provides the first experimental proof of plasma neutralized ballistic transport of a space-charge dominated ion beam, the information about higher order aberration effects on the spot size, the validation of numerical tools based on excellent agreement between measurements and numerical simulations over a broad parameter regime, and the development of new diagnostics to study the ion beam dynamics. The theoretical and experimental results are presented on the beam dynamics in the ion diode, downstream quadrupole lattice, and final neutralized transport.by Enrique Henestroza.Ph.D

FreDist: Automatic construction of distributional thesauri for French

International audienceIn this article we present FreDist, a freely available software package for the automatic construction of distributional thesauri from text corpora, as well as an evaluation of various distributional similarity metrics for French. Following from the work of Lin (1998) and Curran (2004), we use a large corpus of journalistic text and implement different choices for the type of lexical context relation, the weight function, and the measure function needed to build a distributional thesaurus. Using the EuroWordNet and \wolf wordnet resources for French as gold-standard references for our evaluation, we obtain the novel result that combining bigram and syntactic dependency context relations results in higher quality distributional thesauri. In addition, we hope that our software package and a joint release of our best thesauri for French will be useful to the NLP community

EXTRACTION COMPRESSION AND ACCELERATION OF HIGH LINE CHARGE DENSITY ION BEAMS

High Energy Density Physics (HEDP) applications require high line charge density ion beams. An efficient method to obtain this type of beams is to extract a long pulse, high current beam from a gun at high energy, and let the beam pass through a decelerating field to compress it. The low energy beam-bunch is loaded into a solenoid and matched to a Brillouin flow. The Brillouin equilibrium is independent of the energy if the relationship between the beam size (a), solenoid magnetic field strength (B) and line charge density is such that (Ba){sup 2} is proportional to the line charge density. Thus it is possible to accelerate a matched beam at constant line charge density. An experiment, NDCX-1c is being designed to test the feasibility of this type of injectors, where we will extract a 1 microsecond, 100 mA, potassium beam at 160 keV, decelerate it to 55 keV (density {approx}0.2 {micro}C/m), and load it into a 2.5 T solenoid where it will be accelerated to 100-150 keV (head to tail) at constant line charge density. The head-to-tail velocity tilt can be used to increase bunch compression and to control longitudinal beam expansion. We will present the physics design and numerical simulations of the proposed experiment

DARHT-II Injector Transients and the Ferrite Damper

This report summarizes the transient response of the DARHT-II Injector and the design of the ferrite damper. Initial commissioning of the injector revealed a rise time excited 7.8 MHz oscillation on the diode voltage and stalk current leading to a 7.8 MHz modulation of the beam current, position, and energy. Commissioning also revealed that the use of the crowbar to decrease the voltage fall time excited a spectrum of radio frequency modes which caused concern that there might be significant transient RF electric field stresses imposed on the high voltage column insulators. Based on the experience of damping the induction cell RF modes with ferrite, the concept of a ferrite damper was developed to address the crowbar-excited oscillations as well as the rise-time-excited 7.8 MHz oscillations. After the Project decided to discontinue the use of the crowbar, further development of the concept focused exclusively on damping the oscillations excited by the rise time. The design was completed and the ferrite damper was installed in the DARHT-II Injector in February 2006. The organization of this report is as follows. The suite of injector diagnostics are described in Section 2. The data and modeling of the injector transients excited on the rise-time and also by the crowbar are discussed in Section 3; the objective is a concise summary of the present state of understanding. The design of the ferrite damper, and the small scale circuit simulations used to evaluate the ferrite material options and select the key design parameters like the cross sectional area and the optimum gap width, are presented in Section 4. The details of the mechanical design and the installation of the ferrite damper are covered in Section 5. A brief summary of the performance of the ferrite damper following its installation in the injector is presented in Section 6

The DAHRT-II electron injector

The injector for the second axis of the Dual-Axis Radiographic Hydrotest Facility (DARHT) is being constructed at LBNL. The injector consists of a single gap diode extracting a 2 microseconds, 2kA, 3.2 MeV electron beam from a 6.5- inches diameter thermionic dispenser cathode. The injector is powered through an insulatoing column by a Marx generator. There is also the possibility of extracting a beam current of 4 kA. The focusing system for the electron beam consists of a Pierce electrostatic focusing electrode at the cathode and three solenoidal focusing magnets positioned between the anode and induction accelerator input. The off-energy components (beam-head) during the 400 ns energy rise time are overfocused, leading to beam envelope mismatch and growth, resulting in the possibility of beam hitting the accelerator tube walls. The anode focusing magnets can be tuned to avoid the beam spill in the 2kA case. To allow beam-head control for the 4kA case we are considering the introduction of time-varying magnetic focusing field along the accelerator axis generated by a single-loop solenoid magnet positioned in the anode beam tube. We will present the final design as well as beam dynamics calculations of the injector

Analyse syntaxique probabiliste en dépendances : approches efficaces à large contexte avec ressources lexicales distributionnelles

This thesis explores ways to improve the accuracy and coverage of efficient statistical dependency parsing. We employ transition-based parsing with models learned using Support Vector Machines (Cortes and Vapnik, 1995), and our experiments are carried out on French. Transition-based parsing is very fast due to the computational efficiency of its underlying algorithms, which are based on a local optimization of attachment decisions. Our first research thread is thus to increase the syntactic context used. From the arc-eager transition system (Nivre, 2008) we propose a variant that simultaneously considers multiple candidate governors for right-directed attachments. We also test parse correction, inspired by Hall and Novák (2005), which revises each attachment in a parse by considering multiple alternative governors in the local syntactic neighborhood. We find that multiple-candidate approaches slightly improve parsing accuracy overall as well as for prepositional phrase attachment and coordination, two linguistic phenomena that exhibit high syntactic ambiguity. Our second research thread explores semi-supervised approaches for improving parsing accuracy and coverage. We test self-training within the journalistic domain as well as for adaptation to the medical domain, using a two-stage parsing approach based on that of McClosky et al. (2006). We then turn to lexical modeling over a large corpus: we model generalized lexical classes to reduce data sparseness, and prepositional phrase attachment preference to improve disambiguation. We find that semi-supervised approaches can sometimes improve parsing accuracy and coverage, without increasing time complexity.Cette thèse présente des méthodes pour améliorer l'analyse syntaxique probabiliste en dépendances. Nous employons l'analyse à base de transitions avec une modélisation effectuée par des machines à vecteurs supports (Cortes and Vapnik, 1995), et nos expériences sont réalisées sur le français. L'analyse a base de transitions est rapide, de par la faible complexité des algorithmes sous-jacents, eux mêmes fondés sur une optimisation locale des décisions d'attachement. Ainsi notre premier fil directeur est d'élargir le contexte syntaxique utilisé. Partant du système de transitions arc-eager (Nivre, 2008), nous proposons une variante qui considère simultanément plusieurs gouverneurs candidats pour les attachements à droite. Nous testons aussi la correction des analyses, inspirée par Hall and Novák (2005), qui révise chaque attachement en choisissant parmi plusieurs gouverneurs alternatifs dans le voisinage syntaxique. Nos approches améliorent légèrement la précision globale ainsi que celles de l'attachement des groupes prépositionnels et de la coordination. Notre deuxième fil explore des approches semi-supervisées. Nous testons l'auto-entrainement avec un analyseur en deux étapes, basé sur McClosky et al. (2006), pour le domaine journalistique ainsi que pour l'adaptation au domaine médical. Nous passons ensuite à la modélisation lexicale à base de corpus, avec des classes lexicales généralisées pour réduire la dispersion des données, et des préférences lexicales de l'attachement des groupes prépositionnels pour aider à la désambiguïsation. Nos approches améliorent, dans certains cas, la précision et la couverture de l'analyseur, sans augmenter sa complexité théorique