bluuebunny/crossref_metadata_2025 huggingface dataset:
When robots fail: The complex processes of learning and development
Authors: Marin Ludovic , Oullier Olivier
Published: April 2008

Although robots can contribute to the understanding of biological behavior, they fail to model the processes by which humans cope with their environment. Both development and learning are characterized by complex relationships that require constant modification. Given present technology, robots can only model behaviors in specific situations and during discrete stages. Robots cannot master the complex relationships that are the hallmark of human behavior.
Embodiment and complex systems
Authors: Metta Giorgio , Sandini Giulio
Published: April 2008

In agreement with the target article, we would like to point out a few aspects related to embodiment which further support the position of biorobotics. We argue that, especially when complex systems are considered, modeling through a physical implementation can provide hints to comprehend the whole picture behind the specific set of experimental data.
A multiplicity of constraints: How children learn word meaning
Authors: Westbury Chris , Nicoladis Elena
Published: April 2008

This book is an excellent and accessible overview of the position that children learn the meanings of words by applying a variety of nonlinguistic cognitive tools to the problem. We take issue with Bloom's emphasis on Theory of Mind as an explanatory mechanism for language learning; and with his claim that only unitary objects are nameable.
Controversies in the study of word learning
Authors: Bloom Paul
Published: April 2008

How Children Learn the Meanings of Words (HCLMW) defends the theory that words are learned through sophisticated and early-emerging cognitive abilities that have evolved for other purposes; there is no dedicated mental mechanism that is special to word learning. The commentators raise a number of challenges to this theory: Does it correctly characterize the nature and development of early abilities? Does it attribute too much to children, or too little? Does it only apply to nouns, or can it also explain the acquisition of words such as verbs and determiners? More general issues are discussed as well, including the role of the input, the relationship between words and concepts, and debates over nativism, adaptationism, and modularity.
Dimensions of modelling: Generality and integrativeness
Authors: Schank Jeffrey C.
Published: April 2008

Webb has articulated a clear, multi-dimensional framework for discussing simulation models and modelling strategies. This framework will likely co-evolve with modelling. As such, it will be important to continue to clarify these dimensions and perhaps add to them. I discuss the dimension of generality and suggest that a dimension of integrativeness may also be needed.
Is there more to “model” than “muddle”?
Authors: Scheutz Matthias
Published: April 2008

Any discussion comparing different models with respect to their quality qua models must presuppose a notion of model, that is, what it is to be a model. While Webb provides seven criteria to assess the quality of various proposed biorobotic models, she does not clarify the very notion of “model of animal behavior” itself.
The methodology of the artificial
Authors: Steels Luc
Published: April 2008

Biorobotics research should not only target “realistic” models of living systems and be judged exclusively from that perspective. It should pay just as much attention to formal models and artificial systems. They allow the examination of assumptions which do not necessarily hold for living systems, but precisely therein lies their value. They generate insight by enabling a comparison between the artificial and the real.
Accessing grammatical gender in German: The impact of gender-marking regularities
Authors: HOHLFELD ANNETTE
Published: April 2008

The present study investigated whether German speakers compute grammatical gender on the basis of gender-marking regularities. To this purpose two experiments were run. In Experiment 1, participants had to assign the definite article to German nouns in an online task; in the second experiment, participants were confronted with German nouns as well as nonwords in an untimed gender assignment task. In the online experiment, which required the repetition of a visually presented noun with its corresponding definite article as fast as possible, reaction times show that the assignment of the definite determiner to a noun is not facilitated by gender-marking regularities. In an offline gender assignment task, however, participants profited from gender cues during gender assignment to nonwords.
Perception of lexical stress by brain-damaged individuals: Effects on lexical–semantic activation
Authors: SHAH AMEE P. , BAUM SHARI R.
Published: April 2008

A semantic priming, lexical-decision study was conducted to examine the ability of left- and right-brain damaged individuals to perceive lexical-stress cues and map them onto lexical–semantic representations. Correctly and incorrectly stressed primes were paired with related and unrelated target words to tap implicit processing of lexical prosody. Results conformed with previous studies involving implicit perception of lexical stress, in that the left-hemisphere damaged individuals showed preserved sensitivity to lexical stress patterns as indicated by priming patterns mirroring those of the normal controls. An increased sensitivity to the varying stress patterns of the primes was demonstrated by the right-hemisphere damaged patient group, however. Results are discussed in relation to current theories of prosodic lateralization, with a particular focus on the nature of task demands in lexical stress perception studies.
Evaluating the effects of chronological age and sentence duration on degree of perceived foreign accent
Authors: MACKAY IAN R. A. , FLEGE JAMES E. , IMAI SATOMI
Published: April 2008

Immigrants' age of arrival (AOA) in a country where a second language (L2) must be learned has consistently been shown to affect the degree of perceived L2 foreign accent. Although the effect of AOA appears strong, AOA is typically correlated with other variables that might influence degree of foreign accent. This study examined the pronunciation of English by native Italian immigrants to Canada who differed in AOA. As in previous research, those who arrived as young adults (late learners) were somewhat older at the time of testing, and produced somewhat longer English sentences, than those who arrived in Canada when they were children (early learners). The results of Experiment 1 showed that the greater chronological age of early than late learners was not responsible for the late learners' stronger foreign accents. Experiment 2 suggested that the late learners' longer L2 sentences were not responsible for observed early–late foreign accent differences. A principle components analysis revealed that variation in AOA and language use, but not chronological age or sentence duration, accounted for a significant amount of variance in the foreign accent ratings. The findings of the study were interpreted to mean that AOA effects on foreign accent are due to the development of the native language phonetic system rather than to maturational constraints on L2 speech learning.
The role of learner and input variables in learning inflectional morphology
Authors: BROOKS PATRICIA J. , KEMPE VERA , SIONOV ARIEL
Published: April 2008

To examine effects of input and learner characteristics on morphology acquisition, 60 adult English speakers learned to inflect masculine and feminine Russian nouns in nominative, dative, and genitive cases. By varying training vocabulary size (i.e., type variability), holding constant the number of learning trials, we tested whether learners required a “critical mass” of vocabulary to generalize case marking patterns to new nouns. Cattell's Culture-Fair IQ Test mediated the effect of type variability on success in generalizing case marking to new vocabulary: only participants with above-median Culture-Fair Test scores showed the predicted critical mass effect of better generalization with larger training vocabulary. These results demonstrate how individual differences in central executive functioning and attention allocation capacity can affect adult second language learning.
Learning about the letter name subset of the vocabulary: Evidence from US and Brazilian preschoolers
Authors: TREIMAN REBECCA , KESSLER BRETT , POLLO TATIANA CURY
Published: April 2008

To examine the factors that affect the learning of letter names, an important foundation for literacy, we asked 318 US and 369 Brazilian preschoolers to identify each uppercase letter. Similarity of letter shape was the major determinant of confusion errors in both countries, and children were especially likely to interchange letters that were similar in shape as well as name. Errors were also affected by letter frequency, both general frequency and occurrence of letters in children's own names. Differences in letter names and letter frequencies between English and Portuguese led to certain differences in the patterns of performance for children in the two countries. Other differences appeared to reflect US children's greater familiarity with the conventional order of the alphabet. Boys were overrepresented at the low end of the continuum of letter name knowledge, suggesting that some boys begin formal reading instruction lacking important foundational skills.
Complexities and constraints in nonword repetition and word learning
Authors: GATHERCOLE SUSAN E.
Published: April 2008

Because words represent the building blocks upon which the facility to produce and comprehend language at all levels is based, the capacity of a child to learn words has immense impact on his or her developing abilities to communicate and engage properly with the outside world. Both the Keynote Article and the Commentaries in this issue demonstrate that this capacity to acquire vocabulary is neither singular nor simple. Children may fail to learn new words in as rapid and efficient manner as their peers for many reasons: they may, for example, have inadequate environmental experience of either the spoken and printed form of the language (Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991), or they may have poor abilities to produce the sound contrasts of the language (Mirak & Rescorla, 1998). The focus of the present discussion lies somewhere in between these extremes of influence, in the intervening perceptual and cognitive processes that constitute the speech processing and word learning system. Here, too, complexities abound. The developing language system is characterized by dependencies between the multiple processes involved in processing and learning language (Bishop, 1997), rarely evincing the dramatic dissociations in adults with acquired language disorders that have served cognitive neuropsychology so well in its bid to identify a modular structure of the language system. Weaknesses in perceptual analysis of the sound structure of the language, in the storage of the resulting mental representations, and in the availability of existing representations that can support the processing of new words, often coexist within an individual. As a consequence, it can be extremely difficult to tease apart the developmental underpinnings of language acquisition. In this article, I argue that real progress toward understanding vocabulary acquisition requires a substantial and systematic body of research evidence designed to provide strong empirical challenges to existing (and new) hypotheses and theories. Whereas mere description is unlikely to lead to major advances, systematic experimental analysis and the specification of detailed theoretical accounts should result in a more complete understanding of the complexities and constraints of new word learning.
Clarifying the phonological processing account of nonword repetition
Authors: Bowey Judith A.
Published: April 2008

Individual differences in nonword repetition (NWR) show a particularly strong association with vocabulary acquisition for both first- (L1) and second-language (L2) learners, and they serve as a behavioral marker for specific language impairment (SLI) in children (Gathercole, 2006). However, this association is susceptible to alternative explanations.
The developmental trajectory of nonword repetition
Authors: Chiat Shula
Published: April 2008

In line with the original presentation of nonword repetition as a measure of phonological short-term memory (Gathercole & Baddeley, 1989), the theoretical account Gathercole (2006) puts forward in her Keynote Article focuses on phonological storage as the key capacity common to nonword repetition and vocabulary acquisition. However, evidence that nonword repetition is influenced by a variety of factors other than item length has led Gathercole to qualify this account. In line with arguments put forward by Snowling, Chiat, and Hulme (1991), one of Gathercole's current claims is that nonword repetition and word learning are constrained by “the quality of temporary storage of phonological representations, and this quality is multiply determined.” Phonological storage is not just a quantity-limited capacity.
Commentary on Keynote
Authors: Montgomery James , Evans Julia
Published: April 2008

In her Keynote, Gathercole (2006) provides a comprehensive review regarding the nature of the nonword repetition (NWR) task and a compelling argument for the utility of the task as a robust index of children's phonological short-term storage capacity. She further argues that temporary phonological storage acts as a primitive learning mechanism that plays an instrumental role in the lexical learning of young children, including children with specific language impairment (SLI). In this Commentary, we focus our remarks on two related themes concerning the relation of NWR and SLI: (a) performance on NWR tasks provides a window into these children's more general cognitive (dis)abilities and (b) the robustness of NWR as a predictor of these children's lexical knowledge and learning abilities.
Nonword repetition and language learning disorders: A developmental contingency framework
Authors: Snowling Margaret J.
Published: April 2008

In 1990 Gathercole and Baddeley proposed a strong hypothesis that has generated a wealth of research in the field of language development and disorder. The hypothesis was that phonological memory, as indexed by nonword repetition, is causally related to vocabulary development. Support for the hypothesis came from an impressive range of longitudinal, correlational, and laboratory training studies, and from studies of specific language impairment (SLI). However, more recently, Gathercole, Tiffany, Briscoe, Thorn, and The ALSPAC Team (2005), directly tested the causal hypothesis by following a cohort of children from age 5 to 8 years. Contrary to prediction, children with poor nonword repetition abilities at age 5 had normal vocabulary at the age of 8.
Profile effects in early bilingual language and literacy
Authors: OLLER D. KIMBROUGH , PEARSON BARBARA Z. , COBO-LEWIS ALAN B.
Published: April 2008

Bilingual children's language and literacy is stronger in some domains than others. Reanalysis of data from a broad-scale study of monolingual English and bilingual Spanish–English learners in Miami provided a clear demonstration of “profile effects,” where bilingual children perform at varying levels compared to monolinguals across different test types. The profile effects were strong and consistent across conditions of socioeconomic status, language in the home, and school setting (two way or English immersion). The profile effects indicated comparable performance of bilingual and monolingual children in basic reading tasks, but lower vocabulary scores for the bilinguals in both languages. Other test types showed intermediate scores in bilinguals, again with substantial consistency across groups. These profiles are interpreted as primarily due to the “distributed characteristic” of bilingual lexical knowledge, the tendency for bilingual individuals to know some words in one language but not the other and vice versa.
An integrated account of the effects of acoustic variability in first language and second language: Evidence from amplitude, fundamental frequency, and speaking rate variability
Authors: SOMMERS MITCHELL S. , BARCROFT JOE
Published: April 2008

This study examined how three different sources of stimulus variability—overall amplitude, fundamental frequency, and speaking rate—affect second language (L2) vocabulary learning. Native English speakers learned Spanish words in presentation formats with no variability, moderate variability, and high variability. Dependent measures were accuracy and latency of picture-to-Spanish and Spanish-to-English recall. The findings indicated that variability sources that do not affect first language (L1) word identification (amplitude, fundamental frequency) do not affect L2 vocabulary learning. Conversely, variability in speaking rate, which negatively affects L1 word identification, positively affected L2 vocabulary learning. These findings are consistent with an integrated account in which sources of variability that affect phonetically relevant properties of speech impede L1 speech processing but improve L2 vocabulary learning.
“The Seaman Feels Him-self a Man”
Authors: Glasco Jeffrey D.
Published: April 2008

To date, most interpretations regarding the agency of maritime workers in the late eighteenth century have posited the seamen as a united working-class body. However, the application of gender as a mode of analysis illustrates that a division existed amongst the working-class seamen in their vision of what it meant to be a worker, specifically a maritime worker. These differing ideas of masculinity drove a wedge between the seamen of the British Royal Navy, thus weakening their class solidarity. One faction of seamen favored the existing social contract accepted a society based on inequality, even amongst men, but in return demanded that their contributions as workers and as men be acknowledged by their rulers through both material and symbolic rewards. Similarly, another element of the seamen sought a more revolutionary solution in seeking rewards for their masculine contributions, which included acknowledgement of their roles as workers and rights as men through political equality. What it meant to be a seaman and the contestation of that definition motivated and limited the historical agency of seamen in the greatest labor dispute during the Age of Sail: the British seamen's Spithead and Nore mutinies of 1797. As a new approach to labor history, the examination of the power of internalized gendered definitions to limit the actions of workers' agency offers powerful new insights.
Defense Workers' Struggles for Patriotic Control: The Labor-Management-State Contests over Defense Production at Brewster, 1940–1944
Authors: Lee Joong-Jae
Published: April 2008

Labor historians have heretofore presented bifurcated portrayals of the relationship between defense workers and the wartime, corporatist state during the Second World War. While liberal CIO leaders energetically tried to establish labor's greater representation in wartime mobilization and politics through patriotic social unionism, militant rank and filers turned out to be antistate wildcatters. In contrast, this local study of Brewster workers producing naval aircraft suggests that the wartime fetish with patriotic productivity had converging impacts on the relationship of both international union leaders and rank and filers with the state. Industrial mobilization by the military simultaneously demanded and impeded orderly expansion of production, which in the process manufactured faltering companies like Brewster. Brewster workers, criticizing corporate and military mismanagement and calling for state intervention as a political remedy in their political letters and confidential reports, intensified their contests for joint control of production, employment, and planning. However, their struggles for patriotic control were contingent upon the Navy's continuous demands for Brewster planes, skills, and facilities and thus could not survive reconversion downsizing in defense production. Union leaders retreated into organizing new shops in metropolitan New York, which primarily involved routine interplays between few union leaders and NLRB officials. The reconversion at Brewster marked the postwar bureaucratization of defense workers' relationship with the state.
From Peasant to Worker: Migration, Masculinity, and the Making of Mexican Workers in the US
Authors: Cohen Deborah
Published: April 2008

This article examines Mexican migration to the US during the Bracero Program, the unofficial name for the series of US-Mexico agreements that brought Mexican men to work in US agricultural fields from 1942 to 1964. Juxtaposing Mexican and US states' goals for the Program to migrants' understandings of their journeys, the article shows how this migration disrupted men's subjectivities, even as it simultaneously provided the mechanisms to resecure gender and class subjectivities and claims in crucial way. Revealed, ultimately, is what was forged in the wake of this migration: a new kind of historical actor, transnationally gendered and classed.
“They don't even look like women workers”: Femininity and Class in Twentieth-Century Latin America
Authors: Weinstein Barbara
Published: April 2008

Recent research on consumer culture and working-class femininity in the United States has argued that attention to fashionable clothing and dime novels did not undermine female working-class identities, but rather provided key resources for creating those identities. In this essay I consider whether we can see a similar process of appropriation by working-class women in Latin America. There women employed in factories had to contend with widespread denigration of the female factory worker. Looking first at the employer-run “Centers for Domestic Instruction” in São Paulo, I argue that “proper femininity” in these centers—frequented by large numbers of working-class women—reflected middle-class notions of the skilled housewife, and situated working-class women as nearly middle class. What we see is a process of “approximation,” not appropriation. I then look at the case of Argentina (especially Greater Buenos Aires) where Peronism also promoted “traditional” roles for working-class women but where Eva Perón emerges as a working-class heroine. The figure of Evita—widely reviled by women of the middle and upper classes—becomes a means to construct an alternative, class-based femininity for working-class women.
Vera Shlakman, Economic History of a Factory Town, A Study of Chicopee, Massachusetts (1935)
Authors: Kessler-Harris Alice
Published: April 2008

Vera Shlakman had an extraordinary effect on my work and on that of a generation of labor historians. Quietly, unobtrusively her interpretive insights and the methodological innovations she introduced paved the way to a more eclectic and integrated discipline. A full seventy years after its publication in 1935, her Economic History of a Factory Town: A Study of Chicopee, Massachusetts still provides an intellectual and conceptual guide, not only to a changing field, but to the persistent questions it raises.
Photon kinetic theory in plasmas and in optics
Authors: MENDONÇA J.T.
Published: April 2008

Recent advances of the photon kinetic theory are illustrated here,
with examples taken from plasma physics (forward Raman scattering), and
from nonlinear optics (self-phase modulation). This theory can provide
a unified view of nonlinear optical phenomena involving photon beams
with an arbitrary spectrum.
Fast ignition of fusion pellets with superintense lasers: Concepts,
problems, and prospectives
Authors: MULSER P. , BAUER D.
Published: April 2008

The concept of fast ignition of precompressed pellets for inertial
confinement fusion is presented and the main approaches are discussed.
Numerical simulations of fast coronal ignition and the peculiarities of
this scheme are considered in detail. Particular attention is devoted
to the energy transport in the pellet corona. It is shown that fast
coronal ignition will be successful only if the energy deposition by
the fast electrons is anomalous over a sufficiently extended overdense
region. Alternative schemes are briefly discussed.
Cluster structure effects on the interaction of an ultrashort
intense laser field with large clusters
Authors: SHOKRI B. , NIKNAM A.R. , KRAINOV V.
Published: April 2008

The structure of large clusters such as neutral clusters, metallic
clusters, and clusters ion is investigated. Furthermore, the electron
distribution of large clusters when they are irradiated by an intense
ultrashort laser pulse is treated. The cluster excitation results from
the interaction of the electron subsystem with the laser field.
Analyzing the quantum metallic cluster proves that its properties
differ essentially from the properties of a classical small metallic
sphere. The eigen frequency of the surface oscillation of a cluster is
obtained and it is shown that it is lower than Mie frequency because of
Thomas–Fermi screening.
Effect of a magnetic field on the growth rate of the
Rayleigh–Taylor instability of a laser-accelerated thin ablative
surface
Authors: RUDRAIAH N. , KRISHNAMURTHY B.S. , JALAJA A.S. , DESAI TARA
Published: April 2008

The Rayleigh–Taylor instability (RTI) of a laser-accelerated
ablative surface of a thin plasma layer in an inertial fusion energy
(IFE) target with incompressible electrically conducting plasma in the
presence of a transverse magnetic field is investigated using linear
stability analysis. A simple theory based on Stokes-lubrication
approximation is proposed. It is shown that the effect of a transverse
magnetic field is to reduce the growth rate of RTI considerably over
the value it would have in the absence of a magnetic field. This is
useful in the extraction of IFE efficiently.
Interconnection between hydro and PIC codes for fast ignition
simulations
Authors: SAKAGAMI H. , MIMA K.
Published: April 2008

Relativistic laser–plasma interaction, subsequent superhot
electron transport, superhot electron energy deposition, and the
overall implosion process are key subjects for fast ignition. All these
phenomena couple with each other, and more studies by simulations are
essential. We have a plan to simulate the whole of fast ignition
self-consistently with four individual codes. Four codes are integrated
into one big system in the Fast Ignition Integrated Interconnecting
code project. In a first stage of this project, we integrate the
Arbitrary Lagrangian Eulerian (ALE) hydro code with the collective
particle in cell (PIC) code. The PIC code obtains density profile at
maximum compression from the ALE hydro code to introduce imploded
plasma into a PIC system, and we can simulate interaction between
ignition laser and realistic plasma. We have evaluated reflected laser
spectrum and electron energy distribution, and found many differences
between the realistic plasma profile and the conventional one in PIC
simulations.
Skin depth plasma front interaction mechanism with prepulse
suppression to avoid relativistic self-focusing for high-gain laser
fusion
Authors: OSMAN F. , CANG YU , HORA H. , CAO LI-HUA , LIU HONG , HE XIANTU , BADZIAK J. , PARYS A.B. , WOLOWSKI J. , WORYNA E. , JUNGWIRTH K. , KRÁLIKOVA B. , KRÁSA J. , LÁSKA L. , PFEIFER M. , ROHLENA K. , SKÁLA J. , ULLSCHMIED J.
Published: April 2008

Measurements of the ion emission from targets irradiated with
neodymium glass and iodine lasers were analyzed and a very significant
anomaly observed. The fastest ions with high charge number Z,
which usually are of megaelectron volt energy following the
relativistic self-focusing and nonlinear-force acceleration theory,
were reduced to less than 50 times lower energies when 1.2 ps laser
pulses of about 1 J were incident. We clarify this discrepancy by the
model of skin depth plasma front interaction in contrast to the
relativistic self-focusing with filament generation. This was indicated
also from the unique fact that the ion number was independent of the
laser intensity. The skin layer theory prescribes prepulse control and
lower (near relativistic threshold) laser intensities for
nonlinear-force-driven plasma blocks for high-gain ignition similar to
light ion beam fusion.
Guest Editors' Preface: The International Conference on
Ultrashort High-Energy Radiation and Matter
Authors: Batani Dimitri , Wootton Alan J.
Published: April 2008

The International Conference on “Ultrashort High-Energy
Radiation and Matter” has been held in Villa Monastero, Varenna,
Italy, during October 7–10, 2003. The conference has been jointly
organized by the “Piero Caldirola” International Centre for
the Promotion of Science and International School of Plasma Physics,
Milan, Italy, and by the FEMTO Programme of the European Science
Foundation (chaired by Prof. Charles Joachain of the University of
Bruxelles).
Interaction of ultra-intense laser pulses with relativistic ions
Authors: CHIRILĂ C.C. , JOACHAIN C.J. , KYLSTRA N.J. , POTVLIEGE R.M.
Published: April 2008

At high laser intensities, three step recollision processes such as high order harmonic generation and high-order ATI, are normally severely suppressed due to the magnetic field component of the laser pulse. However, if the laser pulse and relativistic ion beam are directed against each other, a significant increase in the frequency and the intensity of the pulse in the rest frame of the ions can occur. By performing calculations based on a Coulomb-corrected nondipole strong field approximation, we have shown that there is a range of intensities, Lorentz factors, and ion charges for which the suppression of the three step recollision processes is not severe, even for ponderomotive energies exceeding 10 keV. As an example, we consider parameters relevant to the accelerator that will be built at GSI-Darmstadt, capable of accelerating multicharged ions to Lorentz factors reaching 30.
Generation of X rays and energetic ions from superintense laser irradiation of micron-sized Ar clusters
Authors: FUKUDA Y. , AKAHANE Y. , AOYAMA M. , INOUE N. , UEDA H. , KISHIMOTO Y. , YAMAKAWA K. , FAENOV A.YA. , MAGUNOV A.I. , PIKUZ T.A. , SKOBELEV I.YU. , ABDALLAH J. , CSANAK G. , BOLDAREV A.S. , GASILOV V.A.
Published: April 2008

High-resolutionK-shell spectra of a plasma created by superintense laser irradiation of micron-sized Ar clusters have been measured with an intensity above 1019W/cm2and a pulse duration of 30 fs. The total photon flux of 2 × 108photons/pulse was achieved for Heα1resonant line of Ar (λ = 3.9491 Å, 3.14 keV). In parallel with X-ray measurements, energy distributions of emitted ions have been measured. The multiply charged ions with kinetic energies up to 800 keV were observed. It is found that hot electrons produced by high contrast laser pulses allow the isochoric heating of clusters and shift the ion balance toward the higher charge states, which enhances both the X-ray line yield of the He-like argon ion and the ion kinetic energy.
Ray-tracing simulations of a bent crystal X-ray optics for imaging using laser–plasma X-ray sources
Authors: LABATE L. , GALIMBERTI M. , GIULIETTI A. , GIULIETTI D. , GIZZI L.A. , KÖSTER P. , LAVILLE S. , TOMASSINI P.
Published: April 2008

Ray-tracing simulations of an optical X-ray system based on a spherically bent crystal operating in Bragg configuration for monochromatic projection imaging of thin samples are presented, obtained using a code developed for that purpose. The code is particularly suited for characterizing experimental arrangements routinely used with laser-produced plasma X-ray sources. In particular, the spatial resolution of the imaging system was investigated and a careful study of the complex pattern of the X-ray backlighting beam was performed.
Photopumping of XUV lasers by XFEL radiation
Authors: LAN KE , FILL ERNST , MEYER-TER-VEHN JÜRGEN
Published: April 2008

Within the next few years X-ray free-electron lasers (XFELs) now under construction are expected to generate highly collimated XUV pulses with 1013photons and a duration of 100 fs. Focusing this radiation to a spot some 10 μm in diameter generates intensities of up to 1016W/cm2. Such pump intensities make feasible the investigation of photopumped XUV lasers using this radiation. We present simulations taking into account two different mechanisms generating the gain: (1) photoionization with subsequent three-body recombination, which takes advantage of the monochromaticity of the pump radiation to generate very cold electrons; (2) inner-shell ionization in which transient inversion is obtained by generating a hole in an otherwise completely filled shell. The simulations show that under appropriate conditions both mechanisms generate very high gain. However, a number of further issues must be considered, such as the propagation of the pump pulse in the medium to be pumped.
Manipulating intense XUV coherent light in the temporal domain
Authors: MERDJI H. , HERGOTT J.-F. , KOVACEV M. , PRIORI E. , SALIÈRES P. , CARRÉ B.
Published: April 2008

In this article, we demonstrate the generation of four phase-locked harmonic pulses separated in time using frequency-domain interferometry. The spectra present a high sensitivity to a change of the relative phase on an attosecond time scale. The spectral resolution and the control of this relative phase could be used to perform high resolution measurements.
Picosecond time-resolved X-ray diffraction from laser-shocked semiconductors
Authors: NAKAMURA KAZUTAKA G. , HIRONAKA YOICHIRO , KAWANO HIDETAKA , KISHIMURA HIROAKI , KONDO KEN-ICHI
Published: April 2008

Ultrashort pulsed hard X rays are generated by focusing an intense femtosecond laser beam onto metal targets. Kαemissions are obtained from a Cu target. Picosecond time-resolved X-ray diffraction is performed to investigate structural dynamics of laser-shocked semiconductors using the laser plasma X-ray pulses. Lattice deformation associated with shock-wave propagation is directly observed. Evolution of strain profiles inside the crystal is determined without disturbance from the time-resolved X-ray diffraction patterns.
Acceleration of high-quality, well-collimated return beam of relativistic electrons by intense laser pulse in a low-density plasma
Authors: BAIWEN LI , ISHIGURO S. , šKORIĆ M.M. , TAKAMARU H. , SATO T.
Published: April 2008

The mechanism of electron acceleration by intense laser pulse interacting with an underdense plasma layer is examined by one-dimensional particle-in-cell (1D-PIC) simulations. The standard dephasing limit and the electron acceleration process are discussed briefly. A new phenomenon, of short high-quality, well-collimated return relativistic electron beam with thermal energy spread, is observed in the direction opposite to laser propagation. The process of the electron beam formation, its characteristics, and the time-history inxandpxspace for test electrons in the beam, are analyzed and exposed clearly. Finally, an estimate for the maximum electron energy appears in a good agreement with simulation results.
Hot electron diagnostics using X-rays and Čerenkov radiation
Authors: STEIN J. , FILL E. , HABS D. , PRETZLER G. , WITTE K.
Published: April 2008

The propagation of laser-generated hot electrons through matter and across narrow vacuum gaps is studied. We use the ATLAS titanium–sapphire laser of Max-Planck-Institut für Quantenoptik to irradiate aluminum and copper foils at intensities of up to 1019W/cm2, generating electrons with temperatures in the megaelectron volt range. After propagating through the target the electrons are detected by means of visible Čerenkov radiation generated in a dielectric or hard X-rays emitted from an X-ray “fluor.” These diagnostics allow the electrons to be characterized with respect to their energy, number, and directionality. We also investigate the propagation of the hot electrons across narrow vacuum gaps, with a width ranging from 500 μm down to 50 μm. The effect of self-generated fields in preventing electrons from crossing the gap is demonstrated. Implications of these experiments with respect to fast ignitor physics, developing optics for fourth-generation light sources and X-ray lasers are indicated.
Microscopy characterization of doped fibers
Authors: MILANI M. , BATANI D. , BALLERINI M. , POZZI A. , SAVOIA C.
Published: April 2008

The use of the Soft X-ray Contact Microscopy technique is discussed as a possible new tool to get information on dopant distribution in the core of single-mode optical fibers with 50 nm spatial resolution.
Evaluation and simulations of a Thomson scattering X-ray source based on ray tracing methods
Authors: XIA BING , LI ZHENG , KANG KEJUN , HUANG WENHUI , HUANG GANG , HE XIAOZHONG , DU YINGCHAO , TANG CHUANXIANG
Published: April 2008

We propose to build up a facility of sub-picosecond hard X-ray pulses based on Thomson scattering between femtosecond laser pulses and relativistic electrons which is a useful tool for the purpose of material investigation, plasma diagnostics, and shock wave measurement. This article reviews the principles and the development of X-ray sources based on Thomson scattering. ThenNew Light Source®, the Thomson scattering X-ray facility we will develop is introduced. The characteristics of a Thomson scattering X-ray source are analyzed. A computer model of the Thomson source to be developed is described in order to provide a source of the rays used in a ray tracing method, which has proved to be an essential computer tool for designing and optimizing the optical system of high brightness X-ray facilities. A code for the ray tracing source model is created based on the Monte Carlo approach. It is able to evaluate the properties and performances of the light source under development using this model. According to the simulation results, we discuss the dependence of imaging quality and source properties including spectral distribution, emittance, flux which depends on the laser and electron beam parameters, in order to check if operation performances are as expected. We also estimate the possibility of measuring the energy spectrum of a Thomson scattering source by using a crystal diffraction method. Ray tracing calculations are performed using SHADOW program package, and a new model of Thomson scattering X-ray source which can be processed in that program is established with additional code.
Bright, point X-ray source based on a commercial portable 40 ps Nd:YAG laser system
Authors: FAENOV ANATOLY , PIKUZ TATIANA , MAGUNOV ALEXANDER , BATANI DIMITRI , LUCCHINI GIANNI , CANOVA FEDERICO , PISELLI MORENO
Published: April 2008

We present some experimental results on X-ray spectra obtained from plasmas produced using a compact Nd:YAG laser system. The beam was focused on different targets (Cu, Al, Ge,…) and both high resolution and low resolution X-ray spectra were recorded.
Fusion and nucleosynthesis by hypervelocity impact using hybrid magnetic fields
Authors: POZWOLSKI ALEXANDRE
Published: April 2008

The electromagnetic acceleration of a metallic ring carrying a high current, combined with the mutual interaction of a concentrical conductor carrying the same current, in the presence of a DC magnetic field, results in a converging beam of particles reaching a velocity above 1000 km/s. Lithium, gold, and uranium rings are considered and transient temperatures ranging from 108to 1010°K are expected. In the presence of deuterium and tritium fusion reactions will occur and the capture of the released neutrons may result in the synthesis of heavier elements. The considered device lends itself to a varied treatment allowing to consider velocities in the range of 2000 km/s.
Angular distribution and forward peaking of laser produced plasma
ions
Authors: RAFIQUE M. SHAHID , KHALEEQ-UR-RAHMAN M. , ANWAR MUHAMMAD SHAHBAZ , ASHFAQ FARYAAL MAHMOOD AFSHAN , SIRAJ KHURRAM
Published: April 2008

This paper represents the results of a study of angular distribution
of laser produced ions (LPI) of Al, Cu, and Ag. The angular distribution
is studied by CR-39 (SSNTD) and ion assisted sputtering experiments. A
Q-Switched Nd:YAG laser (1.064 μm, 1.1 MW) with 10 mJ pulsed energy
was used to produce the Ag ions, which were detected by CR-39 detector
mounted at −17.5°, 0°, 17.5°, 30°, 60°, and
90° from the normal to the target placed at a distance of 9 cm from
the target. Etched CR-39 detectors then observed under the Motic DMB
Series optical microscope. A bunch of ions was detected along the normal
of target due to self generated collimation of ions. This is termed as
Forward Peaking of Laser Produced Ions. Similar results were also observed
from sputtering of polished Al substrate by laser produced ions of Cu and
Sputtering of polished Cu substrate by laser produced ions of Al. The
surface morphology of the ion irradiated samples were observed under the
Scanning Electron microscope (SEM) S 300 Hi-tech. Formation of a circular
damage on the surface of the substrates by irradiation conforms the ions
collimation along the normal or Forward Peaking of ions.
Time resolved imaging studies of the plasma produced by laser
ablation of silicon in O2/Ar atmosphere
Authors: TRUSSO S. , BARLETTA E. , BARRECA F. , FAZIO E. , NERI F.
Published: April 2008

The dynamics of the expansion plasma produced by excimer laser
ablation of a silicon target into oxygen and mixed O2/Ar
atmosphere were studied by means of time-resolved imaging of the expanding
plume. Experiments were performed in pure oxygen, ranging between 0.13 and
13.33 Pa, and at different O2/Ar ratios at a fixed total
pressure of 13.33 Pa. The occurrence of a shock wave (SW) generated by the
supersonic expansion of the plasma was observed at high pressure values.
The presence of the SW had a strong influence on the structure of
SiOx thin films. In fact, silicon dioxide thin films were always
obtained in presence of the SW, irrespective of the oxygen content in the
gaseous mixture. On the contrary, suboxide thin films were obtained when
the expansion occurred at lower pressure values (no SW presence). The
temperature rise following the developing of the SW, is supposed to
enhance the oxygen molecules dissociation by increasing the efficiency of
the silicon oxidation reaction.
Recent highlights of the PALS research program
Authors: JUNGWIRTH K.
Published: April 2008

The Prague Asterix Laser System (PALS) research program covers a broad spectrum of laser–plasma experiments in the range of power densities of 1014-5 × 1016W/cm2, aimed at development and applications of laser plasma-based ion and soft X-ray sources of plasma based ultra-bright XUV lasers in particular. In parallel to these two main lines of research, various principal tasks of laser plasma physics are being studied, such as generation and propagation of laser-induced shock waves, laser ablation, and crater creation processes or laser imprint treatment. Results selected of numerous experimental projects performed at PALS within the period 2002–2004 are surveyed in the paper, experiments with intense soft XUV laser beams being highlighted on the first place.
Inertial fusion features in degenerate plasmas
Authors: LEÓN PABLO T. , ELIEZER SHALOM , PIERA MIREIA , MARTÍNEZ-VAL JOSÉ M.
Published: April 2008

Very high plasma densities can be obtained at the end of the implosion phase in inertial fusion targets, particularly in the so-called fast-ignition scheme (Tabaket al., 1994; Mulser & Bauer, 2004), where a central hot spark is not needed at all. By properly tailoring the fuel compression stage, degenerate states can be reached (Azechiet al., 1991; Nakaiet al., 1991; McCory, 1998). In that case, most of the relevant energy transfer mechanisms involving electrons are affected (Honrubia & Tikhonchuk, 2004; Bibi & Matte, 2004; Bibiet al., 2004). For instance, bremsstrahlung emission is highly suppressed (Eliezeret al., 2003). In fact, a low ignition-temperature regime appears at very high plasma densities, due to radiation leakage reduction (Leónet al., 2001). Stopping power and ion-electron coulomb collisions are also changed in this case, which are important mechanisms to trigger ignition by the incoming fast jet, and to launch the fusion wave from the igniting region into the colder, degenerate plasma. All these points are reviewed in this paper. Although degenerate states would not be easy to obtain by target implosion, they present a very interesting upper limit that deserves more attention in order to complete the understanding on the different domains for inertial confinement fusion.
Opacities and line transfer in high density plasma
Authors: MÍNGUEZ E. , RODRÍGUEZ R. , GIL J.M. , SAUVAN P. , FLORIDO R. , RUBIANO J.G. , MARTEL P. , MANCINI R.
Published: April 2008

In this work, we first presents a review of the work that research
teams have developed in collaboration in order to determine the optical
properties of plasmas during the recent years, and showing the
achievements reached. The second part of this paper is devoted to one of
these improvements, which is to include reabsorption of the radiation in
the calculations of dense optically thick plasmas in non-LTE conditions.
Two models recently developed for this purpose are presented. The
quantitative study was focused on aluminum plasmas, which was obtained
recently at LULI experiments.
Progress in ICF programs at CAEP
Authors: PENG H.S. , ZHANG W.Y. , ZHANG X.M. , TANG Y.J. , ZHENG W.G. , ZHENG Z.J. , WEI X.F. , DING Y.K. , GOU Y. , ZHOU S.P. , PEI W.B.
Published: April 2008

Laser technology developments, including construction of a 286-TW
Ti:Sapphire laser with a focused intensity of
1021W/cm2, installation of the TIL, prototype of
the SG-III, and operation of the SG-II laser are presented. Results of the
experiments on hohlraum physics, indirect-drive implosion, Thomson
scattering, EOS, and X-ray laser are briefly introduced. Simulations and a
code package, LARED, for target physics are outlined.
Theoretical model for heat conduction in metals during interaction
with ultra short laser pulse
Authors: ANWAR MUHAMMAD SHAHBAZ , LATIF ANWAR , IQBAL M. , RAFIQUE M. SHAHID , KHALEEQ-UR-RAHMAN M. , SIDDIQUE SOFIA
Published: April 2008

Theoretical studies have been performed on the interaction of short
laser pulse with metals. The results of the theoretical model indicate
that heat conduction would not be uniform from focal spot or crater at the
surface of target metal, when an ultra short laser will interact with the
metal. The electromagnetic radiations of laser induce electric field
inside the target that is responsible for the induction of current
density, which causes electronic heat conduction in the direction of
current density. Such an effect is dominant for laser pulse having
duration less than of the order of sub-picoseconds. This mode will open a
new significant field of study to discuss laser metal interaction for
ultra short laser pulses.
A spiral strip transformer type electron-beam accelerator
Authors: LIU JINLIANG , LI CHUANLU , ZHANG JIANDE , LI SHIZHONG , WANG XINXIN
Published: April 2008

An electron-beam accelerator, which consists of a primary storage
capacitor system, an air core spiral strip transformer, water Blumlein
line of water dielectric, and a field-emission diode, is described. The
experimental results show that the diode voltage is more than 400 kV,
electron beam current is about 40 kA, and the pulse duration is more than
80 ns. This accelerator is very compact and works stably and reliably.