24,214 research outputs found
Controlled Shock Shells and Intracluster Fusion Reactions in the Explosion of Large Clusters
The ion phase-space dynamics in the Coulomb explosion of very large ( atoms) deuterium clusters can be tailored using two consecutive
laser pulses with different intensities and an appropriate time delay. For
suitable sets of laser parameters (intensities and delay), large-scale shock
shells form during the explosion, thus highly increasing the probability of
fusion reactions within the single exploding clusters. In order to analyze the
ion dynamics and evaluate the intracluster reaction rate, a one-dimensional
theory is used, which approximately accounts for the electron expulsion from
the clusters. It is found that, for very large clusters (initial radius
100 nm), and optimal laser parameters, the intracluster fusion yield becomes
comparable to the intercluster fusion yield. The validity of the results is
confirmed with three-dimensional particle-in-cell simulations.Comment: 25 pages, 11 figures, to appear in Physical Review
Diversidade genética de Dorstenia elata Hook. (Moraceae) em um fragmento de floresta atlântica
Dorstenia é o segundo maior gênero da família Moraceae, contendo espécies
endêmicas com qualidades medicinais e interesse farmacêutico. No Brasil, estão distribuídas entre os diversos biomas, inclusive na Floresta Atlântica, e são consideradas hotspot da biodiversidade (elevado nível de biodiversidade,
endemismo e alto grau de degradação). Contudo, devido a ação humana, esta
floresta vem tendo sua estrutura modificada, causando a redução da densidade
entre as espécies, afetando o modo de reprodução, polinização e distribuição do fluxo gênico entre e dentro de populações. Estes fatos podem levar a uma
diminuição da diversidade genética. Assim, o objetivo do presente trabalho foi
caracterizar a diversidade genética entre e dentro de populações de Dosrtenia elata em um remanescente de Floresta Atlântica no estado do Espírito Santo. Foram coletados três agregados de indivíduos de D. elata no Parque Estadual Mata das Flores (Unidade de Conservação), no município de Castelo. Para a caracterização molecular, foram utilizados 12 primers ISSR (Inter Simple Sequence Repeats), onde os dados obtidos foram analisados e submetidos à analises estatísticas referentes a diversidade entre e dentro de populações. Para análises intrapopulacionais, foi calculada a matriz de distância entre pares de indivíduos, realizada o cálculo da correlação cofenética e gerado um dendrograma de agrupamento das populações, que evidenciaram a formação de dois grupos, o primeiro contendo a população 1 e 2, e o segundo contendo a população 3. Isto pode ter ocorrido em virtude da distância geográfica entre as populações amostradas. Para as análises interpopulacionais, foi realizada a análise de variância molecular, calculado o Índice de Shannon, a heterozigosidade total esperada e o fluxo gênico. De acordo com os resultados obtidos, a maior variabilidade genética foi observada dentro das populações, e o fluxo gênico foi alto tanto entre quanto dentro de populações, indicando que não está havendo perda de diversidade genética devido as barreiras antrópicas ou geográficas, e que o método de dispersão tem grande influência na distribuição da diversidade genética em populações de Dorstenia elata.
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Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Dynamics and control of the expansion of finite-size plasmas produced in ultraintense laser-matter interactions
The strong influence of the electron dynamics provides the possibility of
controlling the expansion of laser-produced plasmas by appropriately shaping
the laser pulse. A simple irradiation scheme is proposed to tailor the
explosion of large deuterium clusters, inducing the formation of shock
structures, capable of driving nuclear fusion reactions. Such a scenario has
been thoroughly investigated, resorting to two- and three-dimensional
particle-in-cell simulations. Furthermore, the intricate dynamics of ions and
electrons during the collisionless expansion of spherical nanoplasmas has been
analyzed in detail using a self-consistent ergodic-kinetic model. This study
clarifies the transition from hydrodynamic-like to Coulomb-explosion regimes
Modelling radiation emission in the transition from the classical to the quantum regime
An emissivity formula is derived using the generalised
Fermi-Weizacker-Williams method of virtual photons which accounts for the
recoil the charged particle experiences as it emits radiation. It is found that
through this derivation the formula obtained by Sokolov et al using QED
perturbation theory is recovered. The corrected emissivity formula is applied
to nonlinear Thomson scattering scenarios in the transition from the classical
to the quantum regime, for small values of the nonlinear quantum parameter
\chi. Good agreement is found between this method and a QED probabilistic
approach for scenarios where both are valid. In addition, signatures of the
quantum corrections are identified and explored.Comment: 11 pages, 4 figures, submitted for publicatio
Radiation in 1.5 GeV and 12 GeV laser wakefield acceleration stages from PIC simulations
WOS:000287176200029 (Nº de Acesso Web of Science)A massivelly parallel post-processing radiation diagnostic for PIC codes is presented, which is then used to study the main features of the radiation from single LWFA stages (1.5 GeV and 12 GeV). This diagnostic also allows to examine radiation signatures associated with the physics of self-injection
Magnetically assisted self-injection and radiation generation for plasma based acceleration
It is shown through analytical modeling and numerical simulations that
external magnetic fields can relax the self-trapping thresholds in plasma based
accelerators. In addition, the transverse location where self-trapping occurs
can be selected by adequate choice of the spatial profile of the external
magnetic field. We also find that magnetic-field assisted self-injection can
lead to the emission of betatron radiation at well defined frequencies. This
controlled injection technique could be explored using state-of-the-art
magnetic fields in current/next generation plasma/laser wakefield accelerator
experiments.Comment: 7 pages, 4 figures, accepted for publication in Plasma Physics and
Controlled Fusio
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