Effects of buried high-Z layers on fast electron propagation

<p>By extending a prior model [A. R. Bell, J.R. Davies, S. M. Guerin, Phys. Rev. E 58, 2471 (1998)], the magnetic field generated during the transport of a fast electron beam driven by an ultraintense laser in a solid target is derived analytically and applied to estimate the effect of such field on fast electron propagation through a buried high-Z layer in a lower-Z target. It is found that the effect gets weaker with the increase of the depth of the buried layer, the divergence of the fast electrons, and the laser intensity, indicating that magnetic field effects on the fast electron divergence as measured from K-a X-ray emission may need to be considered for moderate laser intensities. On the basis of the calculations, some considerations are made on how one can mitigate the effect of the magnetic field generated at the interface.</p>

Enhanced proton beam collimation in the ultra-intense short pulse regime

The collimation of proton beams accelerated during ultra-intense laser irradiation of thin aluminum foils was measured experimentally whilst varying laser contrast. Increasing the laser contrast using a double plasma mirror system resulted in a marked decrease in proton beam divergence (20° to &lt;10°), and the enhanced collimation persisted over a wide range of target thicknesses (50 nm–6 µm), with an increased flux towards thinner targets. Supported by numerical simulation, the larger beam divergence at low contrast is attributed to the presence of a significant plasma scale length on the target front surface. This alters the fast electron generation and injection into the target, affecting the resultant sheath distribution and dynamics at the rear target surface. This result demonstrates that careful control of the laser contrast will be important for future laser-driven ion applications in which control of beam divergence is crucial.

Effects of resistive magnetic field on fast electron divergence measured in experiments

<p>Transport of fast electrons driven by an ultraintense laser through a tracer layer buried in solid targets is studied by particle-in-cell simulations. It is found that intense resistive magnetic fields, having a magnitude of several thousand Tesla, are generated at the interfaces of the materials due to the steep resistivity gradient between the target and tracer layer. Such magnetic fields can significantly inhibit the fast electron propagation. The electrons that can penetrate the first interface are mostly confined in the buried layer by the magnetic fields and cause heating of the tracer layer. The lateral extent of the heated region can be significantly larger than that of the relativistic electron beam. This finding suggests that the relativistic electron divergence inferred from Ká x-ray emission in experiments might be overestimated.</p>

Laser-driven generation of high-quality ultra-relativistic positron beams

An ultra-relativistic electron beam propagating through a high-Z solid triggersan electromagnetic cascade, whereby a large number of high-energy photons andelectron–positron pairs are produced mainly via the bremsstrahlung and Bethe–Heitler processes, respectively. These mechanisms are routinely used to generatepositron beams in conventional accelerators such as the electron–positron collider(LEP). Here we show that the application of similar physical mechanisms to a laserdrivenelectron source allows for the generation of high-quality positron beams in amuch more compact and cheaper configuration. We anticipate that the applicationof these results to the next generation of lasers might open the pathway for therealization of an all-optical high-energy electron–positron collider.

Overview of laser-driven generation of electron–positron beams

<p>Electron–positron (e–p) plasmas are widely thought to be emitted, in the form of ultra-relativistic winds or collimated jets, by some of the most energetic or powerful objects in the Universe, such as black-holes, pulsars, and quasars. These phenomena represent an unmatched astrophysical laboratory to test physics at its limit and, given their immense distance from Earth (some even farther than several billion light years), they also provide a unique window on the very early stages of our Universe. However, due to such gigantic distances, their properties are only inferred from the indirect interpretation of their radiative signatures and from matching numerical models: their generation mechanism and dynamics still pose complicated enigmas to the scientific community. Small-scale reproductions in the laboratory would represent a fundamental step towards a deeper understanding of this exotic state of matter. Here we present recent experimental results concerning the laser-driven production of ultra-relativistic e–p beams. In particular, we focus on the possibility of generating beams that present charge neutrality and that allow for collective effects in their dynamics, necessary ingredients for the testing pair-plasma physics in the laboratory. A brief discussion of the analytical and numerical modelling of the dynamics of these plasmas is also presented in order to provide a summary of the novel plasma physics that can be accessed with these objects. Finally, general considerations on the scalability of laboratory plasmas up to astrophysical scenarios are given.</p>

Driving high energy ion beams with light pressure

State-of-the-art high power lasers can exert immense pressure on thin foils which can be used to accelerate energetic ion beams efficiently at the laser plasma interface.

Atomic data for modelling fusion and astrophysical plasmas

Trends and focii of interest in atomic modelling and data are identified in connection with recent observations and experiments in fusion and astrophysics. In the fusion domain, spectral observations are included of core, beam penetrated and divertor plasma. The helium beam experiments at JET and the studies with very heavy species at ASDEX and JET are noted. In the astrophysics domain, illustrations are given from the SOHO and CHANDRA spacecraft which span from the solar upper atmosphere, through soft x-rays from comets to supernovae remnants. It is shown that non-Maxwellian, dynamic and possibly optically thick regimes must be considered. The generalized collisional-radiative model properly describes the collisional regime of most astrophysical and laboratory fusion plasmas and yields self-consistent derived data for spectral emission, power balance and ionization state studies. The tuning of this method to routine analysis of the spectral observations is described. A forward look is taken as to how such atomic modelling, and the atomic data which underpin it, ought to evolve to deal with the extended conditions and novel environments of the illustrations. It is noted that atomic physics influences most aspects of fusion and astrophysical plasma behaviour but the effectiveness of analysis depends on the quality of the bi-directional pathway from fundamental data production through atomic/plasma model development to the confrontation with experiment. The principal atomic data capability at JET, and other fusion and astrophysical laboratories, is supplied via the Atomic Data and Analysis Structure (ADAS) Project. The close ties between the various experiments and ADAS have helped in this path of communication.

Electron impact fragmentation of thymine:partial ionization cross sections for positive fragments

We have measured mass spectra for positive ions for low-energy electron impact on thymine using a reflectron time-of-flight mass spectrometer. Using computer controlled data acquisition, mass spectra have been acquired for electron impact energies up to 100 eV in steps of 0.5 eV. Ion yield curves for most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The ion yield curves have been normalized by comparing the sum of the ion yields to the average of calculated total ionization cross sections. Appearance energies have been determined. The nearly equal appearance energies of 83 u and 55 u observed in the present work strongly indicate that near threshold the 55 u ion is formed directly by the breakage of two bonds in the ring, rather than from a successive loss of HNCO and CO from the parent ion. Likewise 54 u is not formed by CO loss from 82 u. The appearance energies are in a number of cases consistent with the loss of one or more hydrogen atoms from a heavier fragment, but 70 u is not formed by hydrogen loss from 71 u.

A new avalanche model for solar flares

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Caractérisation photométrique et spectroscopique de compagnons sous-stellaires de faible masse autour d'étoiles de la région de formation Upper Scorpius

Suite à la découverte d’environ 2000 naines brunes au cours des deux dernières décennies, on commence à bien comprendre la physique de ces objets de masse intermédiaire entre les étoiles et les planètes. Malgré tout, les modèles d’atmosphère et d’évolution de ces objets peu massifs peinent toujours à reproduire fidèlement leurs caractéristiques pour les âges les plus jeunes. Ce travail propose la caractérisation de quatre compagnons de masse sous-stellaire (8-30 MJup) en orbite à grande séparation (300-900 UA) autour d'étoiles jeunes (5 Ma) de la région de formation Upper Scorpius. De nouveaux spectres (0,9-2,5 um) et de nouvelles mesures photométriques (YJHKsL') sont présentés et analysés, dans le but de déterminer la masse, température effective, luminosité et gravité de surface de ces compagnons, tout en évaluant la fidélité avec laquelle les spectres synthétiques tirés de deux modèles d’atmosphère récents reproduisent les spectres observés.

A low-inductance, long-life, triggered spark gap switch for Blumlein-driven lasers

A low inductance, triggered spark gap switch suitable for a high-current fast discharge system has been developed. The details of the design and fabrication of this pressurized spark gap, which uses only commonly available materials are described. A transverse discharge Blumlein-driven N2 laser incorporating this device gives a peak output power of 700 kW with a FWHM of 3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser system.

Scattering and absorption by acoustic resonators

by Karl Uno Ingard.