Caractérisation spatio-temporelle d'une chaîne laser à 32.8 nm par plasma laser et perspectives vers une source ultrabrève et intense

Plasma-based soft-x-ray lasers are a promising coherent source complementary to the soft-x-ray free-electron laser. While most of the soft x-ray lasers in the world operate in the ampliifcation of spontaneouse emission regime, we study here a oscillator-amplifier geometry, following the example of power lasers in the VIS/IR range. The amplifier is a 8 times-ionized krypton plasma resulting from optical field ionization of a low-density gaseous target by an intense laser pulse. This amplifier is seeded by a high-order harmonic radiation at the same wavelength. We present here experimental and numerical results focused on the effect of the amplification on the spatial characteristics of the resulting beam (spatial profile, transverse coherence and wavefront) as well as its spectro-temporal properties. We show that, due to spatial filtering of the harmonic beam by the amplifier, the seeded beam exhibits remarkable characteristics such as a strong transverse coherence and a near diffraction-limit wavefront. It is also shown that its spatial profile can be controled from a Gaussian profile to a Bessel profile with several intense rings. This beam is made up of Fourier-limited microjoule picosecond pulses. A lead to enhance this source intensity is to generate the amplifier in a near-critical density plasma. This was achieved by guiding the pump laser pulse in an optically preformed waveguide. Guiding was demonstrated and promising results were obtained in the amplification of spontaneous emission regime.Ce travail s'inscrit dans le cadre du développement des sources XUV impulsionnelles cohérentes, et plus particulièrement des lasers XUV. Alors que la plupart des lasers XUV fonctionnent en régime d'émission spontanée amplifiée, c'est ici une géométrie de type oscillateur-amplificateur inspirée des lasers de puissance qui est étudiée. L'amplificateur, un plasma de krypton ionisé 8 fois par un champ laser intense, est injecté par un rayonnement harmonique d'ordre élevé à la même longueur d'onde. La source ainsi créée est étudiée aussi bien expérimentalement que numériquement, et l'accent est mis sur l'effet de l'amplification du faisceau harmonique par le plasma sur les caractéristiques spatiales du faisceau résultant (profil spatial, cohérence transverse et front d'onde), et ses caractéristiques spectro-temporelles. Il est entre autres démontré que, du fait du filtrage spatial par l'amplificateur, le faisceau possède des qualités optiques dignes du domaine visible (forte cohérence et proche de la limite de diffraction) et que son profil spatial peut être contrôlé entre un profil gaussien et un profil de Bessel. Ce faisceau se compose par ailleurs d'impulsion µJ de durée ps en limite de Fourier. Afin d'augmenter l'intensité de ces impulsions, on se propose également de guider l'impulsion de pompe dans un canal plasma préformé par laser à densité quasi-critique. Le guidage a été démontré et des résultats prometteurs ont ainsi été obtenus en régime non-injecté

Three-dimensional Maxwell-Bloch calculation of the temporal profile of a seeded soft x-ray laser pulse

International audienceWe present three-dimensional modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities , a simplified one-dimensional calculation leads to strong Rabi-like temporal oscillations of the output signal. However, such oscillations have not been observed experimentally. Our three-dimensional calculations demonstrate that this is due to spatial non-uniformities in the plasma gain that cause the Rabi oscillations to dampen dramatically. Large amplitude Rabi-like oscillations are expected to appear only in long and uniform plasma. Such targets require optimized guiding techniques

Sobre a (im)prescritibilidade da pretensão de ressarcimento de danos causados ao erário por ato de improbidade administrativa

TCC(graduação) - Universidade Federal de Santa Catarina. Centro de Ciências Jurídicas. Direito.O presente trabalho objetiva dirimir a dúvida existente acerca da sujeição da pretensão de ressarcimento de danos causados ao erário por ato de improbidade administrativa ao instituto da prescrição. Tal incerteza surge a partir da redação da ressalva contida no artigo 37, § 5º, da Constituição da República Federativa do Brasil, que possibilita variadas interpretações, algumas defendendo a imprescritibilidade de tal pretensão, outras advogando a tese da sujeição de tal pretensão a lapso prescricional. Com o fito de se concluir pelo entendimento mais condizente à ordem jurídico-normativa brasileira, divide-se o estudo em três capítulos principais. No primeiro, são delineados o conceito e a finalidade da prescrição, apresentando-se a garantia da segurança jurídica como sendo seu fundamento constitucional. Em seguida, são explanadas as noções de supremacia do interesse público e segurança jurídica, entendidas por boa parte dos estudiosos como os dois princípios a partir dos quais deve ser analisada a questão. Por fim, no terceiro e último capítulo, depois de serem descritas as principais interpretações que podem derivar da leitura do dispositivo, chega-se ao entendimento pela conformação da pretensão estudada ao manto prescricional. Esta acepção parece melhor se amoldar aos contornos pretendidos pelo ordenamento jurídico-constitucional contemporâneo para a construção de um Direito Administrativo efetivamente democrático, com a sobrevalorização dos direitos fundamentais, individuais e coletivos, e a realização da ideia de supremacia da Constituição. No plano legal e normativo, a compreensão pela prescritibilidade afigura-se coerente à própria construção histórica do comando constitucional, com a delimitação de dois regimes prescricionais distintos para as pretensões originadas com a prática de ato de improbidade. Da mesma forma, a identificação do ressarcimento como consequência, e não sanção, do ato ímprobo, bem como a evolução do tratamento dado à matéria pelo Supremo Tribunal Federal parecem caminhar no mesmo sentido interpretativo

Measurement of the B0_s semileptonic branching ratio to an orbitally excited D_s** state, Br(B0_s -> Ds1(2536) mu nu)

In a data sample of approximately 1.3 fb-1 collected with the D0 detector between 2002 and 2006, the orbitally excited charm state D_s1(2536) has been observed with a measured mass of 2535.7 +/- 0.6 (stat) +/- 0.5 (syst) MeV via the decay mode B0_s -> D_s1(2536) mu nu X. A first measurement is made of the branching ratio product Br(b(bar) -> D_s1(2536) mu nu X).Br(D_s1(2536)->D* K0_S). Assuming that D_s1(2536) production in semileptonic decay is entirely from B0_s, an extraction of the semileptonic branching ratio Br(B0_s -> D_s1(2536) mu nu X) is made.Comment: 7 pages, 2 figures, LaTeX, version with minor changes as accepted by Phys. Rev. Let

Simultaneous measurement of the ratio B(t->Wb)/B(t->Wq) and the top quark pair production cross section with the D0 detector at sqrt(s)=1.96 TeV

We present the first simultaneous measurement of the ratio of branching fractions, R=B(t->Wb)/B(t->Wq), with q being a d, s, or b quark, and the top quark pair production cross section sigma_ttbar in the lepton plus jets channel using 0.9 fb-1 of ppbar collision data at sqrt(s)=1.96 TeV collected with the D0 detector. We extract R and sigma_ttbar by analyzing samples of events with 0, 1 and >= 2 identified b jets. We measure R = 0.97 +0.09-0.08 (stat+syst) and sigma_ttbar = 8.18 +0.90-0.84 (stat+syst)} +/-0.50 (lumi) pb, in agreement with the standard model prediction.Comment: submitted to Phys.Rev.Letter

Search for charged Higgs bosons decaying to top and bottom quarks in ppbar collisions

We describe a search for production of a charged Higgs boson, q \bar{q'} -> H^+, reconstructed in the t\bar{b} final state in the mass range 180 <= M_{H^+} <= 300 GeV. The search was undertaken at the Fermilab Tevatron collider with a center-of-mass energy sqrt{s} = 1.96 TeV and uses 0.9 fb^{-1} of data collected with the D0 detector. We find no evidence for charged Higgs boson production and set upper limits on the production cross section in the Types I, II and III two-Higgs-doublet models (2HDMs). An excluded region in the (M_{H^+},tan\beta) plane for Type I 2HDM is presented.Comment: Submitted to Phys. Rev. Letter

Measurement of the forward-backward charge asymmetry and extraction of sin^2Theta^{eff}_W in ppbar -> Z/\gamma^{*}+X -> e+e+X events produced at \sqrt{s}=1.96 TeV

We present a measurement of the forward-backward charge asymmetry ($A_{FB}$) in $p\bar{p} \to Z/\gamma^{*}+X \to e^+e^-+X$ events at a center-of-mass energy of 1.96 TeV using 1.1 fb$^{-1}$ of data collected with the D0 detector at the Fermilab Tevatron collider. $A_{FB}$ is measured as a function of the invariant mass of the electron-positron pair, and found to be consistent with the standard model prediction. We use the $A_{FB}$ measurement to extract the effective weak mixing angle $sin^2Theta^{eff}_W = 0.2327 \pm 0.0018 (stat.) \pm 0.0006 (syst.)$.Comment: 7 Pages, 1 Figure, 3 Tables, Accepted by Phys. Rev. Let

Measurement of the lifetime of the B_c meson in the semileptonic decay channel

Using approximately 1.3 fb-1 of data collected by the D0 detector between 2002 and 2006, we measure the lifetime of the B_c meson in the B_c -> J/psi mu nu X final state. A simultaneous unbinned likelihood fit to the J/\psi+mu invariant mass and lifetime distributions yields a signal of 881 +/- 80 (stat) candidates and a lifetime measurement of \tau(B_c) = 0.448 +0.038 -0.036 (stat) +/- 0.032 (syst) ps.Comment: 7 pages, 2 figures, submitted to Phys. Rev. Let

Observation of ZZ production in ppbar collisions at sqrt(s) = 1.96 TeV

We present an observation for ZZ -> l+l-l'+l'- (l, l' = e or mu) production in ppbar collisions at a center-of-mass energy of sqrt(s) = 1.96 TeV. Using 1.7 fb-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider, we observe three candidate events with an expected background of 0.14 +0.03 -0.02 events. The significance of this observation is 5.3 standard deviations. The combination of D0 results in this channel, as well as in ZZ -> l+l-nunubar, yields a significance of 5.7 standard deviations and a combined cross section of sigma(ZZ) = 1.60 +/- 0.63 (stat.) +0.16 -0.17 (syst.) pb.Comment: 7 pages, 1 figure, 2 tables Modified slightly following review proces