284 research outputs found

    Some remarks on invariant subspaces in real Banach spaces (revised version)

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    It is proved that a commutative algebra AA of operators on a reflexive real Banach space has an invariant subspace if each operator TAT\in A satisfies the condition 1εT2e1+o(ε) when ε0,\|1- \varepsilon T^2\|_e \le 1 + o(\varepsilon) \text{ when } \varepsilon\searrow 0, where e\|\cdot\|_e is the essential norm. This implies the existence of an invariant subspace for every commutative family of essentially selfadjoint operators on a real Hilbert space

    Parametric frequency mixing in the magneto-elastically driven FMR-oscillator

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    We demonstrate the nonlinear frequency conversion of ferromagnetic resonance (FMR) frequency by optically excited elastic waves in a thin metallic film on dielectric substrates. Time-resolved probing of the magnetization directly witnesses magneto-elastically driven second harmonic generation, sum- and difference frequency mixing from two distinct frequencies, as well as parametric downconversion of each individual drive frequency. Starting from the Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an elastically driven nonlinear parametric oscillator and show that frequency mixing is dominated by the parametric modulation of FMR frequency

    The spall strength limit of matter at ultrahigh strain rates induced by laser shock waves

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    New results concerning the process of dynamic fracture of materials (spallation) by laser-induced shock waves are presented. The Nd-glass laser installations SIRIUS and KAMERTON were used for generation of shock waves with pressure up to 1 Mbar in plane Al alloy targets. The wavelengths of laser radiation were 1.06 and 0.53 μm, the target thickness was changed from 180 to 460 μm, and the laser radiation was focused in a spot with a 1-mm diameter on the surface of AMg6M aluminum alloy targets. Experimental results were compared to predictions of a numerical code which employed a real semiempirical wide-range equation of state. Strain rates in experiments were changed from 106 to 5 × 107 s−1. Two regimes of spallation were evidenced: the already known dynamic regime and a new quasi-stationary regime. An ultimate dynamic strength of 80 kbar was measured. Finally, experiments on targets with artificial spall layers were performed showing material hardening due to shock-wave compression

    Thermal simulations of a C beam stripper for experiments at Spiral2

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    PROBLEMS OF MEASUREMENT OF DENSE PLASMA HEATING IN LASER SHOCK-WAVE COMPRESSION

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    Experimental results of heating measurements of matter carried out in a study of laser-driven shock waves in aluminum (Batani et al. 1997) are discussed. The measured temporal evolution of the "color" temperature of the rear surface of the target is compared with that computed by a numerical code. It has been established that the target preheating can substantially affect optical signal features recorded from the rear side of the target, and consequently influence upon the accuracy of temperature and pressure measurements of the material behind the shock wave front

    Experimental Reconstruction of Lomonosov's Discovery of Venus's Atmosphere with Antique Refractors During the 2012 Transit of Venus

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    In 1761, the Russian polymath Mikhail Vasilievich Lomonosov (1711-1765) discovered the atmosphere of Venus during its transit over the Sun's disc. In this paper we report on experimental reenactments of Lomonosov's discovery with antique refractors during the transit of Venus June 5-6, 2012. We conclude that Lomonosov's telescope was fully adequate to the task of detecting the arc of light around Venus off the Sun's disc during ingress or egress if proper experimental techniques as described by Lomonosov in his 1761 report are employed.Comment: 14 pages, 9 figure
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