6,951 research outputs found
Limits of flexural wave absorption by open lossy resonators: reflection and transmission problems
The limits of flexural wave absorption by open lossy resonators are
analytically and numerically reported in this work for both the reflection and
transmission problems. An experimental validation for the reflection problem is
presented. The reflection and transmission of flexural waves in 1D resonant
thin beams are analyzed by means of the transfer matrix method. The hypotheses,
on which the analytical model relies, are validated by experimental results.
The open lossy resonator, consisting of a finite length beam thinner than the
main beam, presents both energy leakage due to the aperture of the resonators
to the main beam and inherent losses due to the viscoelastic damping. Wave
absorption is found to be limited by the balance between the energy leakage and
the inherent losses of the open lossy resonator. The perfect compensation of
these two elements is known as the critical coupling condition and can be
easily tuned by the geometry of the resonator. On the one hand, the scattering
in the reflection problem is represented by the reflection coefficient. A
single symmetry of the resonance is used to obtain the critical coupling
condition. Therefore the perfect absorption can be obtained in this case. On
the other hand, the transmission problem is represented by two eigenvalues of
the scattering matrix, representing the symmetric and anti-symmetric parts of
the full scattering problem. In the geometry analyzed in this work, only one
kind of symmetry can be critically coupled, and therefore, the maximal
absorption in the transmission problem is limited to 0.5. The results shown in
this work pave the way to the design of resonators for efficient flexural wave
absorption
3D printing of gas jet nozzles for laser-plasma accelerators
Recent results on laser wakefield acceleration in tailored plasma channels
have underlined the importance of controlling the density profile of the gas
target. In particular it was reported that appropriate density tailoring can
result in improved injection, acceleration and collimation of laser-accelerated
electron beams. To achieve such profiles innovative target designs are
required. For this purpose we have reviewed the usage of additive layer
manufacturing, commonly known as 3D printing, in order to produce gas jet
nozzles. Notably we have compared the performance of two industry standard
techniques, namely selective laser sintering (SLS) and stereolithography (SLA).
Furthermore we have used the common fused deposition modeling (FDM) to
reproduce basic gas jet designs and used SLA and SLS for more sophisticated
nozzle designs. The nozzles are characterized interferometrically and used for
electron acceleration experiments with the Salle Jaune terawatt laser at
Laboratoire d'Optique Appliqu\'ee
EP-1796: Definition of thresholds to detect anatomy changes using Delivery Analysis software for Tomotherapy
The tropospheric gas composition of Jupiter's north equatorial belt (NH3, PH3, CH3D, GeH4, H2O) and the Jovian D/H isotropic ratio
The gas composition of the troposphere of Jupiter in the clearest regions of the North Equatorial Belt (NEB) was derived from the Voyager 1 IRIS data. The infrared spectrum for this homogeneous cloud free region was modeled to infer altitude profiles for NH3, PH3, GeH4 and H2O. The Profiles for NH3 and PH3 were found to be depleted in the upper troposphere but otherwise in agreement with their solar values at the 1 bar level. The mole fraction for CH3D was determined to be 3.5(+1.0 or -1.3) x 10 to the minus 7th power. The GeH4 mole fraction of 7+ or -2 x 10 to the minus 10th power at the 2 to 3 bar level is a factor of 10 lower than the solar value. The H2O mole fraction is approximately 1 x 0.00001 at the 2.5 bar level and is increasing to approximately 3 x 0.00001 at 4 bars where it is a factor of 30 lower than solar. Using IRIS infrared values for the mole fractions of CH3D and CH4 a value of D/H = 3.6(+1.0 or -1.4)x 0.00001 is derived. Assuming this Jovian D/H ratio is representative of the protosolar nebula, and correcting for chemical galactic evolution, yields a value of 5.5 - 9.0 x 0.00001 for the primordial D/H ratio and an upper limit of 1.8 to 2.4 x 10 to the minus 31st power cu cm for the present day baryon density
Cephalophus ogilbyi crusalbum Grubb 1978, described from coastal Gabon, is quite common in the ForĂŞt des Abeilles, Central Gabon
La nouvelle forme de céphalophe (Cephalophus ogilbyi crusalbum) décrite en 1978 par Grubb à partir de quelques spécimens de musée était supposée avoir une distribution limitée à la zone côtière du Gabon. De ce fait, elle fut
classée comme rare et comme faisant partie des espèces nécessitant des mesures de protection urgentes. De récentes observations effectuées sur le site d'étude du Camp de la Makandé (Forêt des Abeilles) montrent que cette espèce est en fait commune dans cette région où elle est régulièrement
observée. Les jarrets blancs qui la caractérisent constituent un critère d'identification aisé. Des observations complémentaires faites dans la réserve de la Lopé confirment que C. o. crusalbum est également présent sur la rive gauche de l'Offou
La vallée de l'Escaut à Pont-à -Chin
The sudden opening of a karst pipe in the abandoned bed of the Scheldt River has exposed 8 m ofHolocene deposits; their stratigraphy, archaeological contents, fossil fauna and flora are studied here. A well dated early occupation is from the First Iron Age to Early La Tène (around 650-450 B.C.), a second one from Roman times (around 50-200 A.D.); they permit a close control of the14 C dates, which are systematically too old, due to reworking in the stream. Vertebrate and molluscan assemblages are discussed separatel
Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control
We present an adaptive technique for the formation of multiple co-propagating and stationary filaments in a gaseous medium. Wavefront shaping of the initial beam is performed using a deformable mirror to achieve a complete two-dimensional control of the multi-spot intensity pattern in the laser focus. The spatial organization of these intensity spots yields reliable formation of up to five stable and stationary filaments providing a test bed for fundamental studies on multiple filamentatio
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