It is known that losses of ultra-cold neutrons (UCN) from traps are due to theβ-decay, up scattering and absorption on a surface but we have identified forthe first time a complementary mechanism. We suppose that this arises from an increasein energy of the neutrons resulting in an upper energy spectral cut-off which is abouttwice higher than that for initial stored neutrons. The total probability for such aprocess equals ~10−5 per collision with a trap wall made of stainless steelwithout or with a Fomblin oil and grease coating.

Nd3+-doped KGd(WO4)2 single crystals are investigated as laser mediaand nonlinear Raman shifters. They have been grown by the top nucleated floatingcrystal method. They were Maynly obtained as platelets with two large (010) faces(i.e. perpendicular to the two-fold b'-axis of the monoclinic lattice). Theirspectroscopic properties are given at room and liquid helium temperature:up-conversion fluorescence, absorption from the 4 I 9/2 ground state for an extended Judd-Ofelt parameters analysis, absorption from the 4 F 3/2 excited statenear 1067 nm and Raman spectra have been measured in the direction of the b'-axis intwo orthogonal polarizations.

Under isostatic pressure aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, and thus to modify the elastic properties of aerogels. We show that the elasticity of the material is strongly influenced by structural transformation, and we observe a weakening of the network during the first stage of densification. The structural evolution, followed by Small Angle X-ray Scattering, shows that the densification mechanism is different from the one obtained by a sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, in contrast to viscous sintering.

The magnetic field of an annular MHD thruster made of independent superconductingmodules has been studied with analytical and numerical methods. This configurationallows to obtain large magnetized volumes and high induction levels with rapidlydecreasing stray fields. When some inductors are out of order, the thruster reMaynsstill operational, but the stray fields increase in the vicinity of the failure. Forgiven structural materials and superconductors, it is possible to determine the sizeof the conductor in order to reduce the electromagnetic forces and the peak fieldsupported by the conductors. For an active field of 10 T in a 6 m ray annular activechannel of a thruster with 24 modules, the peak field is exactly 15.6 T in theNb3Sn conductors and the structure has to sustain 108 N/m forces. The necessityto place some magnetic or superconducting shield is discussed, particularly when the thruster is in a degraded regime.

Silicon-On-Insulator waveguide can be used as photoconductor, the light being coupled in the silicon film by a diffraction grating. Nonlinear effects are induced by the photogeneration of electron-hole pairs. This leads to photocurrent variations which are much faster than in linear regime and also much steeper than the incident light pulse. A model is presented which takes into account the refractive index variations arising from both the excess carrier density and the temperature rise induced by carrier recombination and Joule effect. The photocurrent is calculated for various values of the incident light power, incidence angle and pulse duration.

An argon Ion Beam Etching (IBE) simulation model has been developed to investigate the mesa profile evolution in III-V semiconductors' technology. Particular attention hasbeen focused on the sputtering yield angular dependence effect, on the influence ofthe material and 2D-morphology of the mask onto the pattern transfer. Experimentalsputtering yield versus ion incidence angle is injected into the simulation model.The equations which govern the surface evolution, stem from the current method of characteristics. The simulated profiles show that the trenching phenomenon can appearby only considering the variation of the sputtering yield versus the etched surfacecanting. This is obtained when neither the ion reflection nor the electric field linedeviation are taken into account. On the other hand, the slope transfer from the maskto the GaAs and InP substrates is studied.

Electrical traction of vehicle needs accurate control of torque and flux. DC machinesare Maynly used but they are expensive and need heavy Mayntenance. Since some years,the use of field oriented vector control induction machines allows to fulfill the sameobjectives with lower cost. Unfortunately, they may present a decrease of their performances and an instability when their parameters vary with the temperature or the magnetic state and in the presence of converter or measurement noises. Therefore it isnecessary to design effective control laws, which are especially less sensitive to these perturbations and variations. In this work, we have undertaken the conception andthe realization of a voltage vector flux control whose inputs have been uncoupled bycompensating terms. The torque and the flux regulations are realized at two differentlevels: an internal regulation loop for stator currents, and an external one for fluxand torque. In order to achieve robust stability and performance objectives, we haveinvolved several new methods in the doMayn such as H ∞ control designed bygenetic algorithms and reduced order extended Kalman filtering in the synchronousframe. Simulations will show the method efficiency and experimental results validatethe control strategy.

This article deals with a general method for finding the equations of the dynamicresponse of a bimorphous piezoelectric structure through a variational formulation.First, a method of resolution is proposed then a general electric equivalent circuitinvolving the design parameters of the device is deduced, followed by the description ofapplication to a travelling-wave stator.

An electronic speckle-pattern shearing interferometer (ESPSI) is proposed formeasuring the free convection heat transfer coefficient in liquids. The heattransfer coefficient may be deduced by a simple manipulation of the specklepatterns. Theory of the method as well as its application are presented. Themethod is robust and easy to use for non-skilled operators.

We describe how a commercial silicon cantilever can be used both as a force probe and a local scatterer of light. This tip can thus be used as the key element of an apertureless Scanning Near-field Optical Microscope (SNOM) combined with an Atomic Force Microscope (AFM). We use a Total Internal Reflection (TIR) illumination to provide a Fresnel evanescent wave with well-defined characteristics. We show how the weak signal issued from the near-field scattered by the tip can be extracted from the background noise. Finally we present how a signal related to the near field can also be obtained when the tip is oscillated at a frequency close to the resonance frequency of the cantilever. These results are compared with a simulation of the lock-in detection. Finally we describe the role of the vibration amplitude on the signal.

By using a simple model based on a linear susceptibility approach and taking into account the dynamic nature of the molecule's electronic structure, the electric fielddiffracted by the near-field probe is evaluated in the presence or absence of a thinsilver layer deposited on the surface of a bulk fluorescent material. This modelingpoints out that the far-field signal emitted by the molecules situated far from thetip is important in the absence of metal coating and can reduce the near-fielddetection sensitivity. It shows that surface metal coating enhances the near-fieldcontribution and decreases the far-field one. Cathodoluminescence images obtained on afluorite (CaF2:Eu) sample confirm the results predicted by modeling.

This study concerns the multiple scattering of light in random granular systems and the analysis of the image formed by the light scattered by a random medium. We developed an imagery method with high grey level resolution for visualization and analysis of the intensity profile in the backscattering spot image. We further present statistical models for the radiative transfer in granular media only involving the photon mean path length, the asymmetry factor and the absorption probability of photons. A renormalization of the photon trajectory in the medium gives a photon transport equation for anisotropic scattering and leads to an analytical expression for the intensity profile in the backscattering spot. We finally compare the results from imagery experiments with the predictions from numerical simulations and statistical models.

A procedure has been developed for the accurate measurement of film and substrate optical parameters from the multiple sample single-wavelength ellipsometric data. The dimensional reduction of the unknowns from newly formulated ellipsometric functions, the root selection and the thickness-dependent integer deduction enhance the rapidity of finding solutions and the convergence from a wide range of initial guesses while avoiding undesirable solutions. An error analysis carried out shows that the procedure is very resistant to the propagation of angular errors and allows the estimation of optimum film thickness ranges under which the parameters can be accurately found. The standard SiO2/Si structure is particularly studied using the procedure that is further illustrated with the experimental data on Ni/BK7-glass structures. The SiO2 film refractive index and thickness are thus shown to be accurately determined when sought along with the substrate optical constants. Moreover, the film and substrate real indexes are not altered in the presence of an interface layer between the film and the substrate while its existence is indicated by a systematic lowering of the Si substrate extinction coefficient. The procedure can be efficiently used in the continuous real-time optical characterization of films growing on substrates.

The pumping power dependent locations of the degeneration points were calculated for a plane-concave cavity. Such a simple experimental method for determining the degeneration points and their shifts due to thermal lensing is proposed. The laser beam patterns near the degeneration points were observed. For the pumping configuration under investigation the beam parameter product at the degeneration points increases considerably.

A study on Au/n-Si Schottky barrier diodes (SBDs) parameters with andwithout thin native oxide layer fabricated on n-type Si grown by LPE (Liquid-PhaseEpitaxy) technique has been made. The native oxide layer with different thicknesses onchemically cleaned Si surface was obtained by exposing the Si surfaces to clean roomair before metal evaporation. The native oxide thicknesses of samples D2, D3, D4 andD5 are in the form D2 < D3 < D4 ≤ D5 depending on the exposing time. It has been seenthat the value of the barrier height Φ b of samples D2 (0.64 eV), D3 (0.66 eV), D4(0.69 eV) and D5 (0.69 eV) increases with increasing the exposure time and tends tothat of the initial sample D1 (the initial sample, 0.74 eV), and thus also their I − Vcurves. Especially, the experimental results related to the exposure time of thesurfaces to clean air are close in agreement with recently results reported for theHF-treated n-Si surface during initial oxidation in air. Furthermore, it has beendetermined experimentally that ageing of the Au contacts on the oxidized epilayer Sileads to barrier height values close to those measured for Au on chemically cleanedsurfaces.

In this paper, dielectric bi-anisotropic effects on the resonant frequencies of dielectric ring resonators (DRR) are studied in depth. The resulting electromagnetic boundary problem is solved numerically using the mode matching method (MM). In order to validate this numerical model, the obtained results are compared with those of the literature for a uniaxial electric anisotropic dielectric ring resonator.

A comparative study on the emitter-base recombination current in different GaAlAs/GaInP/GaAs HBT structures including uniform base HBTs, and composition graded base HBTs, has been carried out. We have demonstrated that a graded base is not sufficient to prevent recombination on the base surface and that a thin GaInP ledge on the base surface reMayns necessary to retain a high enough current gain for small emitter high-frequency devices.

The electric field measurements in corona discharge provides a complementaryresults for the corona discharge study because the classical theory based on thecurrent and voltage data is unsatisfactory. A new method of current density and fieldmeasurement with a linear biased probe, during the corona discharge, is developed inthis paper. The work has elucidated the phenomenon of probe end-effects using a newprofile of probe adapted to linear geometry. The probe functions were verified and the present theoretical model, which simulates the biased probe by the coplanar planessystem, is thus verified. Other tests have been carried out with a wire-to-planeelectrode system at various parameters. It has been shown that the measured field atthe plane increases linearly with the corona voltage and it decreases as we move awayfrom the centre of plane. The electric field at the plane decreases also as the distancebetween the wire and the plane increases.

Results on the stability regions of alpha (α) to gamma (γ) transition for CO2 gas laser mixture 3He:1CO2:1N2 RF capacitative dischargeswith and without the addition of Xe are presented in terms of current, voltage andpower characteristics. Measurements have also been made of the mean electron energy,neutral gas temperature and time averaged visible emission for different electrodeseparations, frequencies and intermediate pressures. It is found that the Xe additionhas increased the neutral gas temperature and transition input power, while it hasdecreased the mean electron energy and transition voltage. The time averaged visibleemission has shown strong CO and NO emission from the discharge.

A method for determination of probability of electrical breakdown of gases ispresented in this paper and results are compared with theory. The method is based onthe measurements of breakdown time delay at different afterglow periods (relaxationtimes), where the secondary electron yield initiating breakdown is caused by residualactive states reMayning from the preceding glow. Measurements are carried out at lowpd (gas pressure times electrode spacing) of nitrogen and higher overvoltages up to2.1U s (U s: the static breakdown voltage). The experimentallydetermined breakdown probability is fitted by theoretical expressions for both theTownsend and streamer model of electrical breakdown, with particular attention to thehigher overvoltages. On the basis of our measurements and theoretical fit it can beconcluded that the Townsend mechanism is applicable at low pd value over the wholerange of overvoltage applied and a value of a secondary emission coefficient forgold-plated copper cathode is obtained. With the decrease of the breakdown probabilityand electron yield the distributions of delay times become broader and centered atlarger delay times, and the positive asymmetry and roundness increase.