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.

The generalized impedances allow to characterize a one-way thermal system whose two faces are accessible. From experimental measurements of the flux densities and variations in temperature in the access faces of a homogeneous material, the two generalized impedances of storage and transfer are calculated in the frequential field. The theory of the thermal quadripole enables to determine a theoretical expression of these impedances. After a sensitivity study which underlines the accessible parameters and the optimal frequency band, an optimization procedure of the setting of the ideal model of one of the two impedances on the corresponding experimental curve allows to identify the effusivity and the thermal diffusivity of material. The method is applied to the study of a sand with three water contents.

We present a set of complementary experimental and numerical tools for studying miscible fluid displacements in porous media with large scale heterogeneities.Experiments are realized in transparent 2D Hele-Shaw cells allowing opticalobservations and in 3D packings of glass beads with an acoustical technique forimaging fluid displacements. Permeability heterogeneities are modeled by spatialvariations of either the local aperture of the Hele-Shaw cell or the diameter of thegrains used in the packing. The Hele-Shaw cell model provides high resolution maps ofthe invasion front location at regular time intervals and of the flow lines: thevelocity field is determined by combining these informations. Acoustical images ofrelative concentration distributions in the 3D packing are in agreement with Hele-Shawcell data and can be obtained in a broader range of experimental situations. Suchexperiments realized with a stabilizing density contrast between invading and displacedfluids demonstrate a strong reduction of the front width at low flow velocities, asimilar reduction is obtained at high velocities with a stabilizing viscositycontrast. The technique is also applicable to study fluid displacements in naturalopaque media. Numerical simulations by a Boltzmann lattice technique using aStokes-like diffusive term to smooth out the effect of permeability discontinuitiesprovide complementary informations. They are shown to give similar results asexperiments for same flow parameter values and to allow for a fast exploration of abroad range of fluid properties and flow situations.

The improvement of the lateral resolution of 3D atom probe requires the ion trajectories to be determined accurately. For the first time, the atomic structure of the emitter (a sharp tip) and the gradual change of its topology during evaporation are taken into account. Atoms submitted to the highest field are removed one by one. Ion trajectories are then simulated step by step after each atom has been removed from the sample surface. The recurrent use of a simulation software (SIMION 3D) for each elemental step makes it possible to model the image transfer function of 3D atom-probe for a non-stationary shape of the emitter. This dynamic model, applied to atom probe data, is shown to correct for the major aberrations present at the centre of low-index poles. The well-known depleted zone present at the pole centre is shown to disappear using this model. An almost constant density of atoms is then observed over the entire analysis area. For the first time, a physical interpretation of these depleted zones is provided and confirmed through simulation and experiments.

From an analysis of the static friction regime of the tip of an atomic force microscope, a method is proposed for determining the viscoelastic modulus of materials which are viscoelastic at the temperature of the experiment. Present experiments consist in displacing the sample in its plane, periodically or at a constant velocity, while measuring the tangential force response of the contact to this shear solicitation. The proposed model permits to determine the relaxation and complex moduli at a sub-micrometric scale. Several experimental confirmations of the description are presented and it is shown that the local moduli measurements compare favorably with macroscopical results.

An experiment of high-frequency X-ray beam modulation for fast digital data transfer by X-rays is presented in this paper. The modulation of the beam is performed by diffraction on surface acoustic waves whose time-structure contains already the information to be transmitted. It has been possible to connect two computers via an X-ray beam and to perform digital data transfers at a speed of afew kbits/s.