Showing papers in "European Physical Journal-applied Physics in 2001"

Numerical modeling of the transmission of breakdown plasma generated in water during laser shock processing

Abstract: The transmission of breakdown plasma generated in water during laser shock processing experiments was investigated theoretically. A numerical model based on a rate equation formalism has been developed to calculate the characteristics (peak irradiance and duration) of the laser pulses transmitted through the breakdown plasmas generated in water. Results are in good agreement with previous experimental data for 25 ns--1064 nm laser pulses. Above 10 GW/cm 2 , the transmitted peak irradiance saturates and the transmission starts to decrease. The results have been extended to shorter wavelengths (532 and 355 nm). At 1064 nm, the breakdown process is dominated by avalanche ionization, whereas for shorter wavelengths, multiphoton ionization plays the major role.

About the secondary electron yield and the sign of charging of electron irradiated insulators

Abstract: Following the total yield approach (used to predict the sign of charging of electron irradiated insulators) a positive charging is expected when the primary beam energy E p is situated in the energy range where the number of outgoing electrons, , is larger than the number of incoming electrons, I 0. But a negative charging is often experimentally observed when a positive charging is predicted. The present paper is an attempt to elucidate this experimental fact. The arguments being developed are based on the use of a dynamic double layer model (+ for the secondary electron mission, SEE; − for the incident electron implantation) which explains a negative charging via the influence of an evolving S-shape potential function, V(z), which induces a partial freezing of the nominal (uncharged) SEE, δ°, combined to a progressive compression of the negative space charge below the surface. The observed large difference in the measurements of the yield by using pulse excitation methods, or by permanent irradiation methods, δ° or δ is then explained. Furthermore, the influence of an oblique incidence is also deduced.

Void fraction prevision in LCM parts

Abstract: The formation of defects during the LCM processing of composite parts depends on various injection parameters Industrial users need to realize pieces with good physical and mechanical properties and appearance This requires to predict what is named a “processability window” This term defines a range of parameters which will ensure a nearly absence of defects Knowing that most of the defects created during an injection are voids, a bibliographical background about the formation and removal of these voids is presented An original model of void fraction prediction is developed This model is based on an empirical analysis of void formation and of the flow behaviour An experimental qualitative validation of the model is presentedThe proposed model can be used as an effective prediction tool at the design stage of a composite part

Precise in situ measurements of isotopic abundances with pulse counting of sputtered ions

Abstract: Ion counting with an electron multiplier (EM) is necessary when sputtered ions are used for in situ and precise isotopic abundance measurements (10 −4 $(1\sigma)$ ) on small sample volume (about 100 μ m 3 for pure silicon). Measurements were performed on silicon samples bombarded with Cs + ions by extracting negative secondary monatomic Si − ions. Pulse-height distributions ( PHD ) and isotopic ratios were used as diagnostic tools for repeatability studies. Repeatability could be greatly improved by determining the optimal position of the impact area on the conversion dynode and by addressing each isotopic beam properly focused on this area (adaptive optics). A simplified model based on Poisson's laws was developed to fit PHD s and allowed us to calculate quantum detection efficiencies versus thresholds. EM isotopic discriminations were determined with the resulting semiempirical algorithm so as to reconstruct the lost information and get data independent of threshold setting. To reach consistent results, quasi-simultaneous arrivals ( QSA ) on the conversion dynode had to be assumed and modelled using direct ionisation yields Si − / Cs + at different collection efficiencies. The QSA corrected data fitted well on the terrestrial isotopic fractionation line. Dead time uncertainties and possible emission non-linear isotopic fractionation processes were examined. PHD s from other elements and polyatomic ions were also discussed.

A virtual non contact-atomic force microscope (NC-AFM): Simulation and comparison with analytical models

Abstract: A virtual NC − AFM machine has been built using the Matlab language. The virtual NC − AFM is identical to a real hybrid machine built with Digital Instruments and Omicron blocks. The role of every subset is described in detail, and special attention is paid to the parameters of the Automatic Gain Control (AGC) which controls the amplitude of the oscillations. The virtual machine is a powerful tool, that allows to solve, without any approximation, non linear coupled differential equations describing the physics of the tip-surface interaction. The machine is then used to study the frequency shift and damping signal in the approach-retract mode. Two types of situations are analyzed: the first one corresponds to the case where no dissipative force is introduced in the tip-surface interaction, the second one deals with dissipative forces. These last ones are here introduced by assuming a mechanical deformation of the surface under the tip. Interesting results are then obtained, in particular the fact that unstability may occur under some particular conditions. Most of the results obtained by simulation are then compared with those of analytical models already published in the literature.

A deflated iterative solver for magnetostatic finite element models with large differences in permeability

Abstract: The presence of materials with a relative large difference in permeability has a harmful influence on the convergence of Krylov subspace iterative solvers. Some slow converging components are not cured by preconditioning and correspond to eigenvectors reflecting the domains with relatively low permeable material. Approximations for those eigenvectors are determined using physical knowledge of the problem. The iterative solution process is split up in a small problem counting for the separated eigenmodes and a full-size problem out of which the slow converging modes are removed. This deflated preconditioned solver is faster converging compared to more common approaches, such as the incomplete Cholesky conjugate gradient method.

Comparative determination of the electron temperature in Ar- and N2-plasmas with electrostatic probes, optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS

Abstract: The electron temperature T e as the often most essential parameter for applications of low pressures plasma has been derived from comparative in situ measurements with a single and a double electrostatic probe, and with optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS as remote techniques. Electrodeless rf discharges maintained by electron cyclotron wave resonance ECWR in pure Ar and N 2 in the pressure regime from 10 −2 to 1 Pa have been used as sample plasmas. The evaluation of the OES- and EDMS-signals is described in detail. The pressure dependence of the T e -results derived therefrom is found to compare well with the data from the probe measurements, and with calculations from a charge carrier balance equation. By matching the OES data to the absolute T e -values from the probe measurements, numerical expressions have been obtained by which T e can be quantitatively calculated from the intensity ratios between selected emission lines from the Ar- and the N 2 -plasma. Furthermore, the EDMS-results are also shown to deliver quantitative information about T e .

AC conductivity and dielectric properties of In2S3 films

Abstract: Stoichiometric In 2 S 3 films were prepared by thermal evaporation technique onto clean glass substrates. According to X-ray investigations, the as-deposited films were in amorphous state. Both the ac conductivity and dielectric constants were measured in the frequency range 100 Hz−100 kHz at different temperatures. Different parameters such as the frequency exponent parameter s , the density of states near the Fermi level , the activation energy and the optical band gap E g of In 2 S 3 amorphous thin films were estimated. The hopping conduction was recognized as the conduction mechanism for the investigated films.

Influence of Cr3+ ions on the structure and certain physical properties of PbO-As2O3 glasses

Abstract: PbO-As2O3 glasses containing different concentrations of Cr2O3 (ranging from 0 to 0.4 mol% ) were prepared. The differential thermal analysis, IR and optical absorption spectral studies of these glasses were carried out; from these studies, the variations in the values of glass transition and crystallization temperatures, optical band gap and the shifting of symmetrical bending vibrational frequency band were observed with increase in concentration of Cr2O3. Magnetic susceptibilities for these glasses were also determined and Cr3+ ion concentration was estimated. In addition, detailed studies on dielectric properties viz., dielectric constant e, loss tan δ, ac conductivity and dielectric breakdown strength were investigated. Using the results of these studies, the structural modifications taking place in the glass network were discussed in the light of Cr3+ ion concentration.

Identification of the plasma magnetic contour from external magnetic measurements by means of equivalent currents

Abstract: One of the main research issues in the thermonuclear fusion area is the identification of plasma contour starting from external magnetic measurements Possible approaches to this inverse problem make use of equivalent currents to represent the plasma internal current density, regularizing in this way the magnetic field reconstruction Of course, the choice of the representation basis for such equivalent currents is critical The paper aims at analyzing the effect of the number and position of base currents on the performance of the identification algorithm

A nonlinear observer for induction motors

Abstract: In this paper, we give nonlinear observers for the estimation of the rotor flux and the load torque in an induction motor. The observers are designed on the basis of the standard alpha-beta Park's model. The performance of these observers are shown via experimental measurements.

Modelling of negative point-to-plane corona loudspeaker

Abstract: In this paper, electrical and acoustical characteristics of negative point-to-plane corona discharge loudspeakers are investigated. In the first part, the electrical behaviour of point-to-plane corona discharges is modelled by a three-parameters equivalent circuit. An experimental set-up for estimating these electrical parameters has been developed and improved. Based on the experimental results, evolution of the parameters with discharge conditions has been traced. In the second part, the electrode gap in negative point-to-plane corona discharges is divided into an ionisation region near the point, and a drift region. In each region, interactions between charged and neutral particles in the ionised gas lead to a perturbation of surrounding air, and so generate an acoustic field. For each region, seen as a separate acoustic source, an acoustical model is developed. An experimental set-up for measuring acoustic pressure has been developed, and allows us to confirm expectations based on directivity pattern, monopolar and dipolar directivities being associated to the ionisation and drift region respectively.

Reflection and transmission of optical narrow-extent pulses by axially excited chiral sculptured thin films

Abstract: Chiral sculptured thin films (STFs) have unidirectionally periodic electromagnetic constitutive properties and therefore exhibit the circular Bragg phenomenon. The time-domain Maxwell equations are solved using finite difference calculus in order to establish the spatiotemporal anatomy of the action of axially excited, chiral STF slabs on optical narrow-extent pulses (NEPs) modulating circularly polarized carrier waves. A Lorentzian model was adopted for the permittivity dyadics of the chiral STFs. The time-domain manifestation of the circular Bragg phenomenon is focussed on. First, on examining the refraction of NEPs by a chiral STF half-space, a light pipe and the pulse bleeding phenomenon are shown to occur -when the handednesses of the carrier wave and the chiral STF coincide and the carrier wavelength is in the vicinity of the center-wavelength of the Bragg regime. Next, pulse bleeding inside a chiral STF slab is shown to be responsible for the long wakes of reflected pulses and low energy contents of transmitted pulses, when the incident wave spectrums significantly overlap with the Bragg regime and the carrier waves have the same handedness as the chiral STF slab. Thus, a chiral STF slab can drastically affect the shapes, amplitudes, and spectral components of femtosecond pulses.

Orientation control of rhomboedral PZT thin films on Pt/Ti/SiO2/Si substrates

Abstract: Highly (111)- and (001)-oriented rhomboedral PZT thin films have been grown at 500 °C on platinized silicon substrates by in situ RF magnetron sputtering. Crystallization of the perovskite phase was possible provided that a thin TiO x buffer layer was deposited prior to the PZT. Control of PZT films orientation is demonstrated by changing the ratio in the plasma gas during the TiO x sputtering and its consequences on electrical properties of the ferroelectric samples are presented. The structural properties of the TiO x buffer layer were studied by means of transmission electronic microscopy in order to understand the relation between the TiO x seeding and the orientation control of the PZT film.

Fabrication of refractive microlens arrays by visible irradiation of acrylic monomers: influence of photonic parameters

Abstract: The fabrication of refractive microlenses with self-developing photopolymers is reported. A spatially controlled illumination of the photosensitive layer induced an inhomogeneous photopolymerization involving formation of 3-D polymer network, mass-transport process of reactive species and bending of the surface. The process exhibited a completely self-processing character without any chemical post-treatment to reveal the relief. The lens arrays displayed diameters ranging from less than 100 μ m to 1 mm and focal lengths from 100 μ m to a few millimeters, depending on photonic, optical and physico-chemical parameters. The paper focuses on the importance of photonic parameters in the generation of microlens arrays and discusses the flexibility of this technique in the visible range.

Time-domain simulation of the circular Bragg phenomenon exhibited by axially excited chiral sculptured thin films

Abstract: We present simulations of the transmission and reflection of narrow-extent pulses incident upon a chiral sculptured thin film (STF) along its axis of spirality, when the circular Bragg phenomenon is excited. Even though the frequency-domain reflection and transmission spectrums of a sufficiently thick chiral STF slab acquire final shapes within the Bragg regime, the shape and the duration of the transmitted pulse change with slab thickness over the entire range of our simulations. The emergence of a multiple-hump structure in the transmitted pulse is relevant to the use of chiral STFs in digital optics communication.

Electrical and optical properties of amorphous Ga2Te3 films

Abstract: Ga 2 Te 3 has been prepared in bulk and thin film forms. The composition of films has been checked using energy dispersive X-ray (EDX) spectroscopy technique. X-ray diffraction (XRD) measurements have showed that the Ga 2 Te 3 films evaporated at room temperature substrates were amorphous. Investigation of the I - V characteristics in amorphous Ga 2 Te 3 films reveals that it is typical for a memory switch. The thickness dependence of the mean value of the switching voltage V th is linear in the investigated range and V th decreases exponentially with temperature in the range (298−393 K). The switching voltage activation energy ( e ) calculated from the temperature dependence of V th is found to be 0.277 eV. Electrical conduction activation energy () is found to be (0.564 eV). The agreement between the obtained value of the ratio (0.49) and its value derived theoretically (0.5) suggests that the switching phenomenon in amorphous Ga 2 Te 3 films is explained according to an electrothermal model for the switching process. The transmittance ( T ) of Ga 2 Te 3 thin films, has been measured over the wavelength range 400−2500 nm. From an analysis of the transmittance data, the optical constants, the refractive index ( n ) and the extinction coefficient ( k ), have been determined. Similarly the absorption coefficient ( α ) measurements, have been evaluated. Allowed nondirect transitions with optical energy gap () of 1.15 eV have been obtained.

Determination of effective electron yield from swarm and time delay measurements

Abstract: In this paper the dependence of the effective secondary emission coefficient-effective electron yield γ eff in nitrogen on the reduced field (the ratio of the electric field and the gas density E/N ) for various cathode surfaces is determined. Two different methods are applied: swarm measurements (from breakdown voltage) and time delay measurements. In the latter technique, first the breakdown probability is determined as a function of voltage and then γ eff is derived from it. The results of applying both methods are in good agreement for the γ eff versus E/N dependence. The measurements were made for copper cathode, untreated and treated by gas discharge and also several thousand electrical breakdowns, gold-plated copper and steel cathodes. Secondary electron yield γ eff is of the order of a few percent at moderate and high E/N , and slightly increases with increasing E/N up to several kTd. At low E/N , a characteristic peak appears (at about 600 Td for copper). The γ eff value increases when copper cathode is treated by gas discharges and becomes stable after several thousand breakdowns, agreeing well with other breakdown results in the literature. The chosen values for the Townsend primary ionization coefficient, obtained from best fits to available experimental data in the literature and the choice of the equilibration distance from the cathode significantly influence determination of γ eff . Finally, our results are compared with the results of other authors for different cathode materials and a good agreement is found.

Structure and morphology of nanometer-sized Pd clusters grown at high temperature on natural graphite single crystals

Abstract: Performing both ex situ transmission electron microscopy (TEM) and in situ scanning tunnelling microscopy (STM) experiments on the same samples, we have characterized in detail a model catalyst (Pd/graphite). The Pd clusters were epitaxially grown at high temperature on clean natural graphite substrates under ultra high vacuum (UHV) conditions. For the chosen growth conditions the density of clusters is rather low (10 9 cm -2 ), and their size is typically few tens of nanometers. TEM diffraction studies reveal that most of the clusters (92%) are in a same epitaxial orientation which is defined by: (111) Pd ∥ (00.1) Gr and [112] Pd ∥ [11.0] Gr . Moreover, both STM imaging and TEM observations show that the clusters have truncated tetrahedron shapes. The combination of TEM and STM characterizations of the same samples appears to be a very efficient way to get a detailed knowledge of the global properties of a collection of supported clusters (spatial and size distributions) as well as of their individual properties (structure, morphology).

Efficient magneto-optical trapping of a metastable helium gas

Abstract: This article presents a new experiment aiming at BEC of metastable helium atoms. It describes the design of a high flux discharge source of atoms and a robust laser system using a DBR diode coupled with a high power Yb doped fiber amplifier for manipulating the beam of metastable atoms. The atoms are trapped in a small quartz cell in an extreme high vacuum. The trapping design uses an additional laser (repumper) and allows the capture of a large number of metastable helium atoms (approximately 10 9 ) in a geometry favorable for loading a tight magnetostatic trap.

Metallic foam processing from the liquid state: The competition between solidification and drainage

Abstract: A model is developed to describe the formation of metallic foams in which liquid drainage acts to collapse the foam before it can freeze. Numerical solution of the foam drainage equation, combined with the equations of heat conduction, provides new insight into the competition between these two processes. It also stimulates and confirms a theoretical analysis which leads to criteria for creating uniform samples of frozen metal foam. The analysis suggests new experiments to clarify the role of the various processes leading to foam formation.

Simulation of I-V characteristics of organic thin film transistor : Application to the dihexylquaterthiophene

Abstract: Metal-Insulator-Semiconductor Field-Effect-Transistors based on Dihexyl- quaterthiophene (DH4T), has been realized. Unlike conventional MISFET, these devices work through the modulation of an accumulation layer at the semiconductor-insulator interface. An analytical model that describes the operation of organic thin-film-transistors based on a simple trap distribution, with a single shallow trap level located between the valence-band edge and the Fermi level, has been used to determine some microscopic parameters such as the mobility, the density of traps and the corresponding level of traps.

Electrical breakdown studies of pressurised nitrogen in non uniform fields

Abstract: The aim of the present work is to provide data to understand better and quantify the dielectric strength of pure compressed N 2 (1 P γ -ray irradiated N 2 and that of the surface roughness of the HV electrode. In negative polarity, the free electrons are provided from the cathode whereas they are provided from the gas under positive polarity. Voltage waveform influences on the U 50 breakdown voltage are dealt with in a large pressure range (1 P U 50 value is pointed out in both polarities. Finally, the reduced guiding field of negative streamer is experimentally measured.

Particulate growth in a 13.56 MHz radiofrequency methane plasma: influence of the flow rate and the incident rf power

Abstract: The generation and the behavior of amorphous hydrogenated carbon particulates in a pure methane rf discharge have been studied for different methane flow rates (2−14 sccm) and different incident rf powers (40−120 W). Laser light scattering provides informations on their localization in the discharge. The 90° laser light scattered is used to determine the appearance times. The higher the incident rf power, the shorter the particulate appearance time: conversely the higher the methane flow rate, the higher the appearance time. The appearance times have been correlated to the inverse of the residence times of CH 4 molecules. Good correlations have been observed between the particulate generation and the time evolutions of both laser beam extinction and dc self bias voltage. Whatever our experimental conditions, the influence of the methane flow rate seems lower than the rf power one. An increase of the rf power leads to a more important generation of particulates in the plasma.

Coupling between Marangoni and Rosensweig instabilities

Abstract: Two semi-infinite incompressible and immiscible viscous fluids of infinite lateral extent are considered in the gravity field. One of them is ferromagnetic and the other is a usual Newtonian liquid. Such a system is submitted to a magnetic field normal to the reference flat surface, which separates the two bulks. Developing the linear stability analysis, we show that the Rosensweig critical values for transverse waves remain unaffected by a solutal or thermal Marangoni instability.

A transfer standard of the Mètre: an air wavelength reference

Abstract: This paper presents the principle of an air wavelength standard for high accuracy length metrology. According to the definition of the Metre, nanometric accuracy by interferometric measurement techniques can be reached only for measurements made under vacuum or by taking into account the fluctuations of the refractive index of air. We have developed a new type of laser source whose wavelength is insensitive to slow fluctuations of the refractive index of air. The sensitivity of our air wavelength standard to some characteristics (such as temperature, pressure, ageing behaviour...) has been measured. Results show that the relative uncertainty level of the wavelength of our source is below 10−8.

Infrared near-field study of a localised absorption in a thin film

Abstract: We study the conditions to perform micro-spectroscopy with a sub-wavelength lateral resolution in the wavelength spectral range from 3 to 20 μ m, taking advantage of the infrared spectral signature of different chemical species. We utilised CLIO, a free electron laser, as the photon source. The transmitted photons were collected by either fluoride or chalcogenide glass fibres. Fibre tips were obtained through chemical etching by organic solvents. Metallisation of the tips permits to achieve lateral resolution of the order of the tip size. However, parasitic propagation of the light in the film reduces the contrast between irradiated and non-irradiated zones. We exemplify our set up with near-field infrared spectra obtained for polymer thin films deposited onto silicon wafers.

Model of the homogeneous electrical discharge

Abstract: This paper is aimed at the modelling of an electrical discharge controlled by mean of a dielectric barrier, at atmospheric pressure; we focus our study on the homogeneous “regime”. This model is based on the use of equivalent electrical circuits and is taking into account the main phenomena of the discharge. We also offer a method for the determination of the parameters of the model; the comparison of the experimental waveforms with the simulated ones is validating the accuracy of the proposed model in the case of discharges in nitrogen or helium. Our model is exploited to point out the drastic importance of the electrical characteristics of the power supply with respect to the main waveforms of the discharge. (including computer-aided circuit design and analysis)

A method of calculating the propagation of electromagnetic fields both in waveguides and in free space, using the “Fast Fourier Transform”

Abstract: The following calculation method permits the evaluation of the transverse distribution of the field amplitude Ψ(x,y,z) propagating in a waveguide. This method is based on a standard method for propagation in free space, which is using the Fast Fourier Transform (FFT). It is very simple in practice, and it does not require a mode decomposition for the waveguide propagation. It uses the properties of periodicity and symmetry of the FFT in the transverse plane in order to take into account the influence of the waveguide. The advantage of this calculation method is the full compatibility with the case of a guided wave (waveguide propagation), an unguided wave (free space propagation) and any intermediate regime where the guiding occurs softly or only in a limited part of the waveguide; for example when an input wave is not guided at the entrance of the waveguide, and becomes guided at the end. In practice, these conditions are observed experimentally for example in the infrared “Free Electron Lasers”, which are broadband tunable lasers, and some of them are using a waveguide, in the center of the optical cavity, coupled with free space areas. The calculation method presented here works in this special configuration. On the other hand, although not using a mode decomposition, this method allows also a rapid decomposition in eigenmodes TE pq and TM pq . Using only one symmetry operator and one FFT, it gives the whole set of mode amplitudes.

Evolution of overlimiting electron beam instability

Abstract: The evolution of initial perturbations in a plasma-filled system with a high current relativistic electron beam is considered. The beam current is assumed to be comparable or higher than the limiting vacuum current. The approach is based on equation for slowly varying amplitude of the induced waveform, which enable us to separate out the most intrinsic peculiarities of the overlimiting electron beam instabilities. An equation for slowly varying amplitude is derived and solved. An analytical expression describing space structure and development dynamics of the fields is obtained and analyzed. The well-known maximal growth rate of beam instability is actually the growth rate in the peak of induced wave train. Results are valid both for instability due to aperiodic modulation of the beam density in medium with negative dielectric constant and for instability due to excitation of beam wave with negative energy. Comparison with the case of a sublimiting beam is carried out.