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Three-dimensional direct numerical simulations are used to study the energy cascade rate in isothermal compressible magnetohydrodynamic turbulence. Our analysis is guided by a two-point exact law derived recently for this problem in which flux, source, hybrid and mixed terms are present. The relative importance of each term is studied for different initial subsonic Mach numbers
and different magnetic guide fields
. The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field
, unlike the other terms whose moduli increase significantly with
. In particular, for strong
the source and hybrid terms are dominant at small scales with almost the same amplitude but with a different sign. A statistical analysis undertaken with an isotropic decomposition based on the SO(3) rotation group is shown to generate spurious results in the presence of
, when compared with an axisymmetric decomposition better suited to the geometry of the problem. Our numerical results are compared with previous analyses made with in situ measurements in the solar wind and the terrestrial magnetosheath.
Three-dimensional numerical simulation is used to investigate intermittency in incompressible weak magnetohydrodynamic turbulence with a strong uniform magnetic field
and zero cross-helicity. At leading order, this asymptotic regime is achieved via three-wave resonant interactions with the scattering of a wave on a 2D mode for which
. When the interactions with the 2D modes are artificially reduced, we show numerically that the system exhibits an energy spectrum with
, whereas the expected exact solution with
is recovered with the full nonlinear system. In the latter case, strong intermittency is found when the vector separation of structure functions is taken transverse to
. This result may be explained by the influence of the 2D modes whose regime belongs to strong turbulence. In addition to shedding light on the origin of this intermittency, we derive a log-Poisson law,
, which fits the data perfectly and highlights the important role of parallel current sheets.
We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 µJ are focused to intensities ranging between 1013 and 1017 W/cm2. We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations.
Une démarche de développement d’un modèle probabiliste à deux échelles pour la fatigue
HCF des aciers est proposée. Elle est basée sur l’utilisation de mesure
d’auto-échauffement sous chargements cycliques et est validée sur la base de la prévision
des courbes de Wöhler d’un acier dual-phase pour différents rapport de charge.
The influence of oxygen treatment on carrier transport properties of pure ZnO and ZnO:Cl thin films grown by MOCVD were studied. The experimentally obtained values of carrier concentrations after oxyden treatment at different temperatures, were compared with the the results obtained from thermodynemical analysis of the system: ZnO:Cl-Oxyen vapour pressure, using method of quasi-chemical reactions (QCR).
High power RF device performance
decreases as operation temperature increases (e.g. decreasing electron
mobility affects cut-off frequencies and degrades device reliability).
Therefore determination of device temperature is a key issue for device
topology optimisation. In this work the temperature variation of AlGaN/GaN
high-electron-mobility transistors grown either on silicon or sapphire
substrate under bias operation was measured by micro Raman scattering
spectroscopy. Temperature measurements up to power dissipation of 16 W for
4 mm development devices were carried out and a peak temperature of 650 K was
determined. The difference of thermal resistance for similar devices grown
on the two different substrates was assessed. The thermal resistances of
different device topologies were compared to optimise the component design.
The growth of ZnO films deposited by Closed Space Vapor Transport (CSVT) on sapphire substrates has been investigated. Deposition on R oriented sapphire substrates gives rise to a-(11-20) oriented ZnO films. Under optimised conditions, flat surfaces can be achieved and rocking curves with full half width below 500 arcsec are observed. The electrical properties of the films were studied. Hall measurements reveal that the measured n-carrier concentration decreases linearly upon the thickness of the sample. This is interpreted as interface conduction probably related to diffusion of aluminium from the substrate. On thinnest films, the n-carrier concentration can be dramatically decreased with thermal annealing under oxygen. Furthermore, the effect of this annealing under oxygen is found to be completely reversible after a further thermal annealing under oxygen free atmosphere.
The high power RF device performance decreases as the operation
temperature increases (e.g. fall of electron mobility impacting
the cut-off frequencies and degradation of device reliability).
Therefore the determination of device temperature is a key issue
for device topology optimisation. In this work the temperature
variation of AlGaN/GaN high-electron-mobility transistors grown
either on silicon or sapphire substrates under bias operation was
measured by micro Raman scattering spectroscopy. The differences
in thermal resistance for similar devices grown on the two
different substrates were assessed. The thermal resistances of
different device topologies were compared in order to optimise
the component design. The temperature measurement across the gate
and along the component fingers were made to quantify the thermal
gradient of the device. Temperature measurement up to a power
dissipation of 16 W for a 4 mm development device was carried out
and the peak temperature of 650 K was determined.
The one-dimensional MHD system first introduced by J.H. Thomas [Phys.
Fluids 11, 1245 (1968)] as a model of the dynamo effect is thoroughly studied in
the limit of large magnetic Prandtl number. The focus is on two types of localized solutions involving shocks (antishocks) and hollow (bump) waves. Numerical
simulations suggest phenomenological rules concerning their generation, stability
and basin of attraction. Their topology, amplitude and thickness are compared
favourably with those of the meromorphic travelling waves, which are obtained
exactly, and respectively those of asymptotic descriptions involving rational
or degenerate elliptic functions. The meromorphy bars the existence of certain
configurations, while others are explained by assuming imaginary residues. These
explanations are tested using the numerical amplitude and phase of the Fourier transforms
as probes of the analyticity properties. Theoretically, the proof of the partial integrability
backs up the role ascribed to meromorphy. Practically, predictions are
derived for MHD plasmas.
We derive a weak turbulence formalism for incompressible magnetohydrodynamics.
Three-wave interactions lead to a system of kinetic equations for the
spectral densities of energy and helicity. The kinetic equations conserve energy in
all wavevector planes normal to the applied magnetic field B0ê∥.
Numerically and analytically, we find energy spectra
E± ∼ kn±⊥,
such that n+ + n− = −4, where E±
are the spectra of the Elsässer variables z± = v ± b in the
two-dimensional case (k∥ = 0). The constants of the spectra are computed exactly and found to
depend on the amount of correlation between the velocity and the magnetic field. Comparison
with several numerical simulations and models is also made.
An investigation of the decay laws of energy and of higher moments of the
Elsässer fields z±=v±b
in the self-similar regime of magnetohydrodynamic
(MHD) turbulence is presented, using phenomenological models as well as two-dimensional numerical simulations with periodic boundary conditions and up
to 20482 grid points. The results are compared with the generalization of the
parameter-free model derived by Galtier et al. [Phys. Rev. Lett.79, 2807 (1997)],
which takes into account the slowing down of the dynamics due to the
propagation of Alfvén waves. The new model developed here allows for a study
in terms of one parameter governing the wavenumber dependence of the energy
spectrum at scales of the order of (and larger than) the integral scale of the flow.
The one-dimensional compressible case is also dealt with in two of its simplest
configurations. Computations are performed for a standard Laplacian diffusion
as well as with a hyperdiffusive algorithm. The results are sensitive to the
amount of correlation between the velocity and the magnetic field, but rather
insensitive to all other parameters such as the initial ratio of kinetic to magnetic
energy or the presence or absence of a uniform component of the magnetic field.
In all cases, the decay is significantly slower than for neutral fluids in a way that
favours for MHD flows the phenomenology of Iroshnikov
[Soviet Astron.7, 566
(1963)] and Kraichnan [Phys. Fluids8, 1385 (1965)]
as opposed to that of
Kolmogorov [Dokl. Akad. Nauk. SSSR31, 538 (1941)].
The temporal evolution of q-moments of the generalized vorticities
〈[mid ]ω±[mid ]q〉
=〈[mid ]ω±j[mid ]q〉
up to order q=10 is also given, and is compared with the prediction of the model.
Less agreement obtains as q grows – a fact probably due to intermittency and
the development of coherent structures in the form of eddies, and of vorticity
and current sheets.
We present a methodology suitable for SEM and TEM analysis on Heterojunction Bipolar Transistors (HBT) developed for microwave applications. The Focused Ion Beam (FIB) technique has been investigated for SEM observations. It is found that best results are obtained when the active areas are micromachined from the back side and at a low incidence angle. The help of backscattered electron imaging for metallurgical assessment is emphasized on that kind of devices. A combination of both precision dimpling and FIB micromachining is found to be a good solution for the achievement of TEM slabs with an acceptable thickness. Finally we present the design of a dual beam system which uses Ar+ or Ga+ ions for highest TEM slab quality built into a SEM for an accurate localization and monitoring of the milling process. The respective advantages and limitations of this method are stressed in term of precision of localization of device features, surface roughness, contamination and slab thickness. The possibility to apply this technique on 0.1 µm silicon based technology will be discussed.
GaInN/GaN heterostructures and quantum wells have been grown by low pressure metalorganic vapor phase epitaxy on sapphire using an AIN nucleation layer. We found a significant In incorporation only for growth temperatures of 700°C, although still very high In/Ga ratios in the gas phase had to be adjusted. The In content could be increased by reducing the H2/N2 flow ratio in the main carrier gas. GaInN layers typically show two lines in low temperature photoluminescence which are identified as excitonic-like (high energy peak) and impurity-related-like (low energy) by time-resolved spectroscopy. Quantum wells with a thickness between 8 and 0.5 nm showed only one emission line. The peak of the thinnest wells shows excitonic-like behaviour, whereas we found a smooth transition to an impurity-related-like type with increasing thickness. By scanning transmission electron microscopy studies we found indications for composition fluctuations in these thicker quantum wells which may cause localization effects for the excitons and thus be responsible for the observed optical spectra.
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