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 $M_{S}$ and different magnetic guide fields $\boldsymbol{B}_{0}$ . The dominant contribution to the energy cascade rate comes from the compressible flux, which depends weakly on the magnetic guide field $\boldsymbol{B}_{0}$ , unlike the other terms whose moduli increase significantly with $M_{S}$ and $\boldsymbol{B}_{0}$ . In particular, for strong $\boldsymbol{B}_{0}$ 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 $\boldsymbol{B}_{0}$ , 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 $\boldsymbol{b}_{\mathbf{0}}$ 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 $k_{\Vert }=0$. When the interactions with the 2D modes are artificially reduced, we show numerically that the system exhibits an energy spectrum with $k_{\bot }^{-3/2}$, whereas the expected exact solution with $k_{\bot }^{-2}$ 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 $\boldsymbol{b}_{\mathbf{0}}$. 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, ${\it\zeta}_{p}=p/8+1-(1/4)^{p/2}$, 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.

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 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.