Modelling and numerical simulation of the bi-temperature Euler equations in toroidal geometry

National audienceDans les tokamaks comme ITER, la matière est à l’état de plasma chaud en interaction avec des ondesélectromagnétiques. La modélisation et la simulation numérique du transport des électrons et des ions,particules chargées qui constituent ce plasma, sont un processus-clé de la réussite de ce projet.La dynamique des particules chargées peut être décrite par un modèle d’Euler bi-températures, celle desélectrons et celle des ions. Ce modèle prend également en compte les échanges d’énergies entre les deuxespèces de particules. Nous utilisons une forme conservative du modèle bi-températuresqui suppose que le saut d’entropie des électrons est nul au travers des chocs. Cette forme des équationspermet d’utiliser pour l’approximation numérique du système résultant un schéma de relaxation.Toute cette physique complexe a lieu dans un tokamak dont la géométrie est toroïdale. L’écriture deséquations du modèle sous forme conservative en coordonnées non cartésiennes impose des précautionspour être approchée par la méthode des volumes finis. Des prismes dans un maillage non-structuré engéométrie courbe sont également utilisés pour l’approximation du modèle.Dans cet exposé, nous présenterons des formulations non-conservative et conservative des équationsd’Euler bi-températures avec des termes d’échanges d’énergies. Un schéma de relaxation et le traite-ment de la géométrie toroïdale seront montrés. Enfin, des exemples de simulations numériques utilisantcette méthode numérique seront présentés

Strongly enhanced effective mass in dilute two-dimensional electron systems: System-independent origin

We measure the effective mass in a dilute two-dimensional electron system in (111)-silicon by analyzing temperature dependence of the Shubnikov-de Haas oscillations in the low-temperature limit. A strong enhancement of the effective mass with decreasing electron density is observed. The mass renormalization as a function of the interaction parameter r_s is in good agreement with that reported for (100)-silicon, which shows that the relative mass enhancement is system- and disorder-independent being determined by electron-electron interactions only.Comment: As publishe

Transport properties of a quantum Hall ferromagnet in parabolic wells

Abstract We report on the observation of an anomalous resistance peak in a tilted magnetic ÿeld corresponding to the ÿlling factors 2 and 4 in parabolic wells with di erent widths. This phenomenon is due to unpolarized ferromagnetic transitions in quantum Hall ferromagnets. We ÿnd that the e ect is isotropic when an in-plane magnetic ÿeld is applied along and perpendicular to the current direction. Isotropic domain-wall transport is responsible for the magnetoresistance peak.

Magnetotransport of a quasi-three-dimensional electron gas in the lowest Landau level

We have observed features in the magnetoresistance of a wide parabolic quantum well in the presence of the in-plane magnetic field at field three times larger than fundamental field corresponding to the depopulation of the last Landau level. The magnetoresistance structures shift with a specific sample parameter, such as potential width. We suggested the formation of correlated states of a three-dimensional electron gas at Landau filling factor 1/3, in analogy with a two-dimensional fractional quantum Hall effect

Reduced pressure chemical vapour deposition of SiGe virtual substrates for high mobility devices

We have studied the strain state, film and surface morphology of SiGe virtual substrates grown by reduced pressure chemical vapour deposition (RP-CVD). The macroscopic strain relaxation and the Ge composition of those virtual substrates have been estimated in high resolution x-ray diffraction, using Omega-2Theta scans around the (004) and (224) orders. Typically, linearly graded Si0.7Ge0.3 virtual substrates 5 mum thick are 96% relaxed. From transmission electron microscopy, we confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. The threading dislocations density obtained for Ge concentrations of 20% and 27% is indeed typically of the order of (7.5 +/- 2.5) x 10(5) cm(-2). The surface roughness of the relaxed SiGe virtual substrates increases significantly as the Ge concentration approaches 30%. We find for the technologically important Ge concentration of 30% a surface root mean square roughness of 5 nm, with an undulation wavelength for the cross-hatch of the order of 1 mum. We have also studied the electronic quality of our RP-CVD grown SiGe virtual substrates. We have grown a MODFET-like heterostructure for this purpose, with a buried tensile-strained Si channel 8 nm thick embedded inside SiGe 31%. We have obtained a well-behaved two-dimensional electron gas in the Si channel, with electron sheet densities and mobilities at 1.45 K of 5.7 x 10(11) cm(-2) and 180000 cm(2) V-1 s(-1), respectively