THEORETICAL AND EXPERIMENTAL STUDIES OF THE STABILITY IN CVD Nb3Sn SUPERCONDUCTING COMPOSITE TAPES

La stabilité dynamique de rubans composites de Nb3Sn a été étudiée. En considérant la limite d'élasticité des matériaux et un modèle à deux dimensions, un critère de stabilité a été déterminé théoriquement. En utilisant ce critère, on peut montrer qu'il existe une épaisseur optimale pour la couche de cuivre, épaisseur pour laquelle un maximum de densité de courant critique peut être obtenu. Les résultats théoriques sont en bon accord avec les expériences faites avec des bobines maquettes et avec les résultats publiés. En se basant sur la stabilité dynamique, l'unification des critères de stabilité cryostatique, dynamique et adiabatique est discuté.The dynamic stabi1ity of CVD Nb3Sn superconducting composite tapes has been studied. Considering plastic yield of the materials and based on a two-dimensional model, the stability criterion has been obtained theoretically. Using the above criterion, one can show that there exists an optimum thickness of the copper cladding, at which a maximum of the overall critical current density can be reached. The theoretical results are in good agreement with our experiments with simulation coils or with the reported results. Based on the theory of the dynamic stability, the unification of the cryostatic, dynamic and adiabatic stability has been discussed

Ground state description of bound polarons in parabolic quantum wires

The Feynman-Haken variational path integral theory is, for the first time, generalized to calculate the ground-state energy of an electron coupled simultaneously to a Coulomb potential and to a longitudinal-optical (LO) phonon field in parabolic quantum wires. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron-phonon coupling constant than the Coulomb binding parameter and monotonically stronger as the effective wire radius decreases. We apply our calculations to several semiconductor quantum wires and find that the polaronic correction can be considerably large

C-Structures in Mesospheric Na and K Layers and Their Relations with Dynamical and Convective Instabilities

We analyzed the C-structures in the mesospheric metal layers. We used two datasets: one from a narrow band Sodium (Na) Density and Temperature LIDAR and the other from a high-resolution dual band Na and Potassium (K) LIDAR, both operated at São José dos Campos, Brazil (23° S, 46° W). We also investigated the Es layer occurrence and wind shear influences forming these structures. We found three C-type events over 82 analyzed nights in the first data set. They all showed lower temperatures inside C-structures compared to the borders. The squared Brunt-Väissälä frequency analyses showed positive values in the region of C-structures. In two out of three cases, dynamical instability was present (Ri < 0.25). We compared these results with the nine simultaneous C-type events identified in the 185 nights from the second data set. They showed height and time simultaneity correspondence as observed in the Na and K layers. Our results showed a low correlation between Es occurrence and C-structures. Additionally, strong wind shears in the altitude and time where C-structures appeared were always present. The advection of a metal cloud to the LIDAR station and a wind distortion seems to be the plausible mechanism that can explain all the observations