Effect of combining a DC bias current with an AC transport current on AC losses in a High Temperature Superconductor

Creating complex flux configurations by superposing a dc current or magnetic field onto the ac current in a type II superconducting tape should lead to a variety of peculiar behaviors. An example is the appearance of the Clem valley, a minimum in the ac losses as a function of the dc bias amplitude, which has been theoretically studied by LeBlanc et al., in the continuation of Clem's calculations. These situations have been investigated by applying a dc current to a silver-gold sheathed Bi-2223 tape at 77 K (critical current 29 A), in addition to the usual ac transport current. The ac losses were measured by the null calorimetric method to ensure that the total losses were being accounted for. These were recorded for different values of the ac and dc currents, leading to the observation of two different behaviors depending on the ac current. Our revelation of the Clem valley is, to our knowledge, the first experimental validation of this phenomenon in high temperature superconductors, and may provide a simple way of reducing the ac loss in industrial applications of these materials.Comment: 4 pages,4 figures - ASC 9

Proximity, candidates, and presidential power: how directly elected presidents shape the legislative party system

The impact of presidential coattails on the legislative party system is a highly intuitive idea. The coattails effect is believed to depend on the number of presidential candidates and the size of the presidential prize. This article proposes a different way of understanding this relationship. We argue that the strategic behavior of political parties and the way in which the number of presidential contenders shapes the legislative party system can only be predicted for an intermediate range of presidential power. Outside this range the effect is indeterminate. We test our proposition on democracies with direct presidential elections from 1945-2011. Our results confirm that the number of presidential candidates is an important determinant of the legislative party system and show that whether or not legislative elections are held close to presidential elections has little influence on party-system fragmentation in countries with directly elected presidents

Développement d'une méthode calorimétrique de mesure des pertes ac pour des rubans supraconducteurs à haute température critique

Le travail de recherche effectué dans le cadre de ce projet de doctorat a permis la mise au point d'une méthode de mesure des pertes ac destinée à l'étude des supraconducteurs à haute température critique. Pour le choix des principes de cette méthode, nous nous sommes inspirés de travaux antérieurs réalisés sur les supraconducteurs conventionnels, afin de proposer une alternative à la technique électrique, présentant lors du début de cette thèse des problèmes liés à la variation du résultat des mesures selon la position des contacts de tension sur la surface de l'échantillon, et de pouvoir mesurer les pertes ac dans des conditions simulant la réalité des futures applications industrielles des rubans supraconducteurs: en particulier, cette méthode utilise la technique calorimétrique, associée à une calibration simultanée et in situ. La validité de la méthode a été vérifiée de manière théorique et expérimentale: d'une part, des mesures ont été réalisées sur des échantillons de Bi-2223 recouverts d'argent ou d'alliage d'argent-or et comparées avec les prédictions théoriques données par Norris, nous indiquant la nature majoritairement hystérétique des pertes ac dans nos échantillons; d'autre part, une mesure électrique a été réalisée in situ dont les résultats correspondent parfaitement à ceux donnés par notre méthode calorimétrique. Par ailleurs, nous avons comparé la dépendance en courant et en fréquence des pertes ac d'un échantillon avant et après qu'il ait été endommagé. Ces mesures semblent indiquer une relation entre la valeur du coefficient de la loi de puissance modélisant la dépendance des pertes avec le courant, et les inhomogénéités longitudinales du courant critique induites par l'endommagement. De plus, la variation en fréquence montre qu'au niveau des grosses fractures transverses créées par l'endommagement dans le coeur supraconducteur, le courant se partage localement de manière à peu près équivalente entre les quelques grains de matière supraconductrice qui restent fixés à l'interface coeur-enveloppe, et le revêtement en alliage d'argent. L'intérêt d'une méthode calorimétrique par rapport à la technique électrique, plus rapide, plus sensible et maintenant fiable, réside dans la possibilité de réaliser des mesures de pertes ac dans des environnements complexes, reproduisant la situation présente par exemple dans un câble de transport d'énergie ou dans un transformateur. En particulier, la superposition d'un courant dc en plus du courant ac habituel nous a permis d'observer expérimentalement, pour la première fois à notre connaissance, un comportement particulier des pertes ac en fonction de la valeur du courant dc décrit théoriquement par LeBlanc. Nous avons pu en déduire la présence d'un courant d'écrantage Meissner de 16 A, ce qui nous permet de déterminer les conditions dans lesquelles une réduction du niveau de pertes ac pourrait être obtenue par application d'un courant dc, phénomène dénommé"vallée de Clem"

Cure versus Flow in Dispersed Chip-Underfill Materials

The relative stability of chip-underfill composite materials was modeled as a function of glass filler concentration between 10 and 70 wt.-%, filler particle size (between 5 and 25 microns), and the curing temperature of the resin (150 vs. 180 °C), yielding different dynamic viscosity profiles. The stability was gauged using a modified sigmoidal chemorheology model for the dynamic viscosity, and incorporating the time-dependent viscosity into a model for Stokes' law of sedimentation. We also incorporated a hindered sedimentation term, due to filler concentration due to the higher loadings. Several important findings were observed. First, it appears to be the high concentration of filler that is maintaining the stability of these dispersions during cure. Smaller concentrations of the same particles were predicted to have a larger sedimentation velocity leading to stratification in the resin with time. Second, higher cure temperatures led to a shorter period of sedimentation in a pre-cured state and resulted in less sedimentation, even though there was probably a slightly smaller viscosity in the pre-cured condition. While these process models adequately describe the physics of the competitive processes of cure and sedimentation, a full picture may be incomplete without a larger determination of how this also affects polymerization shrinkage and residual shear stress upon cure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61220/1/828_ftp.pd

Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars

The lightcurves of variable DA stars are usually multi-periodic and non-sinusoidal, so that their Fourier transforms show peaks at eigenfrequencies of the pulsation modes and at sums and differences of these frequencies. These combination frequencies provide extra information about the pulsations, both physical and geometrical, that is lost unless they are analyzed. Several theories provide a context for this analysis by predicting combination frequency amplitudes. In these theories, the combination frequencies arise from nonlinear mixing of oscillation modes in the outer layers of the white dwarf, so their analysis cannot yield direct information on the global structure of the star as eigenmodes provide. However, their sensitivity to mode geometry does make them a useful tool for identifying the spherical degree of the modes that mix to produce them. In this paper, we analyze data from eight hot, low-amplitude DAV white dwarfs and measure the amplitudes of combination frequencies present. By comparing these amplitudes to the predictions of the theory of Goldreich & Wu, we have verified that the theory is crudely consistent with the measurements. We have also investigated to what extent the combination frequencies can be used to measure the spherical degree (ell) of the modes that produce them. We find that modes with ell > 2 are easily identifiable as high ell based on their combination frequencies alone. Distinguishing between ell=1 and 2 is also possible using harmonics. These results will be useful for conducting seismological analysis of large ensembles of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky Survey. Because this method relies only on photometry at optical wavelengths, it can be applied to faint stars using 4 m class telescopes.Comment: 73 pages, 22 figures, accepted in the Ap

High Resolution Spectroscopy of the Pulsating White Dwarf G29-38

We present the analysis of time-resolved, high resolution spectra of the cool white dwarf pulsator, G29-38. From measuring the Doppler shifts of the H-alpha core, we detect velocity changes as large as 16.5 km/s and conclude that they are due to the horizontal motions associated with the g-mode pulsations on the star. We detect seven pulsation modes from the velocity time-series and identify the same modes in the flux variations. We discuss the properties of these modes and use the advantage of having both velocity and flux measurements of the pulsations to test the convective driving theory proposed for DAV stars. Our data show limited agreement with the expected relationships between the amplitude and phases of the velocity and flux modes. Unexpectedly, the velocity curve shows evidence for harmonic distortion, in the form of a peak in the Fourier transform whose frequency is the exact sum of the two largest frequencies. Combination frequencies are a characteristic feature of the Fourier transforms of light curves of G29-38, but before now have not been detected in the velocities, nor does published theory predict that they should exist. We compare our velocity combination frequency to combination frequencies found in the analysis of light curves of G29-38, and discuss what might account for the existence of velocity combinations with the properties we observe. We also use our high-resolution spectra to determine if either rotation or pulsation can explain the truncated shape observed for the DAV star's line core. We are able to eliminate both mechanisms: the average spectrum does not fit the rotationally broadened model and the time-series of spectra provides proof that the pulsations do not significantly truncate the line.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June

Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B: New constraints on mode identification and pulsation properties

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, l, of the modes, most of which have l=1. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of 2.6+/-1.0 km/s, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective.Comment: 16 pages, 12 figs.; accepted for publication in A&