The Detection of Defects in a Niobium Tri-layer Process

Niobium (Nb) LTS processes are emerging as the technology for future ultra high-speed systems especially in the digital domain. As the number of Josephson Junctions (JJ) per chip has recently increased to around 90000, the quality of the process has to be assured so as to realize these complex circuits. Until now, very little or no information is available in the literature on how to achieve this. In this paper we present an approach and results of a study conducted on an RSFQ process. Measurements and SEM inspection were carried out on sample chips and a list of possible defects has been identified and described in detail. We have also developed test-structures for detection of the top-ranking defects, which will be used for yield analysis and the determination of the probability distribution of faults in the process. A test chip has been designed, based on the results of this study, and certain types of defects were introduced in the design to study the behavior of faulty junctions and interconnections

Air cavities at the inner cylinder of turbulent Taylor-Couette flow

Air cavities, i.e. air layers developed behind cavitators, are seen as a promising drag reducing method in the maritime industry. Here we utilize the Taylor-Couette (TC) geometry, i.e. the flow between two concentric, independently rotating cylinders, to study the effect of air cavities in this closed setup, which is well-accessible for drag measurements and optical flow visualizations. We show that stable air cavities can be formed, and that the cavity size increases with Reynolds number and void fraction. The streamwise cavity length strongly depends on the axial position due to buoyancy forces acting on the air. Strong secondary flows, which are introduced by a counter-rotating outer cylinder, clearly decrease the stability of the cavities, as air is captured in the Taylor rolls rather than in the cavity. Surprisingly, we observed that local air injection is not necessary to sustain the air cavities; as long as air is present in the system it is found to be captured in the cavity. We show that the drag is decreased significantly as compared to the case without air, but with the geometric modifications imposed on the TC system by the cavitators. As the void fraction increases, the drag of the system is decreased. However, the cavitators itself significantly increase the drag due to their hydrodynamic resistance (pressure drag): In fact, a net drag increase is found when compared to the standard smooth-wall TC case. Therefore, one must first overcome the added drag created by the cavitators before one obtains a net drag reduction.Comment: 14 pages, 13 figure

The influence of wall roughness on bubble drag reduction in Taylor-Couette turbulence

We experimentally study the influence of wall roughness on bubble drag reduction in turbulent Taylor-Couette flow, i.e.\ the flow between two concentric, independently rotating cylinders. We measure the drag in the system for the cases with and without air, and add roughness by installing transverse ribs on either one or both of the cylinders. For the smooth wall case (no ribs) and the case of ribs on the inner cylinder only, we observe strong drag reduction up to $DR=33\%$ and $DR=23\%$, respectively, for a void fraction of $\alpha=6\%$. However, with ribs mounted on both cylinders or on the outer cylinder only, the drag reduction is weak, less than $DR=11\%$, and thus quite close to the trivial effect of reduced effective density. Flow visualizations show that stable turbulent Taylor vortices --- large scale vortical structures --- are induced in these two cases, i.e. the cases with ribs on the outer cylinder. These strong secondary flows move the bubbles away from the boundary layer, making the bubbles less effective than what had previously been observed for the smooth-wall case. Measurements with counter-rotating smooth cylinders, a regime in which pronounced Taylor rolls are also induced, confirm that it is really the Taylor vortices that weaken the bubble drag reduction mechanism. Our findings show that, although bubble drag reduction can indeed be effective for smooth walls, its effect can be spoiled by e.g.\ biofouling and omnipresent wall roughness, as the roughness can induce strong secondary flows.Comment: 10 pages, 5 figure

The extraction of hadronic parameters from experiments on pionium

Experimental values of the lifetime of the 1s level of pionium and of the difference between the energies of the 2s and 2p levels yield values of the a(0c) and a(cc) elements of the s-wave scattering matrix for the 2-channel (pi+ pi-, pi0 pi0) system at the pi+ pi- threshold. We develop a method for obtaining the isospin invariant quanties a20 - a00 and 2a00 + a20 from a(0c) and a(cc). We emphasize that the isospin invariant scattering lengths a00 and a20 universally used in the literature cannot be considered to be purely hadronic quantities.Comment: 17 pages, Revtex, 1 postscript figure, new version of figure which removes ghostscript problem

Self-similar decay of high Reynolds number Taylor-Couette turbulence

We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e. the turbulent flow between two coaxial rotating cylinders. To do so, the rotation of the inner cylinder (Re$_i=2 \times 10^6$, the outer cylinder is at rest) is stopped within 12 s, thus fully removing the energy input to the system. Using a combination of laser Doppler anemometry and particle image velocimetry measurements, six decay decades of the kinetic energy could be captured. First, in the absence of cylinder rotation, the flow-velocity during the decay does not develop any height dependence in contrast to the well-known Taylor vortex state. Second, the radial profile of the azimuthal velocity is found to be self-similar. Nonetheless, the decay of this wall-bounded inhomogeneous turbulent flow does not follow a strict power law as for decaying turbulent homogeneous isotropic flows, but it is faster, due to the strong viscous drag applied by the bounding walls. We theoretically describe the decay in a quantitative way by taking the effects of additional friction at the walls into account.Comment: 7 pages, 6 figure

Efficient Computation of Multiple Density-Based Clustering Hierarchies

HDBSCAN*, a state-of-the-art density-based hierarchical clustering method, produces a hierarchical organization of clusters in a dataset w.r.t. a parameter mpts. While the performance of HDBSCAN* is robust w.r.t. mpts in the sense that a small change in mpts typically leads to only a small or no change in the clustering structure, choosing a "good" mpts value can be challenging: depending on the data distribution, a high or low value for mpts may be more appropriate, and certain data clusters may reveal themselves at different values of mpts. To explore results for a range of mpts values, however, one has to run HDBSCAN* for each value in the range independently, which is computationally inefficient. In this paper, we propose an efficient approach to compute all HDBSCAN* hierarchies for a range of mpts values by replacing the graph used by HDBSCAN* with a much smaller graph that is guaranteed to contain the required information. An extensive experimental evaluation shows that with our approach one can obtain over one hundred hierarchies for the computational cost equivalent to running HDBSCAN* about 2 times.Comment: A short version of this paper appears at IEEE ICDM 2017. Corrected typos. Revised abstrac

Periodically driven Taylor-Couette turbulence

We study periodically driven Taylor-Couette turbulence, i.e. the flow confined between two concentric, independently rotating cylinders. Here, the inner cylinder is driven sinusoidally while the outer cylinder is kept at rest (time-averaged Reynolds number is $Re_i = 5 \times 10^5$). Using particle image velocimetry (PIV), we measure the velocity over a wide range of modulation periods, corresponding to a change in Womersley number in the range $15 \leq Wo \leq 114$. To understand how the flow responds to a given modulation, we calculate the phase delay and amplitude response of the azimuthal velocity. In agreement with earlier theoretical and numerical work, we find that for large modulation periods the system follows the given modulation of the driving, i.e. the system behaves quasi-stationary. For smaller modulation periods, the flow cannot follow the modulation, and the flow velocity responds with a phase delay and a smaller amplitude response to the given modulation. If we compare our results with numerical and theoretical results for the laminar case, we find that the scalings of the phase delay and the amplitude response are similar. However, the local response in the bulk of the flow is independent of the distance to the modulated boundary. Apparently, the turbulent mixing is strong enough to prevent the flow from having radius-dependent responses to the given modulation.Comment: 12 pages, 6 figure