Characterization of gravity current formation for the use in detonation refraction experiments

Detonation propagation through an interface is being studied at Caltech. In these experiments, the interface shape is determined by the gravity currents. This report presents an experimental study of the formation and the development of these gravity currents by an analog system in a water channel using water and salt water to simulate the density differences in detonation experiments. The major parameters such as the Reynolds number and the density difference were matched in both experiments to be able to compare the gravity current in the water channel and the gravity current in the Galcit detonation tube. In the present study, the gravity current was generated by the removal of a plate, and was visualized by adding food dye. The results confirm previous studies; Keulegan demonstrated in 1957 that the velocity of the gravity current is a function of the square root of the density difference. The interface is affect by the retracting of the plate which creates the wake effects. The Kelvin-Helmholtz instabilities on the upstream side of the gravity current create the visualized mixing zones. Finally, this study revealed how the gravity current interface into the Galcit detonation tube should develop and where the mixing zones should occur

Anxiety-potentiated amygdala-medial frontal coupling and attentional control

Anxiety disorders can be treated both pharmacologically and psychologically, but many individuals either fail to respond to treatment or relapse. Improving outcomes is difficult, in part because we have incomplete understanding of the neurobiological mechanisms underlying current treatments. In a sequence of studies, we have identified 'affective bias-related' amygdala-medial cortical coupling as a candidate substrate underlying adaptive anxiety (that is, anxiety elicited by threat of shock in healthy individuals) and shown that it is also chronically engaged in maladaptive anxiety disorders. We have provided evidence that this circuit can be modulated pharmacologically, but whether this mechanism can be shifted by simple psychological instruction is unknown. In this functional magnetic resonance imaging study, we extend a previously used translational anxiety induction (threat of shock) in healthy subjects (N=43) and cognitive task to include an element of instructed attentional control. Replicating our previous findings, we show that induced anxiety engages 'affective bias-related' amygdala-dorsal medial frontal coupling during the processing of emotional faces. By contrast, instructing subjects to attend to neutral shapes (and ignore faces) disengages this circuitry and increases putative 'attentional control-related' coupling between the amygdala and a more rostral prefrontal region. These neural coupling changes are accompanied by corresponding modulation of behavioural performance. Taken together, these findings serve to further highlight the potential role of amygdala-medial frontal coupling in the pathogenesis of anxiety and highlight a mechanism by which it can be modulated via psychological instructions. This, in turn, generates hypotheses for future work exploring the mechanisms underlying psychological therapeutic interventions for anxiety

Instability of ion kinetic waves in a weakly ionized plasma

The fundamental higher-order Landau plasma modes are known to be generally heavily damped. We show that these modes for the ion component in a weakly ionized plasma can be substantially modified by ion-neutral collisions and a dc electric field driving ion flow so that some of them can become unstable. This instability is expected to naturally occur in presheaths of gas discharges at sufficiently small pressures and thus affect sheaths and discharge structures.Comment: Published in Phys. Rev. E, see http://link.aps.org/doi/10.1103/PhysRevE.85.02641

Magpie: towards a semantic web browser

Web browsing involves two tasks: finding the right web page and then making sense of its content. So far, research has focused on supporting the task of finding web resources through ‘standard’ information retrieval mechanisms, or semantics-enhanced search. Much less attention has been paid to the second problem. In this paper we describe Magpie, a tool which supports the interpretation of web pages. Magpie offers complementary knowledge sources, which a reader can call upon to quickly gain access to any background knowledge relevant to a web resource. Magpie automatically associates an ontologybased semantic layer to web resources, allowing relevant services to be invoked within a standard web browser. Hence, Magpie may be seen as a step towards a semantic web browser. The functionality of Magpie is illustrated using examples of how it has been integrated with our lab’s web resources

The dorsal medial prefrontal (anterior cingulate) cortex–amygdala aversive amplification circuit in unmedicated generalised and social anxiety disorders: an observational study

We have delineated, across four prior studies, the role of positive dorsal medial prefrontal/anterior cingulate cortex (dmPFC/ACC)-amygdala circuit coupling during aversive processing in healthy individuals under stress. This translational circuit, termed the 'aversive amplification circuit', is thought to drive adaptive, harm-avoidant behavior in threatening environments. Here, in a natural progression of this prior work, we confirm that this circuit also plays a role in the pathological manifestation of anxiety disorders

Electron surface layer at the interface of a plasma and a dielectric wall

We study the potential and the charge distribution across the interface of a plasma and a dielectric wall. For this purpose, the charge bound to the wall is modelled as a quasi-stationary electron surface layer which satisfies Poisson's equation and minimizes the grand canonical potential of the wall-thermalized excess electrons constituting the wall charge. Based on an effective model for a graded interface taking into account the image potential and the offset of the conduction band to the potential just outside the dielectric, we specifically calculate the potential and the electron distribution for magnesium oxide, silicon dioxide and sapphire surfaces in contact with a helium discharge. Depending on the electron affinity of the surface, we find two vastly different behaviors. For negative electron affinity, electrons do not penetrate into the wall and an external surface charge is formed in the image potential, while for positive electron affinity, electrons penetrate into the wall and a space charge layer develops in the interior of the dielectric. We also investigate how the electron surface layer merges with the bulk of the dielectric.Comment: 15 pages, 9 figures, accepted versio

A planar calculus for infinite index subfactors

We develop an analog of Jones' planar calculus for II_1-factor bimodules with arbitrary left and right von Neumann dimension. We generalize to bimodules Burns' results on rotations and extremality for infinite index subfactors. These results are obtained without Jones' basic construction and the resulting Jones projections.Comment: 56 pages, many figure

Rotational symmetry of self-similar solutions to the Ricci flow

Let (M,g) be a three-dimensional steady gradient Ricci soliton which is non-flat and \kappa-noncollapsed. We prove that (M,g) is isometric to the Bryant soliton up to scaling. This solves a problem mentioned in Perelman's first paper.Comment: Final version, to appear in Invent. Mat