Parametric instabilities in magnetized multicomponent plasmas

This paper investigates the excitation of various natural modes in a magnetized bi-ion or dusty plasma. The excitation is provided by parametrically pumping the magnetic field. Here two ion-like species are allowed to be fully mobile. This generalizes our previous work where the second heavy species was taken to be stationary. Their collection of charge from the background neutral plasma modifies the dispersion properties of the pump and excited waves. The introduction of an extra mobile species adds extra modes to both these types of waves. We firstly investigate the pump wave in detail, in the case where the background magnetic field is perpendicular to the direction of propagation of the pump wave. Then we derive the dispersion equation relating the pump to the excited wave for modes propagating parallel to the background magnetic field. It is found that there are a total of twelve resonant interactions allowed, whose various growth rates are calculated and discussed.Comment: Published in May 2004; this is a late submission to the archive. 14 pages, 8 figure

The Application of Infrared Thermographic Inspection Techniques to the Space Shuttle Thermal Protection System

The Nondestructive Evaluation Sciences Branch at NASA s Langley Research Center has been actively involved in the development of thermographic inspection techniques for more than 15 years. Since the Space Shuttle Columbia accident, NASA has focused on the improvement of advanced NDE techniques for the Reinforced Carbon-Carbon (RCC) panels that comprise the orbiter s wing leading edge. Various nondestructive inspection techniques have been used in the examination of the RCC, but thermography has emerged as an effective inspection alternative to more traditional methods. Thermography is a non-contact inspection method as compared to ultrasonic techniques which typically require the use of a coupling medium between the transducer and material. Like radiographic techniques, thermography can be used to inspect large areas, but has the advantage of minimal safety concerns and the ability for single-sided measurements. Principal Component Analysis (PCA) has been shown effective for reducing thermographic NDE data. A typical implementation of PCA is when the eigenvectors are generated from the data set being analyzed. Although it is a powerful tool for enhancing the visibility of defects in thermal data, PCA can be computationally intense and time consuming when applied to the large data sets typical in thermography. Additionally, PCA can experience problems when very large defects are present (defects that dominate the field-of-view), since the calculation of the eigenvectors is now governed by the presence of the defect, not the "good" material. To increase the processing speed and to minimize the negative effects of large defects, an alternative method of PCA is being pursued where a fixed set of eigenvectors, generated from an analytic model of the thermal response of the material under examination, is used to process the thermal data from the RCC materials. Details of a one-dimensional analytic model and a two-dimensional finite-element model will be presented. An overview of the PCA process as well as a quantitative signal-to-noise comparison of the results of performing both embodiments of PCA on thermographic data from various RCC specimens will be shown. Finally, a number of different applications of this technology to various RCC components will be presented

Parametric instability in dark molecular clouds

The present work investigates the parametric instability of parallel propagating circularly polarized Alfven(pump) waves in a weakly ionized molecular cloud. It is shown that the relative drift between the plasma particles gives rise to the Hall effect resulting in the modified pump wave characteristics. Although the linearized fluid equations with periodic coefficients are difficult to solve analytically, it is shown that a linear transformation can remove the periodic dependence. The resulting linearized equations with constant coefficients are used to derive an algebraic dispersion relation. The growth rate of the parametric instability is a sensitive function of the amplitude of the pump wave as well as to the ratio of the pump and the modified dust-cyclotron frequencies. The instability is insensitive to the plasma-beta The results are applied to the molecular clouds.Comment: 27 page, 5 figures, accepted in Ap

The effects of polymer molecular weight on filament thinning and drop breakup in microchannels

We investigate the effects of fluid elasticity on the dynamics of filament thinning and drop breakup processes in a cross-slot microchannel. Elasticity effects are examined using dilute aqueous polymeric solutions of molecular weight (MW) ranging from 1.5×103 to 1.8×107. Results for polymeric fluids are compared to those for a viscous Newtonian fluid. The shearing or continuous phase that induces breakup is mineral oil. All fluids possess similar shear-viscosity (~0.2 Pa s) so that the viscosity ratio between the oil and aqueous phases is close to unity. Measurements of filament thickness as a function of time show different thinning behavior for the different aqueous fluids. For Newtonian fluids, the thinning process shows a single exponential decay of the filament thickness. For low MW fluids (103, 104 and 105), the thinning process also shows a single exponential decay, but with a decay rate that is slower than for the Newtonian fluid. The decay time increases with polymer MW. For high MW (106 and 107) fluids, the initial exponential decay crosses over to a second exponential decay in which elastic stresses are important. We show that the decay rate of the filament thickness in this exponential decay regime can be used to measure the steady extensional viscosity of the fluids. At late times, all fluids cross over to an algebraic decay which is driven mainly by surface tension

Random division of an interval

The well-known relation between random division of an interval and the Poisson process is interpreted as a Laplace transformation. With the use of this interpretation a number of (in part known) results is derived very easily

Psycholinguistic norms for more than 300 lexical manual signs in German Sign Language (DGS)

Sign languages provide researchers with an opportunity to ask empirical questions about the human language faculty that go beyond considerations specific to speech and writing. Whereas psycholinguists working with spoken and written language stimuli routinely control their materials for parameters such as lexical frequency and age of acquisition (AoA), no such information or normed stimulus sets are currently available to researchers working with German Sign Language (DGS). Our contribution presents the first norms for iconicity, familiarity, AoA, and transparency for DGS. The normed stimulus set consists of more than 300 clips of manual DGS signs accom- panied by mouthings and non-manual components. Norms for the signs in the clips are derived from ratings by a total of 30 deaf signers in Leipzig, Göttingen, and Hamburg, as well as 30 hearing non-signers and native speakers of German in Leipzig. The rating procedure was implemented in a browser to ensure functionality and a similar procedure across locations and participants (Figure 1a), yet all participants performed the ratings on site in the presence of an experimenter. Deaf signers performed a total of three tasks in which they rated stimulus clips for iconicity, AoA, and familiarity. Such subjective measures of AoA and familiarity have been shown to be good proxies for corpus measures in studies of other spoken and sign languages (Vinson, Cormier, Denmark, Schembri, & Vigliocco, 2008). Hearing non-signers performed two tasks in which they first guessed the meaning of the signs in the clips to determine transparency and in the second task rated iconicity given the meaning. In addition to empirical norming data (e.g., Figure 1b), we provide information about German and English correspondences of signs. The stimulus set has been annotated in machine-readable form with regard to lexico-semantic as well as phonological properties of signs: one-handed vs. two-handed, place of articulation, path movement, symmetry, most likely lexical class, animacy, verb type, (potential) homonymy, and potential dialectal variation. Information about sign on- and offset for all stimulus clips and a number of quantitative measures of movement are also available. These were derived from automated motion tracking by fitting a pose-estimation model (Figure 1c) to the clips using OpenPose (Wei, Ramakrishna, Kanade, & Sheikh, 2016) which allows us to quantify and automatically track movement (velocity and acceleration) beyond annotation (Figure 1d). In this presentation, we will focus on providing an overview of the derived norms and attempt to put them in perspective of published empirical norms for other sign languages, for example, ASL and BSL (Vinson et al., 2008; Caselli, Sehyr, Cohen-Goldberg, & Emmorey, 2017), as well as comparable information for spoken languages. This includes a comparison of our subjective rating data with regard to frequency and AoA obtained using DGS signs with norms for other sign languages as well as with similar measures for German and English. We also discuss the relationship of mean iconicity ratings between deaf signers and hearing non-signers, as well as the relation of iconicity and transparency. Our norms and stimulus set are intended to control for psychologically relevant param- eters in future psycho- and neurolinguistic studies of DGS beyond the work of our own labs. Consequently, the norms, stimulus clips, cleaned raw data, and the R scripts used for analysis will be made available for download through the Open Science Framework. References Caselli, N. K., Sehyr, Z. S., Cohen-Goldberg, A. M., & Emmorey, K. (2017). ASL-LEX: A lexical database of American Sign Language. Behavior Research Methods, 49(2), 784-801. doi: 10.3758/ s13428-016-0742-0 Vinson, D. P., Cormier, K., Denmark, T., Schembri, A., & Vigliocco, G. (2008). The British Sign Language (BSL) norms for age of acquisition, familiarity, and iconicity. Behavior Research Methods, 40(4), 1079-1087. doi: 10.3758/BRM.40.4.1079 Wei, S.-E., Ramakrishna, V., Kanade, T., & Sheikh, Y. (2016). Convolutional pose machines. arXiv:1602.00134 [cs]

Origin of the Canonical Ensemble: Thermalization with Decoherence

We solve the time-dependent Schrodinger equation for the combination of a spin system interacting with a spin bath environment. In particular, we focus on the time development of the reduced density matrix of the spin system. Under normal circumstances we show that the environment drives the reduced density matrix to a fully decoherent state, and furthermore the diagonal elements of the reduced density matrix approach those expected for the system in the canonical ensemble. We show one exception to the normal case is if the spin system cannot exchange energy with the spin bath. Our demonstration does not rely on time-averaging of observables nor does it assume that the coupling between system and bath is weak. Our findings show that the canonical ensemble is a state that may result from pure quantum dynamics, suggesting that quantum mechanics may be regarded as the foundation of quantum statistical mechanics.Comment: 12 pages, 4 figures, accepted for publication by J. Phys. Soc. Jp

On rr-Simple kk-Path

An $r$-simple $k$-path is a {path} in the graph of length $k$ that passes through each vertex at most $r$ times. The $r$-SIMPLE $k$-PATH problem, given a graph $G$ as input, asks whether there exists an $r$-simple $k$-path in $G$. We first show that this problem is NP-Complete. We then show that there is a graph $G$ that contains an $r$-simple $k$-path and no simple path of length greater than $4\log k/\log r$. So this, in a sense, motivates this problem especially when one's goal is to find a short path that visits many vertices in the graph while bounding the number of visits at each vertex. We then give a randomized algorithm that runs in time $$\mathrm{poly}(n)\cdot 2^{O( k\cdot \log r/r)}$$ that solves the $r$-SIMPLE $k$-PATH on a graph with $n$ vertices with one-sided error. We also show that a randomized algorithm with running time $\mathrm{poly}(n)\cdot 2^{(c/2)k/ r}$ with $c<1$ gives a randomized algorithm with running time \poly(n)\cdot 2^{cn} for the Hamiltonian path problem in a directed graph - an outstanding open problem. So in a sense our algorithm is optimal up to an $O(\log r)$ factor