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Periodic Oscillation (periodic + oscillation)
Selected AbstractsInvestigation of the Start Transient in a Hall ThrusterCONTRIBUTIONS TO PLASMA PHYSICS, Issue 9-10 2008H. Liu Abstract A two dimensional axisymmetric fully kinetic Particle-in-Cell/Monte Carlo Collision (PIC-MCC) model is used to describe the ignition process in a Hall thruster. A current peak and latter the periodic oscillation of current and electric potential are found. The corresponding evolutions of plasma density, electric potential and atom density during the ignition process are introduced in the paper. In addition, influences of mass flow rate and discharge potential on current peak are modeled and analyzed. The simulated results are consistent with former experimental results. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Double and triple frequency oscillations observed in resonant tunneling spectroscopy through edge states around an antidotPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2005K. Arai Abstract Resonant tunneling through edge states bounded around an antidot in quantum Hall (QH) regime has been investigated and we have found multiple frequency oscillations in a resonant tunneling through an antidot. We have measured resistance as a function of magnetic field and front gate voltage and found the periodic oscillation in the , = 2 to 1 transition regime. It is related that discrete energy levels are formed in the edge states around the antidot [1]. Not only single peaks but also some split peaks are seen in our data. Fourier power spectrum of the data shows double and triple frequency peak. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The patterns of spontaneous Ca2+ signals generated by ventral spinal neurons in vitro show time-dependent refinementEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 8 2009Sara Sibilla Abstract Embryonic spinal neurons maintained in organotypic slice culture are known to mimic certain maturation-dependent signalling changes. With such a model we investigated, in embryonic mouse spinal segments, the age-dependent spatio-temporal control of intracellular Ca2+ signalling generated by neuronal populations in ventral circuits and its relation with electrical activity. We used Ca2+ imaging to monitor areas located within the ventral spinal horn at 1 and 2 weeks of in vitro growth. Primitive patterns of spontaneous neuronal Ca2+ transients (detected at 1 week) were typically synchronous. Remarkably, such transients originated from widespread propagating waves that became organized into large-scale rhythmic bursts. These activities were associated with the generation of synaptically mediated inward currents under whole-cell patch-clamp. Such patterns disappeared during longer culture of spinal segments: at 2 weeks in culture, only a subset of ventral neurons displayed spontaneous, asynchronous and repetitive Ca2+ oscillations dissociated from background synaptic activity. We observed that the emergence of oscillations was a restricted phenomenon arising together with the transformation of ventral network electrophysiological bursting into asynchronous synaptic discharges. This change was accompanied by the appearance of discrete calbindin immunoreactivity against an unchanged background of calretinin-positive cells. It is attractive to assume that periodic oscillations of Ca2+ confer a summative ability to these cells to shape the plasticity of local circuits through different changes (phasic or tonic) in intracellular Ca2+. [source] Removing undersampling artifacts in DCE-MRI studies using independent components analysisMAGNETIC RESONANCE IN MEDICINE, Issue 4 2008A.L. Martel Abstract In breast MRI mammography both high temporal resolution and high spatial resolution have been shown to be important in improving specificity. Adaptive methods such as projection reconstruction time-resolved imaging of contrast kinetics (PR-TRICKS) allow images to be reconstructed at various temporal and spatial resolutions from the same data set. The main disadvantage is that the undersampling, which is necessary to produce high temporal resolution images, leads to the presence of streak artifacts in the images. We present a novel method of removing these artifacts using independent components analysis (ICA) and demonstrate that this results in a significant improvement in image quality for both simulation studies and for patient dynamic contrast-enhanced (DCE)-MRI images. We also investigate the effect of artifacts on two quantitative measures of contrast enhancement. Using simulation studies we demonstrate that streak artifacts lead to pronounced periodic oscillations in pixel concentration curves which, in turn, lead to increased errors and introduce bias into heuristic measurements. ICA filtering significantly reduces this bias and improves accuracy. Pharmacokinetic modeling was more robust and there was no evidence of bias due to the presence of streak artifacts. ICA filtering did not significantly reduce the errors in the estimated pharmacokinetic parameters; however, the chi-squared error was greatly reduced after ICA filtering. Magn Reson Med, 2008. © 2008 Wiley-Liss, Inc. [source] | ||||||||||