23,270 research outputs found
Square root meadows
Let Q_0 denote the rational numbers expanded to a meadow by totalizing
inversion such that 0^{-1}=0. Q_0 can be expanded by a total sign function s
that extracts the sign of a rational number. In this paper we discuss an
extension Q_0(s ,\sqrt) of the signed rationals in which every number has a
unique square root.Comment: 9 page
Time-of-Flight Based Calibration of an Ultrasonic Computed Tomography System
The paper presents a novel method for calibration of measuring geometry and of individual signal delays of transducers in ultrasonic computed tomography (USCT) systems via computational processing of multiple time-of-flight measurements of ultrasonic (US) impulses. The positions and time-delay parameters of thousands of ultrasonic transducers inside the USCT tank are calibrated by this approach with a high precision required for the tomographic reconstruction; such accuracy cannot be provided by any other known method. Although utilising similar basic principles as the global positioning system (GPS), the method is importantly generalised in treating all transducer parameters as the to-be calibrated (floating) unknowns, without any a-priori known positions and delays. The calibration is formulated as a non-linear least-squares problem, minimizing the differences between the calculated and measured time-of-arrivals of ultrasonic pulses. The paper provides detailed derivation of the method, and compares two implemented approaches (earlier calibration of individual transducers with the new approach calibrating rigid transducer arrays) via detailed simulations, aimed at testing the convergence properties and noise robustness of both approaches. Calibration using real US signals is described and, as an illustration of the utility of the presented method, a comparison is shown of two image reconstructions using the tomographic US data from a concrete experimental USCT system measuring a 3D phantom, without and after the calibration
Exotic branes and non-geometric backgrounds
When string/M-theory is compactified to lower dimensions, the U-duality
symmetry predicts so-called exotic branes whose higher dimensional origin
cannot be explained by the standard string/M-theory branes. We argue that
exotic branes can be understood in higher dimensions as non-geometric
backgrounds or U-folds, and that they are important for the physics of systems
which originally contain no exotic charges, since the supertube effect
generically produces such exotic charges. We discuss the implications of exotic
backgrounds for black hole microstate (non-)geometries.Comment: 4 pages. v2: journal version. The discussion on "double puff-up"
revise
Surface plasma resonance in small rare gas clusters by mixing IR and VUV laser pulses
The ionization dynamics of a Xenon cluster with 40 atoms is analyzed under a
pum p probe scenario of laser pulses where an infrared laser pulse of 50 fs
length follows with a well defined time delay a VUV pulse of the same length
and peak intensity. The mechanism of resonant energy absorption due to the
coinc idence of the IR laser frequency with the frequency of collective motion
of quasi free electrons in the cluster is mapped out by varying the time delay
between the pulses
Single-Frequency, Thin-Film-Tuned, 0.6W, Diode-Pumped Nd:YVO\u3csub\u3e4\u3c/sub\u3e Laser
Application of a metallic thin-film selector to the single-frequency oscillation of a diode-pumped Nd:YVO4 laser has been investigated theoretically and experimentally. We show that a chromium thin-film selector with a thickness between 8 and 9 nm provides single-frequency output within a power range of 0.6W. Single-frequency operation, slow smooth turning, or chirping was realized by the output coupler movement with a piezoceramic transducer. Chuirping at a repetition rate of 0.5 kHz in the 0.5-10-GHz range was achieved. Physical and technical limitations caused by the wide-gain bandwidith, thermal effects, and mechanical vibrations of cavity elements are discussed
Next-to-next-to-leading logarithmic resummation for transverse thrust
We obtain a prediction for the hadron-collider event-shape variable
transverse thrust in which the terms enhanced in the dijet limit are resummed
to next-to-next-to-leading logarithmic accuracy. Our method exploits
universality properties made manifest in the factorized expression for the
cross section and only requires one-loop calculations. The necessary two-loop
ingredients are extracted using known results and existing numerical codes. Our
technique is general and applicable to other observables as well.Comment: 6 pages, 2 figures. v2: Corrected values in Eq.(15); updated Figure 2
accordingl
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