9,899 research outputs found

    A Criterion for Comparing Measurement Results and Determining Conformity with Specifications

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    In this paper a new criterion for comparing measurement results and determining conformity with specifications is proposed, which essentially is a strategy of estimating the empirical relationships of objects. Comparing with traditional methods given in GUM: 2008 and ISO 14253-1, this criterion improves the resolution of comparison by reducing the sizes of the coverage intervals to be compared. Interval order (a binary relation) is used for comparing the coverage intervals of the measurand and represents the empirical relations. The systematic effects of measurement are classified into two types: monotonic and non-monotonic effects, so that, without correcting the monotonic effects, a biased measurand can be specified to represent the empirical relations. Thereby the uncertainty components arising from the monotonic effects can be removed from the combined uncertainty. A strategy is given for determining the relationships among measurement results and specification limits. An example is given to demonstrate the application of the criterion

    Is the U.S. Stock Market Sufficiently Efficient around Hurricanes?

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    This paper tests the U.S. stock market efficiency around all 18 hurricanes that have hit continental U.S. since 2000. Using an event-study methodology, the study analyzes the effect of those 18 hurricanes on a sample of 60 property-casualty insurance companies before and following the hurricanes’ landfall. The study supports the semi-strong form market efficiency and concludes that market inefficiency only exists during the pre-landfall period. Moreover, a significant negative relationship is found between the wind speed and firms’ risk exposure, which reiterates the market’s ability to differentiate hurricanes by their damaging power and to discriminate P&C insurers by their existence of exposure

    Development and Fabrication of Thermally Conductive Polymer Matrix Composite Foams

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    Advancements in the electronics industry have led to miniaturized components with increased computing power, which resulted in serious heat management issue. Under such technological trend, the development of new multifunctional packaging materials with excellent thermal conductivity and electrical resistivity, which can be used for heat dissipation, is becoming increasingly important. A recent research revealed the possibility of using foaming-induced filler alignment to promote the effective thermal conductivity (keff). In this context, this thesis research aims to develop thermally conductive polymer matrix composite (PMC) foams that can provide a solution to the heat management of new electronic devices. First, an analytical model was constructed to confirm the feasibility of foaming-induced keff enhancement. This model considered filler alignment caused by foaming-induced stress field, and calculated the keff using the concept of thermal resistor network. Second, a comprehensive experimental study was conducted to parametrically reveal the dependency of PMCs keff on foam morphological parameters, including filler size, foam expansion ratio, cell size, and cell population density. Low density polyethylene (LDPE)-hexagonal boron nitride (hBN) composites blown by Expancel microspheres were studied as a case example to prove the concept

    LWFA diagnostics development for ATLAS-300 and nonlinear plasma wavelength scalings

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    LWFA is the abbreviation of Laser WakeField Acceleration (or Accelerator), which is an emerging technology promising a drastic reduction of particle accelerators' size and cost. LWFA relies on the large (~GV/cm) electric field in the plasma wave excited by laser pulses with a relativistic peak intensity in excess of 10^18 W/cm^2. During the course of this thesis work, diagnostic tools, for electrons and for the plasma medium, specially adapted to the conditions of LWFA research were developed for the ATLAS-300 Ti:sapphire laser at the Laboratory for Extreme (LEX) Photonics at Ludwig-Maximilians-Universität München (LMU). For electron diagnostics, the implementation and characterization of scintillating screens and permanent dipole magnets were carried out. On the plasma diagnostics side, a few-cycle probe beam for producing fs-snapshots of the plasma wave was developed. Specifically, the light-emitting efficiency of nine commonly used scintillating screen types were calibrated with electron beams from the ELBE Linac at Helmholtz Zentrum Dresden Rossendorf. Existing methods for transferring the absolute calibration results into cross-calibration with a constant light source were improved upon. Regarding the dipole magnet, the B-field distribution near the gap central plane was measured with a Hall sensor and electron tracking was performed in General Particle Tracer using the measured field map. Scintillating screens and the dipole magnet comprised the electron spectrometer, allowing for energy-resolved detection of electron beams generated in the LWFA. Self-injected electron beams with several hundreds pC of charge and peak energy above 1 GeV were accelerated in a hydrogen-filled varying-length gas cell. Injecting with shock fronts in supersonic gas jets, stable electron beams with spectral density beyond 10 pC/MeV was achieved thanks to a percent-level energy spread. The highlight of this work is the construction and application of a hollow-core fiber based pulse compression setup, serving as the probe beam during the experiments. This setup delivered sub-10 fs probe pulses, with which, shadowgraphic snapshots of the laser plasma interaction were recorded. In particular, laser-driven plasma waves were resolved owing to the ultrashort probe pulse duration. A portion of the probe beam was split out to illuminate a Nomarski interferometer, enabling independent measurement of the average plasma density with Abel inversion technique. A systematic measurement of plasma wavelength at varying densities or laser intensities revealed insufficiency of the current understanding of nonlinear plasma wavelength scaling. An empirical scaling was proposed based on a set particle-in-cell simulations, which relates the nonlinear plasma wavelength not only to the drive laser's peak strength, but also to its spot-size-to-pulse-length aspect ratio. Excellent agreement was found between the measurement and the new scaling law.LWFA steht für Laser WakeField Acceleration (oder Accelerator), einer neuen Technologie, die eine drastische Reduzierung der Größe und Kosten von Teilchenbeschleunigern verspricht. LWFA beruht auf dem großen (~ GV/cm) elektrischen Feld in der Plasmawelle, die durch Laserpulse mit relativistischer Spitzenintensität (10^18 W/cm^2) angeregt wird. Im Rahmen dieser Dissertation wurden die für die LWFA-Forschung am Ti:Saphir Laser ATLAS-300 am Laboratory for Extreme Photonics der Ludwig-Maximilians-Universität München geeignete Diagnostiken entwickelt, sowohl für Charakterisierung der Elektronen als auch zur Untersuchung des Plasmamediums. Zur Elektronendiagnostik wurden Implementierung und Charakterisierung von Szintillationsschirmen und Permanentdipolmagneten durchgeführt. Zur Plasmadiagnose wurde ein Probestrahl mit wenigen Zyklen entwickelt, mit dessen Hilfe fs-Schnappschüsse der Plasmawelle ermöglicht wurden. Konkret wurde die Lichtemissionseffizienz von neun gängigen Szintillationsschirmtypen mit Elektronenstrahlen des ELBE Linac am Helmholtz Zentrum Dresden-Rossendorf kalibriert. Bestehende Verfahren zur Übertragung der absoluten Kalibrierergebnisse in eine Kreuzkalibrierung mit einer Konstantlichtquelle wurden entscheidend verbessert. Bezüglich des Dipolmagneten wurde die Verteilung des B-Feldes in der Nähe der Mittelebene des Spalts mit einem Hall-Sensor gemessen und die Elektronen wurde in General Particle Tracer mit der gemessenen Feldverteilung getrackt. Szintillationsschirme und der Dipolmagnet bildeten das Elektronenspektrometer, das eine energieaufgelöste Nachweis von im LWFA erzeugten Elektronenstrahlen ermöglichte. Selbstinjizierte Elektronenstrahlen mit mehreren hundert pC Ladung und Spitzenenergien über 1 GeV wurden in einer mit Wasserstoff gefüllten Gaszelle unterschiedlicher Länge beschleunigt. Durch die Injektion an Stoßfronten in überschallen Gasstrahlen wurden stabile Elektronenstrahlen mit einer spektralen Dichte über 10 pC/MeV dank einer prozentualen Energieverteilung erreicht. Das Highlight dieser Arbeit ist der Aufbau und die Anwendung eines auf Hohlfasern basierenden Pulskompressionsaufbaus, der während der Experimente als Probestrahl dient. Dieser Aufbau lieferte sub-10 fs Probepulse, mit denen Phasenkontrast-Schnappschüsse der Laser-Plasma-Wechselwirkung aufgezeichnet wurden. Insbesondere wurden lasergetriebene Plasmawellen aufgrund der ultrakurzen Probepulsdauer aufgelöst. Ein Teil des Probestrahls wurde aufgespalten, um ein Nomarski-Interferometer zu beleuchten, was eine unabhängige Messung der durchschnittlichen Plasmadichte mit der Abel-Inversion ermöglicht. Eine systematische Messung der Plasmawellenlänge bei unterschiedlichen Dichten oder Laserintensitäten zeigte, dass das derzeitige Verständnis der Skalierung von nichtlinearen Plasmawellen-länge unzureichend ist. Basierend auf einer Reihe von Particle-in-Cell Simulationen wurde eine empirische Skalierung vorgeschlagen, die die nichtlineare Plasmawellenlänge nicht nur mit der Spitzenstärke des Treiblasers, sondern auch mit seinem Aspektverhältnis von Punktgröße zu Pulslänge in Beziehung setzt. Es wurde eine hervorragende Übereinstimmung zwischen der Messung und dem neuen Skalierungsgesetz festgestellt
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