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Articles 141001 - 141030 of 303072

Full-Text Articles in Physical Sciences and Mathematics

Laser Based Synchrotron Radiation, Kim Ta Phuoc, Frederic Burgy, Jean-Philippe Rousseau, Victor Malka, Antoine Rousse, Rahul Shah, Donald P. Umstadter, Alexander Puhkov, Sergei Kiselev Jul 2015

Laser Based Synchrotron Radiation, Kim Ta Phuoc, Frederic Burgy, Jean-Philippe Rousseau, Victor Malka, Antoine Rousse, Rahul Shah, Donald P. Umstadter, Alexander Puhkov, Sergei Kiselev

Donald P. Umstadter

Beams of x rays in the kiloelectronvolt energy range have been produced from laser-matter interaction. Here, energetic electrons are accelerated by a laser wakefield, and experience betatron oscillations in an ion channel formed in the wake of the intense femtosecond laser pulse. Experiments using a 50 TW laser (30 fs duration) are described, as well as comparisons with numerical simulations. These results pave the way of a new generation of radiation in the x-ray spectral range, with a high collimation and an ultrafast pulse duration, produced by the use of compact laser system.

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Method For Generating A Plasma Wave To Accelerate Electrons, Donald Umstadter, Eric Esarey, Joon K. Kim Jul 2015

Method For Generating A Plasma Wave To Accelerate Electrons, Donald Umstadter, Eric Esarey, Joon K. Kim

Donald P. Umstadter

The invention provides a method and apparatus for generating large amplitude nonlinear plasma waves, driven by an optimized train of independently adjustable, intense laser pulses. In the method, optimal pulse widths, interpulse spacing, and intensity profiles of each pulse are determined for each pulse in a series of pulses. A resonant region of the plasma wave phase space is found where the plasma wave is driven most efficiently by the laser pulses. The accelerator system of the invention comprises several parts: the laser system, also called beam source, which preferably comprises photo cathode electron source and RF-LINAC accelerator; electron photo-cathode …

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Electron Acceleration And The Propagation Of Ultrashort High-Intensity Laser Pulses In Plasmas, Xiaofang Wang, Mohan Krishnan, Ned Saleh, Haiwen Wang, Donald P. Umstadter Jul 2015

Electron Acceleration And The Propagation Of Ultrashort High-Intensity Laser Pulses In Plasmas, Xiaofang Wang, Mohan Krishnan, Ned Saleh, Haiwen Wang, Donald P. Umstadter

Donald P. Umstadter

Reported are interactions of high-intensity laser pulses ( λ = 810 nm and l≤3×1018 W/cm2) with plasmas in a new parameter regime, in which the pulse duration ( τ = 29 fs) corresponds to 0.6–2.6 plasma periods. Relativistic filamentation is observed to cause laser-beam breakup and scattering of the beam out of the vacuum propagation angle. A beam of megaelectronvolt electrons with divergence angle as small as 1° is generated in the forward direction, which is correlated to the growth of the relativistic filamentation. Raman scattering, however, is found to be much less than previous long-pulse results.

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Tabletop Accelerators Are Brighter And Faster, Phil Schewe, Ben Stein, Donald P. Umstadter Jul 2015

Tabletop Accelerators Are Brighter And Faster, Phil Schewe, Ben Stein, Donald P. Umstadter

Donald P. Umstadter

At last week's APS plasma physics meeting, Donald Umstadter of the University of Michigan's Center for Ultrafast Optical Science (734-764-2284, dpu@umich.edu) reported on advances at his lab and elsewhere in tabletop laser accelerators, devices that use light to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. This acceleration rate or "gradient" is up to a thousand times larger than in conventional accelerators because the tabletop laser light can now exert pressures of gigabars, the highest ever achieved, and approaching the pressure of light near the Sun. Not only that, but Umstadter's …

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Development Of A Source Of Quasi-Monochromatic Mev Energy Photons, Donald Umstadter, Sudeep Banerjee, Viswanathan Ramanathan, Nathan Powers, Nathaniel Cunningham, Nathan A. Chandler-Smith Jul 2015

Development Of A Source Of Quasi-Monochromatic Mev Energy Photons, Donald Umstadter, Sudeep Banerjee, Viswanathan Ramanathan, Nathan Powers, Nathaniel Cunningham, Nathan A. Chandler-Smith

Donald P. Umstadter

We report current progress on a project to develop an all-optically-driven x-ray photon source. A laser pulse with 40-50 TW of peak power is focused on a supersonic helium nozzle to drive a relativistic plasma wave. Electron beams with energies of 320 MeV (+/- 28 MeV) are accelerated by means of laser wakefield acceleration. Remarkably, the acceleration region is only 3 mm in length. This accelerator is currently being employed to demonstrate the generation of MeV- energy x-ray by means of all-optical Thomson scattering. By this mechanism, a lower power, laser pulse (from the same laser system) is focused onto …

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Phase Dependence Of Thomson Scattering In An Ultraintense Laser Field, Fei He, Y. Y. Lau, Donald P. Umstadter, Trevor Strickler Jul 2015

Phase Dependence Of Thomson Scattering In An Ultraintense Laser Field, Fei He, Y. Y. Lau, Donald P. Umstadter, Trevor Strickler

Donald P. Umstadter

The Thomson scattering spectra of an electron by an ultraintense laser field are computed. It is found that the electron orbit, and therefore its nonlinear Thomson scattering spectra, depend critically on the amplitude of the ultraintense laser field and on the phase at which the electron sees the laser electric field. Contrary to some customary notions, the Thomson scattering spectra, in general, do not occur at integer multiples of the laser frequency and the maximum frequency is proportional to the first instead of the third power of the electric field strength for the case of an ultraintense laser. The implications …

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Self-Focusing, Channel Formation, And High-Energy Ion Generation In Interaction Of An Intense Short Laser Pulse With A He Jet, G.S. Sarkisov, B. Yu. Bychenkov, V.N. Novikov, V.T. Tikhonchuk, Anatoly Maksimchuk, Shouyuan Chen, R. Wagner, G. Mourou, Donald Umstadter Jul 2015

Self-Focusing, Channel Formation, And High-Energy Ion Generation In Interaction Of An Intense Short Laser Pulse With A He Jet, G.S. Sarkisov, B. Yu. Bychenkov, V.N. Novikov, V.T. Tikhonchuk, Anatoly Maksimchuk, Shouyuan Chen, R. Wagner, G. Mourou, Donald Umstadter

Donald P. Umstadter

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 μm in diameter, with a radial gradient of electron density ∼5×1022 cm-4 and with an on-axis electron density approximately ten times less than its maximum value of 8×1019 cm-3. A high radial velocity of the surrounding gas ionization of ∼3.8×108 cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We …

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Thomson Scattering And Ponderomotive Intermodulation Within Standing Laser Beat Waves In Plasma, Scott Sepke, Y.Y. Lau, James Paul Holloway, Donald P. Umstadter Jul 2015

Thomson Scattering And Ponderomotive Intermodulation Within Standing Laser Beat Waves In Plasma, Scott Sepke, Y.Y. Lau, James Paul Holloway, Donald P. Umstadter

Donald P. Umstadter

Electrons in a standing electromagnetic wave—an optical lattice—tend to oscillate due to the quiver and ponderomotive potentials. For sufficiently intense laser fields (lλ2≤5×1017 W cm-2 µm2) and in plasmas with sufficiently low electron densities (n≤1018 cm-3), these oscillations can occur faster than the plasma can respond. This paper shows that these oscillations result in Thomson scattering of light at both the laser and ponderomotive bounce frequencies and their harmonics as well as at mixtures of these frequencies. We term this mixing ponderomotive intermodulation. Here, the case of counterpropagating laser beams creating a one-dimensional (1D) optical lattice is analyzed. The near-equilibrium …

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A Cone Of Coherent Light, S.-Y. Chen, A. Maksimchuk, E. Esarey, Donald Umstadter Jul 2015

A Cone Of Coherent Light, S.-Y. Chen, A. Maksimchuk, E. Esarey, Donald Umstadter

Donald P. Umstadter

Lasers come in infrared and visible varieties, but none yet in the x-ray band. A compact and powerful source of coherent x rays is the dream of many physicists, who see applications such as making atomic scale, three-dimensional movies of a melting crystal or an operating photosynthesis protein. In a step toward that goal, a research team has detected high frequency coherent light generated by a new process. As they report in the 12 June PRL, intense laser pulses can stimulate free electrons in a plasma to emit coherent light at triple the input frequency in a narrowly-directed cone. The …

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Plasma Density Gratings Induced By Intersecting Laser Pulses In Underdense Plasmas, Z.-M. Sheng, J. Zhang, Donald Umstadter Jul 2015

Plasma Density Gratings Induced By Intersecting Laser Pulses In Underdense Plasmas, Z.-M. Sheng, J. Zhang, Donald Umstadter

Donald P. Umstadter

Electron and ion density gratings induced by two intersecting ultrashort laser pulses at intensities of 1016 W/cm2 or lower are investigated. The ponderomotive force generated by the inhomogeneous intensity distribution in the intersecting region of the interfering pulses produces deep electron and ion density modulations at a wavelength less than a laser wavelength in vacuum. Dependence of the density modulation on the plasma densities, temperatures, and the ion mass, as well as the laser pulse parameters are studied analytically and by particle-in-cell simulations. It is found that the density peaks of such gratings can be a few times that of …

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Observation Of Relativistic Cross-Phase Modulation In High-Intensity Laser-Plasma Interactions, Shouyuan Chen, Matthew Rever, P. Zhang, W. Theobald, Donald Umstadter Jul 2015

Observation Of Relativistic Cross-Phase Modulation In High-Intensity Laser-Plasma Interactions, Shouyuan Chen, Matthew Rever, P. Zhang, W. Theobald, Donald Umstadter

Donald P. Umstadter

A nonlinear optical phenomenon, relativistic cross-phase modulation, is reported. A relativistically intense light beam (I=1.3×1018 W cm-2, λ =1.05 μm) is experimentally observed to cause phase modulation of a lower intensity, copropagating light beam in a plasma. The latter beam is generated when the former undergoes the stimulated Raman forward scattering instability. The bandwidth of the Raman satellite is found to be broadened from 3.8–100 nm when the pump laser power is increased from 0.45–2.4 TW. A signature of relativistic cross-phase modulation, namely, asymmetric spectral broadening of the Raman signal, is observed at a pump power of 2.4 TW. The …

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A Unidirectional, Pulsed Far-Infrared Ring Laser, W.A. Peebles, Donald Umstadter, D.L. Brower, N.C. Luhmann Jul 2015

A Unidirectional, Pulsed Far-Infrared Ring Laser, W.A. Peebles, Donald Umstadter, D.L. Brower, N.C. Luhmann

Donald P. Umstadter

The first study of a pulsed, far-infrared, ring laser is described. Unidirectional, traveling-wave operation is observed, thereby eliminating the spatial hole burning effects present in linear cavities. Single-mode output powers of 100 kW have been obtained.

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Experimental Observation Of Relativistic Nonlinear Thomson Scattering, Szu-Yuan Chen, Anatoly Maksimchuk, Donald Umstadter Jul 2015

Experimental Observation Of Relativistic Nonlinear Thomson Scattering, Szu-Yuan Chen, Anatoly Maksimchuk, Donald Umstadter

Donald P. Umstadter

Classical Thomson scattering—the scattering of low-intensity light by electrons—is a linear process, in that it does not change the frequency of the radiation; moreover, the magnetic-field component of light is not involved. But if the light intensity is extremely high (~1018 Wcm–2), the electrons oscillate during the scattering process with velocities approaching the speed of light. In this relativistic regime, the effect of the magnetic and electric fields on the electron motion should become comparable, and the effective electron mass will increase. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns rather than in …

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Evolution Of A Plasma Waveguide Created During Relativistic-Ponderomotive Self-Channeling Of An Intense Laser Pulse, Shouyuan Chen, G.S. Sarkisov, Anatoly Maksimchuk, R. Wagner, Donald P. Umstadter Jul 2015

Evolution Of A Plasma Waveguide Created During Relativistic-Ponderomotive Self-Channeling Of An Intense Laser Pulse, Shouyuan Chen, G.S. Sarkisov, Anatoly Maksimchuk, R. Wagner, Donald P. Umstadter

Donald P. Umstadter

An on-axis plasma density depression channel was observed during and after the passage of a relativistically and ponderomotively self-guided laser pulse through a plasma. Optical interferometry was used to produce time-resolved plasma density distributions, revealing the formation of a plasma waveguide. These results were complemented by the guiding of a collinear trailing pulse.

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Application Of A Picosecond Soft X-Ray Source To Time-Resolved Plasma Dynamics, J. Workman, M. Nantel, Anatoly Maksimchuk, Donald P. Umstadter Jul 2015

Application Of A Picosecond Soft X-Ray Source To Time-Resolved Plasma Dynamics, J. Workman, M. Nantel, Anatoly Maksimchuk, Donald P. Umstadter

Donald P. Umstadter

We demonstrate the application of an ultrashort x-ray source as an external probe to measure plasma dynamics. The plasma is generated by a 100-fs Ti: sapphire laser focused onto thin metallic films. Time-resolved spectroscopy of the gold x-ray probe transmission through a perturbed 1000 Å aluminum film reveals redshifts of the L-shell photoabsorption edge. We show that the dynamic behavior of this shift is consistent with the relaxation of the aluminum following the compression generated by a shock wave traveling through the film. An analytic plasma model, with comparison to a numerical hydrodynamics model, indicates compression up to 1.4 times …

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Fast Ignitor Concept With Light Ions, V. Yu. Bychenkov, W. Rozmus, Anatoly Maksimchuk, Donald Umstadter, C.E. Capjack Jul 2015

Fast Ignitor Concept With Light Ions, V. Yu. Bychenkov, W. Rozmus, Anatoly Maksimchuk, Donald Umstadter, C.E. Capjack

Donald P. Umstadter

A short-laser-pulse driven ion flux is examined as a fast ignitor candidate for inertial confinement fusion. Ion ranges in a hot precompressed fuel are studied. The ion energy and the corresponding intensity of a short laser pulse are estimated for the optimum ion range and ion energy density flux. It is shown that a lightion beam triggered by a few-hundreds-kJ laser at intensities of ~1021W/cm2 is relevant to the fast ignitor scenario.

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Relativistic Plasma-Wave Excitation By Collinear Optical Mixing, C.E. Clayton, C. Joshi, C. Darrow, Donald P. Umstadter Jul 2015

Relativistic Plasma-Wave Excitation By Collinear Optical Mixing, C.E. Clayton, C. Joshi, C. Darrow, Donald P. Umstadter

Donald P. Umstadter

The relativistic plasma wave excited when the frequency difference between two copropagating CO2 laser beams equals the plasma frequency is detected for the first time. The plasma-wave frequency, wave number, spatial extent, and saturation time are directly measured by use of 7-mrad, collective, ruby Thomson scattering and the forward-scattered ir spectrum. The wave amplitude ñ/n0 is inferred to be (1-3)% which gives a longitudinal electric field of 0.3 to 1 GV/m at a laser intensity of 1.7×1013 W/cm2, in reasonable agreement with theory.

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High-Energy Ion Generation In Interaction Of Short Laser Pulse With High-Density Plasma, Y. Sentoku, V. Y. Bychenkov, K. Flippo, Anatoly Maksimchuk, K. Mima, G. Mourou, Z. M. Sheng, Donald Umstadter Jul 2015

High-Energy Ion Generation In Interaction Of Short Laser Pulse With High-Density Plasma, Y. Sentoku, V. Y. Bychenkov, K. Flippo, Anatoly Maksimchuk, K. Mima, G. Mourou, Z. M. Sheng, Donald Umstadter

Donald P. Umstadter

Multi-MeV ion production from the interaction of a short laser pulse with a high-density plasma, accompanied by an underdense preplasma, has been studied with a particle-in- cell simulation and good agreement is found with experiment. The mechanism primarily responsible for the acceleration of ions is identified. Comparison with experiments sheds light on the ion-energy dependence on laser intensity, preplasma scale length, and relative ion energies for a multi-species plasma. Two regimes of maximum ion-energy dependence on laser intensity, I, have been identified: subrelativistic, μ I ; and relativistic, μ √I. Simulations show that the energy of the accelerated ions versus …

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High-Energy Laser-Accelerated Electron Beans For Long-Range Interrogation, Nathaniel Cunningham, Sudeep Banerjee, Viswanathan Ramanathan, Nathan D. Powers, Nathan A. Chandler-Smith, Randy Vane, David Schultz, Sara Pozzi, Shaun Clark, James Beene, Donald Umstadter Jul 2015

High-Energy Laser-Accelerated Electron Beans For Long-Range Interrogation, Nathaniel Cunningham, Sudeep Banerjee, Viswanathan Ramanathan, Nathan D. Powers, Nathan A. Chandler-Smith, Randy Vane, David Schultz, Sara Pozzi, Shaun Clark, James Beene, Donald Umstadter

Donald P. Umstadter

We are studying the use of 0.1 - 1.0 GeV laser-accelerated electron beams as active interrogation probes for long-standoff radiography or nuclear activation of concealed special nuclear material. Use of beams in this energy range is largely unexplored, but such beams could provide notable advantages over lower-energy beams and x-rays. High-energy laser-accelerated electrons exhibit large penetration range through air and solids, and low beam divergence for both direct beams and secondary Bremsstrahlung x-rays. We present laboratory measurements of radiography and activation using the high power Diocles laser system at the University of Nebraska, as well as MCNP and GEANT Monte …

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Nonlinear Analysis Of Relativistic Harmonic Generation By Intenselasers In Plasmas, Eric Esarey, A. Ting, P. Sprangle, Donald Umstadter, X. Liu Jul 2015

Nonlinear Analysis Of Relativistic Harmonic Generation By Intenselasers In Plasmas, Eric Esarey, A. Ting, P. Sprangle, Donald Umstadter, X. Liu

Donald P. Umstadter

A linearly polarized, ultra-intense laser field induces transverse plasma currents which are highly relativistic and nonlinear, resulting in the generation of coherent harmonic radiation in the forward direction (i.e., copropagating with the incident laser field). A nonlinear cold fluid model, valid for ultrahigh intensities, is formulated and used to analyze relativistic harmonic generation. The plasma density response is included self-consistently and is shown to significantly reduce the current driving the harmonic radiation. Phase detuning severely limits the growth of the harmonic radiation. The effects of diffraction are considered in the mildly relativistic limit. No third-harmonic signal emerges from a uniform …

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Proceedings Of Naacl Hlt 2009: Short Papers , Pages 157–160, Boulder, Colorado, June 2009. C 2009 Association For Computational Linguistics Answer Credibility: A Language Modeling Approach To Answer Validation, Protima Banerjee, Hyoil Han Jul 2015

Proceedings Of Naacl Hlt 2009: Short Papers , Pages 157–160, Boulder, Colorado, June 2009. C 2009 Association For Computational Linguistics Answer Credibility: A Language Modeling Approach To Answer Validation, Protima Banerjee, Hyoil Han

Hyoil Han

Answer Validation is a topic of significant interest within the Question Answering community. In this paper, we propose the use of language modeling methodologies for Answer Validation, using corpus-based methods that do not require the use of external sources. Specifically, we propose a model for Answer Credibility which quantifies the reliability of a source document that contains a candidate answer and the Question’s Context Model.

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Survey Of Word Sense Disambiguation Appr Oaches, Xiaohua Zhou, Hyoil Han Jul 2015

Survey Of Word Sense Disambiguation Appr Oaches, Xiaohua Zhou, Hyoil Han

Hyoil Han

Word Sense Disambiguation (WSD) is an important but challenging technique in the area of natural language processing (NLP). Hundreds of WSD algorithms and systems are available, but less work has been done in regard to choosing the optimal WSD algorithms. This paper summarizes the various knowledge sources used for WSD and classifies existing WSD algorithms according to their techniques. The rationale, tasks, performance, knowledge sources used, computational complexity, assumptions, and suitable applications for each class of WSD algorithms are also discussed. This paper will provide users with general knowledge for choosing WSD algorithms for their specific applications or for further …

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Gravitational And Electromagnetic Waves On The Null Cone, Maria Babiuc-Hamilton Jul 2015

Gravitational And Electromagnetic Waves On The Null Cone, Maria Babiuc-Hamilton

Maria C. Babiuc-Hamilton

Gravitational radiation is properly defined only at future null infinity, but mathematically it is estimated at a finite radius, and then extrapolated. Our group developed a characteristic waveform extraction tool, implemented in an open source code, which computes the gravitational waves infinitely far from their source, in terms of compactified null cones, by numerically solving Einstein equation. We add electromagnetic waves to the null cone, by deriving a formulation of the Einstein-Maxwell equations suitable to be numerically implemented into a characteristic code, that will evolve and calculate both the gravitational waves and their electromagnetic counterparts all the way to infinity.

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Well-Posedness Of Characteristic Evolution In Bondi Coordinates, Maria Babiuc-Hamilton, Jeff Winicour Jul 2015

Well-Posedness Of Characteristic Evolution In Bondi Coordinates, Maria Babiuc-Hamilton, Jeff Winicour

Maria C. Babiuc-Hamilton

Gravitational waves carry information about their source, and their detection will uncover facets of our universe, otherwise invisible. Recently, we made publicly available a waveform computation tool, the PITT code, as part of the Einstein Toolkit open software for relativistic astrophysics. The code implements the “characteristic method,” which computes the gravitational waves infinitely far from their source in terms of compactified light cones. We proved that our code produces waveforms that satisfy the demands of next generation detectors. However, the main problem is that the well-posedness of the Einstein equations in characteristic formulation is not proven. Here we present our …

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High Gravitational Waveform Accuracy At Null Infinity, Maria Babiuc-Hamilton Jul 2015

High Gravitational Waveform Accuracy At Null Infinity, Maria Babiuc-Hamilton

Maria C. Babiuc-Hamilton

The aim of Cauchy-characteristic extraction is to provide a standardized waveform extraction tool for the numerical relativity community. The new extraction tool contains major improvements and corrections to previous versions and displays convergence. The error introduced by CCE satisfies the time domain criteria required for advanced LIGO data analysis. The importance of accurate waveforms to the gravitational wave astronomy has created an urgency for tools like CCE. The source code has been released to the public and is available as part of the Einstein Toolkit. We welcome applications to a variety of generic Cauchy codes implementing Einstein Equations of General …

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Using Radial Basis Functions To Interpolate Along Single-Null Characteristics, Maria Babiuc-Hamilton, Clyde E. Meador Jul 2015

Using Radial Basis Functions To Interpolate Along Single-Null Characteristics, Maria Babiuc-Hamilton, Clyde E. Meador

Maria C. Babiuc-Hamilton

The Cauchy-Characteristic Extraction (CCE) technique is the most precise method available for the computation of the gravitational waves obtained from numerical simulations of binary black hole mergers. This technique utilizes the characteristic evolution to extend the simulation to null infinity, where the waveform is computed in inertial coordinates. Although we recently made CCE publicly available to the numerical relativity community, there is still room for improvement, and the most important is enhancing the overall accuracy of the code, by upgrading the numerical methods used for interpolation and differentiation. One of the most promising ways is to use the Radial Basis …

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Gravitational-Waveform Extraction By The Characteristic Method, Maria Babiuc-Hamilton Jul 2015

Gravitational-Waveform Extraction By The Characteristic Method, Maria Babiuc-Hamilton

Maria C. Babiuc-Hamilton

When a pair of black holes spiral into each other and collide, the very fabric of space-time shakes, and gravitational waves are created. Gravitational waves carry information about their source, and will increase our understanding of relativistic systems in astrophysics. Gravitational wave observatories like LIGO and Virgo are tuned to detect the emission of these waves from the inspiral and merger of binary black holes, neutron stars, supernovae, etc… Problem: any small vibration is detected, so templates are essential to discern the real signal. It is hard to compute the waveforms obtained from numerical simulations accurately – gravitational radiation is …

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Evidence Of Longitudinal Acoustic Phonon Generation In Si Doping Superlattices By Ge Prism-Coupled Thz Laser Radiation, Thomas E. Wilson, Konstantin Korolev, Erich Kasper, Michael Oehme, Jörg Schulze Jul 2015

Evidence Of Longitudinal Acoustic Phonon Generation In Si Doping Superlattices By Ge Prism-Coupled Thz Laser Radiation, Thomas E. Wilson, Konstantin Korolev, Erich Kasper, Michael Oehme, Jörg Schulze

Thomas E. Wilson

We report on the direct excitation of 246 GHz longitudinal acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation of the same frequency. A longitudinally polarized evanescent laser light field is coupled to the superlattice through a germanium prism providing total internal reflection at the superlattice interface. The ballistic phonon signal is detected by a superconducting aluminum bolometer. The sample is immersed in low-temperature liquid helium.

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Bilayer Lift-Off Process For Aluminum Metallization, Thomas E. Wilson, Konstantin Korolev, Nathaniel A. Crow Jul 2015

Bilayer Lift-Off Process For Aluminum Metallization, Thomas E. Wilson, Konstantin Korolev, Nathaniel A. Crow

Thomas E. Wilson

Recently published reports in the literature for bilayer lift-off processes have described recipes for the patterning of metals that have recommended metal-ion-free developers, which do etch aluminum. We report the first measurement of the dissolution rate of a commercial lift-off resist (LOR) in a sodium-based buffered commercial developer that does not etch aluminum. We describe a reliable lift-off recipe that is safe for multiple process steps in patterning thin (<100  nm) and thick aluminum devices with micron-feature sizes. Our patterning recipe consists of an acid cleaning of the substrate, the bilayer (positive photoresist/LOR) deposition and development, the sputtering of …

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Rooted In Hell: Simulating Phragmites Australis, Rachel Nydegger Rozum, Jacob Duncan, James Powell Ph.D. Jul 2015

Rooted In Hell: Simulating Phragmites Australis, Rachel Nydegger Rozum, Jacob Duncan, James Powell Ph.D.

Rachel Nydegger Rozum

Across the estuaries of the east coast and wetlands of the Great Lakes, the invasive grass Phragmites australis outcompetes other vegetation and destroys local ecosystems. Because its roots are tolerant to salinity that other plants find hellish, Phragmites invasions begin with vegetative spread of genetic clones in brackish marshlands.This plant can grow over three meters tall at densities of 50 stems/m2, provides poor wildlife habitat, and is very difficult to eradicate. A discrete life stage model on a yearly time step captures seed survivorship in a seed bank, sexual and asexual recruitment into a juvenile age class, and differential competition …

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