Focal Geometries Research Articles

Modeling of high intensity orbital angular momentum beams for laser–plasma interactions

In this work, we explore the field of high intensity orbital angular momentum (OAM) beams, their generation with spiral phase mirrors, and the theory behind modeling both ideal and realistic beams. We explore OAM beam asymmetries introduced by aberrations in the beam, manufacturing defects, and bandwidth. A full three-dimensional description of the paraxial Laguerre–Gaussian (LG) modes suitable for modeling beams down to f/2 focal geometries is derived. A perturbative approach to modeling asymmetric OAM beams is introduced showing that only three LG modes are sufficient to model a wide variety of OAM asymmetries. The models are compared with experimental results followed by a discussion on the future of high intensity OAM beams in plasma physics.

Modeling and Forecasting Aftershocks Can Be Improved by Incorporating Rupture Geometry in the ETAS Model

AbstractWe implemented an extended version of the space‐time Epidemic‐Type Aftershock Sequence (ETAS) model, which simultaneously incorporates earthquake focal depths and rupture geometries of large earthquakes, and applied it to the 2016 Kumamoto earthquake sequence. Results show that the new model corrects the estimation biases of model parameters in the point source ETAS model. The reconstructed patterns of productivity density of aftershocks, along the mainshock rupture plane, form complementary patterns for coseismic slip in space and show significant spatiotemporal migrations. Large aftershocks tend to nucleate at the edges of high productivity density areas. The decay of direct aftershocks near the mainshock rupture is consistent with static stress changes caused by the mainshock. The forecasting capability of the ETAS model can be enhanced by considering the rupture geometries of mainshocks, especially for forecasting aftershocks in the first 1–2 days. In the simulation test, the incorporation of focal depths improves the forecasting resolution.

Impact of Acoustic Radiation Force Excitation Geometry on Shear Wave Dispersion and Attenuation Estimates

Shear wave elasticity imaging (SWEI) characterizes the mechanical properties of human tissues to differentiate healthy from diseased tissue. Commercial scanners tend to reconstruct shear wave speeds for a region of interest using time-of-flight methods reporting a single shear wave speed (or elastic modulus) to the end user under the assumptions that tissue is elastic and shear wave speeds are not dependent on the frequency content of the shear waves. Human tissues, however, are known to be viscoelastic, resulting in dispersion and attenuation. Shear wave spectroscopy and spectral methods have been previously reported in the literature to quantify shear wave dispersion and attenuation, commonly making an assumption that the acoustic radiation force excitation acts as a cylindrical source with a known geometric shear wave amplitude decay. This work quantifies the bias in shear dispersion and attenuation estimates associated with making this cylindrical wave assumption when applied to shear wave sources with finite depth extents, as commonly occurs with realistic focal geometries, in elastic and viscoelastic media. Bias is quantified using analytically derived shear wave data and shear wave data generated using finite-element method models. Shear wave dispersion and attenuation bias (up to 15% for dispersion and 41% for attenuation) is greater for more tightly focused acoustic radiation force sources with smaller depths of field relative to their lateral extent (height-to-width ratios <16). Dispersion and attenuation errors associated with assuming a cylindrical geometric shear wave decay in SWEI can be appreciable and should be considered when analyzing the viscoelastic properties of tissues with acoustic radiation force source distributions with limited depths of field.

Studies of the Core Conditions of the Earth and Super-Earths Using Intense Ion Beams at FAIR

Abstract Using detailed numerical simulations, we present the design of an experiment that will generate samples of iron under extreme conditions of density and pressure believed to exist in the interior of the Earth and interior of extrasolar Earth-like planets. In the proposed experiment design, an intense uranium beam is used to implode a multilayered cylindrical target that consists of a thin Fe cylinder enclosed in a thick massive W shell. Such intense uranium beams will be available at the heavy-ion synchrotron, SIS100, at the Facility for Antiprotons and Ion Research (FAIR), at Darmstadt, which is under construction and will become operational in the next few years. It is expected that the beam intensity will increase gradually over a couple of years to its maximum design value. Therefore, in our studies, we have considered a wide range of beam parameters, from the initial beam intensity (“Day One”) to the maximum specified value. It is also worth noting that two different focal spot geometries have been used. In one case, a circular focal spot with a Gaussian transverse intensity distribution is considered, whereas in the other case, an annular focal spot is used. With these two beam geometries, one can access different parts of the Fe phase diagram. For example, heating the sample with a circular focal spot generates a hot liquid state, while an annular focal spot can produce a highly compressed liquid or a highly compressed solid phase depending on the beam intensity.

The Influence of Laser Welding Processes on the Weld Seam Quality of Thermoplastic Composites with High Moisture Content

Presently, fiber reinforced materials are used for many industrial applications. Laser transmission welding (LTW) is a suitable method to join these materials. For multiple thermoplastics, the weld seam quality is affected by the amount of moisture content in the joining members. For LTW, heat is applied to the welding members and water in the composite changes phase from liquid to gas. Increasing the gas content can lead to detrimental cavities in the weld seam.Therefore, experiments were performed to investigate the influence of the laser focal geometry on the generation of cavities. Different focal geometries lead to different levels of process heat generation which affects the amount of vaporization. The experiments were conducted with endless glass fiber reinforced polyetherimide and endless carbon fiber reinforced polyetherimide. These samples were tested for their weld seam strength and the results were correlated to the moisture content and laser beam geometry.

Highly ionized xenon and volumetric weighting in restricted focal geometries

The ionization of xenon atoms subjected to 42 fs, 800 nm pulses of radiation from a Ti:Sapphire laser was investigated. In our experiments, a maximum laser intensity of ∼2×1015W/cm2 was used. Xenon ions were measured using a time-of-flight ion mass spectrometer having an entrance slit with dimensions of 12μm×400μm. The observed yields Xen+(n=1−7) were partially free of spatial averaging. The ion yields showed sequential and nonsequential multiple ionization and dip structures following saturation. To investigate the dip structures and to perform a comparison between experimental and simulated data, with the goal of clarifying the effects of residual spatial averaging, we derived a hybrid analytical-numerical solution for the integration kernel in restricted focal geometries. We simulated xenon ionization using Ammosov-Delone-Krainov and Perelomov-Popov-Terent'ev theories and obtained agreement with the results of observations. Since a large number of experiments suffer from spatial averaging, the results presented are important to correctly interpret experimental data by taking into account spatial averaging.

Seismicity and shallow slab geometry in the central Vanuatu subduction zone

AbstractThe Vanuatu arc in the southwest Pacific Ocean is one of the world's most seismically active regions, with almost 39 magnitude 7+ earthquakes in the past 43 years. Convergence rates are around 90–120 mm/yr along most of the arc, but drop to 25–43 mm/yr in the central section, probably due to the subduction of the d'Entrecasteaux ridge. We characterize the slab geometry and tectonic state in this central section by analyzing data from a 10 month deployment of 30 seismometers over this section. We located more than 30,000 events (all less than magnitude 5.5), constructed an improved 1‐D velocity model, calculated focal mechanisms and cluster geometries, and determined the 3‐D geometry of the interplate seismogenic zone. The seismogenic zone has a shallow bulge in front of the d'Entrecasteaux ridge, which could be explained by the ridge's buoyancy contributing to the uplift of the fore‐arc islands. The seismogenic zone extends to ~45 km depth, significantly below the 26–27 km depth of the fore‐arc Moho, indicating that the upper mantle wedge is not significantly serpentinized, which is consistent with the relatively high thermal parameter of the subducting plate. The maximum width of the seismogenic zone is 80 km, indicating an upper earthquake magnitude limit of Mw 7.85 ± 0.4, assuming standard rupture zone aspect ratios. The data also reveal a double seismic zone, 20 to 30 km below the seismogenic zone, which is presumably caused by flexure of the downgoing plate.

Non-linear optical flow cytometry using a scanned, Bessel beam light-sheet.

Modern flow cytometry instruments have become vital tools for high-throughput analysis of single cells. However, as issues with the cellular labeling techniques often used in flow cytometry have become more of a concern, the development of label-free modalities for cellular analysis is increasingly desired. Non-linear optical phenomena (NLO) are of growing interest for label-free analysis because of the ability to measure the intrinsic optical response of biomolecules found in cells. We demonstrate that a light-sheet consisting of a scanned Bessel beam is an optimal excitation geometry for efficiently generating NLO signals in a microfluidic environment. The balance of photon density and cross-sectional area provided by the light-sheet allowed significantly larger two-photon fluorescence intensities to be measured in a model polystyrene microparticle system compared to measurements made using other excitation focal geometries, including a relaxed Gaussian excitation beam often used in conventional flow cytometers.

Geographically Varying Ground-Motion Prediction Using Records from Historical Events

Maps of the macroseismic intensity and ground‐motion measures for historical events are often irregular; the contours of constant seismic intensity or of constant ground‐motion measures do not necessarily fall on a circle or an ellipse. This is the case, for example, for the 1999 Chi‐Chi earthquake (Shin and Teng, 2001), California earthquakes (Toppozada et al. , 2002), Italian earthquakes (De Rubeis et al. , 2005), Greek earthquakes (Schenkova et al. , 2007), the 2008 Wenchuan earthquake (Wang and Xie, 2010; Wang et al. , 2010; Yuan et al. , 2013), and the Tohoku earthquake (Goto and Morikawa, 2012). The ground‐motion records from historical events are used as the basis for developing ground‐motion prediction equations (GMPEs; Boore et al. , 2014). Often the adopted mathematical forms for the GMPEs are based on implicit assumptions that the ground‐motion measures such as the peak ground acceleration (PGA), spectral acceleration (SA), and Arias intensity ( I A) are independent of wave propagation path and/or focal mechanism and geometries (Douglas, 2011; Foulser‐Piggott and Stafford, 2012; Atkinson and Adams, 2013). This is a practical and useful approach for seismic hazard and risk assessments of buildings during their life‐cycles (Hong and Goda, 2006; Goda and Hong, 2008) because available records are insufficient to develop geographically or site‐dependent ground‐motion prediction models for ranges of focal mechanism and geometries. In fact, the GMPEs for randomly oriented horizontal ground‐motion measures are most often developed and used (Boore et al. , 2006; Douglas, 2011) because the GMPEs for the bidirectional horizontal excitations are rarely available and the directions of the principal axes (with respect to the source‐to‐site orientation) of the records from a broad network of recording stations are not very consistent (Hong and Goda, 2007, 2010). Some of the questions for seismic hazard and risk analysis that need to be …

Dynamics of tightly focused femtosecond laser pulses in water

The dynamics of tightly focused ultrashort (40 fs) pulses manifested in terms of supercontinuum emission (SCE) and cavitation-induced bubbles (CIB) resulting from propagation in water over a wide range of input powers (6 mW–1.8 W) are presented. The effect of linear polarization (LP) and circular polarization (CP) on SCE in different external focal geometries (f/6, f/7.5 and f/10) is investigated and the results are discussed. SCE with higher efficiency and a considerable spectral blue shift is observed under tight focusing conditions (f/6) compared to loose focusing conditions (f/10). At higher input powers, CIB along the axis of propagation are observed to be assisting deeper propagation of these short pulses and enhanced SCE.

Space-bandwidth scaling for wide field-of-view imaging

We examine the space-bandwidth product of wide field-of-view imaging systems as the systems scale in size. Our analysis is based on one conducted to examine the behavior of a plano-convex lens imaging onto a flat focal geometry. We extend this to consider systems with monocentric lenses and curved focal geometries. As a means to understand system cost, and not just performance, we also assess the volume and mass associated with these systems. Our analysis indicates monocentric lenses imaging onto a curved detector outperform other systems for the same design constraints but do so at a cost in lens weight.

Stimulated Stokes downconversion in liquid and solid parahydrogen

We report the results of our preliminary investigations into the suitability of condensed-phase parahydrogen as a Raman-shifting medium for infrared cavity ringdown laser absorption spectroscopy. We have observed the conversion of ∼10-ns pulses of 532-nm radiation into first-, second-, and third-order vibrational Stokes radiation in bulk liquid and solid parahydrogen after a single 11-cm pass. Unexpectedly, we find that liquid H2 yields more efficient conversion than solid H2 with certain focal geometries, and that in the case of the solid, a collimated or loosely focused pump geometry is more efficient than a tight focus.

High order harmonic generation optimization with an apertured laser beam

We report a systematic study of high order harmonic generation with an infrared laser apertured by an iris, as a function of the aperture size. Measurements were made of harmonic generation efficiency for different gas species, laser energies and focal geometries. Harmonic efficiencies as a function of aperture show a characteristic peaked shape, which is independent of gas species and harmonic order. A one dimensional, time dependent simulation of harmonic generation in a gas cell, taking into account experimentally measured transverse coherence of the laser, closely reproduces the observed behaviours. We show that the aperture diameter which maximizes harmonic yield is the result of a compromise between considerations of focal geometry and ionization (which favour small apertures); and harmonic dipole amplitude and phase (which favour large apertures).

Multifaceted ultrasound transducer probe system and methods for its use

A High Intensity Focused Ultrasound system is provided for treatment of focal disease. The preferred embodiment includes an intracavity probe having two active ultrasound radiating surfaces with different focal geometries. Selectively energizing the first surface focuses therapeutic energy a first distance from the housing, energizing the second surface focuses therapeutic energy nearer the housing. Preferably, the probe includes a thin, flexible, inelastic membrane which is rigidized with pressure to allow blunt manipulation of tissue. Methods of use of the probe are also provided, particularly for treatment of diseases of the prostate.

Centroid-moment-tensor solutions for deep earthquakes predating the digital era: Discussion and inferences

We present a discussion of 139 new centroid-moment-tensor solutions for deep earthquakes covering the WWSSN (1962–1976) and earlier years, obtained by applying the standard Harvard inversion algorithm to data sets of seismograms digitized from analog records. Our solutions fill several gaps in seismicity reflecting undersampling by the Harvard catalogue. While most focal mechanisms previously published for WWSSN-era earthquakes are in good agreement with our inverted solutions, focal geometries for older events appear unreliable. We confirm that the region of most intense moment release is around 650 km, and that a minimum exists at 500 km. However, the local maximum around 450 km may be less marked than previously suggested. The Bonin–Marianas subduction zone exhibits a different pattern with an overall maximum of seismic activity at 420 km. All other subduction zones demonstrate the potential for seismicity at least at the 10 27 dyn-cm level. Finally, while non-double-couple components are found systematically in our enhanced data set, they correlate neither with depth nor with earthquake size, suggesting that their presence does not necessarily relate to the exiguity of the space available for rupture at the tip of subduction zones.

Isolated deep earthquakes and the fate of subduction in the mantle

The locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere. The occurrence over the last decade of isolated large deep earthquakes outside the main Wadati‐Benioff zones in the Kurile, Izu‐Bonin, and Chile deep seismic areas (in addition to the observed flat distributions of deep seismicity beneath the Banda Sea and Fiji Plateau areas and the large number of deep earthquakes located away from the main slab in the Tonga arc) supports recent tomographic and other seismological studies, leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle. The focal geometries of these isolated earthquakes, which often exhibit near vertical compressional stresses, are distinct from adjacent events in the main Wadati‐Benioff zone. They are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide. These isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle. Conversely, the deep seismicity in the adjacent Wadati‐Benioff zone is associated with the deflection of the subducting lithosphere.

Application of the CMT algorithm to analog recordings of deep earthquakes

The distribution and characteristics of deep earthquakes provide important information on both the large-scale aspects of mantle convection as well as on the small-scale mechanics of apparent shear failure within descending lithospheric plates at large confining pressures. Neither of these processes is well understood, but the detailed knowledge of the spatial and temporal distribution of earthquakes, as well as the focal geometries, scalar moments, and details of the earthquake rupture process, provides constraints which can be used to differentiate between competing models of mantle circulation and theories of deep seismic rupture. With the objective of expanding and improving the database of source parameters for deep earthquakes, we evaluate the feasibility of applying the Harvard centroid moment tensor (CMT) algorithm to earlier (pre-1977) earthquakes recorded on seismographs with analog recording. Abundant records from the World Wide Standardized Seismograph Network (WWSSN) are available for events after 1962, and the data from these high-quality stations with a known standard response are easily digitized and subjected to analysis by the CMT algorithm. For earlier events, no standardized network of seismometers is available. Using a modem deep earthquake as a test case, we show that reliable single-station moment tensor solutions can be obtained from the Press-Ewing instrument at Pasadena. We apply the single-station technique to three historical earthquakes recorded on different types of electromagnetic and mechanical seismographs: the 1957 Java earthquake (Press-Ewing); the 1922 Peru earthquake (Wiechert); and the 1911 Peru-Brazil earthquake (Galitzin). Robust results can be achieved from these data provided the instruments are well calibrated, and an analysis of selected earlier deep earthquakes may quickly double or triple the time span of the database of deep earthquake source parameters.

Proton-induced displacement damage distributions and extremes in silicon microvolumes charge injection device

An analytic approach for determining the pixel-to-pixel distribution of particle-induced damage and damage extremes in microvolumes representative of focal plane array pixel geometries is presented. Comparisons between predicted and measured dark current distributions in a silicon charge injection device (CID) show excellent agreement for 12- and 63-MeV proton-induced damage. The calculated and measured damage extremes are compared using extreme value statistical analysis. The calculations reveal how high-energy recoils from proton-induced nuclear reactions strongly influence the pixel-to-pixel variation in damage as well as the damage extremes. A comparison between Si and GaAs pixels with equal volumes and equal 12-MeV proton fluences indicates that both the average damage and its variance are significantly greater in GaAs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>