Transverse Spread Research Articles

Chirped optical X-shaped pulses in material media

We analyze the properties of chirped optical X-shaped pulses propagating in material media without boundaries. We show that such ("superluminal") pulses may recover their transverse and longitudinal shapes after some propagation distance, whereas the ordinary chirped Gaussian pulses can recover their longitudinal width only (since Gaussian pulses suffer a progressive transverse spreading during their propagation). We therefore propose the use of chirped optical X-type pulses to overcome the problems of both dispersion and diffraction during pulse propagation.

Complex dispersion in simple fractured media

Previous studies noted situations with simple networks of fractured media where dispersion was complex. Certain network geometries provided principal directions of spreading at angles to the direction of mean flow and occasions where transverse spreading greatly exceeded longitudinal spreading. This article is concerned with understanding this complex dispersion in the context of more variable networks and to evaluate the scaling between dispersion and characteristics of the network. The approach involves modeling flow and particle transport through networks consisting of equally spaced finite fractures. Simulation trials examined how changes to the orientation of the overall network, fracture aperture, fracture connectivity, and fracture length influenced mass transport. With these geometries, particle spreading was due to a grid orientation effect and variability related to apertures and connectivity. The grid orientation effect inherently gives rise to the complex dispersion described in previous work. Spreading caused by variability in aperture and connectivity produces the more familiar, elliptical pattern of spreading. In the various model trials we created situations where either or both of these spreading mechanisms could dominate. When both grid orientation and heterogeneity effects were comparable, particle swarms took on a curious hybrid shape reflecting the outcome of two different spreading mechanisms. With sufficient heterogeneity the grid orientation effect was swamped and disappeared. As the block size increases so does transverse dispersion. Further studies are necessary to investigate whether the grid effects associated with the simplified fracture networks can be extrapolated to real fractured media.

Quantification of the spreading effect of auroral proton precipitation

A three‐dimensional Monte Carlo model has been developed to study the transverse beam spreading effect of incident energetic auroral protons during their precipitation in the Earth's upper atmosphere. Energetic protons with an isotropic angular distribution are injected at 700 km altitude. Two types of incident energy spectra, a monoenergetic and a Maxwellian distribution, are considered. Interaction of fast particles with a three‐species atmosphere (O, N2, and O2) is included through charge exchange, electron stripping, ionization, excitation, and elastic scattering collisions. A uniform geomagnetic field is assumed in the model. The spreading effect is simulated for both a fine proton beam and a proton arc of longitudinal and latitudinal extent. It is found that the main dispersion region for a fine proton beam is located in the altitude range of around 250–450 km, where the first few charge exchange collisions play a significant role. In the spreading study for a proton arc, we compare the numerical results with previous studies and give a convincing explanation by analyzing atmospheric scale heights and cross‐section data. For the purpose of the model validity check, we make a comparison of the Monte Carlo simulation with observations and the results from other models.

Stochastic analysis of transverse dispersion in density‐coupled transport in aquifers

Spectral perturbation techniques have been used previously to derive integral expressions for dispersive mixing in concentration‐dependent transport in three‐dimensional, heterogeneous porous media, where fluid density and viscosity are functions of solute concentration. Whereas earlier work focused on evaluating longitudinal dispersivity in isotropic media and incorporating the result in a mean one‐dimensional transport model, the emphasis of this paper is on evaluation of the complete dispersion tensor, including the more general case of anisotropic media. Approximate analytic expressions for all components of the macroscopic dispersivity tensor are derived, and the tensor is shown to be asymmetric. The tensor is separated into its symmetric and antisymmetric parts, where the symmetric part is used to calculate the principal components and principal directions of dispersivity, and the antisymmetric part of the tensor is shown to modify the velocity of the solute body compared to that of the background fluid. An example set of numerical simulations incorporating the tensor illustrates the effect of density‐coupled dispersivity on a sinking plume in an aquifer. The simulations show that the effective transverse vertical spreading in a sinking plume to be significantly greater than would be predicted by a standard density‐coupled transport model that does not incorporate the coupling in the dispersivity tensor.

On the role of the purely transverse Weibel instability in fast ignitor scenarios

The growth rate for the purely transverse Weibel instability is determined from relativistic kinetic theory using a waterbag distribution function in the momenta perpendicular to the main propagation direction of the beam. A parametric study is presented for conditions relevant to the fast ignitor. It is shown that for expected parameters the purely transverse Weibel instability will be significantly suppressed or even eliminated due to the transverse energy spread or emittance.

Ultrashort pulsed bessel beams and spatially induced group-velocity dispersion.

We find a new family of solutions of the nonparaxial wave equation that represents ultrashort pulsed light beam propagation in free space. The spatial and temporal parts of these pulsed beams are separable; the spatial transverse part is described by a Bessel function and remains unchanged during propagation, but the temporal profile can be arbitrary. Therefore the pulsed beam exhibits diffraction-free behavior with no transverse spreading, but the temporal part changes as if in a dispensive medium; the change is dominated by what we call spatially induced group-velocity dispersion. The analytical and numerical investigations show that the even- and odd-order spatially induced dispersions partially compensate for each other so as to give rise to pulse spreading, weakening, asymmetry, and center shift.

Iceberg calving from the Amery Ice Shelf, East Antarctica

AbstractWe investigate the iceberg-calving cycle of the Amery Ice Shelf (AIS), East Antarctica, using evidence acquired between 1936 and 2000. The most recent major iceberg-calving event occurred between late 1963 and early 1964, when a large berg totalling about 10 000 km2 in area broke from the ice front. The rate of forward advance of the ice front is presently 1300–1400ma–1. At this rate of advance, based on the present ice-front position from recent RADARSAT imagery, it would take 20–25 years to attain the 1963 (pre-calve) position, suggesting that the AIS calving cycle has a period of approximately 60–70 years. Two longitudinal (parallel-to-flow) rifts, approximately 25 km apart at the AIS front, are observed in satellite imagery acquired over the last 14+years. These rifts have formed at suture zones in the ice shelf, where neighbouring flow-bands have separated in association with transverse spreading. The rifts were 15 km (rift A) and 26 km (rift B) in length in September 2000, and will probably become the sides of a large tabular iceberg (25 km 625 km). Atransverse (perpendicular-to-flow) fracture, visible at the upstream end of rift A in 1996, had propagated 6 km towards rift B by September 2000; when it meets rift B the iceberg will calve. A satellite image acquired in 1962 shows an embayment of this size in the AIS front, hence we deduce that this calving pattern also occurred during the last calving cycle, and therefore that the calving behaviour of the AIS apparently follows a regular pattern.

Device Scaling Effects on Substrate Enhanced Degradation in MOS Transistors

AbstractThis paper analyzes in detail the substrate enhanced gate current injection mechanism and the resulting hot-carrier degradation in n-channel MOS transistors and compares the results with conventional channel hot carrier injection mechanism. The degradation mechanism is studied for different values of substrate voltage over a wide range of channel length and oxide thickness. Stress and charge pumping measurements are carried out to study the degradation under identical bias (gate, drain, substrate) and gate current condition. The influence of device dimensions on the gate injection efficiency and hot carrier degradation is also studied. Results show that the degradation under negative substrate voltage operation is strongly dependent on the transverse electric field and spread of the interface trap profile. The possible mechanism responsible for such trends is discussed. It is also found that, under identical gate current (programming time in flash memory cells), the degradation is less for higher negative substrate bias, which is helpful in realizing fast and reliable flash memories.

Recent tests of negative electron affinity photocathodes as source for electron lithography and microscopy

We report recent results from tests of laser driven and modulated negative electron affinity photocathodes as high-brightness electron sources. Preliminary tests used an electron column that could image the cathode surface with a resolution of less than 100 nm. We measured a current density of 2.4(±0.2) A/cm2 from an emission area with a diameter [full width at half maximum (FWHM)] of 3.5 μm. However, unwanted field emission in the gun (aided by the presence of Cs) limited the extraction field to about 1 kV/mm and negatively affected quantum efficiency and emission stability. A curve fit of the source angle with accelerating voltage gave a typical transverse energy spread of 44(±2) meV, for an extrapolated brightness at 50 kV of 1.7×106 (±0.2) A/cm2 sr. Indications that space charge above the flat cathode limited the current density are presented; a possible model is discussed. The electron gun has been redesigned for electric fields up to 7 kV/mm (where space-charge effects are unlikely) with no field emission. Lack of access to an imaging column has prevented subsequent measurements of current density and brightness. In a nonimaging column an emission current of 200(±0.4) nA, from a single emission area, has been maintained for more than a week, with no reapplication of cesium. As with other photocathodes, variations of quantum efficiency after sudden changes in illumination intensity are usually present. We suggest that competing effects of Cs mobility and electron trapping explain this behavior. Transverse energy spreads (FWHM) have been studied as a function of extracted current; 20(±5) meV at small currents (<10 nA) and 70 meV at large currents (700 nA), both from an illumination area of less than 2 μm diameter, are typical.

Laser cooling of TeV muons

We show that Compton scattering can be used to cool TeV-scale muon beams, and we derive analytical expressions for the equilibrium transverse angular spread, longitudinal energy spread, and power requirements. We find that a factor of a few thousand reduction in emittance is possible for a 3TeV muon collider.

Effect of friction on deformation during rolling as revealed by embedded pin technique

The paper investigates the effects of friction on the heterogeneity of deformation during rolling through studies using plane strain (2D) and three-dimensional (3D) finite element models designed to simulate the deformation of the embedded pin inserts during rolling. Redundant work due to friction is defined as a path function along the arc of contact. Since deformation during rolling is profoundly influenced by the amount of redundant work, which depends on friction as a path function along the arc of contact, the study has focused especially on methods of representing these frictional effects. The friction studied has been in one of two classes: the coefficient of friction being constant or varying parabolically along the arc of contact. The results show that the values of shear stress and normal pressure along the arc of contact depend upon the friction profile. The magnitude of these frictional effects is revealed by the through thickness variation of the relative pin insert displacement. This displacement changes in the 3D model because transverse spread reduces the amount of displacement along the axial direction. Comparison of the simulations with experimental pin insert shapes shows close agreement between the predicted and experimental results, while revealing the direction of further work needed to provide suitable mechanics models to interpret experimental pin insert data.

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: creation of the emplaced-source and overview of dissolved plume development

A unique field experiment has been undertaken at the CFB Borden research site to investigate the development of dissolved chlorinated solvent plumes from a residual dense non-aqueous phase liquid (DNAPL) source. The “emplaced-source” tracer test methodology involved a controlled emplacement of a block-shaped source of sand containing chlorinated solvents below the water table. The gradual dissolution of this residual DNAPL solvent source under natural aquifer conditions caused dissolved solvent plumes of trichloromethane (TCM), trichloroethene (TCE) and perchloroethene (PCE) to continuously develop down gradient. Source dissolution and 3-D plume development were successfully monitored via 173 multilevel samplers over a 475-day tracer test period prior to site remediation research being initiated. Detailed groundwater level and hydraulic conductivity data were collected. Development of plumes with concentrations spanning 1–700,000 μg/l is described and key processes controlling their migration identified. Plumes were observed to be narrow due to the weakness of transverse dispersion processes and long due to advection and significant longitudinal dispersion, very limited sorptive retardation and negligible, if any, attenuation due to biodegradation or abiotic reaction. TCM was shown to be essentially conservative, TCE very nearly conservative and PCE, consistent with its greater hydrophobicity, more retarded yet having a greater mobility than observed in previous Borden field tests. The absence of biodegradation was ascribed to the prevailing aerobic conditions and lack of any additional biodegradable carbon substrates. The transient groundwater flow regime caused significant transverse lateral plume movement, plume asymmetry and was likely responsible for most of the, albeit limited, transverse horizontal plume spreading. In agreement with the widespread incidence of extensive TCE and PCE plumes throughout the industrialized world, the experiment indicates such solvent plumes are likely to be highly mobile and persistent, at least in aquifers that are aerobic and have low sorption potential (low f oc content).

Ｌｉ原子ビームのレーザコリメーション

Laser collimation of a Li atomic beam has been studied to create an intense collimated atomic beam. The gain of Li atomic intensity in the beam has been measured as functions of laser collimation parameters, such as laser-atom interaction length, laser-frequency detuning and laser power density. The Li intensity gain depends non-linearly on the laser-atom interaction length due to the longitudinal velocity distribution of the thermal Li beam (550°C). In the high laser power regime, the dependence of the Li intensity gain on the laser power and detuning disagrees with the theoretical prediction of the scattering force. These findings are explained by considering the saturation broadening of the Li absorption line. By optimizing the laser collimation parameters, 3.2-fold increase of the atomic intensity is attained for the collimated Li beam with the transverse velocity spread of±5m/s.

Preparation and performance of transmission-mode GaAs photocathodes as sources for cold dc electron beams

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O2 or NF3 in an extremely high vacuum setup (base pressure below 10−12 mbar). Quantum efficiencies of 20%–25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to ≈100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed.

Numerical analysis of flow and transport in a combined heterogeneous vadose zone–groundwater system

Numerical simulations of water flow and solute transport were used to investigate the effect of the recharge (rain/irrigation) at the soil surface on solute spreading and breakthrough in a realistic, three-dimensional, heterogeneous, vadose zone–groundwater flow system. Results of the analyses suggest that smaller recharge at the soil surface increases the relative variability in the response of the unsaturated flow system, decreases the groundwater recharge from the unsaturated zone, and, concurrently, decreases the degree of nonuniformity of the groundwater flow. Consequently, smaller recharge at the soil surface enhances the longitudinal and, especially the transverse spreading of the solute plume and increases the spreading of the expected solute BTC at the lower boundary of the unsaturated zone. Furthermore, smaller recharge at the soil surface decreases the extent of the vertical penetration of the solute plume into the groundwater, slows the peak arrival of the expected BTC, and considerably increases the spreading of the expected solute BTC at a given vertical control plane in the saturated region. It was shown that for the combined flow system considered here in which the solute plume is displaced to a sufficiently large vertical distance from the inlet zone at the soil surface, the variable that controls the transport is the cumulative recharge at the soil surface, and not the recharge rate.

Numerical simulation of hot electron interference in a solid state biprism: Conditions for interference observation

This article describes elucidations of conditions for electron interference observation, device designs, and numerical simulations for the solid-state biprism. The solid-state biprism is solid-state version of the already realized vacuum biprism, and is also proposed as a test ground for electron interference observation and a new conceptual device. Towards the realization of this device, the designed semiconductor device structures show that interference pattern with a pitch of 86 nm and a visibility of 0.6 can be realized with an electron energy of 20 meV, a transverse energy spread of 0.3 meV, and an operation temperature of 4 K.

Are meteoroids really dustballs?

Analysis of light curves of faint meteors seem to suggest that most meteoroids are collections of hundreds to thousands of fundamental grains at least some of which are released prior to the onset of intensive ablation. We would expect these grains, unless extremely uniform in physical properties, to be aerodynamically separated during flight, and therefore to produce wake, which is defined as instantaneous meteor light production from an extended spatial region. We present here theoretical results for wake production as a function of grain mass distribution, height of separation, zenith angle and velocity. In addition, we have obtained observational results from a new study which used short duration intensified CCD detectors to search for wake in sporadic meteors. The system employed coaxial intensified CCD cameras at each of two separated stations, one camera utilizing a rotating shutter and one not at each station. The majority of these nonshower meteors showed no statistically significant wake. However, several examples of apparent transverse separation of the light production regions were found. We also present results of two interesting Leonid meteors from the Mount Allison light curve experiment in NASA's 1998 Leonid Multi-instrument Aircraft Campaign (MAC) program. One of these provide support for the idea of transverse spread in the light production region, up to hundreds of meters, while the other provides a clear case of extreme wake in one Leonid meteor which can only be successfully modeled with very small (≈10 −16– 10 −17 kg ) constituent grains.

Edge radiation and its potential to electron beam diagnostics at HiSOR

The visible edge radiation generated by a relativistic electron beam at fringe fields of bending magnets in storage rings differs significantly from the well-known standard synchrotron radiation. An intensive peak appears in the electromagnetic radiation angular distribution. Its intensity exceeds considerably the intensity of synchrotron radiation. In addition, the radiation generated at two adjacent edges of bending magnets produces an interference pattern. The real electron beam parameters (its transversal sizes and angular spread) have effect on the radiation intensity distribution. It is possible to find the electron beam parameters from the experimentally measured edge radiation distribution. Such electron beam diagnostics as applied to HiSOR storage ring (Hiroshima, Japan) is discussed in the present paper.

Transverse Injection from Circular and Elliptic Nozzles into a Supersonic Crossflow

An investigation of sonictransverse injection from circular and elliptic nozzles into a supersoniccrosse ow using planar Rayleigh/Mie scattering is reported. Instantaneous images allow examination of the structural details, whereas ensemble-averaged images provide transverse penetration and lateral spread data for each jet. Standard deviation images produce information regarding the large-scale mixing/entrainment and reveal the mixing zones. Results show a highly three-dimensional near-e eld interaction dominated by shear-layer eddies and a counterrotating vortex pair. Ensemble-averaged results show that the elliptic jet spreads more rapidly in the lateral direction than the circular jet, cone rming that an axis-switching phenomenon is present. Near-e eld transverse penetration data collapse well with low-speed scaling conventions; however, the jet produced by the elliptic nozzle suffersa20%reductioninpenetrationcomparedto thecircularjet.Compressibilityleveldoesnotplaya signie cant roleintheaveragepenetrationorspreadofthesejets,althoughitstronglyaffectsthemixing/entrainmentandlargescale structure. Analysis of mixing potential from the standard deviation images indicates that the low and high convective Mach number Mc injection cases are signie cantly different; low Mc injection yields better large-scale mixing potential than high Mc injection regardless of the injector geometry.

Evidence for transverse spread in Leonid meteors

We report here evidence for significant transverse spread of the light production region in bright Leonid meteors. One Leonid meteor has an apparent spread in the light production region of about 600 m perpendicular to the flight path for the meteor, that transverse spread persisting for at least 0.3 s. We have also detected short-duration, jet-like features emanating from a bright Leonid meteor recorded in 1998. These jet-like features have maximum spatial dimensions up to 1.9 km. While we cannot definitively rule out instrumental artefacts as a cause for these jet-like features, they may be evidence of motion contributing to the observed spatial spread in the light production region.