Showing papers on "Wavelength published in 1996"

Global Observations of Oceanic Rossby Waves

Abstract: Rossby waves play a critical role in the transient adjustment of ocean circulation to changes in large-scale atmospheric forcing. The TOPEX/POSEIDON satellite altimeter has detected Rossby waves throughout much of the world ocean from sea level signals with ≲10-centimeter amplitude and ≳500-kilometer wavelength. Outside of the tropics, Rossby waves are abruptly amplified by major topographic features. Analysis of 3 years of data reveals discrepancies between observed and theoretical Rossby wave phase speeds that indicate that the standard theory for free, linear Rossby waves is an incomplete description of the observed waves.

Refraction and geometry in Maxwell's equations

Abstract: Computational studies of Maxwell's equations in complex geometries encountered in photonic band structure calculations run into difficulties when several length scales occur, such as the wavelength of light in free space and the skin depth in metal. These problems are remedied by using an adaptive co-ordinate system which expands or contracts length scales as necessary. Here we show that moving to a general co-ordinate transformation is equivalent to renormalizing e and μ. This is an huge simplification because now we need only write one computer code in a Cartesian system, and we can use this same code to handle any co-ordinate system by adjusting the e and μ we feed into the calculation.

On the kinetic dispersion relation for shear Alfvén waves

Abstract: Kinetic Alfven waves have been invoked is association with auroral currents and particle acceleration since the pioneering work of Hasegawa. However, to date, no work has considered the dispersion relation including the full kinetic effects for both electrons and ions. Results from such a calculation are presented, with emphasis on the role of Landua damping in dissipating Alfven waves which propogate from the warm plasma of the outer magnetosphere to the cold plasma present in the ionosphere. It is found that the Landua damping is not important when the perpendicular wavelength is larger than the ion acoustic gyroradius and the electron inertial length. In addition, ion gyroradius effects lead to a reduction in the Landua damping by raising the parallel phase velocity of the wave above the electron thermal speed in the short perpendicular wavelength regime. These results indicate that low-frequency Alfven waves with perpendicular wavelengths greater than the order of 10 km when mapped to the ionosphere will not be significantly affected by Landau damping. While these results based on the local dispersion relation, are strictly valid only for short parallel wavelength Alfven waves, they do give an indication of the importance of Landua damping for longer parallel wavelengthmore » waves such as field line resonances. 26 refs., 5 fig.« less

Enhanced high-harmonic generation using 25 fs laser pulses.

Abstract: We present experimental and theoretical results on high-harmonic generation in noble gases using an 805 nm, 25 fs, titanium-doped sapphire laser. The harmonic energies observed are unexpectedly high when compared with experimental and theoretical results to date for longer excitation pulses. We observe that the efficiency of harmonic production is highest for shorter pulses. Furthermore, the wavelength of the harmonics can be tuned by adjusting the sign of the chirp of the excitation pulse, demonstrating a tunable, ultrashort-pulse, $l25\mathrm{fs}$ soft-x-ray source.

High-frequency single-electron transport in a quasi-one-dimensional GaAs channel induced by surface acoustic waves.

Abstract: We report on an experimental investigation of the direct current induced by transmitting a surface acoustic wave (SAW) with frequency 2.7 GHz through a quasi-one-dimensional (1D) channel defined in a GaAs - AlGaAs heterostructure by a split gate, when the SAW wavelength was approximately equal to the channel length. At low SAW power levels the current reveals oscillatory behaviour as a function of the gate voltage with maxima between the plateaux of quantized 1D conductance. At high SAW power levels, an acoustoelectric current was observed at gate voltages beyond pinch-off. In this region the current displays a step-like behaviour as a function of the gate voltage (or of the SAW power) with the magnitude corresponding to the transfer of one electron per SAW cycle. We interpret this as due to trapping of electrons in the moving SAW-induced potential minima with the number of electrons in each minimum being controlled by the electron - electron interactions. As the number of electrons is reduced, the classical Coulomb charging energy becomes the Mott - Hubbard gap between two electrons and finally the system becomes a sliding Mott insulator with one electron in each well.

X-ray holography with atomic resolution

Abstract: DIFFRACTION methods for crystallographic structure determination suffer from the so-called 'phase problem'; a diffraction pattern provides intensity but not phase information for the scattered beams, and therefore cannot be uniquely inverted to obtain the crystal structure of a sample. Holographic methods1, on the other hand, offer a means of extracting both intensity and phase information. To be useful for crystallographic applications, holography must be implemented with radiation of sufficiently small wavelength to resolve atomic-scale features2. One method, electron-emission holography3–9, uses electron waves and is a powerful tool for studying surface structure; but it cannot image the internal structure of solids because of complications arising from the highly anisotropic nature of electron scattering processes. A proposed alternative method uses X-rays2,10–13, which scatter more isotropically than electrons. Here we demonstrate the efficacy of atomic-scale X-ray holography by obtaining direct images of the three-dimensional arrangement of strontium atoms in the cubic perovskite SrTiO3. With more intense synchrotron sources for illumination, and with the development of improved X-ray detectors, X-ray holography should become a powerful general technique for unambiguous structure determination in condensed matter systems.

A Study of the Interaction between Ultrasound and a Partially Contacting Solid--Solid Interface

Abstract: The measurement of the reflection of ultrasonic waves from a partially contacting solid--solid interface can be used to study the contact conditions at that interface. This paper describes measurements and predictions of the reflection of ultrasonic waves from partially contacting aluminium--aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact region the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient increased with frequency. No measurements were taken in the perfect contact region in which the reflection coefficient is known to be zero at all frequencies as this state is the same as a continuous piece of aluminium. The reflection coefficient variation with frequency was modelled using a spring model, good agreement between experiments and predictions being achieved. Reflection coefficient measurements were then used to study the contact between two aluminium surfaces under repeated loading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. Both elastic and plastic statistical contact models, as well as a numerical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the experimentally determined stiffness variations and the predicted stiffness was within an order of magnitude of the measured value in all cases.

The Mechanism for Frequency Downshift in Nonlinear Wave Evolution

Abstract: It has long been recognized that the frequency downshift in the wave field evolution is a consequence of nonlinear wave-wave interactions and that the frequency downshift is also necessary for the wind wave field to grow. Yet the detailed mechanism for the frequency change is still unknown: Is the process continuous and gradual? Or, is the frequency of a wave train varying gradually and continuously? Recently, Huang et al. (1995) found that the frequency downshift for a narrow band wave train is through wave fusion, an event described as two waves merging to form one wave, or n waves merging to form n—1 waves. The process was seen tobe local, abrupt, and discrete. Such an event cannot be studied by the traditional Fourier analysis. Using a Hilbert transform to produce the phase-amplitude diagram and the Hilbert Spectrum, we found that in addition to the narrow band waves, the wave fusion event could occur in finite band widths and wind wave fields as well; and it is indeed the mechanism responsible for the frequency downshift in nonlinear wave evolution in general. Specifically, the frequency downshift is an accumulation of wave fusion events, which is also the same phenomena of the “lost crest” observed by Lake and Yuen (1978), and the “crest pairing” observed by Ramamonjiarisoa and Mollo-Christensen (1979). We have made quantitative measure of this fusion through Hilbert analysis techniques. Other than the fusion process, the local frequency can have small variations due to the amplitude modulations. Because of the abrupt and discrete localized variations of wave frequency, a new paradigm is needed to describe the nonlinear wave evolution processes.

Distance measuring confocal microscope

Abstract: The present invention is a novel apparatus and method for the quick and accurate determination of surface profile and depth reading within little or no mechanical motion. The presently claimed apparatus comprises a polychromatic light source; a means for focusing the light onto a point of sample target, said means having a known amount of longitudinal chromatic aberration; and a means for detecting the wavelengths of light reflected from the sample target. The light projected onto the sample target is focused according to wavelength due to the longitudinal chromatic aberration. While light from across the spectrum will be reflected, the light returning from the sample target will be most strongly reflected in a wavelength that is focused on a reflective point in the sample. The means for detecting the light in the present invention passes through a substantially pinhole aperture before the light is detected according to wavelength. The purpose of the pinhole aperture is resolution. The pinhole aperture ensures that those wavelengths that focus onto the pinhole are more strongly detected than wavelengths that do not focus on the aperture. Thus, the resolution of wavelength intensity peaks is greatly increased.

Global to microscale evolution of the Pinatubo volcanic aerosol derived from diverse measurements and analyses

Abstract: We assemble data on the Pinatubo aerosol from space, air, and ground measurements, develop a composite picture, and assess the consistency and uncertainties of measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements agree with theoretical calculations in showing a dominant composition of H2SO4-H20 mixture, with H2SO4 weight fraction of 65-80% for most stratospheric temperatures and humidities. Important exceptions are (1) volcanic ash, present at all heights initially and just above the tropopause until at least March 1992, and (2) much smaller H2SO4 fractions at the low temperatures of high-latitude winters and the tropical tropopause. Laboratory spectroscopy and calculations yield wavelength- and temperature-dependent refractive indices for the H2SO4-H20 droplets. These permit derivation of particle size information from measured optical depth spectra, for comparison to impactor and optical-counter measurements. All three techniques paint a generally consistent picture of the evolution of R(sub eff), the effective radius. In the first month after the eruption, although particle numbers increased greatly, R(sub eff) outside the tropical core was similar to preeruption values of approx. 0.1 to 0.2 microns, because numbers of both small (r 0.6 microns) particles increased. In the next 3-6 months, extracore R(sub eff) increased to approx. 0.5 microns, reflecting particle growth through condensation and coagulation. Most data show that R(sub eff) continued to increase for about 1 year after the eruption. R(sub eff) values up to 0.6 - 0.8 microns or more are consistent with 0.38 - 1 micron optical depth spectra in middle to late 1992 and even later. However, in this period, values from in situ measurements are somewhat less. The difference might reflect in situ undersampling of the very few largest particles, insensitivity of optical depth spectra to the smallest particles, or the inability of flat spectra to place an upper limit on particle size. Optical depth spectra extending to wavelengths lambda > 1 micron are required to better constrain R(sub eff), especially for R(sub eff) > 0.4 microns. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with lambda(sub max) 0.3 microns) and relatively flat extinction spectra (0.4 - 1 microns) are among the longest-lived indicators of Pinatubo volcanic influence. They persist for years after the peaks in number, mass, surface area, and optical depth at all wavelengths <= 1 microns. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between global maps of 0.5- and 1.0-micron optical depth derived from the Advanced Very High Resolution Radiometer (AVHRR) and Stratospheric Aerosol and Gas Experiment (SAGE) satellite sensors. However, there are important differences between the AVHRR and SAGE midvisible optical thickness products. We discuss possible reasons for these differences and how they might be resolved.

Optical nonlinearities of Au/SiO2 composite thin films prepared by a sputtering method

Abstract: Third‐order nonlinear optical properties of Au/SiO2 composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak and the maximum value of χ(3) is obtained to be 2.0×10−7 esu. The size‐dependent enhancement of the χ(3) in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of χ(3)/α (α: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constant of the metal particles.

Single optical photon detection with a superconducting tunnel junction

Abstract: THE charge-coupled device (CCD) has become the detector of choice in optical astronomy. CCDs provide a very linear response to detected photons, are very efficient at some wavelengths, and can now provide coverage of a relatively wide field of view1–3. But they become quite inefficient with decreasing wavelength, and they lack intrinsic wavelength and time resolution. The only way to select specific wavelengths is to place filters in front of the detector, which makes the total system less efficient. Time resolution can be achieved only with short exposures, which are possible only with very bright sources. Here we report a superconducting device that can overcome these limitations, and which has performance characteristics far superior to existing photon counting systems4–7. Our superconducting tunnel junction can detect individual photons at rates up to 2.5 kHz in the wavelength range 200–500 nm, with an intrinsic spectral resolution of 45 nm and a quantum efficiency estimated to be about 50 per cent. The theoretical resolution of the present device is ∼ 20 nm, but use of superconductors with lower transition temperature could improve that to 8 nm.

Room-temperature operation of In0.4Ga0.6As/GaAs self-organised quantum dot lasers

Abstract: We report the room-temperature operating characteristics of InGaAs/GaAs self-organised quantum dot lasers grown by molecular beam epitaxy. The emission wavelength is 1.028 µm and Jth = 650 A/cm2 for a 90 µm × 1 mm broad-area laser. Steady-state and time-resolved photoluminescence measurements confirm that lasing occurs through the e1-hh2 higher-order transition, and the spontaneous recombination time for this transition is ≃ 200 ps.

Wavelength conversion by difference frequency generation in AlGaAs waveguides with periodic domain inversion achieved by wafer bonding

Abstract: Wavelength conversion by difference‐frequency generation is achieved in a periodically domain reversed AlGaAs waveguide. The AlGaAs waveguide is epitaxially grown on a template substrate where a periodic crystal domain inversion is achieved using wafer bonding, selective etching, and organometallic chemical vapor deposition. Wavelength conversion experiments on a fabricated buried heterowaveguide showed a 90 nm conversion bandwidth, polarization diversified operation, and polarization independent conversion efficiency. The experimental results also showed linearity and spectral inversion, which imply transparency to signal formats including analog and frequency modulation. Simultaneous conversion of multiple input wavelengths with no measurable cross talk is also demonstrated.

Optical multiplexing device

Abstract: An optical multiplexing device spatially disburses collimated light from a fiber optic waveguide into individual wavelength bands, or multiplexes such individual wavelength bands to a common fiber optic waveguide or other destination. The optical multiplexing device has application for dense channel wavelength division multiplexing (WDM) systems for fiber optic telecommunications, as well as compact optical instrument design. Multiple wavelength light traveling in a fiber optic waveguide is separated into multiple narrow spectral bands directed to individual fiber optic carriers or detectors. An optical block has an optical port for passing the aforesaid multiple wavelength collimated light, and multiple ports arrayed in spaced relation to each other along a multiport surface of the optical block. A continuous, variable thickness, multi-cavity interference filter extends on the multiport surface of the optical block over the aforesaid multiple ports. At each of the multiple ports the continuous interference filter transmits a different wavelength sub-range of the multiple wavelength collimated light passed by the optical port, and reflects other wavelengths. Multicolor light passed to the optical block from the optical port is directed to a first one of the multiple ports on an opposite surface of the optical block. The wavelength sub-range which is "in-band" of such first one of the multiple ports is transmitted through that port by the local portion of the continuous, variable thickness interference filter there, and all other wavelengths are reflected. The light not transmitted through the first port is reflected to strike a second port, at which a second (different) wavelength band is transmitted and all other light again reflected. The reflected optical signals thus cascades in a "multiple-bounce" sequence down the optical block of the multiplexing device, sequentially removing each channel of the multiplexed signal. In reverse operation, individual channels are combined in the optical block and transmitted through the optical port.

Sensitivity of a Spectrally Filtered and Nudged Limited-Area Model to Outer Model Options

Abstract: Numerical filters required to control spatial computational modes in a limited-area model (LAM) that uses the unstaggered. A grid are developed and tested over the complex topography of the Great Basin of the western United States. The filters are founded upon Fourier expansions of forecast deviation fields and function equally effectively for both periodic and aperiodic local structures. Unlike other spatial filters, the approach used here avoids any direct contamination of larger scales. Provided that the shortest resolved wavelength of two grid intervals is removed, the results do not depend strongly on the range of filtered short waves or on the type and order of horizontal space difference approximations. This approach leads naturally to methods in which the large scales predicted by an ambient outer model can be directly incorporated within the complete domain of the inner LAM, rather than just through conditions applied at the lateral boundaries of the LAM. This technique has some similari...

Absorption/Scattering Coefficients and Scattering Phase Functions in Reticulated Porous Ceramics

Abstract: Spectral absorption and scattering coefficients and spectral scattering phase functions have been derived for partially stabilized zirconia (PS ZrO 2 ) and oxide-bonded silicon carbide (OB SiC) reticulated porous ceramics (RPCs) across the wavelength range 0.4-5.0 μm. These spectral radiative properties were investigated and quantified for 10 ppi (pores/inch), 20 ppi, and 65 ppi materials. Radiative properties were recovered from spectral hemispherical reflectance and transmittance measurements using inverse analysis techniques based upon discrete ordinates radiative models. Two dual-parameter phase functions were investigated for these materials : one based on the physical structure of reticulated porous ceramics and the other a modified Henyey-Greenstein phase function. The modified Henyey- Greenstein phase function provided the most consistent spectral radiative properties. PS ZrO 2 radiative properties exhibited strongly spectrally dependent behavior across the wavelength range studied. OB SiC radiative properties exhibited radiative behavior that was relatively independent of wavelength across the wavelength spectrum studied. OB SiC also demonstrated consistently higher absorption coefficients than PS ZrO 2 at all wavelengths. Spectral scattering albedos of PS ZrO 2 were discovered to be in the range 0.81-0.999 and increased as ppi rating increased, while those for OB SiC were lower in the range 0.55-0.888 and decreased as ppi rating increased. The average extinction efficiencies for 0.4-5.0 μm were discovered to be 1.45 for PS ZrO 2 and 1.70 for OB SiC. Extinction coefficients were discovered to correlate well with geometric optics theoretical models and electromagnetic wave/fiber interaction models based on independent scattering and absorption mechanisms.

Wave transformation across the inner surf zone

Abstract: Sea and swell wave heights observed on transects crossing the mid and inner surf zone on three beaches (a steep concave-up beach, a gently sloped approximately planar beach, and a beach with an approximately flat terrace adjacent to a steep foreshore) were depth limited (i.e., approximately independent of the offshore wave height), consistent with previous observations. The wave evolution is well predicted by a numerical model based on the one-dimensional nonlinear shallow water equations with bore dissipation. The model is initialized with the time series of sea surface elevation and cross-shore current observed at the most offshore sensors (located about 50 to 120 m from the mean shoreline in mean water depths 0.80 to 2.10 m). The model accurately predicts the cross-shore variation of energy at both infragravity (nominally 0.004 < f < 0.05 Hz) and sea swell (here 0.05 < f ≤ 0.18 Hz) frequencies. In models of surf zone hydrodynamics, wave energy dissipation is frequently parameterized in terms of γs, the ratio of the sea swell significant wave height to the local mean water depth. The observed and predicted values of γs increase with increasing beach slope β and decreasing normalized (by a characteristic wavenumber k) water depth kh and are well correlated with β/kh, a measure of the fractional change in water depth over a wavelength. Errors in the predicted individual values of γs, are typically less than 20%. It has been suggested that infragravity motions affect waves in the sea swell band and hence γs, but this speculation is difficult to test with field observations. Numerical simulations suggest that for the range of conditions considered here, γs is insensitive to infragravity energy levels.

Optical filter including a sub-wavelength periodic structure and method of making

Abstract: An optical filter includes a dielectric layer formed within a resonant optical cavity, with the dielectric layer having formed therein a sub-wavelength periodic structure to define, at least in part, a wavelength for transmission of light through the resonant optical cavity. The sub-wavelength periodic structure can be formed either by removing material from the dielectric layer (e.g. by etching through an electron-beam defined mask), or by altering the composition of the layer (e.g. by ion implantation). Different portions of the dielectric layer can be patterned to form one or more optical interference filter elements having different light transmission wavelengths so that the optical filter can filter incident light according to wavelength and/or polarization. For some embodiments, the optical filter can include a detector element in optical alignment with each optical interference filter element to quantify or measure the filtered light for analysis thereof. The optical filter has applications to spectrometry, colorimetry, and chemical sensing.

Dispersive white-light interferometry for absolute distance measurement with dielectric multilayer systems on the target

Abstract: We have extended the use of a dispersive white-light interferometer for absolute distance measurement to include effects of dielectric multilayer systems on the target. The phase of the reflected wave changes as a function of wavelength and layer thickness and causes errors in the interferometric distance measurement. With dispersive white-light interferometry these effects can be measured in situ, and the correct mechanical distance can be determined. The effects of thin films deposited upon the target have been investigated for one and two layers (photoresist and SiO2 upon Si). Experimental results show that the thicknesses of these layers can also be determined with an accuracy of the order of 10 nm.

Method and device for wavelength locking

Abstract: A method and device for wavelength locking is provided, wherein an element having a wavelength dependent characteristic such as a Fabry Perot etalon is used to provide an output signal having an intensity that varies with wavelength The intensity of a reference signal derived from an input signal is compared with an output from the Fabry Perot etalon to provide a feedback signal that corresponds to the frequency of the input signal The system is calibrated before wavelength locking is performed to determine a ratio of intensities that determines a locked state or condition

Plasma production from helicon waves

Abstract: Experimental measurements taken in a large magnetoplasma show that a simple double half‐turn antenna will excite m=1 helicon waves with wavelengths from 10–60 cm. Increased ionization in the center of the downstream plasma is measured when the axial wavelength of the helicon wave becomes less than the characteristic length of the system, typically 50–100 cm. A sharp maximum in the plasma density downstream from the source is measured for a magnetic field of 50 G, where the helicon wave phase velocity is about 3×108 cm s−1. Transport of energy away from the source to the downstream region must occur to create the hot electrons needed for the increased ionization. A simple model shows that electrons in a Maxwellian distribution most likely to ionize for these experimental conditions also have a velocity of around 3×108 cm s−1. This strong correlation suggests that the helicon wave is trapping electrons in the Maxwellian distribution with velocities somewhat slower than the wave and accelerating them into a quasibeam with velocity somewhat faster than the wave. The nonlinear increase in central density downstream as the power is increased for helicon waves with phase velocities close to the optimum electron velocity for ionization lends support to this idea.

Electron acceleration by inertial Alfvén waves

Abstract: Alfven waves reflected by the ionosphere and by inhomogeneities in the Alfven speed can develop an oscillating parallel electric field when electron inertial effects are included. These waves, which have wavelengths of the order of an Earth radius, can develop a coherent structure spanning distances of several Earth radii along geomagnetic field lines. This system has characteristic frequencies in the range of 1 Hz and can exhibit electric fields capable of accelerating electrons in several senses: via Landua resonance, bounce or transit time resonance as discussed by Andre and Eliasson or through the effective potential drop which appears when the transit time of the electrons is much smaller than the wave period, so that the electric fields appear effectively static. A time-dependent model of wave propagation is developed which represents inertial Alfven wave propagation along auroral field lines. The disturbance is modeled as it travels earthward, experiences partial reflections in regions of rapid variation, and finally reflects off a conducting ionosphere to continue propagating antiearthward. The wave experiences partial trapping by the ionospheric and the Alfven speed peaks discussed earlier by Polyakov and Rapoport and Trakhtengerts and Feldstein and later by Lysak. Results of the wave simulation and an accompanyingmore » test particle simulation are presented, which indicate that inertial Alfven waves are a possible mechanism for generating electron conic distributions and field-aligned particle precipitation. The model incorporates conservation of energy by allowing electrons to affect the wave via Landau damping, which appears to enhance the effect of the interactions which heat electron populations. 22 refs., 14 figs.« less

Synchrotron Models for X-Rays from the Supernova Remnant SN 1006

Abstract: Recent observations with the ASCA satellite (Koyama et al. 1995) have finally settled the question of the nature of the X-ray spectrum from the remnant of the supernova of 1006 AD. The bright rims have a featureless power-law spectrum while fainter parts of the remnant show a normal thermal spectrum with the expected lines. I describe model images and spectra that fit the data from the bright rims well, on the premise that the X-rays are synchrotron emission from electrons with energies up to 100 TeV accelerated in the remnant blast wave. The maximum energy to which electrons can be accelerated is limited by the requirement that the acceleration time be less than the smaller of the remnant age or the electrons' radiative loss time. In addition, absence of magnetohydrodynamic waves with sufficiently long wavelength to scatter electrons above some energy would allow electrons to escape freely above that energy, rather than being further accelerated. The maximum energy is thus a function of time and position. I assume that the electron mean free path is proportional to gyroradius, with proportionality constant f. With no internal magnetic field amplification beyond the original shock compression, the observed morphology, spectral shape, and X-ray flux at 4 keV can be fitted well with two models, one with escape, and the other with a perhaps unreasonably low upstream magnetic field. The former model has an external magnetic field strength of 3 μG, f = 10, and a maximum MHD wavelength of 1017 cm. The latter, no-escape model has an external magnetic field strength of 0.6 μG and f = 1. Both models predict upstream emission at a level of a few percent of postshock emission, but with differing morphologies. Models with an upstream magnetic field of 3 μG and without escape overpredict X-rays at 4 keV by over an order of magnitude.

Scanned- and fixed-wavelength absorption diagnostics for combustion measurements using multiplexed diode lasers

Abstract: A multiplexed diode-laser sensor system comprising two diode lasers and fiber-optic components has been developed to nonintrusively monitor temperature and species mole fraction over a single path using both scanned-and fixed-wavelength laser absorption spectroscopy techniques. In the scanned-wavelength method, two InGaAsP lasers were current tuned at a 2-kHz rate across H 2 O transitions near 1343 nm and 1392 nm in the 2v 1 and v 1 + v 3 bands. Gas temperature was determined from the ratio of single-sweep integrated line intensities. Species mole fraction was determined from the measured line intensity and the calculated line strength at the measured temperature. In the fixed-wavelength method, the wavelength of each laser was fixed near the peak of each absorption feature using a computer-controlled laser line-locking scheme. Rapid measurements of gas temperature were obtained from the determination of peak line-intensity ratios. The system was applied to measure temperature and species concentration in the postflame gases of an H 2 -O 2 flame. The good agreement between the laser-based measurements obtained using scanned- and fixed-wavelength methods with those recorded with thermocouples demonstrates the flexibility and utility of the multiplexed diode-laser sensor system and the potential for rapid, continuous measurements of gasdynamic parameters in high-speed or transient flows with difficult optical access.

From Helix to Localized Writhing in the Torsional Post-Buckling of Elastic Rods

Abstract: In this paper we study the competition between helical and localizing modes in the torsional buckling of stretched and twisted elastic rods. Within the Love-Kirchhoff formulation, we make comparative studies of the helical deformation of Love (1927) and the homoclinic localizing solution of Coyne (1990). Plots of the loads against their corresponding deflections allow the energetically preferred mode to be identified: it is found that preference switches from the helix to the localized mode early in the post-buckling range. These plots also allow us to predict the jumps that are observed under a variety of dead and rigid loading processes: these dynamic jumps take the rod from the spatially localized form to the familiar writhing state. Preliminary experiments confirm this preference for the localizing mode. They also reveal a second type of helical deformation, at a shorter wavelength, that is not predicted by the above long-rod analyses. A programme of further experimental and theoretical studies is suggested, and in a companion paper we lay the mathematical foundations for a numerical investigation of the complex and spatially chaotic deformations of a wider class of elastic rods.

Conversion efficiencies from laser-produced plasmas in the extreme ultraviolet regime

Abstract: The conversion efficiency of spectral emission from laser‐irradiated solid targets was investigated for short wavelength source development. The plasma brightness was quantified using absolutely calibrated detectors for 20 materials and spectra were obtained between 50 and 200 A. Laser parameters such as wavelength, pulse length, intensity, and spot size were systematically varied to establish a comprehensive database for source optimization. Qualitative differences in the underlying dominant emission features as a function of atomic number and laser wavelength were observed that accounted for the relatively high spectral conversion efficiencies produced. In the specific case of Sn, a conversion efficiency greater than 0.8%/eV has been observed in the technologically important region of λ=134.0 A using a laser intensity of 1–2×1011 W/cm2.

Phase difference plate and circularly polarizing plate

Abstract: PROBLEM TO BE SOLVED: To obtain a phase difference plate which has decreased phase differences by wavelengths and has the excellent consistency thereof by sticking a quarter-wave plate and a halfwave plate to each other in the state of intersecting their respective optical axes. SOLUTION: This phase difference plate is formed by sticking the quarter- wave plate (a double refractive film of which the phase difference of the double refractive light is a quarter wave) 13 and the halfwave plate (a double refractive film of which the phase difference of the double refractive light is a half wave) 11 to each other in the state of intersecting the respective optical axes at a preset angle. The halfwave plate 11 and the quarter-wave plate 13 are formed by subjecting high-polymer films to stretching treatments. In such a case, a polycarbonate material which is often used as a wavelength material is selected. This phase difference plate is designed to function as the quarter-wave plate improved in the wavelength dispersion characteristic when linearly polarized light of a perpendicular direction (0 deg. direction) is made incident from the halfwave plate 11 side. The phase difference plate is thus capable of converting the light to nearly perfectly circularly polarized light regardless of wavelengths in a range of visible light.