Showing papers on "Doppler broadening published in 1983"

The apparent spectral broadening of VLF transmitter signals during transionospheric propagation

Abstract: ISIS 1 and 2 and ISEE 1 VLF/ELF electric field wave data indicate the existence of a novel phenomenon, in which initially narrow band upgoing signals from ground-based VLF transmitters undergo a significant spectral broadening as they propagate through the ionosphere and protonosphere, up to altitudes in the 600-3800 km range. For transmitter signals in the 10-20 kHz range, the spectral broadening can be as high as 10 percent of the input signal's nominal frequency. In many cases, the bandwidth of the spectrally broadened signals is a strong function of the electric dipole antenna orientation with respect to the local direction of the earth's magnetic field. The unusual dispersion in the components of the spectrally broadened pulses suggests that the spectral broadening may be due to a Doppler shift effect in which the initial signals scatter from irregularities in the F region and couple into quasi-electrostatic modes of short wave length.

An in vitro model and its application for the study of carotid doppler spectral broadening

Abstract: A new in vitro model has been developed for studying the changes in the ultrasound Doppler spectrum that occur in the region of a stenosis. Pulsatile flow in rigid acrylic tubes was produced by means of a modified hemodialysis pump. The Doppler spectral waveforms were measured using a continuous wave Doppler system, a probe of a known field pattern, a real-time high resolution frequency analyzer, and a video display and recording system. The flow velocity waveforms were found to be nearly identical to those seen in the human carotid. Measurements were made to determine the critical stenosis and the results are similar to those reported from in vivo studies. In a preliminary study, the extent of spectral broadening was found to be dependent on the recording site in relation to the stenosis, the severity of the stenosis, and the flow rate. Using qualitative methods it was not possible to determine either the influence of the shape of the stenosis or the phase of the cardiac cycle on spectral broadening.

Measurements of plasma ion temperature and rotation velocity using the He II 4686‐Å line produced by charge transfer

Abstract: Spatially and temporally resolved values of the ion temperature and toroidal rotation velocity near the plasma axis have been obtained in neutral beam‐heated plasmas in the Doublet III tokamak from measurements of the Doppler broadening and Doppler shift of the He II 4686‐A line, excited by charge transfer reactions between fast hydrogen atoms in the beam and thermal helium ions in the plasma. The measurement technique, which uses visible optics and a multichannel detector, promises to be a straightforward method to obtain ion temperature profiles in beam‐heated tokamaks.

Spectral broadening of lower-hybrid waves by time-dependent density fluctuations

Abstract: Lower‐hybrid waves injected into tokamaks are scattered by turbulent time‐dependent drift‐wave density fluctuations. This scattering process gives rise to frequency shifts as well as angular deviations. In tokamaks, most of the frequency spreading occurs in a regime where the angular scattering results from multiple scattering events, each event having a small mean‐square scattering angle. A radiative transport equation governing diffusion in both angle and frequency is derived. The solution is obtained in slab geometry via separation of variables and leads to a Mathieu equation. In the limit where the slab thickness l exceeds the typical distance for diffusion through a large angle l s , an explicit generalization of the diffusion solution to the radiative transfer equation is formed. This solution gives the combined angular and frequency spectrum in the scattering layer. In the thick slab limit, the rms frequency width of lower‐hybrid waves emerging from the layer is given by Δω=(ω d / ξ0ρ i ) N ∥ [(M i /m e )β i ]1 / 2 (l/l s ), where ω d and ξ−1 0 are the characteristic angular frequency and correlation length of the drift‐wave turbulence.

Theory of saturated line shapes in phase-conjugate emission by resonant degenerate four-wave mixing in Doppler-broadened three-level systems

Abstract: We present a theoretical study of phase-conjugate (PC) emission by backward degenerate four-wave mixing (DFWM) processes in a resonant gas medium. In this study, high-intensity (pump) saturation and Doppler broadening are simultaneously taken into account. We develop a general formalism to deal with the saturation effects in the case of a three-level atom (with degenerate frequencies) and obtain analytical expressions of the PC field amplitude (after integration over the velocity distribution) under the following conditions: (1) the probe beam and one of the pump beams have intensities weak enough to be treated at the lowest perturbation order, whereas the other pump intensity is arbitrarily high; (2) the probe crosses the standing pump wave at grazing incidence. It is shown that the saturation induced by the forward pump (copropagating with the probe beam) leads to entirely different behavior of the nonlinear susceptibility from that induced by the backward pump (counterpropagating with the probe beam). This saturation anisotropy is specific to the inhomogeneous broadening of the gas medium, which is also responsible for the frequency splitting experimentally observed in the intensity line shape of the PC emission for large pump intensities. At saturation, this splitting is theoretically shown to be proportional to the Rabi frequency, with a proportionality coefficient critically dependent on the relaxation processes; in the case of a single-relaxation model, the splitting is equal to 1.5 times the Rabi frequency. The PC intensity line shape is demonstrated to be of dispersive origin in the case of backward saturation, whereas for forward saturation, narrow structures are predicted in both real and imaginary parts of the nonlinear susceptibility, also leading to the intensity line-shape splitting. One also finds that the reflectivity of PC mirrors based on resonant DFWM strongly depends on the origin of the saturation. For a saturating forward pump, the PC emission intensity tends toward zero with increasing pump intensities, and it reaches a finite (nonzero) limit in the case of backward saturation. Most of these predictions have been observed experimentally [ BlochD., Phys. Rev. Lett.49, 719 ( 1982)].

Propagation and absorption of electromagnetic waves in fully relativistic plasmas

Abstract: The propagation and absorption of electromagnetic waves in a relativistic Maxwellian plasma are investigated by solving the uniform plasma dispersion relation. Both the Hermitian and the anti‐Hermitian parts of the plasma conductivity tensor σ are calculated relativistically. The Bessel functions occurring in σ are not expanded, and many cyclotron harmonic terms are included at high temperatures. The dispersion relation is solved numerically for perpendicular propagation, k∥ =0, where the relativistic effects are maximum and are not masked by Doppler broadening, which has been more thoroughly investigated. It is found that relativistic broadening has a substantial effect on wave dispersion, shifting the extraordinary mode right‐hand cutoff and the upper‐hybrid resonance to higher magnetic field with increasing temperature. Above a critical temperature, the cutoff disappears entirely. There is a broad range of temperatures, 20 keV≤Te ≤500 keV, for which the wavenumber k⊥ differs significantly from both the cold‐plasma value and the vacuum value. This has important implications for ray tracing in relativistic plasmas. Wave damping rates are calculated and compared to results from a previous formulation using the Poynting theorem, in which only the Hermitian part of σ is calculated relativistically.

Ultrasonic doppler spectral broadening in the diagnosis of internal carotid artery stenosis.

Abstract: In a clinical study, 78 carotid bifurcations were examined by pulsed Doppler ultrasonography and X-ray arteriography. Maximum Doppler frequencies greater than or equal to 3 kHz diagnosed stenoses of greater than or equal to 25% diameter reduction with a sensitivity of 73% and specificity of 79%. A quantitative analysis of the degree of spectral broadening was obtained from the ratio of the maximum to mean frequency at peak systole and improved the sensitivity and specificity to 90% and 98% respectively. All vessels greater than 40% stenosis were correctly classified as stenosed and all less than 20% stenosis were classified as normal.

Limitations of doppler spectral broadening in the early detection of carotid artery disease due to the size of the sample volume

Abstract: In the present study the criteria of spectral broadening used in pulsed Doppler systems to diagnose minor disease in the cervical carotid arteries were evaluated in relation to the size of the sample volume For this purpose the Doppler audio spectra as recorded in asymptomatic volunteers with an ATL Duplex scanner (relatively large sample volume) and a multi-gate pulsed Doppler system (relatively small sample volume) were compared with each other The volunteers, randomly selected from the population registers, varied in age from 20–69 yr Only in the common carotid artery at younger age, were normal spectra recorded, especially with the multi-gate system In the internal carotid artery the number of spectra, supposed to be associated with low grade stenosis, increased significantly, especially when recorded with the Duplex scanner More pronounced spectral broadening was found when the spectra were recorded distally in this artery In the cervical carotid arteries normal spectra were found only when sampled in the middle of the bloodstream These findings indicate that for the detection of vascular lesions in the cervical carotid arteries at an early stage of the disease on the basis of spectral broadening, small sample volumes and sampling in the centre of the vessel are required

A positron study of plastic deformation of silicon

Abstract: Positron lifetime and Doppler broadening experiments have been conducted in silicon which was subjected to various degrees of plastic deformation. With increasing plastic deformation (from 2.5 to 38%) the trapping rate per unit dislocation concentration decreased from about 8 × 1015 to 4.5 × 1015 s−1 as measured at room temperature. Two annealing stages were identified by isochronal annealing, one of which had an activation enthalpy of 3.2 to 3.8 eV as determined by isothermal annealing. Measurements in the temperature range 15 to 673 K show that the positron annihiliation parameters (lifetimes, intensities, and S-parameters) depend strongly on temperature and that the functional form of these dependencies is related to the state of annealing of the deformed samples. These results are disscussed in terms of the presence of vacancy clusters and dislocations, and evidence is presented indicating the existence of the hexavacancy.

Plasma ion temperature measurement from Balmer alpha charge-exchange radiation during neutral injection

Abstract: Measurements are presented of Balmer alpha line profiles from plasma ions which have been neutralized by charge exchange with injected neutral hydrogen beams in the DITE tokamak. Doppler broadening of the H? line is interpreted in terms of the ion temperature and line profiles are compared with neutral particle energy spectra in the regime where the plasma is transparent to the transport of neutrals from the core to the boundary. An asymmetry in the charge-exchange line profile is interpreted in terms of plasma rotation during neutral beam injection. The advantage of this method over conventional neutral analysis is seen in the context of the diagnosis of large, hot and dense tokamak plasmas.

Intensity-fluctuation spectroscopy of optical fields with non-Gaussian statistics

Abstract: Intensity-fluctuation spectroscopy (IPS) is usually considered to be complementary to conventional spectroscopy and capable of removing technical restrictions on the resolving power. However, the information provided by field spectroscopy is identical to that obtained by IFS only for fields with Gaussian statistics. For non-Gaussian fields, IFS yields essentially new information, and the present review is devoted to this aspect of IFS. It surveys experiments concerned with the investigation of the noise spectrum of resonance fluorescence and of coherent forward scattering by an atomic vapor, which provide data on the width and the structure of levels involved in atomic transitions under the conditions of dominant Doppler broadening. Fundamental and technical limitations of the method are examined. Analogous studies of fluctuation spectra of radiation scattered by macroparticles in liquids can be used to determine the time dependence of the particle form factor independently of the characteristics of translational diffusion.

Mercury-199 Trapped Ion Frequency Standard: Recent Theoretical Progress and Experimental Results

Abstract: : The 40.5 GHz hyperfine resonance of 199 Hg ions stored in an rf quadrupole trap has shown to have a very small fractional linewidth, suggesting its use as a high-precision frequency standard. The most significant offset in such a standard would be the second-order Doppler shift resulting from the motion of the stored ions. We have recently analyzed the situation in which the secular motion is cooled to a temperature of about 300 K by the presence of a light background gas at low pressure, resulting in an ion cloud whose density is almost completely determined by the balance of pseudopotential and space-charge forces. Under these circumstances we have found that the second-order Doppler shift can be calculated accurately from the trapping parameters, the temperature, and the total number and mass of the stored ions.

Longitudinal ramsey-fringe spectroscopy in a calcium beam

Abstract: For ultra-high resolution spectroscopic applications such as optical frequency standards, the value of thermal sources such as atomic beams is currently limited by secondorder Doppler broadening. The use of a longitudinal interaction geometry in which an atomic beam crosses the counter-propagating laser fields at a small angle is able to reduce second-order Doppler broadening to an insignificant level as well as to provide long interaction times without the necessity of large-diameter optical beams. We have analyzed this geometry for the case of the long-lived calcium intercombination line, and conclude that when combined with pulsed (Ramsey) excitation, the longitudinal interaction geometry could be used with a thermal calcium beam to create an optical frequency standard with a reproducibility of the order of 10−14 for a few seconds averaging time. Our initial experimental results have demonstrated the first use of the longitudinal geometry.

Fiber-optical Doppler anemometer

Abstract: In a fiber-optical Doppler anemometer for measurement of fluctuating light or the Doppler broadening of laser light scattered by a scattering medium, for example by particles in motion, coherent laser light is passed via a fiber-optical coupler (1) and a submerged fiber-optical probe (2) into the scattering medium, and the light scattered back by the scattering medium is picked up by the submerged fiber-optical probe (2), branched by the fiber-optical coupler (1) and passed to a photodetector (3).

Method for sea surface high frequency radar cross-section estimation using Dopler spectral properties

Abstract: A method for estimating the high frequency radar cross-section of the sea surface. A Doppler spectrum of the sea surface is obtained by correlating returns from high frequency radar signals. Approach and recede Bragg spectral lines and a zero Doppler frequency continuum level are identified in the Doppler spectrum. The amplitude ratio ρ between the Bragg spectral lines and an amplitude ratio ζM between the maximum amplitude Bragg spectral line and the zero Doppler frequency continuum are measured from the Doppler spectrum. The radar cross-section is then determined from the ratios ρ and ζM. In a further method embodiment, the estimated radar cross-section is used to estimate the radar cross-sections of targets appearing in the Doppler spectrum.

Self-broadening at low densities in the spectrum of neon

Abstract: Self-broadening of the transitions at 585.2, 659.9 and 667.8 nm in neutral neon has been studied in emission by means of a scanning Fabry-Perot interferometer. These lines all connect to the strong resonance level (2p53s1P1), and the broadening is dominated by the resonance interaction. The source was a weak discharge run at pressures up to 10 Torr and cooled with liquid nitrogen (77K) or melting ice (273K). Data were recorded and analysed digitally; most spectral profiles were found to be well represented by the Voigt function, though evidence of slight profile distortion, attributed to excitation mechanisms, was found for all three lines under certain conditions of temperature and pressure. The Lorentzian widths varied linearly with pressure. All experiments were carried out with the natural mixture of isotopes and a sample enriched in 20Ne and no significant differences in the broadening were observed. When allowance was made for profile distortion the collisional broadening was found to be independent of the spectral line and of the temperature to within experimental error, in accordance with the prediction of theory for the case of pure resonance broadening. The measured broadening constants were 6.13(16) at 77K and 6.19(28) at 273K, to be compared with the theoretical value of 6.28(25), in all units of 10-20 cm-1 cm3 per atom. The Lorentzian width of lambda 585.2 nm extrapolated to zero pressure at 77K was found to exceed the radiation width of the line, in agreement with earlier workers, but only by 0.7(5) mK which is small enough to be attributed to excitation mechanisms. lambda 585.2 nm shows a small shift to longer wavelength with increasing pressure at both temperatures; this is attributed to higher order terms in the interaction potential.

Parameterization of carbon dioxide 15 μm band absorption and emission

Abstract: It is well known that absorption and emission of the 15/m CO2 band are the most significant components in the determi- nation of terrestrial infrared cooling rates and equilibrium temperature in the stratosphere. Generally, there are three approaches to model the atmospheric infrared radiation transfer due to the 15-tm CO2 band absorption and emis- sion: (1) line-by-line integration over the entire band (-500- 850 cm-); (2) band-by-band summation involving a limited number of sub-bands, using an approximate band model; and (3) broadband parameterization in which the absorption and emission effects are evaluated through some predetermined emissivity (or transmissivity) tables. The line-by-line ap- proach is widely employed for the precise determination of radiative flux and cooling rate profiles. Usually, thousands of absorption lines are required in the numerical integration which takes enormous machine computation time in order to achieve reliable accuracies. The first-order approximation to the tedious and time consuming calculations is to employ an appropriate band model. The average error produced by using a band model depends on the selection of band models, the spectral resolution used, the relative importance of the Doppler broadening, and the pressure-temperature correc- tion scheme used in estimating the effective path length over an inhomogeneous stratification (Drayson, 1967). The broad- band emissivity approach is a second-order approximation which utilizes a set of predetermined values. These values may be fitted by a known mathematical representation. The broadband emissivity approach is particularly useful for applications to infrared radiation energy transfer and climate modeling because of its simplicity and efficiency in numeri- cal computations. The objective of this paper is to construct a set of broadband emissivity values for the 15-/m CO2 band from the line-by-line data. In the line-by-line calculation for cooling rates due to the 15-tm CO2 band, the absorption lines can be grouped into three categories: the fundamental band, the first hot band, and the second hot band (Dickinson, 1973). All these three groups include the contribution from all the isotopes. The first fundamental problem in the flux and cooling rate calculations is the assumption of local thermodynamic equi- librium (LTE) which breaks down as the altitude increases. Dickinson pointed out that a significant deviation from LTE occurs at a height around 65 km for the isotope C13016018

Computer simulations of upper‐hybrid and electron cyclotron resonance heating

Abstract: A 2 1/2 ‐dimensional relativistic electromagnetic particle code is used to investigate the dynamic behavior of electron heating around the electron cyclotron and upper‐hybrid layers when an extraordinary wave is obliquely launched from the high‐field side into a magnetized plasma. With a large angle of incidence most of the radiation wave energy converts into electrostatic electron Bernstein waves at the upper‐hybrid layer. These mode‐converted waves propagate back to the cyclotron layer and deposit their energy in the electrons through resonant interactions dominated first by the Doppler broadening and later by the relativistic mass correction. The line shape for both mechanisms has been observed in the simulations. At a later stage, the relativistic resonance effects shift the peak of the temperature profile to the high‐field side. The heating ultimately causes the extraordinary wave to be substantially absorbed by the high‐energy electrons. The steep temperature gradient created by the electron cyclotron heating eventually reflects a substantial part of the incident wave energy. The diamagnetic effects due to the gradient of the mode‐converted Bernstein wave pressure enhance the spreading of the electron heating from the original electron cyclotron layer.

Helium implantation in metals investigated by monoenergetic positrons

Abstract: Helium-implanted metals and alloys were investigated with a positron beam system capable of producing positrons with energies ranging between 150 eV and 28 keV. Such a system is superior to the classical positron annihilation methods using high energy positrons from radioactive sources, since, for the first time, near-surface defects and defect agglomerates at high concentrations can be studied. Results are reported on crystalline nickel and amorphous nickel alloys irradiated at room temperature with helium up to doses of 1.8 × 1018 cm−2 at different energies. The Doppler broadening technique of the annihilation radiation is applied, and the lineshape parameter Ic is used to characterize the various defect configurations. In some favourable cases information about defect concentrations could be obtained.

Solid State Effects on Thermal Neutron Cross Sections and on Low Energy Resonances

Abstract: The neutron total cross sections of several single crystals (Si, Cu, sapphire), several polycrystalline samples (Cu, Fe, Be, C, Bi, Ta), and a fine powder copper sample have been measured from 0.002 to 5 eV. The Cu powder and polycrystalline Fe, Be and C data exhibit the expected abrupt changes in cross section. The cross section of the single crystal of Si is smooth with only small broad fluctuations. The data on two “single” Cu crystals, the sapphire crystal, cast Bi, and rolled samples of Ta and Cu have many narrow peaks ~ 10−3 eV wide. High resolution (0.3%) transmission measurements were made on the 1.057−eV resonance in 240Pu and the 0.433−eV resonance in 180Ta, both at room and low temperatures to study the effects of crystal binding. Although the changes in Doppler broadening with temperature were apparent, no asymmetries due to a recoilless contribution were observed.

Multi-Channel Mass-Separated Neutral Particle Energy Analyser for Simultaneous Measurements of Hydrogen and Deuterium Atoms Emitted from Tokamak Plasma

Abstract: A mass-separated neutral particle energy analyser able to carry out the simultaneous measurement of hydrogen and deuterium atoms emitted from a tokamak plasma was constructed The analyser was calibrated for the energy and mass separation in the energy range from 04 keV to 9 keV It was installed in the JFT-2 tokamak in order to investigate the behavior of deuterons and protons in a JFT-2 tokamak plasma heated by ion cyclotron wave and neutral beam injection It was found that the energy spectrum could be determined with sufficient accuracy, and the ion temperature obtained from the energy spectrum was in good agreement with the ion temperature deduced from the Doppler broadening of the impurity ion line

Comprehensive theory for line broadening in high-resolution and nonlinear spectroscopies: application to the Na D lines

Abstract: A general method for analysing line broadening caused by Doppler, phaseshift, inelastic and velocity-changing collisional effects is proposed for transitions at optical wavelengths. In a unified approach Poisson statistics are used to describe collision probabilities randomly distributed in a time interval which can terminate the optical coherence in a combination of the above effects; markoffian independence is assumed for each collision. Then the dipole moment correlation function is obtained in a form which is virtually exact in accounting for all effects of isotropic forces, and which is correct to second-order terms in the anisotropic interaction potentials. Phenomena as diverse as standard optical pressure broadening, broadening due to velocity-changing collisions in nonlinear spectroscopies, and Dicke narrowing of the Doppler contour are described. The implications of the theory are discussed in detail and applied to the case of the NaD lines perturbed by He buffer gas. Numerical calculations reveal that the substantially different lineshift and broadening parameters in nonlinear and in standard optical spectroscopies can be reasonably well understood within the limitations posed by present uncertainties in the Na 32P-He interaction potentials. However, the large differences in shift and width between the D2 and D1 lines in saturation spectroscopy cannot be explained within the present theory.

Scattering of an acoustic field by a free jet shear layer

Abstract: This paper deals with scattering of an acoustic field propagating through a large wind-tunnel jet shear layer. For a monochromatic wave, the scattering effects are mainly spectral broadening of the incident peak and amplitude and phase fluctuations of the wave. These effects have been studied both experimentally through a parametric study and theoretically by carrying out analyses based on single scattering methods. The experimental results are compared with predictions deduced from these analyses in which the data concerning the turbulence of the medium have been determined experimentally.

Spontaneous emission and the gas temperature in a pulsed copper vapor laser

Abstract: A study was made of spontaneous emission from a pulsed copper vapor laser, the radiation being recorded both along and at right angles to the axis of a discharge tube 1 cm in diameter and 70-cm long. Self-absorption was found to be the principal mechanism for broadening the spontaneous emission lines from the discharge. The self-absorption was very weak for lines with small oscillator strengths at the beginning of an emission pulse. The gas temperature was estimated from the Doppler broadening of the 540 nm neon line. It was found that a gas temperature of 2500 K was reached for an input power of 2.5 kW.

Observation of saturated absorption resonances in a molecular I2 beam subjected to separated fields

Abstract: The Doppler broadening of the absorption lines of the I2 molecules was suppressed by a combination of the saturated absorption and molecular beam methods. Saturated absorption resonances were observed at the components of the hyperfine structure of the P(13) and R(15) branches of the lΣ+g(X)→3ΠOU+(B) electronic transition in the I2 molecules subjected to 514.5 nm radiation from an argon laser. The optical fields were separated spatially by 3.5 cm along the molecular beam. The advantages of this method of inducing narrow resonances were considered from the point of view of their reproducibility.