Virtual Point Source Research Articles

Control of the thermal lensing effect with different pump light distributions

We built a 32-laser-diode-formed virtual point source pumping system and achieved different pump light distributions from central intense to central uniform and central depressed. Continuous wave TEM(00) operations of a Nd:YAG laser were performed under these pump light distributions and their thermal lensing effects were estimated. Results show that the operation under central depressed pump light distribution has the lowest thermal lensing effect and can provide the highest output power, which agrees with the results derived from the theoretical calculation with the heat conduction equation.

Measurement and correction of ultrasonic wavefront distortion

Wavefront distortion produced by transmission through abdominal wall, breast, and chest wall has been measured in a two-dimensional aperture and the abdominal wall measurements have been compared to distortion simulated using sound speed and density determined from stained cross sections. The results show that wavefront distortion varies with tissue type, scattering contributes significantly to distortion, and arrival time and energy level fluctuations can be large compared to increments in current beamformers. Focus degradation and compensation has also been investigated by reconstructing the time history of a virtual point source without compensation, with time-shift compensation in the aperture, and with backpropagation followed by time-shift compensation. Backpropagation followed by time-shift compensation was found to be more effective than time-shift compensation in the aperture. A comparison of wavefront distortion and compensation in one- and two-dimensional apertures has shown that distortion is smoothed increasingly by one-dimensional apertures with larger elevation dimensions and that one-dimensional compensation becomes less effective than two-dimensional compensation for larger elevations. Pulse-echo studies using different f-number transmitters have shown that wavefront aberration is significantly underestimated with a broad incident beam and demonstrated that a sharply focused incident beam is necessary for good estimation of time shifts and for effective compensation.

Effective source size, yield and beam profile from multi-layered bremsstrahlung targets

Modern conformal radiotherapy benefits from heterogeneous dose delivery using scanned narrow bremsstrahlung beams of high energy in combination with dynamic double focused multi-leaf collimation and purging magnets. When using a purging magnet to remove electrons and positrons the target space is limited and unorthodox thin multi-layered targets are needed. A computational technique has therefore been developed to determine the forward yield and the angular distributions of the bremsstrahlung beam as well as the size and location of the effective and the virtual photon point source for arbitrary multi-layer bremsstrahlung targets. The Gaussian approximation of the diffusion equation for the electrons has been used and convolved with the bremsstrahlung production process. For electrons with arbitrary emittance impinging on targets of any multi-layer and atomic number combination, the model is well applicable, at least for energies in the range 1 - 100 MeV. The intrinsic bremsstrahlung photon profile has been determined accurately by deconvolving the electron multiple scattering process from thin experimental beryllium target profiles. For electron pencil beams incident on a target of high density and atomic number such as tungsten, the size of the effective photon source stays at around a tenth of a millimetre. The effective photon source for low-Z materials such as Be, C and Al is located at depths from 3 - 7 mm in the target, decreasing with increasing atomic number. The effective photon source at off-axis positions then moves out considerably from the central axis, which should be considered when aligning collimators. For high-Z materials such as tungsten, the location of the effective photon source is at a few tenths of a millimetre deep. The virtual photon point source is located only a few tenths of a millimetre upstream of the effective photon source both for high- and low-Z materials. For 50 MeV electrons incident on multi-layered full range targets the radial energy fluence distributions will have a full width at half maximum (FWHM) of 80 to 100 mm at 1 m from the target. The best target composition made of two layers when the space is limited to 15 mm was found to be 9 mm Be followed by 6 mm W. A thin beryllium target () results in a high-intensity bremsstrahlung lobe with a FWHM of about 35 mm at the isocentre. Interestingly, the forward dose rate in such a beam is as high as 62% of the maximum achievable with an optimal target design, even if on average only 1 MeV is lost by the electrons.

Scatter reduction in mammography with air gap.

Scatter reduction by air gaps in mammography was investigated. We have experimentally demonstrated that, independently of the imaging geometry, scatter in air-gap mammography can be well described by a virtual source of scatter (VSS) model. This model postulates that scatter radiation originates from a virtual point source of scatter placed on the central axis between the x-ray source and the exit surface of a patient at distance delta and utilizes only two parameters: delta and (S/P)0. The (S/P)0 parameter represents scatter-to-primary ratio without an air gap and delta is the distance from the exit surface of a patient to the virtual source of scatter. We have experimentally determined the analytical form of the two independent parameters of the VSS model; delta exhibits a linear increase proportional to the radiation field size, does not depend on patient thickness, and is in the 10-30 cm range, while (S/P)0 increases with the field size as a power function and is in the 0.4-1.3 range. In the framework of the VSS model the selectivity, the contrast improvement factor, and the signal-to-noise improvement factor were employed to evaluate performance of air-gap mammography systems. We have demonstrated that selectivity of an air gap rapidly deteriorates at some well-defined critical value of scatter fraction that has profound consequences on air-gap performance. Assuming fixed patient exposure, the results shows that, if a contrast limited detection system (such as film/screen mammography) is used, an air gap system can outperform a grid system only if a very large source-to-patient (SPD) distance is utilized, which might be possible with new laser-based x-ray sources. For the noise limited detection systems (such as digital mammography) even a small SPD (70 cm) and a small air-gap (20 cm) system will outperform a grid system.

Phenomenological description of the axisymmetric air-bubble plume

The axisymmetric air-bubble plume in water may be thought of as emanating from a virtual point source located at some distance below the real source. To give a phenomenological description of this system one has at one's disposal the equation of continuity, the balance equation for momentum, and the balance equation for kinetic energy. The most common way of approach so far has been to make use of the first two of these equations, together with the assumption that the rate of entrainment be proportional to the vertical centerline velocity. This is the theory developed, in particular, by Ditmars & Cederwall (1974). The present paper presents an alternative theory, in which the rate-of-entrainment assumption is abandoned and use is made of the kinetic energy equation together with the assumption that the most dominant component of the Reynolds stress be self-preserved. Very good agreement is found in this way with the large scale experiments of Kobus (1968) and Milgram (1983). The agreement is somewhat better than that found for the plane plume; the analogous theory for this case was developed by one of the present authors (I.B.). The conclusion of our analysis is that the kinetic energy approach stands out as a quite viable alternative for engineering applications in the axisymmetric case.

Continuous-wave TEM_00-mode 265-W-output virtual-point-source diode-array-pumped Nd:YAG laser

We describe a novel side-pumping configuration of the virtual-point-source diode-array-pumped Nd:YAG laser with 32 cw 10-W laser diodes. A new concept of the virtual point source and an image-relay system are proposed for the generation of a Gaussian-like pumping profile with an optically thin gain medium. A cw linearly polarized TEM00-mode, 26.5-W output was obtained at a diode pump power of 171 W. The M2 values at the x and y axes were measured to be Mx2=1.03 and My2=1.20, respectively.

Dynamic properties of human visual evoked and omitted stimulus potentials

Visual evoked potentials (VEPs) and omitted stimulus potentials (OSPs) are re-examined in scalp recordings from 19 healthy subjects. The principal finding is a distinction in form, latency and properties between OSPs in the conditioning stimulus range < 2 Hz, used in previous human studies, and those in the range > 5 Hz, used in previous studies of selected elasmobranchs, teleost fish and reptiles. We cannot find OSPs between 2 and 5 Hz. The high frequency (“fast,” ca. 6- > 40 Hz) and the low frequency (“slow,” ca. 0.3–1.6 Hz) OSPs have different forms and latencies but both tend to a constant latency after the omission, over their frequency ranges, suggesting a temporally specific expectation. Fast OSPs (typically N120, P170–230 and later components including induced rhythms at 10–13 Hz) resemble an OFF effect, and require fixation but not attention to the interstimulus interval. Slow OSPs (usually P500–1100) require attention but not fixation; they are multimodal, unlike the fast OSPs. Based on cited data from fish and reptiles, fast OSPs probably arise in the retina, to be modified at each subsequent level. We have no evidence on the origin of slow OSPs. In both ranges not only large, diffuse flashes, but weak, virtual point sources (colored LEDs) meters away suffice. They are difficult to habituate. Both require very short conditioning periods. The transition from the single, rested VEP to the steady state response (SSR) at different frequencies is described. Around 8–15 Hz in most subjects larger SSRs suggest a resonance. Alternation between large and small SSR amplitude occurs around 4 Hz in some subjects and conditions of attention, and correlates with an illusion that the flash frequency is 2 Hz or is irregular. Jitter of the conditioning intervals greatly reduces the slow OSP but only slightly affects the fast OSP. Differences between scalp loci are described.

Development of a numerical integration area source algorithm for industrial source complex long term (ISCLT) model

The Previous model evaluation studies have shown the deficiencies of the virtual point source algorithm for modeling area sources used in the ISCLT2 model. This paper documents the development, testing, and evaluation of a new numerical integration algorithm for modeling area source for the ISCLT2 model. This algorithm, which is based on the numerical integration for area source recently implemented in the ISCST2 model, allows users to handle the complex geometry of irregularly shaped area sources, and allows the calculation of the area source impact for receptors located within and nearby the area source. Detailed performance tests, statistical analyses, and sensitivity analyses have been completed to assure the reliability and reasonableness of the modeling results. The algorithm has been compared with the currently used ISCLT2 virtual point source algorithm, as well as with the numerical integration area source algorithm for the ISCST2 model. The results show that the new numerical integration ISCLT2 area source algorithm performs very well. Using idealized meteorological conditions, the new algorithm achieves very good comparison results when compared with the newly developed ISCST2 area source algorithm. It is also concluded that the currently used ISCLT2 area source algorithm based on the virtual point source approach underestimates the concentration value by a factor of about 2 or 4, especially when the receptors are located inside or near the source.

Revision of attenuation formula considering the effect of fault size to evaluate strong motion spectra in near field

We propose a new distance measure, Equivalent Hypocentral Distance (EHD), which is the distance from a virtual point source that provides the same energy to the site as does a finite-size fault. As EHD includes the effects of fault size, fault geometry, inhomogeneous slip distribution and station location, we can use a far-source attenuation formula to estimate near-source spectra with EHD. We investigated the 1979 Imperial Valley and the 1984 Morgan Hill earthquakes and we found that the attenuation of Fourier spectra in the range from far to near source can be interpreted by the attenuation of body waves predicted by the point-source theory with EHD. We also confirmed that EHD can be applied to the estimation of the maximum accelerations. For near-source data, it is worth emphasizing that EHD does not require any new regression coefficients except those used in far-source attenuation formula. Therefore, if we use EHD, we can estimate the near-source spectrum by using regression coefficients obtained from the far-source data.

Time-shift compensation of ultrasonic pulse focus degradation using least-mean-square error estimates of arrival time.

Focus degradation produced by abdominal wall has been compensated using a least-mean-square error estimate of arrival time. The compensation was performed on data from measurements of ultrasonic pulses from a curved transducer that emits a hemispheric wave and simulates a point source. The pulse waveforms were measured in a two-dimensional aperture after propagation through a water path and after propagation through 14 different specimens of human abdominal wall. Time histories of the virtual point source were reconstructed by removing the time delays produced by geometric path differences and also removing time shifts produced by propagation inhomogeneities in the case of compensation, finding the complex amplitudes of the Fourier harmonics across the aperture, calculating the Fraunhofer diffraction pattern of each harmonic, and summing the patterns. This process used a least-mean-square error solution for the relative delay expressed in terms of the arrival time differences between neighboring points and included an algorithm to determine arrival time differences when correlation based estimates were unsatisfactory due to dissimilarity of neighboring waveforms. Comparisons of reconstructed time histories in the image plane show that the -10-dB effective radius of the focus for reception through abdominal wall without compensation for inhomogeneities averaged 48% greater than the corresponding average effective radius for ideal waveforms, while time-shift compensation reduced the average -10-dB effective radius to a value that is only 4% greater than for reception of ideal waveforms. The comparisons also indicate that the average ratio of energy outside an ellipsoid defined by the -10-dB effective widths to the energy inside that ellipsoid is 1.81 for uncompensated tissue path data and that time-shift compensation reduced this average to 0.93, while the corresponding average for ideal waveforms was found to be 0.35. These results show that time-shift compensation yields a significant improvement over the uncompensated case although other factors must be considered to achieve an ideal diffraction limited focus.

Time-shift compensation of ultrasonic pulse focus degradation using least-mean-square error estimates of time delay

Focus degradation produced by abdominal wall has been compensated using least-mean-square error estimates of time shifts in measurements of ultrasonic pulses from a curved transducer that emits a hemispheric wave and simulates a point source. The pulse waveforms were measured in a two-dimensional aperture after propagation through a water path and after propagation through different specimens of human abdominal wall. Time histories of the virtual point source were reconstructed by removing the time delay produced by geometric path differences and including time shift produced by propagation inhomogeneities in the case of compensation, finding the amplitudes of the Fourier harmonics across the aperture, calculating the Fraunhofer diffraction pattern of each harmonic, and summing the patterns. This process used a least-mean-square error solution for the relative arrival time expressed in terms of the arrival time differences between neighboring points. Comparisons of the time histories in the source plane show that the −30 dB effective radius of the focus can be 90% greater for propagation through abdominal wall than through a water path and that this percentage is reduced to 37% by time-shift compensation.

Ultrasonic focus degradation produced by abdominal wall

Focus degradation produced by human abdominal wall has been studied experimentally using measurements of ultrasonic pulses from a special curved transducer that emits a hemispheric wave and simulates a point source. The pulse waveforms were measured in a two‐dimensional aperture after propagation through a water path and after propagation through five different specimens of human abdominal wall. The time history of the virtual point source was reconstructed from the measurements by removing the time delay produced by geometric path differences, finding the complex amplitudes of the temporal harmonics across the aperture, calculating the Fraunhofer diffraction pattern of each harmonic, and summing the patterns. Comparison of the time history in the plane of the virtual source reconstructed from signals that propagated through the abdominal wall specimens and the time history obtained from signals without the abdominal wall specimens present shows that the main features of the reconstructed source waveform are relatively unaffected by the presence of the human abdominal wall but that significant differences in signals greater than 40 dB down from the peak result from propagation through the specimens employed in this study.

Methods for estimating on-site ambient air concentrations at disposal sites

Currently, Gaussian type dispersion modeling and point source approximation are combined to estimate the ambient air concentrations of pollutants dispersed downwind of an areawide emission source, using the approach of virtual point source approximation. This Gaussian dispersion modeling becomes less accurate as the receptor comes closer to the source, and becomes inapplicable for the estimation of on-site ambient air concentrations at disposal sites. Partial differential equations are solved with appropriate boundary conditions for use in estimating the on-site concentrations in the ambient air impacted by emissions from an area source such as land disposal sites. Two variations of solution techniques are presented, and their predictions are compared.

Screening Models for Estimating Toxic Air Pollution Near a Hazardous Waste Landfill

This paper is directed to environmental scientists concerned with assessing toxic air pollution downwind of hazardous waste landfills to determine whether potential health threats or exceedances of air quality standards exist. The purpose of this paper is to evaluate the performance of four air quality screening models. The emission rate of vinyl chloride from the BKK co-disposal landfill in West Covina, California is estimated. Ambient vinyl chloride concentrations are estimated using a ground level point source model, two virtual point source models, arid the simple box model with meteorological and landfill input data representative of periods when ambient monitoring was conducted. The two virtual point source models are most precise and accurate in estimating 24-hour vinyl chloride concentrations. However, the results could include compensating errors in the emission rate and dispersion calculations because the emission rate estimate could not be independently evaluated.

A comparison between a plume path model and a virtual point source model for a stack plume

Some years ago we developed a theoretical model for the calculation of the rise of a stack plume in the atmosphere. Recently we have extended this model in such a way that the ground level concentration of gases emitted by a stack can also be calculated. The cross-section of the plume, which was originally assumed to be a circle, is now assumed to be an ellipse. The height-to-width ratio of the ellipse is chosen in accordance with full-scale data. The entrainment of air into the plume due to atmospheric turbulence can now be calculated not only for neutral atmospheric conditions, but also for stable and unstable conditions. Predictions of the ground level concentration made with this model have been compared with predictions made with the often used virtual point source model. For neutral and unstable atmospheric conditions the agreement is rather good. For stable conditions the differences can be rather large. The sensitivity of the ground level concentration with respect to the atmospheric stability condition, the stack height, the plume exit momentum flux and the plume exit buoyancy flux has also been investigated with the aid of the model. The influence of the plume exit momentum flux is negligible; however, the influence of the other parameters can be large.

Experiments on Turbulence Characteristics and Multiwavelength Scintillation Phenomena*

Measurements of atmospheric turbulence structure and multiwavelength scintillation statistics are described. The scintillation measurements use coincident virtual point sources, and include log-amplitude variances and covariances, spectra, and receiver-aperture smoothing. These are related to turbulence strength, spectral slope, and inner scale. The saturation of scintillations is found to be wavelength independent. The Kolmogorov atmospheric model breaks down under weak turbulence conditions, and hence the commonly used atmospheric and propagation theories tend to apply under mutually contradictory conditions. The transverse amplitude-correlation length and resultant receiver-aperture smoothing depart from theoretical predictions under strong scintillations. Scintillation spectra show much data spread but averages support the Taylor hypothesis. Short-path optical determinations of turbulence strength are seriously affected by nonzero inner scales of turbulence. Correlations of multiwavelength scintillations vs time indicate nonuniformity of both turbulence spectra and strength over the path.

Extruding Flow Characteristics of Solid-Liquid Mixture through Perforated Nozzle Plate

In this report, the extruding flow characteristics of solid-liquid mixture through perforated nozzle plate were investigated experimentally. Ram type extruders with nozzle plates, of which each has one to five circular holes in symmetrical arrangement, were used as the experimental equipments. The mixture of crushed silicate sand powder and lubricant oil was used as the sample of solid-liquid mixture.The relation between ram extruding pressure, P, and extruding velocity of the ram, V, are found to be linear with an apparent yield value of extruding pressure, Pya, as are shown in Figs. 3-6. It is also found that groups of flow lines have each of their virtual source point (Po, Vo), where all flow curves of a group meet together, as are shown in Figs. 3-6 when the number of hole, nN, is selected as a parameter. Using the ultimate limited value of extruding pressure, Po, and the virtual value of ram velocity, Vo, at the source point, we can obtain the expression of eq. 2, (V-Vo)=K(P-Po) … (2) of which one of the authors has proposed in the preceding report.The experimental results of Po and Vo are plotted against liquid content in Figs. 7 and 8, and the values of K are correlated with nozzle factors, total area of holes A and nozzle diameter DN, and the relation of eq. 9 is obtained, K/A2/3DNn=k31/μpl … (9) where the values of plastic viscosity μpl are measured with the viscometer shown in Fig. 10. In this, the factor of DNn shows the effect of nozzle diameter on the flowability of plastic flow by extrusion, and also reveals the fact that when the particle diameter is small enough, this mixture behaves as a fluid, but when it becomes larger, the phenomenon encountered in the discharging process of solid particles through orifice must be considered.The experimental equation of this extruding flow can be summarized in eq. 10 as follows (V-Vo)=k3nN2/3DN4/3+n/μpl(P-Po) … (10)

An experimental investigation of natural convection wakes above a line heat source

The natural convection wake arising from a heated horizontal line source in liquids and in air is investigated in detail. The three-dimensional effects for a wire with a length to diameter ratio of 250 is observed in air and water with a schlieren system. The temperature field in the plume above wires with length-diameter ratios of 250 and 1200 in liquid silicone ( Pr = 6·7) is determined using a 20 cm Mach—Zehnder interferometer. Various wire heating rates are used yielding Grashof numbers, based on the vertical distance in the plume, in the range from 4 × 10 3 to 1·7 × 10 6. Excellent agreement of the temperature distributions with theory are found for the larger length-diameter ratio wire at a Grashof number around 10 5. A regular natural swaying motion of the plume, observed at a high Grashof number, caused temperature fluctuations across the entire plume width. The applicability of the idea of a virtual line and point source is considered in detail. Discrepancies in the level of the plume centerline temperature distribution in this and in previous experimental work are attributed to three-dimensional effects of the finite diameter, non-infinite length line sources.