Velocity Filter Research Articles

The SHIPTRAP project: A capture and storage facility at GSI for heavy radionuclides from SHIP

SHIPTRAP is an ion trap facility which is being set up to deliver very clean and cool beams of singly-charged recoil ions produced at the SHIP velocity filter at GSI Darmstadt. SHIPTRAP consists of a gas cell for stopping and thermalizing high-energy recoil ions from SHIP, a rf ion guide for extraction of the ions from the gas cell, a linear rf trap for accumulation and bunching of the ions, and a Penning trap for isobaric purification. The physics programme of the SHIPTRAP facility comprises mass spectrometry, nuclear spectroscopy, laser spectroscopy and chemistry of transeinsteinium elements.

Alpha decay of the new isotopes 188,189Po

New neutron-deficient isotopes 188,189Po have been produced in the complete fusion reaction of 52Cr ions with a 142Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In 189Po a ground state to ground state α decay with Eα1= 7540(20) keV, T1/2= 5(1) ms and two fine structure α-decays at Eα2= 7264(15) keV and Eα3= 7316(15) keV have been observed. In 188Po (T1/2= 400+200 −150μs) a ground state to ground state α decay at Eα= 7915(25) keV and a fine structure α decay at Eα= 7350(40) keV have been found. Improved data on the α-decay of 189Bi were obtained.

Modeling the ionospheric effects of ion and electron precipitation in the cusp

The effect of increased ion production due to the precipitation of both ions and electrons in the cusp region has been studied using a computational model of the high‐latitude ionosphere. The modeling assumes that the cusp region is subject to fluxes of precipitating electrons which have a Maxwellian energy distribution at a temperature of 50 eV. In addition to the electrons the modeled cusp ionosphere is also subjected to precipitating ions with a Maxwellian energy distribution, at a temperature of 500 eV and a “low energy cutoff” for ions with an energy less than 900 eV. The results reveal both ion and electron precipitation to be important production sources in the cusp region, but at markedly different altitudes. Electron precipitation produces dominant effects at F2 peak altitudes of around 300 km while the ions, at higher energies, are capable of creating a precipitation “F1 ledge” at altitudes of around 150 km. A simulation of the ion dispersion with latitude, known as the “velocity filter effect,” reveals that this leads to large enhancements in lower‐altitude electron concentration on the equatorward edge of the cusp, the enhancement tailing off sharply in latitude.

Laser-free slow atom source

A slow atom source, which does not rely on lasers, has been developed and characterized. The device, acting as an atomic low-pass velocity filter, utilizes permanent magnets to passively select the slow atoms present in a thermal atomic beam. Slow atoms are guided along a curved, conduction-limited tube by an octupole magnetic field, while fast atoms, unable to follow the curved trajectory, strike the tube wall and are removed from the beam. The performance of the device is demonstrated by loading a magneto-optical trap. Approximately $2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ lithium atoms are loaded with a rate of $\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms/s, while maintaining a background gas pressure of $\ensuremath{\sim}{10}^{\ensuremath{-}11}$ torr. This loading technique provides an exceptionally simple, economical, and robust alternative to laser cooling methods.

A rotating torus phantom for assessing color Doppler accuracy

A rotating torus phantom was designed to assess the accuracy of color Doppler ultrasound. A thin rubber tube was filled with blood analog fluid and joined at the ends to form a torus, then mounted on a disk submerged in water and rotated at constant speeds by a motor. Flow visualization experiments and finite element analyses demonstrated that the fluid accelerates quickly to the speed of the torus and spins as a solid body. The actual fluid velocity was found to be dependent only on the motor speed and location of the sample volume. The phantom was used to assess the accuracy of Doppler-derived velocities during two-dimensional (2-D) color imaging using a commercial ultrasound system. The Doppler-derived velocities averaged 0.81 ± 0.11 of the imposed velocity, with the variations significantly dependent on velocity, pulse-repetition frequency and wall filter frequency ( p < 0.001). The torus phantom was found to have certain advantages over currently available Doppler accuracy phantoms: 1. It has a high maximum velocity; 2. it has low velocity gradients, simplifying the calibration of 2-D color Doppler; and 3. it uses a real moving fluid that gives a realistic backscatter signal.

Polarisation Analysis: What is it? Why do you need it? How do you do it?

Polarisation analysis quantitatively describes the particle motion of a seismic wavefield. It is the fundamental vector processing technique applied to multi-component seismic data. Synthetic and real VSP examples illustrate the application of polarisation analysis via particle-motion hodograms, and an automated single-trace time-domain method.Polarisation analysis can facilitate the extraction of pure P and S-wave sections, removal of unwanted noise events, and recovery of information relating to fracturing, porosity and lithology from multi-component data. Two applications of polarisation analysis are demonstrated, namely the extraction of pure P and S-wave sections, and S-wave splitting analysis.The success of polarisation analysis depends on the accuracy with which the recorded seismic wavefield represents the true vector wavefield. Care must be taken to ensure particle motion is not distorted during acquisition and preprocessing. Sensor response and coupling must be matched across components. Amplitude equalisation must be identical on all components. Frequency and/or velocity filtering should be designed to remove only noise, so as not to modify the underlying signal.The effect of the analysis window length is demonstrated, with the best compromise between resolution and stability being provided by a window approximately equal to the dominant period of the recorded signal. If more than one seismic event exists within the analysis window, single-trace polarisation analysis cannot accurately recover infoimation on the individual wave types. A comparison of polarisation analysis in the t-x and T-p domains highlights the value of utilising both slowness and polarisation information to enhance the accuracy of discriminating between interfering wave types.

Gated time-of-flight plasma composition analyzer for space physics research

We have constructed and tested a prototype instrument for space plasma physics research. The instrument is capable of measuring total ion velocity distribution functions as well as velocity distribution functions for individual mass-per-charge (m/q) species. The instrument consists of a deflection system, a toroidal energy-per-charge (E/q) analyzer, and a hemispherical condenser that serves either as the second stage of E/q analysis or as a gated time-of-flight velocity filter. The deflection system subtends a full 360° in azimuth and allows one to select ions for analysis in the toroidal section according to their elevation angle with respect to the axis of the instrument. The hemispherical condenser accepts a fraction of the ions transmitted by the toroidal analyzer through an electronic shutter at its entrance plane. With the shutter open, total ion distribution functions are measured. By appropriately gating the shutter and measuring the time of flight of ions introduced into the hemisphere, ion velocities are measured and m/q determined. The dimensions of the instrument have been chosen for measurements of the interplanetary solar wind at distances of 4–20 solar radii (Rsun) from the surface of the sun, consistent with the requirements of a solar probe mission. It can also be adapted to make measurements at 1 a.u.

Filtering coherent noise during prestack depth migration

We can often suppress short‐period multiples by predictive deconvolution. We can often suppress coherent noise with significantly different moveout by time‐invariant dip filtering on common‐shot, common‐receiver or NMO-corrected common‐midpoint gathers. Unfortunately, even time variant dip filtering on NMO-corrected data breaks down in the presence of strong lateral velocity variation where the underlying NMO correction breaks down. Underattenuated multiples, converted waves, and diffracted head waves can significantly impede and/or degrade prestack migration‐driven velocity analysis and amplitude variation with offset analysis as well as the quality of the final stacked image. Generalization of time‐variant dip filtering based on conventional NMO corrections of common‐midpoint gathers also breaks down for less conventional data processing situations where we wish to enhance data having nonhyperbolic moveout, such as converted wave energy or long‐offset P-wave reflections in structurally deformed anisotropic media. We present a methodology that defines a depth‐variant velocity filter based on an approximation to the true velocity/depth structure of the earth developed by the interpreter/processor during the normal course of their prestack imaging work flow. Velocity filtering in the depth domain requires the design and calibration of two new least‐squares transforms: a constrained least‐squares common offset Kirchhoff depth migration transform and a transform in residual migration‐velocity moveout space. Each of these new least‐squares transforms can be considered to be generalizations of the well‐known discrete Radon transform commonly used in the oil and gas exploration industry.

Properties of He+ beams observed by Geotail in the lobe/mantle regions: Comparison with O+ beams

In the lobe/mantle regions of Earth's magnetotail (&lt;210 RE), the Geotail spacecraft sometimes observed multicomposition ion flows consisting of both ionospheric (H+/He+/O+) and solar wind (H+/He++) ions. Statistics on the He+ beams show that they were observed under similar conditions to those of the O+ beams reported previously. Namely, the heavy ion beams of ionospheric origin tend to exist during geomagnetically active periods in the mantle‐like regions of high plasma beta, where ions of solar wind origin are the major component. The total duration of identified He+ events amounts to 3% of the total observation time in the lobe/mantle region, while that of O+ events amounts to 13%. A remarkable point is that on a short timescale, the He+ and O+ beams often appear to exist nearly exclusively. That is to say, their densities sometimes vary in an opposite sense, even when He+ and O+ coexist. If the O+ and He+ ions are given the same energy in a source region, the initial distribution function of He+ has twice the peak velocity of that of O+, and their alternating appearance and density anticorrelation in the lobe/mantle are easily explained due to the velocity filter effect. Thus the anticorrelation of their densities may suggest that ionospheric ions have undergone an energization which leads to a different velocity for different ion species. A mechanism leading to the same velocity, on the other hand, would require an alternating enhancement of the He+ and O+ fluxes in a source region to explain the anticorrelation of the densities.

Comparison of 2D filters for suppressing noise in common shot gathers

From the point of view of reflection signal fidelity, the best way of removing strong regular noise from a common shot gather is to estimate the noise and then subtract it from the data. This minimizes the distortion of the signal. The Karhunen±Loeive (K±L) transform is the basis of such a method, and may be used to extract high-amplitude coherent noise from common shot gathers acquired in seismic reflection surveys on land. In this paper we compare the results of filtering synthetic and real data examples in the frequency±wavenumber (f±k) domain, by hyperbolic velocity filtering in the intercept-slowness (tau-p) domain, and by extracting the noise using the K±L transform. The f±k and hyperbolic velocity filters cause serious distortion of the reflection signals, especially when the difference between the apparent velocities of signal and noise is small. In contrast, the K±L transform enables the noise to be extracted with minimal distortion of the reflection signals. It is a common experience in processing land seismic reflection data to be confronted with data containing weak reflection signals submerged by strong, regular noise such as groundroll or air waves. Although the stack-array approach to the choice of acquisition parameters is a powerful technique for suppressing these types of noise in processed sections, applications such as AVO analysis of the prestack data require that the noise be suppressed with minimum distortion of the reflection signals. For such prestack applications, the choice of filtering method to remove the noise is very important. Other types of source-generated noise, commonly observed in land data, are reverberant refraction events parallel to the first breaks. They are even more difficult to suppress than groundroll and airwaves because their apparent velocities are close to the apparent velocities of the reflection events at far offsets. We show synthetic examples to evaluate the performance of the three filtering methods when the apparent velocities of strong, regular noise are similar to, and different from, the apparent velocities of reflection signals. We then show a real data example which illustrates the superiority of the K±L transform method of noise extraction for suppressing the reverberant refracted arrivals.

Filter Performance and Velocity Distribution Relation in Magnetic Filtration of Non-Newtonian Liquids

Fluid flow regime and the velocity distribution are effective parameters for systems in which various transport phenomena take place. Besides the properties of the contaminating particles and filter system in the case of high gradient magnetic filtration, the rheological properties of the fluid are also important for both design and efficiency of the process. This paper presents a theoretical study about estimation of the velocity distribution in a magnetic filter and the dependence of filter performance on this distribution. A model is presented to estimate the velocity distribution and filter performance in magnetic filtration of Newtonian or weak non-Newtonian liquids. The model is essentially based on the balance of forces acting on particles captured and accumulated in the filter. Model predictions and experimental data given in the literature for Newtonian liquids are in a good agreement.

Quantification of tailward cold O+ beams in the lobe/mantle regions with Geotail data: Constraints on polar O+ outflows

Over a wide range of the tailward distances in the lobe/mantle regions (|XGSM′|≤210 RE), the Geotail spacecraft sometimes observed cold O+ beams (COBs) streaming tailward at a velocity nearly equal to that of the major proton component. The discovery of the ionospheric ions in the distant lobe/mantle leads us to a reexamination of the conventional view which expects these O+ ions to descend into the plasma sheet because of the E×B drift. In order to clarify the quantitative property of the COBs, we have projected COB parameters onto the polar ionosphere with a simple model which takes into account the velocity filter effect alone. The statistical data set from October 1993 to March 1995 in a solar‐minimum period is used in the calculation. The calculated average COB flux in the lobe/mantle corresponds to at least ∼108 cm−2s−1 O+ flux at the 1000‐km altitude. From the comparison with polar outflows, it is suggested that the cusp/cleft originating O+ ions sometimes need significant additional energization(s) so as to supply the COBs in the distant tail. When we assume the typical flux and temperature of polar O+ outflows, the required bulk energy of upward flowing ions (UFIs) turns out to be of the order of 3 keV, which is much higher than the typical energy of cusp/cleft outflows (&lt;1 keV). The energy difference between the tail lobe/mantle and the cusp/cleft O+ provides clues to the supply and/or energization mechanisms of the ionospheric ions. Calculations with individual COB events suggest that the rate of COB events in which the ionospheric O+ outflow flux exceeds 109 cm−2s−1 increases with the tailward distance, and it corresponds to 27% of all events in the distant tail of |XGSM′|≥150 RE.

Filtering non-solenoidal modes in numerical solutions of incompressible flows

SUMMARY Solving the incompressible Navier‐Stokes equations requires special care if the velocity field is not discretely divergence-free. Approximate projection methods and many pressure Poisson equation methods fall into this category. The approximate projection operator does not dampen high frequency modes that represent a local decoupling of the velocity field. For robust behavior, filtering is necessary. This is especially true in two instances that were studied: long-term integrations and large density jumps. Projection-based filters and velocity-based filters are derived and discussed. A cell-centered velocity filter, in conjunction with a vertex-projection filter, was found to be the most effective in the widest range of cases. © 1998 John Wiley & Sons, Ltd.

Improving seismic data quality in the Gippsland Basin (Australia)

Deep seismic exploration in the Gippsland Basin is hindered by strong noise below the Latrobe Group coal sequence. The reflectivity method provides a means for constructing detailed and accurate synthetic seismograms, often from little more than a partial sonic log. The noise contributions to the synthetics can then be interpreted using additional synthetics computed from variations upon the depth model and by exercising control over the wave types modeled. This approach revealed three types of persistent noise in progressively deeper parts of the subcoal image: (1) mode‐converted interbed multiples (generated within the coal sequence), (2) S-wave reflections and long‐period multiples (generated between the coal sequence and the Miocene carbonates), and (3) surface‐related multiples. The noise interpretation can also be performed upon semblance analyses of the elastic synthetics to guide a velocity analysis away from a well. This procedure helped to avoid picking the interformation long‐period multiples, whose stacking velocities were only 5 to 10% below those of the weak target zone primaries. An improve subcoal image was obtained by making full use of the versatile noise suppression offered by a τ-p domain processing stream. By separating the strong linear events at the far offsets, it is possible to stack a larger portion of the target zone reflections, provided hyperbolic velocity filtering (HVF) is applied to suppress the transform artifacts. Hyperbolic velocity filtering can be incorporated into a point‐source τ-p transform to suppress S-wave reflections and guided waves while preserving plane‐wave amplitudes to assist the subsequent deconvolution of the mode‐converted interbed multiples. Stacking in the τ-p domain is achieved using an elliptical moveout correction that reduces wavelet stretch and approximates the exact reflection traveltime better than NMO. Two regional seismic lines were reprocessed in this manner and cointerpreted with the modeling studies performed at nearby wells to avoid the noise events that still remained. Several new events appeared in the immediate target zone, passing the low‐frequency character expected following transmission through a coal sequence.

Modeling and observations of dayside auroral hydrogen emission Doppler profiles

We present a three‐dimensional, collision by collision model of the transport and energy degradation of energetic protons incident on the atmosphere. A test of the model was carried out using NOAA 12 satellite measurements of incoming energetic protons as input and simultaneous spectrometric measurements of the resulting dayside auroral hydrogen emission. There is a good match between the calculated and the observed emission profiles. The emission peak of the narrow and symmetric emission profile is positioned 4 Å to the blue of the unshifted line center, which is characteristic of the monoenergetic proton precipitation associated with the magnetospheric cusp/cleft, and the “velocity filter” resulting from dayside magnetic field line merging and convection. The intensities of the emission was reproduced using a proton number flux of approximately one‐half of that which was obtained by NOAA. The portion of the emission profile to the red of the unshifted line was well within the instrumental broadening. From this we conclude that the contribution of upward moving hydrogen atoms to the observed dayside H emission must be of the order of 10 percent.

Reconnection event at the dayside magnetopause on January 10, 1997

Geotail was at the magnetopause near the sub‐solar point when the magnetic cloud struck the earth's magnetosphere on January 10, 1997. Reeonnection between the southward IMF and the magnetospheric field lines occurred at the dayside magnetopause as observed by Geotail. The signatures of the reconnection were, i) accelerated plasma flow from the reconnection site which reached up to 500km/s, ii) ion heating through the reconnection process, and iii) a velocity filter effect seen in the ion distribution function in the magnetosheath boundary layer. This reconnection may be related to the large substorm onset observed at geosynchronous orbit.

Dayside auroral hydrogen emission as an aeronomic signature of magnetospheric boundary layer processes

Doppler spectral profiles of dayside auroral hydrogen show sudden increases in precipitating proton energy by a broadening of the spectral profile towards shorter wave‐lengths. This coincides with the equatorward motion of the dayside aurora. A model of the expected spectral character and spatial distribution of the H emission shows that the H emission spectrum is a direct measure of the reconnection process at the dayside magnetopause, i.e., 1) the narrow, symmetric spectral profiles are a measure of the magnetospheric “velocity filter”, 2) the sudden spectral shift of the H emission profiles reflect changes of ion energy due to an increase in the rate of reconnection, 3) spectral H emission observations could be used to map the spatial distribution of the precipitating ion energy, the total reconnection rate, the reconnection sites on the magnetopause, and the direction and intensity of convection.

Multiple suppression using a local coherence filter

Generally, when velocity filters are applied to prestack seismic data, they can suppress long‐period multiples, but they can also deteriorate the primary amplitudes. Since the apparent velocity differences between primaries and multiples can be negligible within the near‐offset regions of data gathers, a velocity filter will be ineffective in these regions, tending to remove or distort primary signal. The normal solution to this problem is to mute or remove near‐offset data, but this approach is detrimental to low‐fold data. In this paper, we derive a filter that can be used to address the problem by responding to average apparent velocity differences rather than to instantaneous apparent velocity differences between primaries and multiples. The filter, called a local coherence filter, is essentially a finite‐difference operator whose step size is equivalent to a spatial prediction distance. Within the local coherence filter’s small application window, an NMO-corrected multiple is predictable while an overcorrected primary is not predictable. The step size of its difference operator gives the filter the ability to discriminate between primaries and long‐period multiples in regions of a data gather where velocity filters fail. This paper derives the local coherence filter and compares it to the f-k filter in both model and real‐data applications. Results demonstrate that the local coherence filter is more effective in suppressing long‐period multiples without distorting the primary reflections.

Utility of the proximal color flow jet width in the assessment of regurgitant/stenotic offices — effect of low velocity filter and comparison to actual vena contractasize: a numerical study

Utility of the proximal color flow jet width in the assessment of regurgitant/stenotic offices — effect of low velocity filter and comparison to actual vena contractasize: a numerical study

Compact sputter source for deposition of small size-selected clusters

We report on the development of a caesium ion sputter source for the production of small mass-selected atomic clusters. It has been designed to be compact and portable, enabling it to be attached to ultrahigh vacuum (UHV) systems for use in deposition experiments. The sputter source, developed from a high energy (∼20 keV) atomic ion beam source, has been modified by introducing a beam extraction section, resulting in a cluster beam energy of 1.5 kV. This allows mass selection to be performed with a compact Wien velocity filter, greatly reducing the dimensions of the source. The source produces sufficient intensities of small clusters (1 – 15 atoms/cluster) for UHV deposition experiments, for example, a deposition rate of 0.002 monolayers/min for Al7− focused onto a 1 cm2 area has been obtained. It is possible to produce beams of many materials, especially the noble metals and carbon. We show mass spectra for Agn −, Aln− , and Cn− clusters.