Velocity Filter Research Articles

Bifurcated cusp ion signatures: Evidence for re‐reconnection?

Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) ion composition measurements during a pass of the POLAR spacecraft through the Earth's magnetospheric cusp show a bifurcated ion signature. High energy (several keV/e) solar wind ions are observed simultaneously with a low energy (up to several hundred eV/e) component. The high energy component exhibits an energy‐latitude dispersion consistent with magnetic reconnection at the dayside magnetopause and velocity filtering in the cusp. The low energy component exhibits no such dispersion. Simultaneous observations of the solar wind ion composition from the WIND spacecraft indicate that the high and low energy components in the cusp have a higher He2+/H+ density ratio than that in the solar wind. Additional solar wind data show that the IMF was southward and the solar wind plasma and magnetic field were relatively steady. These POLAR and WIND observations are interpreted as evidence for continuous re‐reconnection of magnetospheric field lines that thread the high latitude cusp.

Processing Through to Stack in the τ-ρ Domain

Improved seismic sections can be obtained by processing through to stack in the τ-ρ domain. This approach provides significant improvements over routine processing, using hyperbolic velocity filtering (HVF) to suppress the τ-ρ transform artefacts. HVF is a form of time and offset variant filtering that only allows each point in the t-x domain to contribute to a small number of p traces during a τ-ρ transform. HVF can be incorporated into a point-source τ-ρ transform, extending previous implementations that applied HVF during the slant stack. This assists gap deconvolution by providing plane-wave amplitudes in a τ-ρ gather where reverberations are exactly periodic along p traces and where S-wave reflections and long period multiples are suppressed. The τ-ρ domain also offers improved velocity analysis through the use of wide-angle reflections and because primary reflection ellipses never cross each other. The τ-ρ transform of a CMP gather can then be stacked with further advantages resulting from the elliptical moveout correction, which minimises wavelet stretch and approximates the exact reflection traveltime better than NMO. Examples from the Carnarvon and Gippsland Basins confirm that the cumulative theoretical advantages of τ-ρ domain processing are achieved in practice.

Concept of a high-resolution online mass separator for the Munich fission fragment accelerator

A fission fragment accelerator combined with the Munich high-flux reactor FRM-II is under design for the delivery of intense beams of mass separated very neutron-rich fission fragments with energies at the Coulomb barrier. They can be used to produce very heavy neutron-rich nuclei in fusion reactions. The large neutron excess will result in much longer lifetimes of the produced heavy elements, compared to neutron-deficient ones produced in reactions with stable nuclear beams. Thus fast α-chains can no longer be used to identify the heavy reaction products. A new separator will be used consisting of a velocity filter, an ion guide system (IGISOL) and a Penning trap. The velocity filter separates the primary beam and results in a strong background suppression. The IGISOL system improves the emittance and enriches the 1 + ions in the beam, which will then be efficiently transferred into a Penning trap. In the Penning trap the ions are bunched, stopped and mass separated with high resolving power ( R ≈ 10 5).

Discussion 2: On the convection and velocity filter

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.

GEOTAIL observation at the dayside magnetopause—Confirmation of reconnection events

Geotail has surveyed the dayside magnetopause in the equatorial plane and studied the dayside reconnection of the interplanetary magnetic field (IMF) with the magnetospheric field lines. When the IMF is directed southward leakage of ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) are observed. Reconnection is shown to be responsible for the leakage. A variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions is explained by the “velocity filter effect” model with a finite source region. In addition to such conditions, reconnection occurs also when the IMF has low-inclination angles, i.e., the condition between the southward and northward IMF conditions. Between the magnetosphere and the magnetosheath, two types of boundary region develop, i.e., the inner-LLBL and the outer-LLBL. The inner-LLBL is characterized by the bi-directional cold electrons and trapped cold ions coexisting with hot magnetospheric plasma. The field lines are regarded to be closed. The outer-LLBL is, on the other hand, open to the magnetosheath. It is characterized by the uni-directional cold electrons escaping to the magnetosheath. Newly penetrating solar wind ions are overtaking the trapped cold ions. The formation of the outer-LLBL is explained by the high-latitude reconnection at the equatorward regions of the cusp.

On the origins of the upward shift of elevated (bimodal) ion conics in velocity space

We present a statistical study of elevated (bimodal) ion conics observed by the Exos D (Akebono) satellite from 0900 to 1500 magnetic local time (MLT). We especially focus on a comparative analysis of elevated conics with standard conics and ion beams. Elevated conics are observed mainly above 6,000 km, while standard conics begin to appear at lower altitudes. The energy of elevated conics is usually higher than that of standard ion conics. This is consistent with the idea that the elevated conics evolve from standard conics through some acceleration process while they flow up to high altitudes. Since the elevated conics studied here received additional energization as they evolve from standard conics, the velocity filter mechanism previously invoked to explain elevated conics is inconsistent with our results. Our observation suggests that two kinds of acceleration processes are responsible for the upward shift in the velocity distribution of elevated conics. The elevated conics with a large cone angle and small upward shift in velocity space are most likely to be caused by the perpendicular energization extended along the field line. The MLT occurrence of this subset of elevated conics is similar to that of standard conics. The ion conics with a cone angle near 60° are found to be the most energetic, which strongly suggests the extended energization. On the other hand, the elevated conics with a small cone angle and large upward shift are mainly due to the parallel acceleration acting in tandem with perpendicular energization. This subset of elevated conics is found more often in the afternoon sector than are standard conics, which evokes rather some similarity to ion beams.

A recoil separator for the measurement of radiative capture reactions

A recoil separator in combination with a windowless gas target has been designed for the measurement of the radiative capture reaction p(7Be, γ)8B. The separator consists of momentum and velocity filters and a ΔE−E detector telescope. The setup was tested quantitatively using the p(12C, γ)13N reaction at the effective energy Ecm = 841 keV. Projectile fluxes were measured directly with Faraday cups and indirectly with elastic backscattering into Si detectors, while the 13N recoil flux was measured directly with the ΔE−E telescope. A suppression of the 12C beam particles by a factor 2 × 10−10 was observed when the system was tuned for the recoil 13N's. Special emphasis was given to the charge state probabilities of the 13N recoils. Possible improvements of the system for the measurement of other capture reactions are discussed.

Electric field structure and ion precipitation in the polar region associated with northward interplanetary magnetic field

We investigated electric field and charged particle data of the EXOS D (Akebono) satellite and studied magnetospheric plasma flow and its relation with particle precipitation in the dayside polar region during northward interplanetary magnetic field. Sunward flow characterizes the convection in this region and suggests that reconnection takes place at the high‐latitude magnetopause. Sheath ion precipitation is often observed associated with the sunward flow. When it shows energy dispersion caused by the velocity filter effect, it has noteworthy characteristics: (1) the ion precipitation region has a long scale length (>200 km at the 120‐km altitude) along the flow, and (2) the upstream edge of this region is located at latitudes of >80°. We consider that ion precipitation with energy dispersion is a characteristic phenomenon when the Alfven Mach number MaA. is high at the reconnection site, and we propose a new interpretation for the transportation of sheath plasma into the magnetosphere during high MaA. In this model the injection of the sheath ion occurs on the field lines that have penetrated into the magnetosphere after the reconnection. When the sheath ions associated with the sunward flow are observed at latitudes of <80°, on the other hand, the length of the ion precipitation region along the flow is generally short (100–150 km). The signature of these ions indicates that they have entered through the entry layer.

Generation of a fast atomic-oxygen beam from O− ions by resonant cavity radiation

An apparatus has been developed for producing a beam of ground-electronic-state oxygen atoms with energies variable from 4 to 1000 eV with a 1.5 eV FWHM energy distribution. The technique involves extraction of negative ions from a low-voltage gas-discharge source, mass selection of the extracted O− with a Wien-type velocity filter, O− acceleration or deceleration and focusing by electrostatic ion optics, and electron detachment from O− by intracavity laser radiation. A 25 W argon-ion-laser cavity has been extended to include the ion-beam vacuum chamber so that the intracavity radiation intersects the O− ion trajectories normally. Depending on the laser configuration in use, ion-neutralization efficiencies between 5% and 25% have been achieved at 5 eV O− energy. Thus, 5 eV O-atom fluxes of ∼1011 atoms/s (∼1012 atoms/cm2 s) have been achieved for O− currents of ∼10−7 A. The advantages and limitations of the technique are discussed, and preliminary measurements of the secondary-negative-charge production from low-energy O-atom impact on copper and stainless-steel surfaces are presented.

Pulsed laser deposition of thin InxGa1-xAs films

An investigation was made of the distribution of the velocities of the droplets and of the atomic component in a plasma plume formed during laser ablation of InAs and GaAs semiconductor targets. The velocity of atoms was at least 6 times greater than the velocity of droplets. A mechanical velocity filter for the removal of droplets from the plasma plume was constructed. The use of a composite target made it possible to grow epitaxial InxGa1-xAs films of different compositions.

Leakage Ions from the LLBL to MSBL: Confirmation of Reconnection Events at the Dayside Magnetopause.

This letter reports on leakage ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) as observed by Geotail at the dayside magnetopause. A reconnection of the interplanetary magnetic field (IMF) with magnetospheric field lines is shown to be responsible for the leakage. Based on analysis of a variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions, we propose a “velocity filter effect” model with a finite source region to explain this variation. The location of the reconnection region is subsequently calculated to be 2.2RE north of the geomagnetic equatorial plane under the proposed velocity filter effect model.

Ion-implantation system for near surface studies

An ion-implantation system designed and fabricated indigenously is described. It consists of a versatile ion source, Einzel lens system, EXH velocity filter, dose measuring equipment, and target chamber. The ion-implantation system fabricated has been used to prepare ion-implanted luminescent phosphors by implanting Sb+ ions into CaS.

Point target detection in consecutive frame staring infrared imagery with evolving cloud clutter

The problem of detection of aircraft at long range in a background of evolving cloud clutter is treated. A staring infrared camera is favored for this application due to its passive nature, day/night operation, and rapid frame rate. The rapid frame rate increases the frame-to-frame correlation of the evolving cloud clutter; cloud-clutter leakage is a prime source of false alarms. Targets of opportunity in daytime imagery were used to develop and compare two algorithm approaches: banks of spatio-temporal velocity filters followed by dynamic-programming-based stage-to-stage association, and a simple recursive temporal filter arrived at from a singular-value decomposition analysis of the data. To quantify the relative performance of the two approaches, we modify conventional metrics for signal-to-clutter gains in order to make them more germane to consecutive frame real data processing. The temporal filter, in responding preferentially to pixels influenced by moving point targets over those influenced by drifting clouds, achieves impressive cloud-clutter suppression without requiring subpixel frame registration. The velocity filter technique is roughly half as effective in clutter suppression but is twice as sensitive to weak targets in white noise (close to blue sky conditions). The real-time hardware implementation of the temporal filter is far more practical.

Ground roll and polarization

Ground roll is the major source of coherent noise on land seismic data. It is current practice to attenuate ground roll through the use of geophone arrays in the field, and by velocity filtering during processing. However, data recorded using multi-component geophones provide an additional means of attenuating the ground roll through polarization filtering. The basis of polarization filtering for roll attenuation is to utilize the phase difference between the components. Body waves are assumed to exhibit linear motion (in phase) whilst ground roll is assumed to exhibit elliptical motion (out of phase).

Passive sonar detection and localization by matched velocity filtering

This paper presents a new bearings-only method of detecting and tracking low signal-to-noise ratio (SNR) wideband targets on a constant course and velocity trajectory. A track-before-detect strategy based on matched velocity filtering is adopted using spatial images constructed from a sequence of power bearing map (PBM) estimates accumulated during a track. To lower the threshold SNR for detection, a discrete bank of matched velocity filters integrates the PBM images over a range of hypothesized trajectories, such an approach eliminates the need to estimate the number of targets since signal detection is determined by comparing the output of each matched filter (MF) to a decision threshold. The distribution of the MF output is derived based on a single point target in diffuse noise assumption. Receiver operating characteristic curves show a definite detection gain under low SNR conditions for matched velocity filtering (track-before-detect) over detection from a single PBM.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interacting acceleration compensation algorithm for tracking maneuvering targets

The two-stage Kalman estimator has been studied for state estimation in the presence of random bias and applied to the tracking of maneuvering targets by treating the target acceleration as a bias vector. Since the target acceleration is considered a bias, the first stage contains a constant velocity motion model and estimates the target position and velocity, while the second stage estimates the target acceleration when a maneuver is detected, the acceleration estimate is used to correct the estimates of the first stage. The interacting acceleration compensation (IAC) algorithm is proposed to overcome the requirement of explicit maneuver detection of the two-stage estimator. The IAC algorithm is viewed as a two-stage estimator having two acceleration models: the zero acceleration of the constant velocity model and a constant acceleration model. The interacting multiple model (IMM) algorithm is used to compute the acceleration estimates that compensate the estimate of the constant velocity filter. Simulation results indicate the tracking performance of the IAC algorithm approaches that of a comparative IMM algorithm while requiring approximately 50% of the computations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recent experiments with radioactive ion beams at GSI

Abstract A review is given over recent experiments performed by means of A·keV to A·GeV radioactive ion beams at GSI Darmstadt. Nuclear structure information on exotic nuclei is discussed as obtained by using an online isotope separator, a velocity filter or a magnetic beam-line spectrometer.

Pulsed-laser deposition of smooth high-T c superconducting films using a synchronous velocity filter

Pulsed-laser deposition of smooth high-Tc superconducting films almost free from droplets and precipitates with the use of velocity filtration of plasma particles is reported. We have removed droplets from laser-induced plasma by using a shutter technique; a reduction of the droplet density by a factor of 105 has been achieved. We have applied the technique to the preparation of high quality YBa2Cu3O7−δ films on (100)-oriented SrTiO3, MgO, Y2O3-stabilized ZrO2 (YSZ) substrates and, furthermore, on (11̄02)-oriented sapphire covered with (100) sublayers of Si and (100) YSZ buffer layers.

Dereverberation of marine reflection seismic data by a spatial combination of predictive deconvolution and velocity filtering

Contamination of seismic reflection records at early times by first-order water reverberations can be especially severe during survey operations over hard and flat sea floors on the continental shelf or in lake environments. A new dereverberation scheme based on two classical techniques — predictive deconvolution and velocity filtering — has been developed to address this problem. The techniques are combined spatially to take advantage of their complementary offset- and time-dependent properties. Stage I of the scheme consists of applying predictive deconvolution at short offset. The data are previously conditioned by a normal moveout correction with the water velocity which restores the periodicity of the reverberations in the offset-time plane and enhances the performance of deconvolution. Stage II of the scheme involves velocity filtering in the common-midpoint domain which is particularly effective at long offset where the moveout difference between primary reflections and reverberations is largest. The dereverberation scheme is well suited for the initial processing of large volumes of data due to the general availability of cost-effective deconvolution and velocity filtering algorithms in seismic processing software packages. Practical implementation issues are illustrated by a field example from the GLIMPCE survey in Lake Superior.

?Slow? residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesARes>Atar,ZRes>Ztar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions20Ne+208Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/νCN≅0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ≅23% (at E/A=8.6 MeV/u) and ≅8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.