Low Relative Velocities Research Articles

Electron cooling experiments at the heavy ion storage ring TSR

At low relative velocities the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to increase the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force, which was achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Moreover, electron cooling of very hot ion beams is usually rather slow. Two different methods were investigated to study the longitudinal cooling times. The first method uses an induction accelerator to push the ions into the effective velocity range of the longitudinal electron cooling force. In the second method the mean electron velocity is swept over the velocity distribution of the ion beam in order to decrease the collection time.

Role of Frictional Force on the Polishing Rate of Cu Chemical Mechanical Polishing

In the chemical mechanical polishing (CMP) of low-dielectric-constant (low-k) materials and Cu for ultralarge-scale integrated circuits, one of the major issues is the adhesion failure of the film caused by frictional force. An in situ quantitative measurement technique for frictional force during CMP has been developed. We have examined frictional force with respect to process parameters and slurries. A low frictional force and a high polishing rate (PR), which are desirable for a low-k/Cu CMP process, have been achieved simultaneously by optimizing slurry flow rate. The groove structures of the polishing pad did not affect frictional force at a low relative velocity. Frictional force and polishing rate increased with increasing glycine and H2O2 concentration in the slurry, and saturated at concentrations above 0.3 M and 3 vol%, respectively. A modified Preston's equation describing polishing rate as a function of frictional force instead of polishing pressure was proposed.

Dispersion Number Studies in CMP of Interlayer Dielectric Films

Determining the factors that cause flow nonidealities during chemical mechanical planarization (CMP) is critical for controlling and optimizing the process. This study explores aspects of the fluid dynamics of CMP on interlayer dielectric films. The residence time distribution of slurry under the wafer was experimentally determined and used to calculate the dispersion number of the fluid in the wafer-pad region based on a dispersion model for nonideal reactors. Furthermore, lubrication theory was employed to explain trends in flow behavior as operating conditions were varied. The results indicate that at low wafer pressure and high relative pad-wafer velocity, the slurry flow exhibits nearly ideal plug flow behavior. As pressure increases and velocity decreases, flow begins to deviate from ideal behavior and the slurry becomes increasingly more mixed beneath the wafer. These phenomena were found to be the result of variable slurry film thickness between the pad and the wafer, as measured by changes in the coefficient of friction between the pad and the wafer. © 2003 The Electrochemical Society. All rights reserved.

Evidence for subthermal rotational populations in stored molecular ions through state-dependent dissociative recombination.

We demonstrate that the dissociative recombination of D2H+ with low-energy electrons depends on the rotational energy of the molecular ion such that highly excited ions have a larger rate coefficient than colder ones. Observations on an ion beam continuously interacting with electrons at low relative velocity indicate that excited rotational levels are preferentially depleted which, in competition with radiative heating due to blackbody radiation, provides an opportunity for controlling the rotational temperature of stored molecules.

Dispersive electron cooling experiments at the heavy ion storage ring TSR

At low relative velocities, the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to transfer heat from the horizontal into the longitudinal degree of freedom, thereby increasing the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force which can be achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Operating the heavy ion test storage ring TSR at a moderate dispersion of D S =1.81 m , we investigated the dispersive electron cooling process and found good agreement with the expectations from fundamental considerations.

Scintillation Caustics in Planetary Occultation Light Curves

We revisit the GSC 5249-01240 light curve obtained during its occultation by Saturn's north polar region. In addition to refractive scintillations, the power spectrum of intensity fluctuations shows an enhancement of power between refractive and diffractive regimes. We identify this excess power as due to high-amplitude spikes in the light curve and suggest that these spikes are due to caustics associated with ray-crossing situations. The flux variation in individual spikes follows the expected caustic behavior including diffraction fringes, which we have observed for the first time in a planetary occultation light curve. The presence of caustics in scintillation light curves requires an inner scale cutoff to the power spectrum of underlying density fluctuations associated with turbulence. Another possibility is the presence of gravity waves in the atmosphere. While occultation light curves previously showed the existence of refractive scintillations, a combination of small projected stellar size and a low relative velocity during the event have allowed us to identify caustics in this occultation. This has led us to reexamine previous data sets, in which we have also found likely examples of caustics.

Influence of velocity in nanoscale friction processes

Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities, the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped; the viscous force is then predominant in the energy-dissipation process.

Star Formation History of ω Centauri Imprinted in Elemental Abundance Patterns

The star formation history of the globular cluster ω Centauri is investigated in the context of an inhomogeneous chemical evolution model in which supernovae induce star formation. The proposed model explains recent observations for ω Cen stars and divides star formation into three epochs. At the end of the first epoch, ~70% of the gas was expelled by supernovae. Asymptotic giant branch stars then supplied s-process elements to the remaining gas during the first interval of ~300 Myr. This explains the observed sudden increase in Ba/Fe ratios in ω Cen stars at [Fe/H] ~ -1.6. Supernovae at the end of the second epoch were unable to expel the gas. Eventually, Type Ia supernovae initiated supernova-induced star formation, and the remaining gas was stripped when the cluster passed through the newly formed disk of the Milky Way. The formation of ω Cen is also discussed in the framework of globular cluster formation triggered by cloud-cloud collisions. In this scenario, the relative velocity of clouds in the collision determines the later chemical evolution in the clusters. A head-on collision of protocluster clouds with a low relative velocity would have converted less than 1% of the gas into stars and promoted the subsequent chemical evolution by supernova-driven star formation. This is consistent with present observed form of ω Cen. In contrast, the other Galactic globular clusters are expected to have formed from more intense head-on collisions, and the resultant clouds would have been too thin for supernovae to accumulate enough gas to form the next generation of stars. This explains the absence of chemical evolution in these other globular clusters.

Ram Pressure Stripping in Planetary Nebulae

We present two-dimensional numerical simulations of the evolution of a low-mass star moving supersonically through its surrounding interstellar medium (ISM). We show that the ejecta of a moving star with a systemic velocity of 20 km s-1 will interact with the ISM and will form bow shock structures qualitatively similar to what is observed. We find that, owing to ram pressure stripping, most of the mass ejected during the asymptotic giant branch (AGB) phase is left downstream of the moving star. As a consequence, the formation of the planetary nebula is highly influenced, even at the low relative velocity of the star. The models are based on the predictions of stellar evolution calculations. Therefore, the density and velocity of the AGB and post-AGB winds are time dependent and give rise to the formation of shock regions inside the cavity formed by the previous winds. As a result, the stand-off distance is also time dependent and cannot be determined by simple analytical arguments.

Dispersion Number Studies in ChemicalMechanical Planarization

AbstractThis study explores aspects of the fluid dynamics of CMP processes. The residence time distribution of slurry under the wafer is experimentally determined and used to calculate the Dispersion Number (Δ) of the fluid in the wafer-pad region based on a dispersion model for non-ideal reactors. Furthermore, lubrication theory is used to explain flow behaviors at various operating conditions. Results indicate that at low wafer pressure and high relative pad-wafer velocity, the slurry exhibits nearly ideal plug flow behavior. As pressure increases and velocity decreases, flow begins to deviate from ideality and the slurry becomes increasingly more mixed beneath the wafer. These phenomena are confirmed to be the result of variable slurry film thicknesses between the pad and the wafer, as measured by changes in the coefficient of friction (COF) in the pad-wafer interface.

Tidal Effects in Clusters of Galaxies

High-redshift clusters of galaxies show an over-abundance of spirals by a factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative to the nearby clusters. This morphological evolution can be explained by tidal interactions with neighboring galaxies and with the hierarchically growing cluster halo. The efficiency of tidal interactions depends on the size and structure of the cluster, as well as on the epoch of its formation. I simulate the formation and evolution of Virgo-type clusters in three cosmologies: a critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model Omega_0=0.4 with a cosmological constant. The orbits of identified halos are traced with a high temporal resolution (~10^7 yr). Halos with low relative velocities merge only shortly after entering the cluster; after virialization mergers are suppressed. The dynamical evolution of galaxies is determined by the tidal field along their trajectories. The maxima of the tidal force do not always correspond to closest approach to the cluster center. They are produced to a large extent by the local density structures, such as the massive galaxies and the unvirialized remnants of infalling groups of galaxies. Collisions of galaxies are intensified by the substructure, with about 10 encounters within 10 kpc per galaxy in the Hubble time. These very close encounters add an important amount (10-50%) of the total heating rate. The integrated effect of tidal interactions is insufficient to transform a spiral galaxy into an elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in the low Omega_0 clusters.

Fragmentation of positronium in collision with He atoms.

The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.

Erratum: “Final-State Interaction in Multichannel Quantum Systems and Pair Correlations of Nonidentical and Identical Particles at Low Relative Velocities” [Physics of Atomic Nuclei 61, 2050–2063 (1998)

Erratum: “Final-State Interaction in Multichannel Quantum Systems and Pair Correlations of Nonidentical and Identical Particles at Low Relative Velocities” [Physics of Atomic Nuclei 61, 2050–2063 (1998)

小形工具による揺動速度制御研磨加工 第２報 高速・はみ出し加工時における形状

As ULSI feature dimensions continue to shrink, and the number of interconnect layers increases from 4 to 8, the resulting surface topography causes serious problems in device patterning. CMP has become a standard technology used in semiconductor manufacturing to planarize the surface topography. We have developed an oscillation-speed-control-type sequential grinding and polishing machine and have proceeded to simulate the polishing process. A previous paper showed theoretically that it is possible to polish within±1% uniformity with the optimum oscillation speed control. This paper presents removal rates measured under high relative velocity and high pressure, and a comparison between experiment and simulation for polishing profiles. It is shown that the removal rate begins to saturate gradually from low relative velocity and that Preston's equation can be used under high pressure caused when the tool overhangs from the wafer. In addition, the experimental result of profiles polished at normal and reverse rotations with disc and ring tools agreed well with the simulation result calculated with a modified equation of relative velocity under high velocity. However, it did not agree when the tool overhung the wafer by a significant amount.

Two-fragment correlation functions for quasiprojectile source and midrapidity component at intermediate energies

Two-fragment reduced-velocity correlation functions of intermediate-mass fragments emitted from midrapidity component and quasiprojectile (QP) sources formed in ${}^{58}\mathrm{Ni}{+}^{12}\mathrm{C}$ and ${}^{58}\mathrm{Ni}{+}^{197}\mathrm{Au}$ reactions at 34.5 MeV/nucleon have been studied. For the midrapidity component, they show a stronger Coulomb suppression at low relative velocities than for the QP source, suggesting a shorter emission time for it than for the QP source. Comparing the experimental correlation functions with the prediction of many-body Coulomb trajectory code, the emission times of a QP source formed in both reactions were extracted as a function of the excitation energy. The variation of emission time of the QP source with the excitation energy is independent of the reaction system. It decreases monotonically with the excitation energy in the range of $(2--6)A$ MeV from several hundred $\mathrm{fm}/c$ to about $100 \mathrm{fm}/c.$ Above an excitation energy of $6A$ MeV, it becomes very short and saturates, suggesting that the QP source undergoes a multifragmentationlike breakup. The influence of the quasitarget fragment Coulomb interaction on QP emission time is also considered.

Depth thresholds of motion parallax as a function of head movement velocity

The lower parallactic depth threshold is determined by (a) the ratio of relative image velocity to head velocity when the head moves fast (>13 cm/s) and (b) the motion threshold when the head moves slow (<13 cm/s). These two results are explained by a single system that codes the ratio of relative image velocity to head velocity, using the same image velocity signal as that used for motion perception. In this explanation, ratios coded from low relative image velocities, which are slightly higher than the motion threshold, produce a perception of depth only when the head moves slowly.

First positron cooling of antiprotons

Positrons are used to cool antiprotons for the first time. The oppositely charged positrons and antiprotons are first simultaneously accumulated in separate Penning trap volumes, and then are spatially merged in a nested Penning trap. The antiprotons cool until they reach a low relative velocity with respect to the cold positrons, the situation expected to be optimal for the production of cold antihydrogen.

Competitive accretion in embedded stellar clusters

We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi-Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity that results means that the tidal radius is smaller than the Bondi-Hoyle radius in these systems. In contrast, when the stars dominate the potential and are virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus Bondi-Hoyle accretion is a better fit to the accretion rates.

Rotor-stator interaction in a centrifugal pump equipped with a vaned diffuser

This paper provides the results of a detailed flow investigation within a centrifugal pump equipped with a vaned diffuser. The measurements made with a laser Doppler velocimeter were carried out at two operating points. Unsteady velocity measurements obtained in phase with the impeller angular position gave access to the relative flow within the impeller channels. Measurements were obtained in the impeller and the diffuser at different measuring planes relative to the diffuser vanes. Results are presented as animations reconstituting a temporal evolution of the flow permitting a better comprehension of the complex flow structure existing between the two interacting blade rows. The analysis of the relative flow field indicates the presence of a jet wake structure. The wake is characterized by low relative velocities localized in the suction side-shroud corner. At the design flowrate, the presence of the vanes seems to have a limited effect on the impeller flow structure, except when the suction side of the blades is facing the diffuser vanes. At the impeller discharge, the time-resolved sequences show that the mixing process of the unsteady and periodic flow leaving the impeller is affected by the presence of the diffuser.

Finger instability in radiatively driven dusty plasmas

In this work the dynamics of a three-fluid plasma consisting of electrons, ions and charged dust particles are considered. The plasma is illuminated by a radiation field, which, through absorbtion, accelerates the dust particles. By direct and Coulomb collisions the gained momentum is partially transmitted to electrons and ions. A steady state can be achieved by an external gravitational field acting against the radiation. After presenting the steady-state flow, linear perturbations of dynamical variables with the wave vector perpendicular to the radiation flux are considered. A linear analysis is performed by reducing the full three-fluid description to a suitable one-fluid system of equations, assuming the plasma to be quasineutral and omitting plasma species separation. It is demonstrated that when the drag force between ions and dust particles is a decreasing function of their relative velocity, the steady state is unstable against the formation of parallel layered streams with high and low relative flow velocities, i.e., multiple current sheets. The temporal evolution of this instability is then investigated by means of numerical simulations in the framework of a complete three-fluid model.