Showing papers on "Critical ionization velocity published in 1989"

Velocity versus temperature relation for solidification and melting of silicon: A molecular-dynamics study.

Abstract: We report on molecular-dynamics simulations to determine the steady-state velocity versus temperature relation for (001) solidification and melting of silicon using the Stillinger-Weber potential to model the interactions between the silicon atoms. Down to 250 K of undercooling, the simulation values show good agreement with experimental results. The slope of the temperature velocity relation near the melting point is reported as (-15\ifmmode\pm\else\textpm\fi{}5 K)/(m/s) whereas we find the slope of the transition-state curve that we fitted to the simulation values has a slope (-9.8 K)/(m/s) at the melting point.However, since Stillinger-Weber amorphous silicon is not formed easily by cooling the melt our simulations do not show the growth into amorphous silicon at the critical velocity of \ensuremath{\approxeq}15 m/s (undercooling below \ensuremath{\approxeq}250 K) that is observed experimentally. Instead in our simulations the velocity of growth into crystalline silicon increases to a maximum of 19.4 m/s at 1350 K and then decreases to at least 1050 K. At 1000 K, the system grows a few incomplete planes, which we suggest may be associated with the growth of amorphous silicon. At 950 K, we did not detect any growth during the 115 ps of the simulation. We also did not observe growth at 1600 or 1700 K over the 115 ps of these simulations. The simulation velocity versus temperature relation shows asymmetry in the solidification and melting portions of the curve with no discontinuity in slope at the melting point. We fitted our simulation velocity versus temperature relation by using the transition-state theory parametrized to Stillinger-Weber silicon. The transition-state theory gives a good qualitative description of the simulation values at all temperatures.It is interesting that the transition-state theory also fits the experimental values for crystallization even though it is parametrized for Stillinger-Weber silicon. The results of these epitaxial-growth studies of silicon are important for studies attempting to model the growth of layered structures by atom deposition with use of molecular dynamics with the Stillinger-Weber potential. Our results show that, even if the deposited atoms have kinetic energies 60% below the melting temperature, the growth of the crystal may be caused by liquid epitaxial growth with one (001) plane grown every 10 ps.

Nonstationary Vibration of a Rotating Shaft with Nonlinear Spring Characteristics During Acceleration Through a Critical Speed : A Critical Speed of a 1/2-Order Subharmonic Oscillation

Abstract: This paper deals with the nonstationary oscillations of a flexible rotating shaft with nonlinear spring characteristics. In particular, we investigate a phenomenon during constant acceleration and deceleration passing through a critical speed of a 1/2-order subharmonic oscillation of forward precession. In numerical simulations, we examined the influence of the angular acceleration λ and the initial angular position ψ0 of a rotor unbalance on the maximum amplitude of the subharmonic oscillation. As a result, the following points are clarified: (1) the maximum amplitude depends markedly on λ and ψ0; (2) in order to always pass through this critical speed with finite amplitude during acceleration, an angular acceleration greater than a certain value λ0 is necessary; and (3) when the angular acceleration is less than this critical value λ0 (0<λ<λ0), the shaft's ability to pass through this critical speed depends on ψ0. We ascertained the validity of these theoretical results by experiments.

Barrier failure by critical accumulation of viscous oil

Abstract: A barrier may fail to contain an oil slick because of drainage failure, droplet entrainment failure, or critical accumulation Drainage and entrainment failure are well known The present paper introduces the phenomenon of critical accumulation, that is, the unstable reduction of the slick to an infinitely small length when a layer of highly viscous oil is exposed to a relative oil-water flow velocity exceeding a certain critical velocity Uca The reduction to an infinitely small slick length causes all the oil to pass underneath the barrier independent of the barrier draft Experiments on barrier failure were performed in two laboratory flumes using a wide range of oil types and oil viscosities Critical accumulation always occurred with oil viscosities v0 ≤ 3,000 cs The critical relative velocity was Uca ≃ 015 m/s for oil with 3,000 cs < v0 < 20,000 cs, with a slight increase in Uca for higher viscosities Uca is independent of other oil and hydrodynamic parameters The findings of the study

An overview of atomic and molecular processes in critical velocity ionization

Abstract: The authors present an overview of the time development of some atomic and molecular processes in critical ionization velocity (CIV). In the preonset stage, metastable states play an important role: they provide an energy pooling mechanism allowing low-energy electrons to participate in the ionization process; they may explain the low energy threshold was well as the fast time scale in the onset of CIV. For a sustaining CIV to occur, Townsend's criterion has to be satisfied. The kinetic energies of the neutrals are transformed to plasma wave energies via beam-plasma instabilities, and the plasma waves that heat the electron result in a tail formation. Excitation of neutrals with subsequent radiation is an important energy loss mechanism. Finite size also limits the instability growth rate. In the propagation of CIV, ion-molecule reactions and molecular dissociative recombination are important. >

Velocity-dependent dissipation from free vortices and bound vortex pairs below the Kosterlitz-Thouless transition.

Abstract: We have developed a model for the onset of nonlinear dissipation in thin superfluid helium films below the static transition temperature ({ital T}{sub KT}) by extending the linear response theory of Ambegaokar, Halperin, Nelson, and Siggia to include flow velocity. Contributions from both vortex pairs and free vortices are included, the relative contributions of which are controlled by two weakly coupled adjustable parameters: the vortex diffusivity {ital D} and a free vortex creation time {tau}{sub {ital f}}. The model does not predict a critical velocity in the usual sense except at {ital T}=0; however, we define a characteristic velocity for the onset of nonlinear dissipation to compare with experiments. At {ital T}=0 we find a critical velocity given by the Feynman criterion, where the frequency-dependent diffusion length coincides with the zero in the vortex pair energy. Applications of this model to experiments with ac and dc flows are discussed.

Local Scour at Vertical Piles under Wave Action

Abstract: Scour around vertical piles under wave action in uniform, cohesionless sediment was investigated experimentally. Experimental data show that the development of scour hole starts when the maximum horizontal velocity near the bed exceeds half the critical velocity for sediment entrainment. The relative equilibrium scour depth is a function of both the horizontal bed velocity and excursion length.

Laboratory simulation of cometary neutral gas ionization

Abstract: The laboratory simulation of the interaction of the solar wind with a comet is used to study the cometary neutral gas ionization. The experiment is carried out in the UCR T-1 facility with an ice ball as the comet model. Photographs and data are taken with a variety of values of the solar wind velocity, interplanetary magnetic field (IMF), and comet configurations. The results show that the cometary neutral gas ionization depends on both the velocity of the solar wind and the interplanetary magnetic field. The plasma cloud surrounding the comet is visible only when the solar wind velocity and IMF are both above certain minimum values. This velocity dependent phenomena is explained by Alfven's critical ionization velocity effect. The critical magnetic field may be explained by assuming two stream lower hybrid instability as a triggering mechanism for the ionization of the neutral gas by plasma flow. Critical upper and lower limits for the magnetic field, required by anomalous ionization, are also derived that satisfy the experimental observations.

Pulsating slurry flow in pipelines

Abstract: An experimental study on pulsating turbulent flow of sand-water suspension was carried out. The objective was to investigate the effect of pulsating flow parameters, such as, frequency and amplitude on the critical velocity, the pressure drop per unit length of pipeline and hence the energy requirements for hydraulic transportation of a unit mass of solids. The apparatus was constructed as a closed loop of 11.4 m length and 3.3 cm inner diameter of steel tubing. Solid volumetric concentrations of up to 20% were used in turbulent flow at a mean Reynolds number of 33,000–82,000. Pulsation was generated using compressed air in a controlled pulsation unit. Frequencies of 0.1–1.0 Hz and amplitude ratios of up to 30% were used. Instantaneous pressure drop and flow rate curves were digitized to calculate the energy dissipation associated with pulsation. The critical velocity in pulsating flow was found to be less than that for the corresponding steady flow at the same volumetric concentration. Energy dissipation for pulsating flow was found to be a function of both frequency and amplitude of pulsation. A possible energy saving was indicated at frequencies of 0.4–0.8 Hz and moderate amplitudes ratios of less than 25%.

Magnetic field-aligned plasma expansion in critical ionization velocity space experiments

Abstract: The temporal evolution of a plasma cloud released in an ambient plasma is studied. Time-dependent Vlasov equations for both electrons and ions, as well as the self-consistent electric field parallel to the ambient magnetic field, are solved. The initial cloud is considered to consist of cold, warm, and hot electrons with temperatures of approximately 0.2 eV, 2 eV, and 10 eV, respectively. It is found that the minor hot electrons escape the cloud; their velocity distribution function shows the typical time-of-flight dispersion feature, i.e. the average drift velocity of the escaping electrons is proportional to the distance from the cloud. The major warm electrons expand along the magnetic field lines with the corresponding ion-acoustic speed. The combined effect of the escaping hot electrons and the expanding warm ones sets up an electric potential structure that accelerates the ambient electrons into the cloud. Thus, the energy loss due to the electron escape is partly replenished. The electric field distribution in the potential structure depends on the stage of the evolution; before the rarefaction waves propagating from the edges of the cloud reach its center, the electric fields point into the cloud. After this stage the cloud divides into two subclouds, each having its own bipolar electric field. The effects of collisions on the evolution of plasma clouds are also discussed. The relevance of the results seen from the calculations are discussed in the context of space experiments on critical ionization velocity. >

The role of convective flow processes in determining plasma armature characteristics

Abstract: As railgun performance proceeds toward higher velocity ranges and/or more exotic geometries, entrainment of the armature in turbulent boundary layers at the wall or differential transverse force loading on the armature can lead to the development of convective flow within the armature. The effects of such flow were investigated and were shown to give significant departure from the usual zero-flow models. Specifically, such effects lead to: (1) peaking of the armature current toward the projectile/armature interface at sufficiently high projectile velocities, in agreement with B/sub z/ measurements; (2) a possible upper limit on total armature mass; and (3) transition to an armature resistance that is relatively independent of resistivity but is dependent on the bulk convection velocity. >

The influence of the multiply charged ion velocity on electron emission

Abstract: As is well established experimentally the electron emission yield γ q is directly proportional to the total neutralization energy W q of multiply charged ions interacting with a solid. If one increases the ion velocity γ q still scales with W q but the slope ( k ) changes. We study the velocity dependence of k , k ( v ), and relate it to a simple model for the ion neutralization. This enables us to define a critical velocity v c , to set the velocity-scale, and to extract it from experimental results.

Group velocity measurement from the propagation of the ionization front in a surface-wave-produced plasma

Abstract: During the first instant, previous to steady-state in a surface-wave-produced plasma, an ionization front advance front the launcher to the plasma column end. The velocity of the ionization front is much slower than the group velocity of the surface wave, this give a reflection of the incident signal on the moving ionization front. In this paper, we use this effect to calculate the surface wave group velocity.

The plasma universe

Abstract: The term "Plasma Universe", coined by Hannes Alfven, emphasizes the fact that plasma phenomena discovered in the laboratory and in accessible regions of space, must be important also in the rest of the universe, which consists almost entirely of matter in the plasma state. Relevant aspects of this concept will be discussed. They include the response of the plasma to electric currents, the support of magnetic-field aligned electric fields, violation of the frozen-field condition, rapid release of magnetically stored energy, acceleration of charged particles, chemical separation, filamentary and cellular structures, and critical velocity interaction.

Collisional and wave-particle interactions in critical velocity ionization

Abstract: Alfven's critical ionization velocity (CIV) process involves collective and collisional interactions of a magnetoplasma streaming through a neutral gas. Numerical simulations of CIV are reported using particle-in-cell plasma codes including various collisional interactions. Fast electron heating is observed. The interplay between collisional and collective interactions renders the hot electron tail shorter than in non-CIV situations with collisionless wave-particle interactions without a neutral gas. Metastable states serve in the role of energy pooling and foster rapid ionization near the critical velocity. Line excitation is a severe energy loss mechanism. Charge exchange replenishes ion energy and is important during tight energy budget situations near the critical velocity.