Showing papers on "Drift velocity published in 1990"

Fluctuation spectrum and transport from ion temperature gradient driven modes in sheared magnetic fields

Abstract: The ion temperature gradient driven mode or ηi‐mode turbulence is reinvestigated based on two‐component compressible fluid equations with the polarization drift velocity and adiabatic electrons. The scaling of the anomalous ion heat conductivity with magnetic shear s=Ln/Ls and the excess of ηi over the critical value ηi,c for marginal stability is found to vary as χi=g(ρs/Ln)(cTi/eB) (ηi−ηi,c)exp(−αs), where g≂1 and α≂5.

Electron lifetime detector for liquid argon

Abstract: A method has been developed for a fast and reproducible measurement of the electronegative contamination of liquid argon. The method is based on the transport of electrons extracted by a NdYag UV laser beam from a metallic photocathode. The transit time of the electrons from the emitting surface to a collecting plate depends on the applied electric field and ranges from a few tens of microseconds to milliseconds. The measurement of the purity is based on the ratio of collected to emitted charges. The operation can be performed in a few minutes and gives values of the electron lifetime with a typical error of a few percent. Filling and emptying the vessel can be done in less than an hour.

Particle simulation of magnetic reconnection in the magnetotail configuration

Abstract: We investigate the time development of a magnetotail like plasma sheet equilibrium, using the two-dimensional implicit electromagnetic particle code VENUS. As a starting point, we treat the simplified case of equal ion and electron masses. It is shown that a tearing instability develops if the magnetic field component perpendicular to the sheet is small enough and if the ion gyroradius is comparable to the plasma sheet thickness. An earlier estimate of the stability criterion has been confirmed approximately. In contrast to the case of a one-dimensional sheet equilibrium, the instability does not saturate. Instead, the unstable dynamics continues to evolve by formation, acceleration, and ejection of a plasmoid structure. The analysis of the distribution function in the nonlinear stage shows anisotropic heating and the generation of a bulk flow in the tail direction, caused by the development of the plasmoid structure. There is a local enhancement of the drift velocity in the cross-tail direction in the vicinity of the neutral line. Detailed investigation of the particles at the neutral line presents evidence that the current is significantly carried by resonant particles, which is typical for a kinetic tearing instability. We briefly address the case of an ion to electron mass ratio of 10, where the ion tearing instability still develops. Compared to the case of equal masses, electron stabilization leads to smaller growth rate and to an enhanced wavelength, according to theory.

Drift compensation for scanning probe microscopes using an enhanced probe positioning system

Abstract: This invention is an enhanced probe positioning technique for Scanning Tunneling Microscopes, Atomic Force Microscopes, and other scanning probe microscopes. The invention has particular application for drift compensation. The invention adds a controllable motion to the probe that is totally independent of the scanning or other probe positioning. If the drift velocity is known, the invention can be used to compensate for the drift. In addition, several implementations are shown for measuring drift velocity. One method has the operator identify a significant feature of the acquired image on separate frames of data. The shift of this pattern or feature, along with the time between frames, can be used to calculate the drift velocity. Two methods are described that utilize the frequency shift of the image spatial spectrum due to the effect of the drift velocity on bi-directional scans. Another method is described that derives drift velocity and direction from the correlation of separate frames of data. The invention can also be used to compensate for predicted drift, such as the drift after a scan area offset.

Electron Collision Cross-Sections Determined from Beam and Swarm Data by Boltzmann Analysis

Abstract: Whenever free electrons collide with atoms and molecules, a wide variety of kinetic processes may take place. Electron collision cross-sections, of interest in plasma processing of VLSI production, carbonization of surfaces and so on, modeling of gas lasers, physics of gaseous dielectrics, analysis of high current discharge switches, applications for space science and radiation physics and chemistry, have been determined from available electron beam and electron swarm data utilizing the Boltzmann analysis. The goal of electron collision studies is to provide absolute values of cross-sections for all processes involved, from elastic to inelastic scattering, as a function of incident electron energy and scattering angle. In spite of many experimental and theoretical studies, electron impact cross-sections for atoms and molecules have not been systematically quantified.

Kinetic equilibria of plasma shear layers

Abstract: The analysis of plasma beam and shear problems in magnetic fields is usually based on a hydromagnetic fluid model. In a low‐density collisionless plasma, however, the kinetic effects of the plasma, such as finite Larmor radius effects, are not yet clearly understood. In this paper, the kinetic equilibria of plasma shears in a uniform and fixed magnetic field, with full ion motion, are discussed by solving the Vlasov equation with a given electric field and drift velocity. In this model, the ion density profile through the plasma shear layer is quite different from the one predicted by a hydromagnetic model. As a result of a complicated ion gyromotion through the shear layer, single‐ and double‐humped ion density profiles are obtained. The dependence on the temperature and the strength of the shear will be discussed. The results show a significant difference between positive and negative shears.

The bootstrap current in stellarators

Abstract: A method of calculating the bootstrap current in asymmetric devices, such as the stellarator, is developed. The features that determine the bootstrap current are the variation of the magnetic‐field strength within a magnetic surface and the rotational transform. The shape of the magnetic surfaces is shown to be irrelevant. If the fraction of trapped particles is small, the device dependence of the bootstrap current is determined by a single constant which can be calculated by integrations of the field strength along the field lines. The relative dependence of the bootstrap current on the density and the temperature gradients is the same in any device with a small fraction of trapped particles as in a large aspect ratio tokamak.

Heating Current Induced Conversion between 2×1 and 1×2 Domains at Vicinal (001) Si Surfaces–Can it be Explained by Electromigration of Si Adatoms?

Abstract: Besides randomly walking on the crystal surface the adatoms are assumed to have a drift velocity c=DsF/kT, where Ds is the surface diffusion coefficient and F is a force, related in some way to the specimen heating current. In the limiting case of narrow terraces (small interstep distance l) the major process on the whole surface is the surface transport, which results an absence of noticeable conversion of the domain structure when Fl/kT≈0.1. Making use of the agreement between this conclusion and the experimentally observed [H. Kahata and K. Yagi: Jpn. J. Appl. Phys. 28 (1989) L858] lack of conversion (under reversal of the current direction) at l≈40–80 nm and T=700°C, the force F is estimated to be in the range 10-14–10-13 N. Experimental verification of the predicted stability of vicinal surfaces with l<80 nm when F has step-up direction and instability (decay to large terraces, separated by steps of multiple height) when F has step-down direction can be a crucial test of the hypothesis for an electromigration of Si adatoms.

Galvanotaxis of human granulocytes: electric field jump studies

Abstract: The static and dynamic responses of human granulocytes to an electric field were investigated. The trajectories of the cells were determined from digitized pictures (phase contrast). The basic results are: (i) The track velocity is a constant as shown by means of the velocity autocorrelation function. (ii) The chemokinetic signal transduction/response mechanism is described in analogy to enzyme kinetics. The model predicts a single gaussian for the track velocity distribution density as measured. (iii) The mean drift velocity induced by an electric field, is the product of the mean track velocity and the polar order parameter. (iv) The galvanotactic dose-response curve was determined and described by using a generating function. This function is linear in E for E less than EO = 0.78 V/mm with a galvanotaxis coefficient KG of (-0.22 V/mm)-1 at 2.5 mM Ca++. For E greater than EO the galvanotactic response is diminished. This inhibition is described by a second term in the generating function (-KG.KI(E-EO)) with an inhibition coefficient KI of 3.5 (v) The characteristic time involved in directed movement is a function of the applied electric field strength: about 30 s at low field strengths and below 10 s at high field strengths. The characteristic time is 32.4 s if the cells have to make a large change in direction of movement even at large field strength (E-jump). (vi) The lag-time between signal recognition and cellular response was 8.3 s. (vii) The galvanotactic response is Ca++ dependent. The granulocytes move towards the anode at 2.5 mM Ca++ towards the cathode at 0.1 mM Ca++. (viii) The directed movement of granulocytes can be described by a proportional-integral controller.

Poloidal and parallel plasma viscosities in tokamak geometry

Abstract: The poloidal and parallel plasma viscosities in tokamak geometry in Hamada coordinates are calculated from the drift kinetic equation, including a large mass flow velocity without imposing the usual constraint that VpB/(vtiBp) be small. Here, Vp is the poloidal plasma flow velocity, vti is the ion thermal speed, B is the magnetic field strength, and Bp is the poloidal magnetic field strength. With this extended validity, the poloidal and parallel viscosities are useful in modeling the radial electric field in the edge region of a tokamak in the enhanced confinement regime.

Velocity electric field relationship for minority electrons in highly doped p‐GaAs

Abstract: Using the time‐of‐flight method, the drift velocity for minority electrons in highly doped p‐GaAs was successfully measured at room temperature. The obtained velocity electric field relationships indicate that the velocity does not decrease but slightly increases and then saturates with increasing hole concentration above 1×1019 cm−3. Such behavior is well explained by the effects of degeneracy, which reduces the electron‐hole interaction, and the hole distribution function dependence on electric field from the Monte Carlo calculation.

Near-field characteristics of a turbulent coflowing jet

Abstract: Author(s): Gladnick, PG; Enotiadis, AC; Larue, JC; Samuelsen, GS | Abstract: The near-field evolution of the velocity and concentration fields for an axisymmetric jet flow of CFC-12 issuing into a coannular jet flow of air is presented. Results based on measurements of the time resolved velocity (two components) and (separately) the concentration, obtained using laser anemometry and Rayleigh scattering systems, show that the transport of momentum and mass in the near field depends on the large scale structure which forms in the shear layer at the edge of the jet. The type of instability and hence the flow development is shown to depend primarily on the ratio of the coflow to jet velocity (m), density ratio, and the jet exit velocity profile. Specifically, for velocity ratios less than and greater than unity, the statistical properties of the velocity and concentration fields are compatible with the existence of an annular vortex ring with positive vorticity for m l 1 and negative vorticity for m g 1. For a velocity ratio equal to unity, the results are consistent with the presence of pairs of counter-rotating vortices that are typical of a wake flow. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.

Time-dependent electron swarm parameters in RF fields in CH4 and H2

Abstract: The temporal behaviour of electron swarm transport in an RF field has been investigated for the electron non-conservative case in CH4, H2 and their mixture by using the first three terms in Legendre polynomials with a fourier expansion in time for the electron velocity distribution from the time-dependent Boltzmann equation. Calculations have been made for reduced RMS electric fields ER/N in the range 15 to 70 Td and reduced angular frequencies omega /N in the range 105/N0 to 1010/N0 cm2 s-1, where N0 is the number density at 1 Torr and 273 K. The temporal modulation of the isotropic part of the velocity distribution is essentially determined by the ratio between omega -1 and an energy relaxation time tau e. It is shown that in molecular gases with rotational and vibrational collisions at very low energy, the asymptotic form of the energy distribution at very high frequency field differs from a Maxwellian as in atomic gases. The phenomenon of negative differential conductivity in CH4 has been studied as a parameter of omega /N. The gas mixture effect in an RF field is also discussed.

Equatorial fountain effect and dynamo drift signatures from AE-E observations

Abstract: Vertical and horizontal ion drift velocities for 1977-1979 from the Unified Abstract files of the satellite Atmosphere Explorer E were used in preliminary studies of the behavior of the low-latitude ({minus}20{degree} to 20{degree} dip latitude) F region. Sample diurnal variations obtained during 1978-1979 equinox time periods are very similar to those measured by the incoherent scatter radar at Jicamarca, Peru. Latitude profiles of the vector drift values clearly show the classic fountain effect responsible for the Appleton anomaly during the day. Investigation of longitude effects revealed that the average perpendicular (approximately vertical) drift velocity V{sub perp} is independent of longitude, implying that the average east-west electric field is proportional to B and thus is likely derived from classical dynamo winds.

Negative hydrogen ion densities and drift velocities in a multicusp ion source

Abstract: We have determined densities of negative hydrogen ions in a discharge by a laser detachment technique. We measured the electron density, the electron temperature, and the positive ion density using a Langmuir probe. We also performed extraction measurements. Combination of H− density measurements and extraction measurements yields information about the H− drift velocity. It was found that the velocity scaled with the square root of the electron temperature. All measurements were done as a function of discharge voltage, discharge current, and gas pressure. The densities are compatible with a semiquantitative model in which H− is produced by dissociative attachment of plasma electrons to vibrationally excited molecules and destroyed by wall collisions at very low pressure and collisions with H atoms, positive ions and/or hot thermal electrons at higher pressure.

Drift of vortex in the myocardium

Abstract: The behavior of a vortex in rabbit myocardium with artificial inhomogeneity was studied using mapping technique. The inhomogeneity was created by perfusion of a part of the preparation with quinidine solution. Quinidine increased the refractory period of the myocardium and diminished the conduction velocity. It has been found that the vortex drifts along the border of the inhomogeneity because of the difference of refractory periods. The drift velocity was about 4 cm/s, which was five times less than the wave velocity. The direction of the drift was determined by the vector (----omega X----delta R), where ----omega is the angular velocity of vortex rotation, and ----delta R is a gradient of the refractory period.

Spatial structure of particle-orbit loss regions in l=2 helical systems

Abstract: Particle orbits and loss regions in both configuration and velocity spaces are studied on the basis of adiabatic invariants and guiding‐center drift equations. The boundary of the loss region in the pitch angle major radius plane (χ0‐ρ) is determined from the condition of whether the drift surfaces for helically trapped and transition particles hit the limiter or not. Two typical loss boundaries on the χ0‐ρ plane are obtained. One determines the boundary of absolute confinement where no particles are lost. The other gives the boundary of the confined region of deeply trapped particles with v∥=0. An analytical form describing the latter boundary is also derived. Effects of ripple modulation and electrostatic potential on these loss boundaries are discussed for the improvement of particle confinement.

Plasma Reactor and method for semiconductor processing

Abstract: Magnetic confinement of electrons in a plasma reactor (200,300) is effected by using electro-magnetic coils (232,234,332,334) and other magnets (246) which generate respective magnetic fields (B,230) which are mutually opposed and substantially orthogonal on their common axis (236) to the major plane (242) of a wafer (202) being processed, instead of being aligned and parallel to the major plane as in prior magnetically enhanced plasma reactors. The respective magnetic fields (B,230) combine to yield a net magnetic field which is nearly parallel to the wafer (202) away from the magnetic axis (236) so that electrons are confined in the usual manner. In addition, a magnetic mirror (244) provides confinement near the magnetic axis (236). The E x B cross product defines a circumferential drift velocity urging electrons about a closed path about the magnetic axis (236). The magnetic and cross-product forces on plasma electrons have a rotational symmetry which enhances reaction uniformity across the wafer (202).

Convection within an experimental wave flume

Abstract: The velocity field beneath a series of progressive gravity waves was investigated using laser Doppler anemometry. The drift velocity was observed to vary with respect to both time (t) and distance ...

Electromigration and mechanical stress in aluminium conductor tracks passivated by anodisation

Abstract: The electromigration lifetime of integrated circuit metallisation is commonly found to increase if the metallisation is covered with a passivation layer such as glass. We have investigated the electromigration behaviour of aluminium conductor stripes under different thicknesses of oxide passivation formed by anodisation. The ionic drift velocity of anodised stripes was measured using the Blech-Kinsbron edge displacement technique (Thin Solid Films 25 327 (1975)) and it was found that the drift velocity decreased with increasing anodisation thickness. Stripes tested with a reversed current drifted backwards with an initial velocity which exceeded the original forward velocity. These results are related to a change in the self-diffusivity of aluminium as a result of high compressive stresses imposed by the anodised layer, and the distribution of stress in a drifting stripe is discussed. A general conclusion is that passivation layers can support compressive stresses sufficient to retard mechanical failure processes in metallisation in two distinct ways, either by raising the threshold stress for the nucleation of damage such as hillocks or whiskers, or by reducing the diffusivity in the metallisation.

STARE and EISCAT measurements: Evidence for the limitation of STARE Doppler velocity observations by the ion acoustic velocity

Abstract: Measurements of the phase velocity of ionospheric plasma waves using the STARE radar system are compared with the electron drift velocity and the ion acoustic velocity calculated from data measured by EISCAT. This comparison shows that for those flow angles that are smaller than the largest flow angle (θs) for which the component of the electron drift velocity along the radar line of sight equals the ion acoustic velocity, the phase velocity observed by STARE tends to be limited by, or lower than, the ion acoustic velocity. Using observations with flow angles larger than θs, one can infer the electron drift from STARE measurements by the well-known cosine relation.

Analytical model for I-V characteristics of JFETs with heavily doped channels

Abstract: A theoretical analysis of the I - V characteristics of junction field-effect transistors (JFETs) of types useful for electronic and optoelectronic integrated circuits, has been carried out in some detail. Attempts have been made to study the effects of various device parameters, including carrier degeneracy due to heavy doping of the channel, velocity overshoot of electrons in the channel and shortening of gate length, on the I - V characteristics of these JFETs. A new theoretical model for drift velocity v in semiconductors has been proposed for this purpose. A comparison of results from this model with those from the existing model of Trofimenkoff and with available experimental data for v attests at least to the high-field accuracy of the proposed model. Unlike almost all other models, the simple functional form of the proposed model provides it with additional advantages for application to the analytical I - V study of FETs with both uniformly and nonuniformly doped channels. Use of this model in the framework of the Lehovec-Zuleeg procedure for short-channel FETs appears to provide reasonably good results. Wherever experimental data are available, they agree at least qualitatively with results from the present model. The latter demonstrates that a very heavy doping in the p -type gate does not yield higher transconductance. A submicron gate and a heavy doping (higher than the nulled degeneracy level) in the channel are, on the other hand, necessary for higher drain saturation current and transconductance. A semiconductor alloy with the highest possible electron saturation velocity is the most suitable for high-speed JFETs.

Momentum transfer cross section of argon deduced from electon drift velocity date

Abstract: The momentum transfer cross section of argon is deduced from experimental electron drift velocity data over the energy range from 0.02 to 100 eV. A method proposed earlier by Suzuki et al., which is improved here by adding a minimising procedure of the fractional difference between calculated and experimental electron drift velocities, is used. In an E/N range below 0.1 Td, the electron drift velocity data of Robertson at a gas temperature of 89.6 K are analysed and the cross section for the low-energy region is deduced

Minority electron transport property in p‐GaAs under high electric field

Abstract: Time‐of‐flight and photoluminescence measurements are performed for p‐GaAs samples at room temperature to investigate minority electron transport properties under high electric field. Reduction of drift velocity, absence of negative differential resistance, and thermally nonequilibrium states between electrons and holes were observed. These features were different from those for majority electrons and indicate strong dependence on hole concentration. Through studies of the energy loss rate and energy and momentum relaxation times, and by performing Monte Carlo calculations including electron‐hole interaction, it is clarified that the energy and momentum transfers by electron‐hole interaction have significant effects on minority electron transport property.

Monte Carlo simulation of electron swarms in nitrogen in uniform E*B fields

Abstract: The motion of electrons in nitrogen in uniform E*B fields is simulated using the Monte Carlo technique for 240 >

A study of the electron image due to ionizing events in a two-dimensional liquid argon TPC with a 24 cm drift gap

Abstract: We have tested a liquid argon time projection chamber with a novel wire configuration based on electrostatic focussing which allows the realization of a nondestructive detection of the electron image produced by ionizing events. The chamber was tested in a 5 GeV pion beam at the CERN proton synchrotron. The measured pulse shapes at both 200 V/cm and 500 V/cm were in very good agreement with the expected shapes, calculated taking into account the electron lifetime, the response of the electronics and the longitudinal diffusion of the electron cloud. The measured electron drift velocity was in good agreement with the results of other workers as well as with our previous measurements. We have also analysed a sample of events containing delta rays in order to study the behaviour of low-energy electrons in the liquid argon. We find that for electron energies greater than 5 MeV the measured energy spectrum agrees very well with the predicted spectrum after corrections for acceptance and energy loss, hence demonstrating the feasibility of recognizing low-energy electrons in liquid argon.