Showing papers on "Drift velocity published in 1982"

Markov-chain simulation of particle dispersion in inhomogeneous flows: The mean drift velocity induced by a gradient in Eulerian velocity variance

Abstract: The Langevin equation is used to derive the Markov equation for the vertical velocity of a fluid particle moving in turbulent flow. It is shown that if the Eulerian velocity variance Σ wE is not constant with height, there is an associated vertical pressure gradient which appears as a force-like term in the Markov equation. The correct form of the Markov equation is: w(t + δt) = aw(t) + bΣ wEζ + (1 − a)T L ∂(Σ wE 2)/∂z, where w(t) is the vertical velocity at time t, ζ a random number from a Gaussian distribution with zero mean and unit variance, T L the Lagrangian integral time scale for vertical velocity, a = exp(−δt/T L), and b = (1 − a 2)1/2. This equation can be used for inhomogeneous turbulence in which the mean wind speed, Σ wE and T L vary with height. A two-dimensional numerical simulation shows that when this equation is used, an initially uniform distribution of tracer remains uniform.

The drift velocity of water waves

Abstract: The important role of viscosity in producing second-order Eulerian drift currents in the presence of small-amplitude water waves was first recognized by Longuet-Higgins (1953).The theoretical and experimental background is first reviewed. It is then shown that, contrary to previous belief, the presence of surface contamination must greatly enhance the drift velocity of short waves. We then solve an initial-value problem for the drift current associated with temporally decaying waves, thereby resolving questions raised by the work of Liu & Davis (1977), whose solution exhibits anomalous singularities. Next, the steady drift velocity of spatially decaying waves is calculated and shown to bear a close resemblance to Longuet-Higgins’ ‘conduction solution’ for unattenuated waves.Finally, we establish that unidirectional drift currents of both surface and inter-facial waves are sure to be unstable to spanwise-periodic disturbances; the instability mechanism being identical to that first proposed by Craik (1977), and recently developed by Leibovich & Paolucci (1981), to explain the generation of Langmuir circulations.

Monte‐Carlo Studies of the Electron Mobility in SiO2

Abstract: The motion of quasi-free electrons in SiO2 is simulated by means of the Monte-Carlo-method and the Frohlich theory of the electron—LO-phonon scattering. From the field dependence of the drift velocity an electron mobility of μ ≈ 20 cm2/Vs is deduced. The energy distributions of hot electrons indicate an electronic breakdown at the field strength F0 ≈ 2 × 106 V/cm. For higher excited electrons with initial energies E0 ≈ (1 to 5) eV a “partial breakdown” even at lower field strength F < F0 is observed.

The electron velocity-field characteristic for n-In 0.53 Ga 0.47 As at 300 K

Abstract: Electron drift velocities in n-type In 0.53 Ga 0.47 As have been measured at 300 K using a microwave time-of-flight technique at electric field strengths from about 10 kV/cm to 104 kV/cm. The electron velocity-field characteristic for n-type In 0.53 Ga 0.47 As exhibits a larger negative differential mobility and a lower high field velocity than is obtained with GaAs.

Effects of inertia on the diffusional deposition of small particles to spheres and cylinders at low Reynolds numbers

Abstract: A formalism that accounts for inertial and diffusive effects in the dynamics of a dilute gas-particle suspension is introduced. The treatment is purely deterministic away from a very thin Brownian diffusion sublayer, while, within the sublayer, inertial effects are small, permitting a near-equilibrium expansion in powers of the Stokes number (particle relaxation time divided by flow characteristic residence time). This expansion provides phenomenological expressions for the particle velocity including two terms : the standard Brownian diffusion, and an additional inertial drift velocity which is closely related to the pressure diffusion term of the Chapman-Enskog expansion. As an example, the general formalism is applied in detail to the case of Stokes flow about a sphere, and sketched for the similar case of a cylinder. Two competing mechanisms are seen to affect the total rate of particle capture by the sphere : (if the stagnation-point region is considerably enriched in particles owing to the high compressibility of the particle phase, which leads to locally enhanced deposition; (ii) centrifugal forces tend to deplete the Brownian diffusion sublayer of particles, reducing diffusion rates away from the stagnation point to the surface. The first effect is seen to dominate over the second except in a very narrow zone of small Stokes numbers. Our method bridges the gap between Levich's solution for the ‘pure-diffusion’ limit and Michael's treatment in the ‘pure-inertia’ limit.

High field temperature dependent electron drift velocities in GaAs

Abstract: Electron drift velocities in (100) GaAs have been measured at various temperatures from 95 to 385 K for electric field strengths from about 15 to 160 kV/cm at most temperatures and as high as 236 kV/cm at 300 K using a microwave time‐of‐flight technique. The velocity decreases monotonically with increasing electric field strength at all temperatures over the entire field range investigated.

Electron drift velocity measurement in compositionally graded AlxGa1−xAs by time‐resolved optical picosecond reflectivity

Abstract: We have measured for the first time the velocity of minority‐carrier electrons in a heavily doped (p = 2×1018 cm−3) molecular beam epitaxy grown compositionally graded (0.12 eV/μm) AlxGa1−x As layer. The drift velocity is determined to be ve = 2.3×106 cm/s in a quasi‐electric field of 1.2 kV/cm.

Low field mobility, effective saturation velocity and performance of submicron GaAs MESFETs

Abstract: Analytical model is proposed which related the transconductance of submicron GaAs MESFET’s to a low field mobility, effective electron saturation velocity and device geometry and doping. The model predicts that the effective saturation velocity determines the performance of the devices at relatively high pinch-off voltages (Vpo > 5 V). At smaller pinch-off voltages (especially for enhancement mode devices) the low field mobility becomes increasingly important leading to additional advantages of GaAs devices over Si devices. Another prediction is a higher transconductance in thinner and higher doped devices. This effect is also more important for devices with low pinch-off voltage. The obtained results may be used to deduce the effective values of the electron drift velocity in GaAs MESFET’s as a function of the gate length.

Adiabatic charged particle motion in rapidly rotating magnetospheres

Abstract: Some aspects of adiabatic drift theory are investigated in the regime where the E x B drift velocity is comparable with or larger than the gyro velocity. Particles undergo five drifts in addition to the E x B gradient, and line curvature drifts have three more terms in the parallel equation of motion. The case of the rapidly rotating rigid magnetic field configuration is found to be an exception due to the corotating particle with a nearly periodic motion. The guiding center drift velocity and parallel differential equation of motion are obtained, and the drift velocity in the rotating frame is found to consist of the expected field gradient and line curvature drift, plus centrifugal and coriolis force induced drifts. It is shown that the second invariant is conserved in the rotating frame by the four drifts, so that a particle slowly drifts around on its drift shell and returns to its original field line. Thus, there is no long term energy change, and any energy change is periodic on the bounce and drift time scale.

Temperature dependent electron velocity-field characteristics for In 0.53 Ga 0.47 AS at high electric fields

Abstract: The static electron velocity-field characteristics for n-In0.53Gan0.47AS have been measured at temperatures from 95 K to 300 K over a range of electric field strengths from about 10 kV/cm to beyond 100 kV/cm. The velocity decreases monotonically with increasing electric field at all temperatures over the range of field strengths investigated. The electron velocity-field curves for In0.53Ga0.47As exhibit a larger peak-to-valley ratio, lower high-field velocity, and a smaller high-field temperature dependence than those for GaAs measured using the same experimental technique.

Low energy electron collision phenomena in HgBr2

Abstract: Cross sections for mercuric bromide (HgBr2) dissociative attachment and ionization have been measured using an electron beam experiment. The dominant products of these reactions, as identified by mass analysis, are Br− and HgBr+2. A complementary electron swarm experiment was used to determine the ion production coefficients in a variety of gas mixtures containing HgBr2. The measured ion production coefficients were found to be in excellent agreement with the attachment and ionization coefficients computed using the measured cross sections. Additionally, analysis of the variations in the measured HgBr2 ion production coefficients along with measured electron drift velocity variations is found to be consistent with the interpretation that vibrational excitation of HgBr2 is dominated by a resonance in the 3–5 eV electron energy range for which dissociative attachment is observed to occur.

Semiconductor structures for repeated velocity overshoot

Abstract: We discuss conditions necessary for obtaining repeated velocity overshoot in semiconductors. Two classes of structures which provide these conditions are considered. These structures offer the potential for achieving average drift velocities well in excess of the maximum steady-state velocity over distances ranging from submicron to tens of microns.

Ps 6 spatial and temporal structure from STARE and riometer observations

Abstract: At the time of ground-based observations of Ps 6 events, the two-dimensional electron drift velocity maps (or the corresponding electric field maps) deduced from the Scandinavian twin auroral radar experiment (STARE) measurements exhibit a characteristic wavelike oscillation pattern. This pattern extends 3–4° in latitude and moves eastward roughly along L shells at a speed typically in the range 800–1300 m/s. The pattern has a precursor, a core, and a recovery zone. In the core, the electron drift attains some of the largest values ever seen by STARE (>2000 m/s). The electric field divergence indicates the presence of an east-west field-aligned current (FAC) pair, the more spatially confined leading FAC being downward, the more broadly spread following FAC being upward. A multiple narrow beam 51.4-MHz riometer at Tromso indicates a broad absorption zone associated with the trailing upward FAC. By using the Biot-Savart Law to integrate the magnetic flux density due to the STARE velocity vectors (elemental currents) and by comparing the result with the ground-based magnetometer records, it can be inferred that the closure current for the FAC pair has both Pedersen and Hall components, thereby indicating the presence of conductivity gradients. Furthermore, the STARE flows predict the observed ground D component variation very well and indicate integrated Hall conductivity of ∼50 mhos.

In0.53Ga0.47As n‐channel native oxide inversion mode field‐effect transistor

Abstract: We describe a native oxide insulated gate, inversion mode In0.53Ga0.47As field‐effect transistor. Drain characteristics measured with pulsed gate voltage yield gm of 33 mS/mm and an effective saturated drift velocity for inversion layer electrons of 2.6×106 cm/s. The presence of a slow drift in drain current is found to reduce the transconductance to 20 mS/mm for low‐frequency measurements.

Fluid Model of Plasma Outside a Hollow Cathode Neutralizer

Abstract: The present study analyzes the capability of a fluid model of electron transport to explain observed properties of the external plasma of a hollow cathode neutralizer used to neutralize a beam emerging from an ion thruster. Calculations reported here show that when the effective collision frequency in such a model is near the plasma frequency, the resulting electric potential and electron temperature variations are in qualitative agreement with values measured in the plume mode of the hollow cathode. Both theory and experiment show strong variations of temperature and potential within a few centimeters of the cathode orifice. E E J k L m n n, P Q V r R T u V_ y+ V dn Nomenclature kinetic energy of an electron electric field net current density Boltzmann's constant scale length for variation of macroscopic plasma properties electron mass electron density density of ions (+) and electrons (-); / = + , scalar electron pressure magnitude of electron charge heat flux position vector collisional drag force between electrons and ions electron temperature v.v+ drift velocity of electrons drift velocity of ions jitter velocity of electrons electron density fluctuation plasma resistivity kT thermal conductivity of plasma coefficient defined by power law dependence of K on 6, K — mK 1 Om~l, with m a pure number mean free path for pair collisions between electrons Debye length effective collision frequency electron-ion collision frequency = 7J0 = electric potential (jo = frequency of oscillation of fluctuating field cjp = electron plasma frequency

Electron drift velocities in xenon

Abstract: The electron drift velocity has been measured in xenon over the range of reduced field strength 1 Td

Calculation of SF6 transport coefficients from revised data

Abstract: The transport coefficients of SF6 have been recalculated in the range of reduced field between 35 and 225 V cm-1 Torr-1 using the latest cross-section data. While fitting of calculated and experimental ionisation coefficients has been used to determine missing data on excitation cross-sections, calculations of the drift velocity and the diffusion and attachment coefficients provided a means of checking the consistency of the model and the data. The calculated drift velocity proved to be about 20% lower than the experimental value. The diffusion coefficients agreed well but the attachment coefficients compared poorly with measured values. Good fitting was obtained by reducing the width of the SF6- attachment cross-section to 30% and increasing the high-energy attachment cross-sections by about 30%.

Dynamics Explorer observations of equatorial spread F: Evidence for drift waves

Abstract: Recent DE-2 data from the Langmuir probe, vector electric field, and ion drift meter instruments are employed to study equatorial spread F in the frequency regime of the low frequency drift and the lower hybrid drift instabilities. Strong electron density gradients observed along the orbital path through the topside equatorial F region correspond to regions of high electric field waves and large ion drift velocities. The electric field waves are seen in two distinct wavelength ranges which correspond to the parameter regimes of the low frequency drift and the lower hybrid drift instabilities. In the smaller of the two wavelength ranges the lower hybrid drift instability is found to be unstable, based on the ion drift velocity and the other plasma parameters measured on DE-2, and using published theory. Thus there is experimental evidence that the lower hybrid drift instability may produce the observed short wavelength waves without invoking a cascading mechanism.

Electron motion in one-dimensional semiconductors

Abstract: The electron mobility, hot electron effects, and the role of polarons are studied theoretically in a one-dimensional semiconductor. A particular semiconducting polymer, the bis(P-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol, PDATS, is used as an example. Interaction with acoustic phonons alone is considered in the deformation potential approximation. The dependence of drift velocity on electric field is qualitatively different depending upon whether the phonons are one-dimensional (as for one isolated chain) or three-dimensional (as for a chain in a medium). And in both cases the results are qualitatively different from the established results for a three-dimensional semiconductor.

High‐field drift velocity of holes in inversion layers on silicon

Abstract: We report measurements of the room‐temperature drift velocity of holes along the (100) Si‐SiO2 interface (drift direction [011]) as obtained by a time‐of‐flight technique. Our measured values range up to 60% higher than previously reported values at comparable tangential and normal electric fields. An empirical equation that fits the low‐field mobility and the high‐field velocity as a function of tangential and normal fields is presented.

Current carrying properties of double layers and low frequency auroral fluctuations

Abstract: A summary is given of a systematic parametric study in which the ion-transit time effects on double-layer fluctuations are investigated in a series of numerical simulations involving several lengths of finite extent plasmas. Typical values of ion drift velocities both smaller and larger than the ion-thermal speed on the high potential end of the simulation plasma are given. It is shown that the link between the electron-current fluctuations and ion-transit time is the Langmuir criterion for the existence of double layers. The time period of the recurring phenomena depends on the ion dynamics; for ions entering the simulation plasma in such a way that the ion drift velocity is less than approximately the ion thermal speed, the time period is governed by the ion transit time from the location of ion injection to the location of the double layer.

Semiempirical expression for direct transconductance and equivalent saturated velocity in short-gate-length MESFETs

Abstract: A very simple and straightforward relationship has been derived between the direct transconductance and the apparent electron saturation drift velocity in GaAs MESFETs. From a comparison with experimental data it was found that this velocity may exceed the usual equilibrium velocity value. A new semiempirical relationship between the apparent non equilibrium velocity and the gate length is suggested. This relationship provides good agreement with both published experimental data and published Monte-Carlo numerical simulations. The practical implications on the saturated current at a given gate voltage, and on the maximum available transconductance per unit gate width for a given gate length and a given epilayer thickness are outlined. Moreover, the validity of present results for ion-implanted MESFETs is tested.

Variations in electron transport in argon with temperature near the Ramsauer-Townsend minimum

Abstract: A previously developed three-term expansion method has been extended to study the electron swarm in argon in the energy range of the Ramsauer-Townsend minimum. The swarm parameters of electrons have been computed at temperatures of 89.6, 293 and 1000K in the range 1*10-21

Electron drift velocity in GaAs using a variable frequency microwave time-of-flight technique

Abstract: A modified form of microwave time-of-flight technique has been developed which uses a variable frequency microwave deflection system to modulate the electron beam. This enables electron drift velocities to be measured absoslutely over a wide range of values whereas previous microwave time-of-flight measurements have given only relative velocities and require at least one point of normalisation. This new technique has been applied to measure the drift velocity of electrons in n -type GaAs for electric fields up to 135 kV cm −1 and over the temperature range 77–400 K. For fields above 10 kV cm −1 , excellent agreement is obtained with conventional time-of-flight measurements.