Values Of Drift Velocity Research Articles

Simultaneous measurement of local drift velocities and electron densities of plasma jets

A non-perturbing probe for drift velocities in high-density plasma flows is described. Coherent Thomson scattering has been used for direct and local measurements. Two pulsed laser beams probe the plasma flow alternately from two different directions yielding measurable Doppler shifts. The high repetition frequency of the laser system permits the detection of small relative Doppler shifts of the scattered light pulses by means of a correlation technique. The influence of fluctuations of the scattered light intensity is eliminated by employing two detection units with different spectral characteristics. Local values of drift velocities and electron densities can be measured simultaneously at particular points in the plasma flow. Drift velocities between 0.3 and 2.7 km s-1 and electron densities between 1022 and 1023 m-3, depending on the discharge conditions of the plasma thruster (current, gas composition), are found. The laser Doppler technique presented provides a spatial resolution better than 1 mm3.

Analytical determination of the emitter saturation current in ultra-shallow silicon junctions

Simple analytical relations expressing the influence of electric field strength E prevailing in the heavily-doped quasi-neutral layer of ultra-shallow (x j = 01 to 0.3 μm) silicon junctions on the emitter component of the reverse saturation current density J 0 are obtained. It is shown physically and is demonstrated mathematically that, owing to the peculiarities of such junctions, an additional field-based current term appears in the Shockley relation for the emitter component of the saturation current. For the usual values of drift velocities encountered in the heavily-doped layer of ultra-shallow junctions, the field-based term enhances substantially the emitter component of the saturation current. The particular forms of the analytical expression obtained for different practical cases of interest are detailed. The electric-field-free case is also investigated, and simple analytical expressions for emitter saturation current are derived as a function of practical values of the surface recombination vel...

Boltzmann equation analysis of electron collision cross sections and swarm parameters for krypton

A set of electron gas-atom collision cross sections of krypton, giving good agreement between calculated and experimental values of drift velocity and Townsend first ionisation coefficient is obtained for the electron energy range 1< epsilon <30 eV by Boltzmann equation analysis. It is found that slight modification near the onset of excitation cross section alters the calculated Townsend first ionisation coefficient significantly; the difference of up to 100% between the values of the experimental Townsend first ionisation coefficient by Kruithof and by Dutton et al. can be accounted for by this modification.

Drift velocity and ionization coefficient for holes in single-valley semiconductors

An analytical theory has been developed for drift velocity ( V d ) and ionization coefficient ( α h) of holes in silicon. Based on Boltzmann transport equation, expressions for drift velocity ( V d ) and ionization coefficient ( α h) are derived. The theoretical approach is based on calculation of the collision operator for ionization probability, approximated by a delta function. It is observed that the values of drift velocity ( V d ) and ionization coefficient ( α h) are in good agreement with experimental results for ionization length (l io = 70 A ̊ ) and ionization energy ( ϵ i = 2.5 eV). This confirms the validity of the developed theoretical model for drift velocity and ionization coefficient of holes.

Drifting effect of electrons on fully non-linear ion acoustic waves in a magnetoplasma

Ion acoustic solitons affected by the drift motion of the electrons along the direction of the magnetic field have been investigated. The ranges of the existence of solitons of different velocities and the limiting value of drift velocity are reported.

Monte Carlo calculation of electron transport coefficients in counting gas mixtures: II. Mixtures containing neon and carbon dioxide

In this, the second part of a comprehensive study of electron drift and diffusion in counting gas mixtures, we report the results of calculations for pure Ne, pure CO 2 and the mixtures Ne-20% CO 2, Ne-10% CH 4, Ne-20% CH 4, A-10% CO 2, A-20% CO 2 and A-50% CO 2. Values of drift velocity and of the transverse and longitudinal diffusion coefficients computed by Monte Carlo methods are shown to be in good agreement with the available experimental measurements in the range 0 ⩽ E p ⩽ 2.0 V cm −1 Torr −1 .

The influence of unattached electrons on the collection efficiency of ionisation chambers for the measurement of radiation pulses of high dose rate

Due to their high mobility, electrons represent the major component in the initial phase of the current in an ionisation chamber irradiated with short pulses of ionising radiation. The authors have developed 'fast' ionisation chambers and, by means of these, have directly measured the electron component of the chamber current. The actual number of electrons collected appeared to be significantly greater than that calculated when using attachment coefficient and drift velocity values found in the literature. For example, approximately four times the expected value is collected in room air. The deficit of negative ions resulting from the collection of electrons is incorporated within a modified version of Boag's recombination equation. The recombination loss up to an exposure per pulse of, at least, 2.58*10-2C kg-1(100R) can be calculated in this manner.

Measuring plasma drift velocities in tokamak edge plasmas using probes

Plasma drift or rotation is caused at the edge of tokamaks by various mechanisms including flow to surfaces and neutral injection. These drift velocities can be measured using electrostatic probes and a theory is presented providing a method of probe analysis to deduce drift velocity, plasma density, and temperature. Initial probe experiments using this probe analysis yield credible values of drift velocity, but comparison with an independent technique, e.g., spectroscopic Doppler shifts, is sought. OFF

Transport properties ofn-type metalorganic chemical-vapor-depositedAlxGa1−xAs (0≤x≤0.6)

Temperature-dependent measurements in the range $20\ensuremath{\le}T\ensuremath{\le}600$ K have been made on undoped and Si-doped metalorganic chemical-vapor-deposition---grown ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As} (0\ensuremath{\le}x\ensuremath{\le}0.6)$ on GaAs. Data obtained from Hall and high-field measurements with the use of the probe technique have been analyzed. The transport parameters in the samples have been obtained from analysis of data and the relative importance of the various scattering mechanisms in different composition ranges have been elucidated. It is found that space-charge scattering plays an important role in limiting electron mobility at 300 K for $0&lt;x\ensuremath{\le}0.3$, and intervalley scattering plays the dominant role in the composition range $0.3\ensuremath{\le}x\ensuremath{\le}0.5$. Donor levels, with their activation energy ${E}_{D}$ increasing with $x$ up to 0.133 eV at $x=0.35$, are present in the undoped samples. The dominant donor level in the Sidoped samples also exhibits a similar trend, with ${E}_{D}=0.095$ eV for $x=0.35$. High values of drift velocity in samples with $x\ensuremath{\ge}0.4$ at 300 K and similar features observed in some samples with $x&lt;0.4$ at high temperatures have been attributed to electron transfer from the substrate to the epitaxial layers.

High-field transport in organometallic VPE AlxGa1-xAs transferred-electron devices

The velocity-field characteristics of hot electrons in planar organometallic vapor phase epitaxial (VPE) Al x Ga 1-x As (0.1 ≤ x ≤ 0.6) transferred-electron devices have been measured by the probe technique. The characteristics in layers with x ≤ 0.30 show normal subthreshold behavior. For x > 0.30, the characteristics exhibit high values of drift velocity. The effects are more enhanced when an undoped 0.4-µm GaAs buffer layer is grown first on the GaAs: Cr substrate before the ternary layer is grown. Monotonically increasing drift velocities with electric field, with a value of 2 × 107cm/s at 6 kV/cm, have been measured in these devices. These effects and overshoots in the pulsed current-time profiles indicate the onset of real-space electron transfer at the heterointerface at high fields.

Electron Collection by Blunt Probes in the Lower Ionosphere

In order to determine electron densities from /V (current-voltage) data, an analysis of electron collection processes in the weakly ionized, high-pressure ionosphere plasma between 40 and 80 km has been carried out. The diffusion layer concept of ion collection is not applicable. At collector voltages above those for Langmuir saturation, an approximate continuum solution to the electron flux determination is based on experimental values of drift velocity vs El P. Correction for larger \en at higher altitudes is included. Electron flux is found to be determined at distances far from the collector, and flow effects are found to be minimal. Electron densities from blunt probe data are compared with those taken simultaneously with other instruments.

The calculation of electron energy distributions and attachment coefficient for electron swarms in oxygen

Electron energy distributions have been calculated for electron swarms in oxygen for 15 less-than-or-eq, slant E/N less-than-or-eq, slant 152 Td. These distributions satisfy the experimentally measured values of both drift velocity and the diffusion coefficient, and allowed the theoretical value of the rate of attachment (ηv) to be calculated to an accuracy of ±3%.The calculated values of η/N are dependent upon the measured values of the drift velocity, but since the latter show a large variation, the η/N values have been represented by two limiting curves. The important feature of these curves is that η/N quickly rises to its maximum value at about E/N=30 Td, from which it only gradually reduces as E/N increases. It has also been possible to obtain values of the momentum transfer cross section in oxygen for electron energies up to 10 eV. This cross section was found to have a simple form and to be very close in value to the total cross section.

Acoustoelectric Effects in Indium Antimonide

The acoustoelectric interaction in InSb is developed and investigated on the basis of a hydrodynamical theory. Comparison of numerical results obtained from this theory with the corresponding numerical results from more detailed microscopic theories reveals that the essential features of the microscopic theories are also present in the simpler hydrodynamical theory. In particular, the hydrodynamical theory accounts for the existence of two distinct modes of acoustic domain formation and for the reduction of peak acoustic gain at low values of magnetic field. Valuable insight into the physical basis of the acoustoelectric effect is obtained and it is shown that in the limit of large electron drift velocities and small transverse magnetic field strengths, the Mode I-type acoustoelectric interaction arises from a drift-enhanced quenching of electron diffusion effects. Moreover it is found that the frequency of maximum acoustoelectric gain, as given by the hydrodynamical theory of the present paper, does not occur at the fixed frequency f0 = (ωRωD)1/2/2π, but rather, it is dependent upon the exact values of drift velocity and magnetic field strength. Several computer-generated plots are presented and discussed in order to fully illustrate the results of this investigation.

A test for dispersion in F-region drifts observed by the radio star scintillation method

Records of scintillations of the source Cassiopeia A made on 38 Mc/s using three spaced interferometers have been analysed to determine whether Fourier components of different frequencies give different values of drift velocity. Such an effect is found to occur, and in most cases the speed of drift increases with frequency, while the direction of drift does not change. This effect is related to a systematic ‘skewness’ of the cross-correlation functions. The dispersion could be produced by the propagation of some type of dispersive wave through the F-region of the ionosphere, but it is considered more likely that it is produced by a variation of drift velocity along the line of sight.

Role of Electric Field Generated in the Equatorial Electrojet Region

Adopting an atmospheric model for the sunspot maximum day, the height variation of various ionospheric conductivities have been computed. The electric currents are calculated from yearly averaged magnetic data for the sunspot maximum period in 1958. Using the height integrated Cowling conductivity, electric fields for five selected equatorial and non-equatorial stations have been obtained from the calculated electric currents. The electric currents and electric fields thus obtained have been compared with the electric field derived from the rocket measured electrojet current density. It is shown that such comparisons give good overall agreement of the results obtained from magnetic data and the rocket measurements. The electron temparature is calculated from Joule heating considerations. The resulting electron temparature has been found to be smaller than the rocket measured values, The vertical electron drift velocity produced by the electric field in the electrojet region has been calculated and is found to be higher than the earlier calculation of Hirono and Kitamura (1956) and compares well with the drift velocity values obtained from continuity equation.

Rotational Excitation and Momentum Transfer Cross Sections for Electrons inH2andN2from Transport Coefficients

Rotational excitation and momentum transfer cross sections for low-energy electrons in hydrogen and nitrogen are obtained from a comparison of theoretical and experimental values for the mobility and the diffusion coefficient. The theoretical values of the transport coefficients were obtained by calculating accurate electron energy distribution functions using an assumed set of elastic and inelastic cross sections. The discrete nature of the energy loss occurring in a rotational or vibrational excitation collision was included in the theory, as were collisions of the second kind with thermally excited molecules. The resulting values of drift velocity and characteristic energy $\frac{D}{\ensuremath{\mu}}$ were compared with experimental data and adjustments made in the assumed cross sections until good agreement was obtained. The momentum transfer cross sections found in this manner agree well with several recent analyses valid in restricted energy ranges. The final values of the rotational excitation cross sections are about twice the values computed using the theory of Gerjuoy and Stein and the latest available value for the molecular electric quadrupole moments. In hydrogen, the analysis has been extended to energies for which vibrational excitation is important. A vibrational cross section with a maximum of roughly 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 3 eV is consistent with the measurements.