Investigations on the Electron Component in an Argon-Hexamethyldisiloxane (HMDS) Low Pressure Glow Discharge by Probe Diagnostics

Probe measurements were carried out in the positive column of an argon glow discharge in a flow system with different admixtures of hexamethyldisiloxane (HMDS). From this the mean electron energy n_e, number density n_e and gradient E were determined. Some results concerning the predominant ionization mechanism and the ion distribution in the gas phase could be obtained. By means of probes it is also possible to get some information about the deposition mechanism in polymerizing plasmas.     

Investigation of the Beam Plasma Discharge in Electronegative Gases Using the Second Derivative of Langmuir Probe Characteristics

The distribution of the charge carriers is measured and the influence of the attachment processes on the electron energy distribution function is demonstrated for beam plasma discharges in SF₆, CF₄ and O₂ with the aid of the second derivative of Langmuir probe characteristics. The structuration of the plasma into regions of predominating negative ions and regions of predominating electrons is determined by the the established radial T_e-profile. The dimension of the quasi electronfree plasma changes significantly as well at transition from the turbulent heating mechanism into the electron impact plasma generation as by occuring low-frequency instabilities. With increasing n_?/n_e a deficit of low energetic electrons appears in the electron energy distribution parallel to the formation of the negative ion peak. The saturation currents at n_e/n_?=0 yield the mass ratios between negative and positive ions.     

Experimental study on influence of cooling on glow discharge parameters

The influence of cooling of the discharge tube walls by the liquid nitrogen on some basic parameters of the neon glow discharge positive column is studied. By probe measurements, the longitudinal electrical fieldE _z, the electron concentrationn _e and the electron temperatureT _e are determined. The equation for determination of the radial profile of the relative electron concentration is derived and, on base of experimentally obtained quantities of the radial potentialV(r), this radial profile is computed.     

Radiation losses of a theta pinch plasma in the wave-length range 10–200 Å

In order to measure the radiation losses of a theta pinch plasma (electron densityn _e=1?5×10¹⁶ cm^?3, electron temperatureT _e=150?350 eV), a grazing incidence spectrograph is absolutely calibrated in the range 10–200 Å. This is done in two steps: First the measured intensity ratios of lines emitted by hydrogen-like ions are compared with their calculated values thus yielding the relative sensitivity of the instrument. The result is confirmed by incorporating well known intensity ratios of lithium-like ions. Secondly absolute calibration is possible by hanging the spectrograph on an absolutely calibrated monochromator via the branching-ratios of lithium-like ions. Radiation losses from the plasma turn out to be negligible as compared with heat conduction losses and the total energy radiated is found to be small compared with the energy content of the electrons, if the impurity concentration does not exceed 0.5 percent. The radiation is found to be predominantly emitted by the resonance lines of the oxygen and carbon ions O VI, O VII, O VIII, CV, CVI whereas continuum radiation and the contribution from other ions are negligible small. However, in discharge where the oxygen concentration reaches 5 percent, radiation losses can exceed the losses by heat conduction during the early phase of the discharge. But still the final electron temperature is not significantly influenced by the impurities.     

Measurement of electron energy and density in an H. F. electrodeless ring discharge

The electron temperatureT _e and the electron density were measured as functions of the radial distance in a 10 Mc/s electrodeless ring discharge in hydrogen in the pressure range 0.1–0.3 Torr. It was found thatT _e remains nearly constant along the radius of the cylindrical vessel. The measured values ofT _e have been compared with those observed by other workers and an estimate of the effective electric field in the discharge has been made. From this estimate it has been inferred that even after the first stage of the ring discharge is well established the longitudinal component of the electric field remains of considerable importance. The radial density distribution of the electrons was found to be different from those in an uniform electric field. This deviation in the radial density distribution has been attributed to the influence of the azimuthal electric field.     

Zur Radialstruktur der diffusionsbestimmten positiven Säule der Sauerstoff-Entladung bei niederen Gasdrucken

The radial distribution of the electrons, negative and positive ions in the cylindrical positive column of the direct current discharge in 0₂ has been calculated. The negative ions are formed by dissociative attachment (0₂ + e →^? + 0), the destruction of negative ions is effected by the reverse process (0^? + 0 0₂ + e). The pressure is assumed to be low enough that the radial variation of 0-concentration can be neglected. By use of the condition of the quasineutrality and the ambipolarity for the radial current from the balance equations of the electrons and the negative ions two differential equations of second order are derived, which has been solved numerically for different values of the ratio z_j/z_a and β[0]. Numerical results show that the radial distributions of the charge carriers are not-proportional. If z_j ≈? z_a the radial distribution of the electron density is enlarged, compared to a Bessel distribution. The radial distribution of the negative ions is also enlarged in the centre of the column, but near the wall of the tube a lack in the density exists. The effective diffusion coefficient of the negative ions is negative, i.e., they are moving from the wall of the tube to the centre of the discharge. The enlargement of the radial distributions of the charge carriers, which results in an improvement of the economy of the balance of electrons, increases if z_j/z_a and β/[0] are decreased.     

Das Neutralgasprofil und die Elektronentemperatur in der Freifallsäule bei hohem Ionisationsgrad

The neutral gas flux from the wall into the column - connected with the ion flux to the wall - and the anisotropy of the velocity distribution of the neutral gas are included into the theory of the steady-state positive column. d/l_n and v_n0/v₀ occur as characteristic parameters. d denotes the radius of the column, l_n the mean free path of the neutral atoms for ionization, v_n0 the mean velocity of the neutral atoms re-emitted from the wall, v₀ = (2kT_e/M)^1/2, M the ion mass, T_e the electron temperature. On the axis the neutral gas density N_n is decreasing, if d/l_n is rising. At the wall N_n is increasing for v_n0 ? v₀, but it is almost constant or decreasing for v_n0 ? v₀ at the same time. In the plasma the total number of the atoms and the ions is taken as constant. In the case of small v_n0 the degree of ionization is high only for d ? l_n. However, it is already high for d ≈ l_n in the case of high v_n0. Therefore, the radial profiles of the neutral gas densities of different gases in a column can differ from each other. Almost full ionization can be reached near the axis. These results hold, too, if the initial velocity of the ions and a magnetic field are taken into account. d/l_n, the degree of ionization, and the electron temperature are given as functions of the electric power input and of the total number of the neutrals and the ions. The velocities of the re-emitted atoms depend on the accomodation coefficient of the ions recombining at the wall. With rising d/l_n an increasing number of the neutral atoms is re-emitted with superthermal velocities and the total number of the neutrals and the ions can be decreased. The anisotropy of the velocity distribution of the neutral atoms and the ions must be taken into account for the interpretations of spectroscopical measurements.     

[Russian Text Ignored].

Radial distributions of charged particles in an oxygen discharge depending on the relative concentration of negative ions are experimentally investigated. Corresponding change of the selfconsistent field along the radius of the discharge positive column at different pressures and currents are given.     

Branching of electron current and quantum effects in two-dimensional dislocation barrier of n-type semiconductor

The current flow through the dislocation wall of the bicrystal (or intergrain) boundary in semiconductor is considered. It is known that the broken bonds along the dislocations make up the acceptor levels (being offset by energy ?_d from the bottom of the conductance band) whose population by electrons is defined by the bulk donor concentration n_D, the ?_d-value and the dislocation spacing D_y. At low enough n_D-value (n_D<e²/?_dD_y⁴), electrons captured at the dislocation acceptors produce the periodic 2D-lattice with two comparable periods of the order of n_s^−1/2 where n_s∼(?_dN_D/e²)^1/2 is 2D-density of captured electrons. Charges of those electrons result in the formation of the dislocation potential barrier whose height (with mean value in order of ?_d) is periodically modulated in the wall plane. Naturally, the electron current prefers to flow along the paths with the lowest barrier height, i.e. through the saddle points of the 3D-potential relief positioned in the dislocation wall plane. At low enough temperatures that leads to separation of the electron current into numerous branches. Provided both 2D-lattice periods of the captured charges are smaller as compared with the electron mean free path, all those narrow and short branches make up a filamentary 1D-conductors with the ballistic transport. As this takes place, effects associated with the 1D-conductance quantization define the whole system resistance and could be observed experimentally. In particular, exponentially strong temperature dependence of the whole system resistance vanishes.     

Distribution Radiale de la Densité des Atomes Métastables dans les Plasmas Produits par un Champ Electrique Continu ou de Haute-Fréquence

The radial density distribution of metastable Ar(³P₂) and resonant Ar(¹P₁) atoms is determined experimentally in two types of electrical discharges of cylindrical shape: the positive column plasma of a DC discharge and the plasma produced by an electromagnetic surface wave. The exitation and deexcitation rates for Ar(³P₂) by electron collisions are determined as a function of radius from the measured radial population density profile using a population density balance equation. These rate coefficients are obtained for various electron density values on the axis. The published values for these coefficients in the positive column plasma assume that they are independent of the discharge current. In this work, it is shown that these coefficients actually decrease as the electron density increases. In a more general way, the results obtained indicate that the examination of the radial density distributions of exited atoms is a powerful method for determining the kinetics of their creation and destruction.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂−N₂−He−H₂O laser mixture the principal ions are CO ₃ ⁻ , CO ₄ ⁻ and H⁻ with the total negative ion densityn ₋ given by 0.1n _e <n ₋<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ⁻ and NO ₃ ⁻ are formed in significant amounts andn ₋ can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn ₋ in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

Influence of magnetic impurities on the heat capacity of nuclear spins

It is found that over a wide range of temperatures and magnetic fields even a small concentration of magnetic impurities in a sample leads to a T ⁻¹ temperature dependence of the nuclear heat capacity. This effect is due to nuclear spin polarization by the magnetic impurities. The parameter that controls the theory turns out not to be the impurity concentration n _imp but instead the quantity n _imp μ _e /μ _n , where μ _e and μ _n are the magnetic moments of an electron and a nucleus, respectively. The ratio of μ _e and μ _n is of order of 10³. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 341–345 (10 September 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Reaktionskinetik der Ozonbildung in der Sauerstoffglimmentladung

Hg-light absorption at λ = 2537 ǎ by O₃ has been used to measure the concentration of O₃ in the positive column of an oxygen glow discharge under the nonstationary conditions of the Q-form (tube diameter 3:4 cm). For all gas pressures investigated (p = 7 — 12 torr) the O₃ density (n₃) as a function of the current reaches a remarkable maximum at i ≈ 15 mA. n₃ ^max was found to increase almost linearly with the gas pressure. At i = 80 mA, on the other hand, n₃ increases according to a quadratic relation. In order to understand the dependence of n₃ on the pressure and on the current density (or equivalently on the electron density n_e) a kinetic model previously developed has been used involving the following particles . For example, it can be shown that in the region of the maximum of n₃ one main production process of ozone is the associative detachment reaction . The calculated maximum of the ozone density is higher than the measured one and appears at higher electron density.     

Spectral diagnostics of a unipolar high-frequency discharge excited in nitrogen and argon at pressures from 1 to 12 atm

By methods of spectral diagnostics, the temperature of neutral gas and the electron temperature and density have been determined in the channel of a unipolar high-frequency discharge excited at very high pressures. In nitrogen the h.f. discharge was excited at pressures of 1–5 atm, in argon at pressures of 1–12 atm. In the discharge excited in argon, the electron temperature does not change with increasing pressure and isT _e =(6–7)×10³ °K; the electron density increases with increasing pressure. It can be demonstrated that the electron velocity distribution is given by a Maxwellian distribution function although the plasma of a unipolar high-frequency discharge is non-isothermal (T _e –T _n 5×10³ °K).In conclusion, the author thanks Prof. Dr. V. Truneek for stimulating remarks and his kind interest in this work.     

Comments on the Bohm Criterion and on the Determination of the Ion Velocity at the Sheath Edge in Non‐Maxwellian Plasma

This work is devoted to the study of the Bohm criterion in the general case of the electron energy distribution function (EEDF). Investigations are performed by means of a Monte Carlo integration method. We resolve the cold fluid equation system describing the ion motion within the sheath, assuming collisionless conditions, singly charged and mono kinetic incoming ions (BOHM model). Results confirm that the limit ion velocity at the sheath edge to assure a monotone electric field with a positive charge over the entire sheath is v_i ≥ (kT_e/M_i) or ε_i ≥ 1/3 <ε_e> in the case of Maxwellian electrons. We show that in the case of a Druyvesteyn electron energy distribution, this limit is larger, it is ε_i ≥ 0.6 <ε_e>. The study is also extended to other distributions functions. Because of the large controversy in recent publications, concerning the boundary conditions at the sheath entrance, we discuss the collisionless conditions at the sheath edge according to the plasma parameters. It is shown that in a collisionless sheath, the condition n_i(χ) ≥ n_e(χ) can be used to determine the limit ion velocity at the sheath edge of the negatively biased collector (Langmuir probe for instance) (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rydberg states in the D layer of the atmosphere and the GPS positioning errors

The noncoherent radiation in the frequency range 0.8–8.0 (GHz) formed in the D layer of the ionosphere at high solar activity due to transitions between Rydberg states is considered. The emitting layer thickness located 80–110 km above ground surface is estimated. A complicated irregular behavior of the frequency dependence of the radiation intensity for different values of å electron concentration n _e and temperature T _e due to different characteristics of electron scattering on the nitrogen and oxygen molecules is revealed. The dependences of the flux power of UHF radiation from the D layer in the indicated frequency range on the concentration and temperature of free electrons are calculated. It is shown that, at a frequency of ν = 1.44 GHz, the UHF radiation spectrum features a characteristic waist point, the position of which is almost independent of the electron temperature T _e ; i.e., a one-parameter dependence of the power flux on the electron n _e density takes place. In the frequency range of 4.0–8.0 GHz, the radiation spectrum exhibits a family of curves that, for each value of n _e and a wide range of T _e , give rise to a relationship known as the “bottleneck.” It was found that, with increasing frequency, the bottleneck moves upwards along a curve described by a quadratic dependence on the radiation frequency. For a frequency of ～5 GHz, and a certain range of temperature T _e and electron concentration within 5 · 10³ cm^?3 < n _e < 2 · 10⁴ cm^?3, an almost linear dependence of the UHF radiation power on n _e is observed. A comparative analysis of GPS signal delays at frequencies ν _f ⁽¹⁾ = 1.57 and ν _f ⁽²⁾ ≈ 5 GHz for various states of the ionosphere is performed. It is shown that, under the same condition, the use of the second frequency is more advantageous and informative. The ways of further development of the theory and experiment in studying the role of quantum resonant properties in the distortion of global satellite positioning system signals and in solving the fundamental problem of their elimination are discussed.     

Effect of Cr3+ ions on the magnetic moment of ferrites of the system CuFe2−x CrxO4

The dependence of the magnetic moment n _0exp of samples of the system CuFe_2−x Cr_xO₄ (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0) on their Cr³⁺ content is examined here for the first time. It is found that the experimental values of the magnetic moment n _0exp are much smaller than the values calculated from the cation distribution obtained previously (n _{0 theor}). It is suggested that this relationship (n _{0 theor}>n _{0 exp}) is due to a decrease in the magnetic moments of the Cr³⁺ ions resulting both from pairing of the t _2g orbitals of these cations in the octahedral sublattice and from a transfer of spin density from the ligands to the e _g orbitals of these ions. For compositions with x>1.0, the noncollinear magnetic structure also leads to an increase in the difference between n _{0 theor} and n _0exp. Fiz. Tverd. Tela (St. Petersburg) 40, 99–100 (January 1998)     

Non-Maxwellian Electron Velocity Distribution Resulting from Electron-SF6 Attachment Collisions

A model reaction system is considered consisting of a heat bath of argon atoms (c) and small concentrations of SF₆-molecules (m) and electrons (e) (number densities: n_e ? n_m ? n_c). Within the model the time behaviour of the electron velocity distribution function is studied under the influence of elastic electron-argon collisions and electron SF₆ attachment collisions. On the basis of the distribution function time-dependent macroscopic quantities (kinetic electron temperature. nonequilibrium rate constant of the attachment reaction) are calculated.     

To the role of positive ions in plasma nitridation

The method similar to that of determining the asymmetric componentf ₁ of electron distribution function was used to prove whether the nitrogen ions produced in discharge may be the very particles which directly influence magnesium nitridation in glow discharge. The amounts of created magnesium nitride on surfaces facing the anode and the cathode, and positive ion current to the plane double-probe were measured. It follows from experimental results that positive ions from bulk plasma are not decisive for nitride formation.The authors would like to thank Dr. V.Krejí and Dr. K.Maek for helpful discussions and critical notices.