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.     

Excited atoms in argon gas discharge plasma

General principles are discussed for a gas discharge plasma involving excited atoms where electron-atom collision processes dominate. It is shown that an optimal kinetic model of this plasma at not large electric field strengths can be based on the rate constants of quenching excited atom states by electron impact. The self-consistent character of atom excitation in gas discharge plasma is important and results in the tail of the energy distribution function of electrons being affected by the excitation process, which in turn influences the excitation rate. These principles are applied to an argon gas discharge plasma where excitation and ionization processes have a stepwise character and proceed via formation of argon atom states with the electron shell 3p ⁵4s.     

Spektroskopische Untersuchungen über das thermodynamische Gleichgewicht der Anregung in den Plasmen kurzdauernder Impulsentladungen

The light emission of gases conducting electric discharges is of considerable complexity. The observed phenomenas are controlled to a great part by the electric parameters of the circuit, as well as by the time constants of the specific plasma properties as the relation times for reaching Maxwellian distribution and thermal equilibrium of electrons, ions, and atoms. The influence of the nonequilibrium between the excitation and the electron temperature on the population of states was studied in short time discharges by means of singulet and triplet lines of elements of the second group of the periodic system. In these discharges a nonequilibrium was found between the excitation and the electron temperature. While the time constant measured for the cooling of the electron gas was found to 7·10^?7 s in a discharge switched off 9·10^?7 s past the ignition the minimal time constant of relaxation of excitation is equal to the life time of the 2p ₂ state — this value of cadmium being 2.4·10^?6 s -.The use of Boltzmann and Saha equation must be considered very carefully in this type of short time discharges.     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

The energy of thermal electrons in electron beam created helium discharges

We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s³S?np³P He I series. Values of kT_e between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He?Hg electron beam created plasma is calculated. The results are applied to the Hg⁺ laser pumping scheme using a electron beam created He?Hg plasma.     

约化场强对氮-氧混合气放电等离子体演化特性的影响

采用零维等离子体动力学模型,计算了不同约化场强条件下N₂/O₂放电等离子体的演化特性.结果表明,平均电子能量与约化场强有着近似的线性关系,在约化场强为100 Td时,平均电子能量约为2.6 eV、最大电子能量达35 eV;约化场强是影响电子能量函数分布的主要因素.气体放电过程结束后,振动激发态氮分子的粒子数浓度不再变化,电子激发态的氮分子、原子和氧原子的粒子数浓度达到一峰值后开始降低;放电结束后的氧原子通过复合反应生成臭氧.约化场强升高,由于低能电子减少的影响,振动激发态氮分子的粒子数浓度降低,当约化场强由50 Td增加75 Td,100 Td时,粒子数浓度由3.83×10¹¹ cm^-3降至1.98×10¹¹ cm^-3和1.77×10¹¹ cm^-3,其他粒子浓度则相应增大. 关键词： 等离子体 约化场强 粒子演化 数值模拟     

Calculation of the parameters of excimer light sources on the basis of a positive column glow discharge

We calculate the concentration of plasma and gas-temperature components in a contracted filament of a glow capillary discharge (R = 0.75 mm) in xenon for pressures of P = 100 and 400 Torr and currents of I = 6–15 mA for cases of with and without cryogenic cooling of the discharge. We find that the gas temperature in the channel of the glow discharge has a value of 1000–2000 K, the concentration of xenon excimers attains a maximum at the boundary of the filament with a value of 10¹⁰–10¹¹ cm⁻³, and the efficiency of electric energy transformation into excimer radiation energy has a value of 0.1–5%.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

Numerical Simulation of Ne–Tl and Ne–Ga Ion Hollow-Cathode Lasers

Results of theoretical and experimental studies of the behavior of the level populations and gain factors for the np ^1,3 P – ns ^1,3 S and np ^1,3 P –> (n-1)d ^1,3 D transitions in Tl II (n = 7) and Ga II (n = 5) are presented. These levels are filled due to the charge exchange in thermal collisions of neon ions with thallium and gallium atoms in thallium--neon and gallium--neon mixtures, respectively. The total pumping rate of all levels of a metal due to the charge exchange in the hollow-cathode discharge plasma at the operating concentrations of metal vapors was shown to be equal to the gas ionization rate. The latter is defined by the number of fast electrons and is independent of the charge-exchange cross section. The exciting and deexciting collisions with slow electrons and gas atoms were taken into account for all Tl II and Ga II levels related to the laser ones, and the radiation trapping by the resonance transitions of metal ions was considered. The partial pumping rates by the charge exchange were found for levels with 0 < E() < 1.5 eV. Theoretical results are compared with experimental data. The behavior of the lasing characteristics is explained for the known laser transitions. The parameters of the as yet unknown transitions in the IR range are predicted.     

Ne(I) spectral lines from a Ne-O2 d.c. glow discharge

Measurements have been made of intensities of the spectral lines emitted from an Ne-O₂ d.c. discharge with small discharge current (1–4 mA) under the following conditions: gas pressures of 2 and 3 torr and oxygen partial pressures (P₀₂) up to 0.1 torr. All of the Ne(I) line intensities observed decrease when O₂ is added. The Ne(I) λ5852 line (1s₂-2p₁) has been studied in detail as a representative example. The population density of the 2p₁ level of neon has been obtained from the intensity measurements as a function of P₀₂. The energy-distribution function of electrons has been determined using Druyvesteyn's method in order to calculate the population density for a corona model. The high-energy tail of the measured distribution function is markedly reduced when O₂ is added. It is shown that inelastic collisions of electrons with O₂ produce large energy losses for the electrons. These cause a decrease in population density of the 2p₁ level when O₂ is added. The population density of the 2p₁ level at a gas pressure of 2 torr is 1.2×10⁴ cm^-3 in pure neon and 5.2×10² cm^-3 in an Ne-O₂ mixture (P₀₂ = 0.01 torr). The electron densities and average electron energies are 3.5×10⁸ cm^-3 and 8.7 eV and 1.7×10⁸ cm^-3 and 5.3 eV, respectively, for the specified two cases.     

Kinetics of luminescence induced by sputtering metallic cadmium by a pulsed fast electron beam in helium

Results of an experimental study of the kinetics of luminescence observed when a metallic cadmium foil is bombarded in a helium medium by a 3-ns pulsed beam of 150-keV fast electrons are reported. The foil was irradiated at gas pressures from 76 to 2280 Torr. At a foil temperature of T = 240° C, the de-excitation time of the Beitler levels of the Cd II ion was measured as a function of the buffer gas pressure and the constant of collision quenching of the 5s22D_5/2 level of Cd II by He atoms was determined as k ≈ 3 × 10^-29 cm⁶/s. The experimental data were compared with calculations performed for the gas—vapor mixture in order to find the fraction of excited Cd II ions in the 5s²2D_5/2 state produced directly as a result of sputtering of metallic cadmium by high-energy electrons and by components of the helium plasma. At a helium buffer gas pressure of P ≤ 2.5 atm and a temperature of the cadmium target of T = 240° C, the value of this quantity was found to be α = 0.28 + 0.23P (where P is the helium pressure in atmospheres).     

Generation of H− ions in a low-pressure xenon-hydrogen discharge. I

A theory of a low-pressure discharge in a xenon-molecular hydrogen mixture is developed. It is shown that, in such a discharge, at an interelectrode distance of L = 1 cm and a total plasma pressure of p ₀ ～ 1 Torr, the density of negative hydrogen ions produced via the dissociative attachment of thermal electrons to vibrationally excited molecules H₂ can reach a value as high as N_H ^? ≥ 10¹² cm^?3. According to calculations, the electron temperature in discharge operating regimes under study attains T _e ≈ 1?2 eV, which corresponds to the maximum of the e-v exchange rate constant of H₂ molecules. This ensures a relatively high rate of vibrational pumping of H₂ molecules in the discharge.     

Level-crossing-spectroscopy with additional optical pumping in the ground state

The level crossing-technique with additional optical pumping in the ground state is demonstrated in the case of the hyperfinestructure of the 6p ¹ P ₁ state of the two odd isotopes of Yb using a natural occuring isotopic mixture. The influences of the pumping process and of a buffer gas on the lc signal structure are discussed in terms of a modified Breit formula.     

Dynamics of electron-hole plasma in semiconductor opening switches for ultradense currents

A physicomathematical model for calculating the dynamics of the electron-hole plasma in semiconductor opening switches for ultradense currents is developed. The model takes account of the real doping profile of a semiconductor p ⁺-p-n-n ⁺ structure and the following elementary processes in the electron-hole plasma: current-carrier diffusion and drift in high electric fields, recombination on deep impurities and Auger recombination, and collisional ionization in a dense plasma. The electrical pumping circuit of the opening switch is calculated by solving the Kirchhoff equations. The motion of the plasma in the semiconductor structure is analyzed on the basis of the model. It is shown that for ultrahigh pumping levels the interruption of the current in the opening switch occurs in the heavily doped regions of the p ⁺-p-n-n ⁺ structure and is due to saturation of the particle drift velocity in high electric fields. Zh. Tekh. Fiz. 67, 64–70 (October 1997)     

Hot electron plasmas trapped in helical magnetic surfaces

Experimental studies on nonneutral (pure electron) plasmas of finite temperature, trapped in helical closed magnetic surfaces have been conducted. The helical electron plasmas are produced with thermal electrons launched from the outside of the last closed flux surface (LCFS). About 150 μs after the electron injection, the plasmas reach equilibrium state. Around the LCFS, a steep gradient of plasma space potential φ _s is formed. The corresponding radial electric field is about 2.5 kV/m. On the other hand, around the magnetic axis of helical magnetic surfaces, φ _s is almost constant, indicating that there are little electrons there. The volume-averaged electron density is on the order of 10¹³ m^–3, smaller than the Brillouin density limit. The confinement time seems to be limited by a disruptive instability, and is so far about 1.5 ms.      

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

On the role of exchange interaction in magnetic ordering and conduction of manganates

A model of chemical bonds in manganates that allows for one-electron covalent σ bonding between manganese and oxygen ions is suggested. One-electron covalent bonding results in the strongly correlated state of electrons due to exchange interaction between the electrons when they are shared by the cation and anion orbitals. The correlated state shows up as the spatial ordering of electrons and the ordering of their spins, causing the spin-ordered electron lattice to form. In this model, electrical conduction in manganates takes place when the electron lattice (more precisely, its part) shifts from one site of localization to another. The conductivity of the material depends on the type of the spin order of electrons in the electron lattice and on the energy of localization, which is defined by the energy of one-electron σ bonding. The model also implies the strong cationic polarization of anions, which facilitates the 3s2p hybridization of anions and the transition of one of the pairs of 2p electrons from the singlet state to the triplet one. The 3s2p hybridization of anions favors the formation of the spin-polarized electron lattice (the electron spins are parallel) and the ferromagnetic ordering of manganese ions. Under these assumptions, the effect of giant magnetoresistance is explained by a change in the conduction mechanism when an external voltage is applied. In this case, the conduction mechanism typical of ionic crystals changes to that specified by the spin-polarized electron lattice.     

Inversion of electron spins in CaO with the reversal of the magnetic field

We have obtained population inversion of a system of polarized (P=80%) electron spins in a solid during a very fast (dB _z dt=4·10⁵ T·s ^–1) reversal of the external magnetic field. The electrons were trapped at oxygen vacancies in CaO single crystals. This method, whch does not exploit any high frequency electromagnetic field, has been for the first time successfully used to achieve an inverted state of electron spins in a solid. The negative temperature of an electron spin ensembleT=–23 mK has been obtained.     

Ionenschallinstabilitäten in Edelgas-Gleichstromentladungen bei niedrigen Drücken. I. Meßanordnung und experimentelle Ergebnisse

An experimental study of self-excited ion acoustic waves with wavelengths greater than the radius of the discharge tube in a de-low pressure column is presented. The propagation of this type of waves was observed over a pressure range of 5 · 10^?4 torr ≦ p ≦ 10^?1 torr and currents of 0,02 A ≦ i ≦ 0,6 A in various gases (argon, neon, helium, and hydrogen) in cylindrical glass tubes with diameters of d = 2;4 and 6 cm. The Dispersion behaviour and the existence range were measured in dependence of the internal parameters of the discharge (characteristic velocities and collision frequencies) and the geometry of the discharge tube. It is shown, that the existence range depends not only on gas pressure and current but is influenced also by the geometry of the discharge tube. The minimum wavelengths which belong to the upper cut-off frequencies correlate with the radius of the discharge tube at currents higher than i ～ 0,2 A and increasing pressure.     

Electron resonance studies of the renner effect

The gas phase electron resonance spectra of NCO in its ground ²Π_3/2 vibronic state and in two excited vibronic states are described. Theoretical analysis of the spectra yields effective g values for the three states. In additon the ¹⁴N magnetic hyperfine and electric quadrupole coupling constants and the electric dipole moment are determined. The theory of the Renner coupling of electronic and vibrational motion is extended, and shown to account for anomalous contributions to the g values. The theory also shows that these contributions are closely related to the Renner coupling constant.