Ionization waves (strata) in a bounded low-temperature plasma

Resonant properties of a positive column with excitation of induced strata are considered. The mechanism of steady-state amplitude buildup is established, and the conditions for the soft and hard modes of intrinsic strata excitation are obtained. The conditions under which strata are suppressed when the system is acted upon asynchronously by modulation of the discharge currents are investigated. The analysis is carried out for a bounded positive column using the generally accepted model [1, 3].Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 11, pp. 1691–1701, November, 1978.     

A Small Microwave Plasma Source for Long Column Production without Magnetic Field

A new HF device is described. It allows the production, without the use of a magnetic field, of long plasma columns from a small HF coupling structure situated at one end of the column. Its operation is based on the propagation of a cold plasma surface wave. This device can work (in argon for example) at pressures from 2 mTorr to 20 Torr with electron densities from 1010 cm-3 to 1013 cm-3, depending on plasma diameter and HF power. Typically, 80W of 500 MHz HF will produce a 25 mn diameter column of 1.8 m length. The plasma is quiescent (low electron density fluctuations), efficient (~ 100% absorbed power), and perfectly reproducible. It can be used as a substitute for a positive column, and some practical applications are foreseen in ion production, laser excitation, gas preionization and spectroscopic sources.     

Stochastic stability of a fractional viscoelastic column under bounded noise excitation

The stability of a viscoelastic column under the excitation of stochastic axial compressive load is investigated in this paper. The material of the column is modeled using a fractional Kelvin–Voigt constitutive relation, which leads to that the equation of motion is governed by a stochastic fractional equation with parametric excitation. The excitation is modeled as a bounded noise, which is a realistic model of stochastic fluctuation in engineering applications. The method of stochastic averaging is used to approximate the responses of the original dynamical system by a new set of averaged variables which are diffusive Markov vector. An eigenvalue problem is formulated from the averaged equations, from which the moment Lyapunov exponent is determined for the column system with small damping and weak excitation. The effects of various parameters on the stochastic stability and significant parametric resonance are discussed and confirmed by simulation results.     

Nonlocal phenomena in the positive column of a glow discharge in molecular gases

The influence of nonlocality of the electron energy distribution function on the parameters of a positive column of a molecular gas discharge at a mean pressure from the range 1 ≤ pR ≤ 10 cm Torr is studied. Under these conditions, the electron energy relaxation length is small compared with the characteristic size of the plasma region; that is, calculation can be carried out in the local approximation. For fast electrons at the periphery of the discharge, where the ambipolar field exceeds the longitudinal one, the local approximation may introduce significant errors into the basic parameters of the positive column. The nonlocality of the electron energy distribution function is found to increase excitation constants from the center of the discharge to its periphery.     

Effect of acoustic flows on the structure of a constricted glow discharge in argon

Numerical experiments on the effect of acoustic flows on the structure of a constricted glow discharge in argon have been performed in the hybrid approximation. The possibility of controlling the combustion regime of the glow discharge with an extended positive column at a high pressure by means of the formation of acoustic flows at the excitation of a standing acoustic wave has been demonstrated. In this case, the discharge transfers from the constricted combustion regime to the diffuse one and becomes stable.     

Der Einfluß der Elektronenverteilungsfunktion auf Mikrowellenemission und Hochfrequenzleitfähigkeit eines schwach ionisierten Entladungsplasmas

We have investigated the influence of the distribution function on the microwave emission and the rf-conductivity. In the positive column of low current discharges through neon and argon the electrons are Druyvesteyn-like distributed, owing to the absence of Coulomb-interactions. In this case the radiation temperature is a function of the dc-field in the positive column, of the collision frequency for momentum transfer and the radiation frequency. With only elastic and excitation terms in the Boltzmann equation we have computed the distribution functions. These results are used to calculate the radiation temperature by means of Bekefis formula. Agreement is obtained between the experimental and theoretical results. The rf-conductivity does not depend very sensitively on changes in the distribution function; nevertheless the high values of the electron collision frequency obtained from the real part of the rf-conductivity could be explained only with a Druyvesteyn-like distribution.     

Relaxation of the pulsed opto-galvanic current signal in a helium positive column

Transient behaviour of an opto-galvanic (OG) current signal induced by pulsed dye laser excitation has been investigated on the He I 388.9-nm (2³S-3³P) transition in a He positive column. The OG current signal is composed of a sharp spike at the early time stage followed by a slowly-varying component. The decay time of the sharp OG current signal has been found to be almost independent of the dc current substaining the discharge and to depend sensitivity on a product of pressure and column-radius. The experimental results show that the dominant decay process of the OG current consists in an ion current to the walls, as has been previously referred to in a mmodel of a cw OG effect in a He positive column. A model for the pulse OG effect in the early time stage is presented on the basis of the experimental results. The decay times of the OG current signals calculated with the model are in agreement with the measurements within an accuracy of 30%.     

稀薄等离子体中激发尾波场的共振条件

研究了在稀薄等离子体中强激光激发尾波场的情况 ,发现尾波场的激发与入射激光的脉冲宽度有共振现象。在光强很小情况下 ,共振所需要的入射激光脉冲宽度为λp 2 ,随着光强的增大共振激光脉冲宽度减小。同时发现在稀薄等离子体中激发的尾波势场与等离子体的密度几乎无关 ,而激发的尾波场最大电场强度与等离子体的密度有关。     

Liquid optical fibers with a multistable core actuated by light radiation pressure

We report on spatiotemporal behavior of self-adapted dielectric liquid columns generated and sustained by light radiation pressure. We show that single- or multivalued liquid column diameter depends on the excitation light beam. When the beam diameter is sufficiently small, we observe a well-defined stationary column diameter. In contrast, at a larger beam diameter, the liquid column experiences complex spatiotemporal dynamics whose statistical analysis evidences an underlying multistable structure. Experimental observations are all supported by a full electromagnetic model that accounts for the wave guiding properties of the liquid column viewed as a step-index liquid-core liquid-cladding optical fiber having an optically tunable core diameter.     

Critical Current of the Ionization Waves and Anomaly of the Positive Column in an Axial Magnetic Field

Influences of an axial magnetic field on the ionization wave and the positive column in rare gas discharges are studied experimentally. The upper critical current I_c for the appearance of ionization waves in the magnetic field B is newly found. As B is gradually increased, the value of I_e slightly increases from the Pupp's value and after passing a prominent maximum, finally become very small. In addition an anomaly takes place in the axial electric field E of the positive column stable for helical instabilities. With increasing the magnetic field the value of E goes through a weak maximum before decreasing. It is concluded that this anomaly, apparently incompatible with classical diffusion theory, is closely related to the appearance of ionization waves in the positive column.     

On the electron energy distribution in the gas discharge positive column: Langmuir paradox

The results of calculations of electron drift characteristics in a dc spatially inhomogeneous periodic electric field are presented. It is shown that the effect of field inhomogeneities on the drift velocity and the average electron energy is insignificant under typical conditions of experiments with gas-discharge plasma at low gas pressures. However, the intensity of the processes of excitation, ionization, and plasma spatial distribution are strongly affected by the inhomogeneity (variance) and field variation behavior. It is shown that the electric field inhomogeneity in the gas discharge positive column leads to maxwellization of the electron energy distribution function.     

Remarks on the Energy Balance of Low Pressure Discharges

Including excitation and de-excitation processes, direct and stepwise processes of the ionization, the particle balance and the energy balance of a positive column are jointly studied. It is shown that even near the axis the energy flux due to the radial electron drift can be only neglected provided that the mean electron energy is considerably less than the ionization energy. In some cases, it appears easier to apply the total energy balance instead of the energy balance of the electron gas alone.     

Determination of Plasma Density Profile and Other Parameters of Cylindrical Plasma Columns with an Electroacoustic Probe

A new diagnostic method based on the excitation of thermal resonances using an electroacoustic probe has been developed to determine the electron density profile and other plasma parameters of a cylindrical warm-plasma column. A coaxial electroacoustic probe is used to excite and detect the dipole and thermal resonances as the plasma density is varied. The electron density profile is expressed functionally in terms of unknown profile parameters. The profile and other parameters of the plasma column are then determined numerically based on some theoretical formulas and experimental results.     

Excitation of multilayered plasma columns with the ring of magnetic currents

The excitation of gaseous plasma column having semiconductor plasma columns on either side is studied with the excitation source being the ring of magnetic currents. The whole structure being cylindricalin shape, contains the conductor along its axis. The effect of radii of all the columns including that of the ring on radiation pattern, is studied which shows a peak of maximum radiation in a certain direction along with some other less ntense peaks. In most cases the peak occurs roughly at 61° at which fine structure is given. Also three intense peaks of radiation occur when radius of the plasma (outer semiconductor) column is 0.058 m.     

Excitation and populations of the 2s2 AND 2p4 levels of neon

The populations of the 2s₂ and 2p₄ levels on neon in a positive column were determined for various pressures and currents. The populations were found by a method which obviates the need to measure absolute line intensities. The populations of the 2s₂ level were computed from temperatures and electron concentrations determined by probe measurements. The results were compared with experimental findings. Conclusions concerning the excitation mechanism of the 2s₂ and 2p₄ level are drawn.     

Non-linear gas oscillations in a pipe

The results of an experimental study of non-linear longitudinal gas oscillations in a closed tube are presented. Forced oscillations greater than in other experiments to date are obtained. A brief representation of the theory holding for excitation frequencies near and equal to the natural frequencies of a gas column is given. The experimental amplitude and the periodic shock wave form are compared with the calculated values. Oscillations whose excitation frequencies are twice as small as the first natural frequency are also studied.     

Near-IR Luminescent Rare Earth Ion–Sensitizer Complexes

Rare earth ions, with relatively long luminescence lifetimes, have significant advantages for application in fields as varied as diagnostics and optical amplification. In diagnostics the long luminescence lifetimes allow for extremely sensitive time-gated detection, where the difference in temporal behavior of scatter and background fluorescence and the long-lived rare earth luminescence is utilized. In optical amplification the long excited-state lifetime makes it easier to obtain population inversion, a requirement for effective stimulated emission. Unfortunately the absorption cross section of rare earth ion transitions is extremely low. However, via sensitized excitation by means of a suitable organic molecule, efficient excitation is obtained. It is shown that excitation in the visible part of the spectrum can be used to excite rare earth ions which luminesce in the near-IR, such as ytterbium, neodymium, and erbium, via a fluorescein-derivative as sensitizer. The advantages of this approach are manifold. Low-cost light sources are available for the visible part of the spectrum, and interferences from the matrix (scatter, absorption) are minimal. Detection in the near-IR is almost interference-free. For optical amplification the wavelength regions around 1300 and 1550 nm, which can be covered with the neodymium and erbium complexes, respectively, are the most important for applications in optical telecommunication.     

泵浦光激发下金刚石纳米柱内部光场的研究

金刚石纳米柱是实现色心单光子源增强的有效结构,而纳米柱结构尺寸决定泵浦光对色心的激发强度.本文为提高其激发效率,采用时域有限差分法(Finite Difference Time Domain,FDTD)研究金刚石纳米柱的光学性质.通过仿真分析了纳米柱直径和高度对内部电场强度的影响.结果表明,在泵浦光波长为532 nm时...     

Dynamics of vibrating systems with tuned liquid column dampers and limited power supply

Tuned liquid column dampers are U-tubes filled with some liquid, acting as an active vibration damper in structures of engineering interest like buildings and bridges. We study the effect of a tuned liquid column damper in a vibrating system consisting of a cart which vibrates under driving by a source with limited power supply (non-ideal excitation). The effect of a liquid damper is studied in some dynamical regimes characterized by coexistence of both periodic and chaotic motion.     

Electron scattering by atoms at intermediate energies II. Theoretical and experimental data for light atoms

Experimental data for the scattering of electrons by selected light atoms at intermediate energies have been collected and compared with the predictions of the theoretical models discussed in part I. The data discussed include elastic scattering and discrete excitation but ionization is not covered. The target atoms include H, He; the alkali metals; the alkaline earths; C, N, O and the rare gases from Ne to Xe, together with certain positive ions isoelectronic with these neutral atoms.