Low-current helium discharge at low pressures

On the basis of good knowledge of the energy distribution function of electrons corresponding to the conditions of low-current discharge the electrons as well as the excited helium atoms are balanced. A model of helium atom possessing four excitation levels (2³S, 2¹S, 2³P, 2¹P) is assumed. The calculated volt-ampere characteristic is compared with an experiment and the relative densities of excited atoms are given. For these quantities approximate formulae are derived. The laws of similarity, ionization mechanism and energy balance of the electron gas are discussed.The authors gratefully acknowledge many valuable discussions with Dr. L. Pekárek, Mr. E. Vokatý and Mr. K. Rohlena. We wish to express our thanks to Mr. J. Moudrý for his help in the computation.     

Phonons and electron-phonon interactions in rare-gas crystals at high pressures

The lattice dynamics of compressed rare-gas crystals is theoretically investigated within the ab initio approach in the framework of the Tolpygo model, which explicitly allows for the deformation of electron shells. The deformation of the electron shells is associated with the retardation of the electron response and treated as a nonadiabaticity (the electron-phonon interaction). This approach and the ab initio short-range repulsive potentials are used to construct the dynamic matrix, which makes it possible to calculate the phonon frequencies and the electron-phonon interaction of crystals in the series Ne-Xe at any point of the Brillouin zone. The contributions of the long-range Coulomb and van der Waals forces to the dynamic matrix are the structure sums that depend only on the lattice type. The structure sums for the face-centered cubic lattice are calculated using the Ewald and Emersleben methods, as well as the direct summation over the vectors of the face-centered cubic lattice. The use of 20 spheres in the last case provides an accuracy of no less than four significant figures. An analysis of the role played by the phonon-electron interaction at five points of high symmetry in the Brillouin zone (X, L, U, K, W) at high pressures demonstrates that not only the longitudinal phonon modes (at the points X and L) but also the transverse phonon modes (at the points U, K, and W) are softened. The inclusion of the electron-phonon interaction at the point X improves agreement between the theoretical and experimental phonon frequencies for the argon crystal.     

Study of a positive corona discharge in argon at different pressures

The corona discharge in argon at atmospheric pressure has been studied by means of a 2D model. The reduced characteristic derived from the experimental data has been described by linear regressions for the different pressures and the two studied inter-electrode distances thus confirming the validity of Townsend's approximation also in case of point to plane configuration and argon as process gas. The model validated this hypothesis which has been attributed to the minor influence of space charge in the ionization zone. Its effect is, on the other hand, more significant in the drift zone where the electric field is greatly enhanced, leading, for higher currents, to the formation of a spark gap. Electron and ion distributions allow the influence of structural (electrode configurations and distance) and operative (pressure and discharge current) parameters to be evaluated including the current loss due to diffusion through different confining boundaries.     

Surface microwave discharge at high air pressures

Microwave discharge on the surface of a quartz antenna at high air pressures is investigated. It is shown that the discharge formation is accompanied by the excitation of shock waves and that the longitudinal velocity of the discharge propagation reaches several kilometers per second. Owing to these circumstances, discharge of this type discharge may find applications in supersonic aerodynamics.     

Experimental determination of dust-particle charge in a discharge plasma at elevated pressures

The charge of dust particles is determined experimentally in a bulk dc discharge plasma in the pressure range 20-100 Pa. The charge is obtained by two independent methods: one based on an analysis of the particle motion in a stable particle flow and another on an analysis of the transition of the flow to an unstable regime. Molecular-dynamics simulations of the particle charging for conditions similar to those of the experiment are also performed. The results of both experimental methods and the simulations demonstrate good agreement. The charge obtained is several times smaller than predicted by the collisionless orbital motion theory, and thus the results serve as an experimental indication that ion-neutral collisions significantly affect particle charging.     

Glow discharge in rare-gas and metal vapour mixture

Conclusion The results are summarized in the form of graphs which illustrate the dependence of the rate coefficients on three parameters: E/N (electric field over total neutral concentration) x_e (ionization degree) and n₁ (relative concentration of cadmium). In such a way, we have tried to point out the main feature of the collisional rates under the conditions of steady-state, weakly ionized plasma. The use of the maxwellian distribution function for an evaluation of the rate coefficients would yield the results which differ substantially from ours. This concerns, particularly, the excitations from the ground state of atom efficacy of which depends on the high energy distribution tail. Since the true distribution function from Ref. [1] was employed in our calculations, it can be inferred that the main source of the inaccuracy in the rate coefficients is essentially given by the cross-sections used. It is believed that the present results can serve for a more quantitative understanding or for obtaining insight into the microscopic phenomena occurring in the positive column of the discharge.     

Glow discharge in rare-gas and metal vapour mixture

Applying the Boltzmann equation to a He-Cd mixture discharge the electron energy distribution functions, kinetic coefficients and collision frequencies are numerically calculated. Calculations are made for a homogeneous and stationary discharge plasma subjected to an externally applied electric field. The collision processes which have been taken into account are elastic and inelastic collisions of electrons with He and Cd atoms as well as mutual encounters of electrons. In this case the electron energy distribution and all the quantities calculated from it are dependent on the reduced electric field, the ionization degree and the relative cadmium concentration.     

Broadband radiation from rare-gas plasmas excited by a modulated rf discharge

An investigation was made of plasma radiation spectra created by a modulated rf discharge having an anomalously low operating voltage, in krypton, xenon, argon, and neon. Broadband radiation was recorded in the ultraviolet and visible spectral regions.Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences. Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 32–36, October, 1995.     

干涉法测量气体低压下声速的实验研究

介绍了干涉法测量气体声速的试验原理和变程干涉声速测量的实验系统,使用本套测量系统测得的气体声速的精度可达5×10-4。根据热力学原理,分析了气体声速与理想气体比定压热容之间的物理关系。在本套实验系统上测得了一批高精度的新环保制冷剂的气相声速数据,并可进一步确定其理想气体比定压热容.     

Two types of current oscillations in a low-current spark discharge initiated in a nonuniform electric field

Experimental data for a spark discharge in the tip-plane electrode configuration in argon are considered for the case when the discharge current is limited by a high-resistivity ballast resistor. It is shown that the current passing in the thin plasma channel of a low-current spark represents a steady sequence of regular pulses. It is found that low-frequency current pulses of the low-current spark are accompanied with high-frequency current oscillations of nanosecond duration.     

Generation of high-energy electrons in a transverse slot-cathode nanosecond discharge at working gas medium pressures

Experimental data for the electrical and optical characteristics of a transverse slot-cathode nanosecond discharge are reported. The discharge is initiated in He at a discharge current of 1–500 A and a working gas pressure in a discharge chamber ranging from 10² to 10⁴ Pa. It is shown that the cathode current density is much (several orders of magnitude) higher than the total current density of an equivalent abnormal discharge. The electrical characteristics of an open discharge and a discharge confined by dielectric walls are found to differ considerably. Electrons passing through the cathode fall region acquire a high energy (on the order of 1 keV) under the given conditions. The fast electron relaxation conditions correlate with the initiation and evolution of the discharge. A pattern of the discharge evolution is derived from experimental data. A way of estimating the coefficient of electron emission from the cathode plasma is suggested.     

Acoustic cavitation at low gas pressures in PZT-based ultrasonic systems

The generation of cavitation-free radicals through evanescent electric field and bulk-streaming was reported when micro-volumes of a liquid were subjected to 10 MHz surface acoustic waves (SAW) on a piezoelectric substrate [Rezk et al., J. Phys. Chem. Lett. 2020, 11, 4655–4661; Rezk et al., Adv. Sci. 2021, 8, 2001983]. In the current study, we have tested a similar hypothesis with PZT-based ultrasonic units (760 kHz and 2 MHz) with varying dissolved gas concentrations, by sonochemiluminescence measurement and iodide dosimetry, to correlate radical generation with dissolved gas concentrations. The dissolved gas concentration was adjusted by controlling the over-head gas pressure. Our study reveals that there is a strong correlation between sonochemical activity and dissolved gas concentration, with negligible sonochemical activity at near-vacuum conditions. We therefore conclude that radical generation is dominated by acoustic cavitation in conventional PZT-based ultrasonic reactors, regardless of the excitation frequency.     

Structural transitions in ice samples at low temperatures and pressures

The structure of ice samples formed in the decay of a water impurity gel at temperatures above 4 K and atmospheric pressure has been examined. The X-ray diffraction analysis indicates that three phases coexist in the initial sample at temperatures of 85–110 K. These phases are amorphous ice occupying up to 30% of the sample volume, cubic-phase ice I _c metastable at low pressures (∼60%), and normal hexagonal ice I _h (≤6%). The characteristic sizes of crystals of the cubic and hexagonal phases are about 6 and 30 nm, respectively. The amorphous phase at annealing above 110 K is gradually transformed to the crystalline phase both cubic and hexagonal. This transition is accompanied by two processes, including a fast increase in the sizes of cubicphase nanocrystals and the partial transition of the cubic phase I _c to the hexagonal one I _h. Hexagonal ice I _h prevails in the bulk of the sample above 200 K.