Electron-optics study of the development of an electric discharge in a gas at high-electric field intensities and with one-electron ignition

A study was made of the luminous region in a discharge gap with copper electrodes, a field of E = 80 kV/cm, air at atmospheric pressure, and a gap width of 4 or 2 mm. The cathode was irradiated with a spark to ignite the initial electrons. The electron current from the cathode was 0.2 · 10⁹ electrons/sec. The exposure time per frame was 3 · 10^–9 sec. With a gap width of = 4 mm, a luminous region is observed at the cathode 2 nsec after voltage is applied to the gap; this region propagates toward the anode, simultaneously increasing in diameter, at a velocity of 10³ cm/sec. A voltage drop is established across the gap approximately 0.5 nsec after the luminous front arrives at the cathode. In narrower gaps, the voltage drop is established across the gap a considerable time after the luminous region has crossed the gap.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 24–27, November, 1969.     

Observation of the luminescence between the electrodes during the rise in the spark current in a vacuum breakdown at constant voltage

The instants corresponding to the appearance of luminescence in three regions of the interelectrode gap (at the anode and cathode and in the center of the gap) were determined by means of a photomultiplier and compared with the onset of the rise in current during a dc spark vacuum breakdown. Only the luminescence at the cathode started at exactly the same moment as the rise in current. It appeared that the irreversible breakdown in the vacuum insulation during a dc breakdown was due to the development of cathode jets, as in the case of a pulsed discharge.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 12–16, March, 1973.The authors wish to thank G. A. Mesyats for interest in this work and R. B. Baksht for active participation in discussing the results.     

Über den zeitlichen Verlauf des Stromes einer Gasentladung in Wasserstoff

The transient growth of currents in a Townsend gas discharge system under uniform dc field conditions in Hydrogen is examined. The discharge is started by 10³ to 10⁶ electrons released from the cathode by an UV light pulse within some 10^?7 seconds. Observed oscillations of the current are found to be due to the motion of the electrons through the gap creating new electrons by photoelectron emission at the cathode due to photons generated in the gap. At sparking threshold conditions (Μ ₀=1) the electron current becomes self-sustaining after a few electron transit times. The positive ion current soon exceeds the electron current and grows linearly with time (Μ ₀=1) until the positive ions of the first generation enter the cathode. For times greater than a positive-ion transit time the current becomes self-sustaining. Neglecting space-charge effects one would not expect a spark to occur. The space-charge of the positive ions, however, causes a distortion of the field changing the ionization efficiency of the electrons. It is shown that an observed rapid growth of current leading to breakdown after some positive-ion transit times is in agreement to this conception.     

Time resolved spectroscopic studies of a FXR discharge

Time resolved spectroscopic studies of some electrode material lines and an impurity line have been performed in a flash X-ray (FXR) discharge. The results indicate that the breakdown takes place in impurities and adsorbed gases released from the electrodes. Thereafter, a front of anode material propagating with a velocity of (1–2)×10⁶ cm/sec moves towards the cathode. A cloud of cathode material is localized outside the cathode during an early stage of the discharge. The anode front and the cathode cloud penetrate each other after current maximum and a “metallic arc” is formed between the electrodes     

Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge

An optogalvanic method is used to measure photoemission coefficient γ_ph in a gas discharge exposed to the resonant radiation of helium atoms. The range of working current j/P _He ² (j is the current density, and P _He is the gas pressure) extends from 2 to 1000 μA/(cm² Torr), and field strength E/N at the cathode varies from 0.45 to 13 kTd. Up to j/P _He ² = 10 μA/(cm²Torr²), photoemission coefficient γ_ph grows and then tends toward saturation at a level of γ_ph = 0.30 ± 0.01. Under the no-discharge conditions, γ_ph = 0.35 ± 0.05. It is concluded that the emissivity of cold cathodes in a gas discharge is governed by adsorption of the working gas on the cathode surface and its implantation into the cathode. With allowance for this factor, the contribution of photoemission to the discharge current is reconsidered. It is shown that, for cathodes with diameter d _c ? l _c (l _c is the length of the cathode layer), a normal or weakly abnormal glow discharge in noble gases is largely of a photoelectron character. In light noble gases, the photoelectron character of the discharge persists even for a strongly abnormal discharge. The energy dependences of coefficients γ of kinetic and potential emission in helium are calculated with allowance for implantation of helium atoms into the cathode and compared with published data. The influence of particle implantation on γ in a vacuum is estimated.     

Analysis of statistical nature of electrical breakdown time delay in nitrogen at 6.6 mbar pressure in presence of positive ions and N(4S) atoms

On the basis of the mean value of electrical breakdown time delay and the standard deviation of electrical breakdown time delay σ dependence on the afterglow period τ, the mechanisms which dominantly influence to breakdown initiation in nitrogen were separated. It was shown that the positive ions formed in mutual collision of long‐lived metastable molecules have a dominant role in breakdown initiation for τ values up to 70 ms. The metastable molecules were derived from previous breakdown and discharge. In this case σ ≪ t_d and the total time delay t_d is approximately equal to formative time t_f which decreases in value with the increase of overvoltage. When positive ions have a dominant role in the breakdown initiation the Gaussian distribution describes data of t_d ≈ t_f very well. For τ > 1 s, N (⁴S) atoms formed in previous breakdown and discharge, have a dominant role in the breakdown initiation. Then, σ ≈ and Laue's distribution, which is valid for statistical time delay t_s, describes t_d data very well (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New ways of estimating the energy of pulsed sources,according to the results from recording acoustic waves in the atmosphere

The problem of developing relations that allow us to increase the accuracy of estimates for energy E of a pulsed source of acoustic waves in the atmosphere is solved by generalizing experimental data on time t _R+ of pressure growth to the peak value P ₊ in the first positive phase of acoustic signals from different sources in a wide range of energies (10^?8 < E < 10¹⁰ kg of TNT) and reduced distances (10 < RE ^?1/3 < 4 × 10⁴ m kg^?1/3). In addition to a new way of estimating energy E of a pulsed source, a way of estimating distance R from the source is also proposed. Innovative science also reveals a change in the law of the increase in parameter t _R+ as distance R from a source grows.     

Photographic appearance of high-current vacuum arcs prior to and during anode spot formation

This article presents the results of research on the photographic appearance of a highcurrent vacuum arc between butt type copper electrodes a of 30–80 mm diameter and a fixed gap of 10 mm. Current pulses of up to 30 kA peak amplitude at an initial value of (di/dt)₀ from 1–10kA/ms and a duration of approximately 14 ms were applied. Arcs were photographed with a high-speed framing camera, mostly at 10⁴ frames/s. A detailed study of discharge modes in phase transition from a high-current diffuse arc to a constricted arc with an anode spot was conducted. Most of the measurements were obtained at a peak current slightly in excess of 10 kA for electrodes of 55 mm diameter. It was found that at peak current exceeding moderately the threshold value of the onset of anode spot formation, the arc is characterized by the following main features: the formation of an anode spot and an anode plasma jet occurs concurrently with a local concentration of cathode spots; the anode spot is, most often, formed on the electrode edge; the coexistence of very varied structures of spots on the cathode; the lack of considerable constriction of the cathode discharge; the pseudo-periodic shrinking and expansion of the area occupied by cathode spots; the existence of a relatively dark space separates the anode plasma jet from the plasma sheath near the cathode surface; the plasma space distribution in the interelectrode gap is non-uniform and non-stationary.This work was supported by State Committee for Scientific Research within the research project No. 3 P40101507.     

Die Entwicklung der Elektronenlawine in den Plasmakanal,untersucht mit Bildverstärker und Wischverschluß

The application of high gain image intensifier and image converter streak shutter techniques to investigations of faintly luminous early stages of gaseous discharges discloses in some detail the development of electron avalanches into anode- and cathode-directed streamers as soon as the critical amplification (n≈10⁸) is attained in the gap. The velocities of both of the streamer branches were measured quantitatively as to the dependence on pressure (p), field (E/p) and gas (Nitrogen and various mixtures), typical values being 0.5 ... 1 · 10⁸ cm/sec. We find that streamer propagation is suitably described in a coordinate-system drifting with the electrons (driftvelocityv _? ≈ 1 · 10⁷ cm/sec) and that the way it depends on the discharge parameters suggests that ionization by electron impact (in the eigen-spacecharge distorted field) and photoionization are the determining effects. — The cathode-directed streamers show a considerable intensification and acceleration as soon as they are encountered by cathodic secondary avalanches, which however are obviously not essential for the initiation of the process in itself. — The streak images give also an impression of persisting excitation and ionization processes in the region of the streamer-paths and of the transition into the beginning of the bright spark, correlated with several ionizing fronts observed moving towards anode or cathode with velocities even >10⁸ cm/sec. (The paper includes also details of the gaseous near UV-radiation which was utilized for registration with image intensifier or converter, especially with respect to the decaytime of the excited states.) The results obtained are essentially a verification and to some extend completion of predictions made on the basis of early cloud chamber investigations.     

Efficiency of pulsed electrode conditioning in a vacuum

It is shown that the efficiency of pulsed electrode conditioning in a vacuum, which is estimated from the state of the electrode surface, rises as conditioning pulses get shorter and may exceed the efficiency of conditioning by dc breakdowns by two orders of magnitude or more. A criterion with which one can judge the cathodic mechanism of breakdown initiation in the steady-state regime is suggested. Under optimal electrode processing conditions (pulse width t _p < 10⁻⁸ s, field strength E ₀ > 10⁸ V/m), the ultimate dielectric strength corresponding to the cathodic mechanism of breakdown initiation in a vacuum can be reached.     

Transition of a diffuse discharge to a spark at nanosecond breakdown of high-pressure nitrogen and air in a nonuniform electric field

The transition of a runaway-electron-induced diffuse discharge initiated in a nonuniform electric field under a high pressure of air and nitrogen to a spark is studied. High-voltage pulses with a rise time of 0.5 ns are applied to a discharge gap with a tubular cathode having a small radius of curvature. It is shown that the leader of the spark discharge propagates toward the tubular cathode along preproduced tracks and may pass from one track to another. For a pulse rise time of about 0.5 ns and a gap length of 12 mm or more, it is found that spark leaders originating at the cathode (which has a small radius of curvature) do not reach the anode and accordingly, do not cause the spark breakdown of the gap. It is confirmed that the spark breakdown of the gap is associated with a spark leader that moves away from the plane electrode after the appearance of a bright spot on it.     

Kinetics of the Late Stage of Disintegration of Supersaturated Solid Solutions with Pronounced Crystalline Structure

The kinetics of disintegration of a supersaturated solid solution is studied in the context of the Lifshits–Slezov–Wagner (LSW) theory for a quasi-coherent interface between the new phase precipitates and the matrix. It is shown that the particle size distribution lies within a very narrow range of relative particle size (r _m/r _c = 6/5) and that the critical or average particle size varies with time as t^1/6 (r _c t ^1/6).     

Efficient extraction of hydrogen ions from high-velocity plasma bunches by a pulsed electric field

The systematized results are presented of an investigation of the process of the efficient extraction of ions, by a pulsed electric field, from the surface of a plasma moving at a velocity of 10⁴ m/sec under the conditions of small accelerating diodes. A mathematical model is analyzed of the propagation of a plasma bunch formed by a laser pulse (E _las<1 J) in a quasicylindrical diode system. The possibilities are considered of increasing the current of the extracted ions by utilizing the phenomenon of magnetic confinement of electrons, and diode systems with a transparent anode. The designs of small acceleration tubes for generating neutron pulses are presented.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 65–74, April, 1996.     

Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (～10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 10¹⁴?10²⁰ cm^?3 and T _e = 0.1 eV?10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm²; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ² T/δt ² > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ～ 10⁴ K and j ～ 10⁸ A/cm²). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm^?3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ～1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micr?xplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.     

Dynamics of the formation of two-dimensional ordered structures

The growth of ordered domains in lattice gas models, which occurs after the system is quenched from infinite temperature to a state below the critical temperatureT _c, is studied by Monte Carlo simulation. For a square lattice with repulsion between nearest and next-nearest neighbors, which in equilibrium exhibits fourfold degenerate (2×1) superstructures, the time-dependent energy E(t), domain size L(t), and structure functionS(q, t) are obtained, both for Glauber dynamics (no conservation law) and the case with conserved density (Kawasaki dynamics). At late times the energy excess and halfwidth of the structure factor decrease proportional tot ^–x, whileL(t) t ^x, where the exponent x=1/2 for Glauber dynamics and x1/3 for Kawasaki dynamics. In addition, the structure factor satisfies a scaling lawS(k,t)=t ^2xS(kt^x). The smaller exponent for the conserved density case is traced back to the excess density contained in the walls between ordered domains which must be redistributed during growth. Quenches toT>T _c, T=T_c (where we estimate dynamic critical exponents) andT=0 are also considered. In the latter case, the system becomes frozen in a glasslike domain pattern far from equilibrium when using Kawasaki dynamics. The generalization of our results to other lattices and structures also is briefly discussed.     

Photoproduction of Φ-mesons at small t-values

The reaction γ + p → Φ + p has been measured using a spark chamber spectrometer and a tagged photon beam in the energy range from 4.6 to 6.7 GeV. Approximately 3500 photoproduced elastic Φ-events have been collected in the t-range between t_min and t = ?0.4 (GeV/c)². Cross sections and t-distributions are presented.     

Free energy of the chiral Potts model in the scaling region

We explicitly calculate the free energy of the general solvableN-state chiral Potts model in the scaling region, forT<T _c . We do this from both of the two available results for the free energy, and verify that they are mutually consistent. Ift=T _c –T, then we find that - _c /t has a Taylor expansion in powers oft ^2/N (together with higher-order non-scaling terms of ordert, ort logt).     

Phenomenological model of the unstable stage of a vacuum spark discharge

A model of the unstable stage of a spark discharge in vacuum is proposed, which describes all typical manifestations of this stage, including current spikes in the diode, an increase in the potential at the cathode flame front, collective acceleration of ions in vacuum and plasma diodes, change in the cathode erosion mechanism, and the emergence of electron microbeams with a high current density at the anode. It is shown that these processes are associated with the formation of a charged electron layer of a spatially inhomogeneous plasma at the cathode flame boundary at the unstable stage of the spark discharge in vacuum. The emergence of this layer is associated with a limited emissive ability of the plasma at the cathode flame front during its expansion in vacuum. This leads to disruption of the plasma (field-induced emission of electron from the boundary region of the flame) and the formation of a short-lived charged plasma, viz., high-density ion cluster at the cathode flame boundary. The estimates obtained using this model are in good agreement with the experimental data on physical processes at the unstable stage of a vacuum spark discharge.     

The Ecton Mechanism for the Generation of Ion Flows in a Vacuum Arc

The principal characteristics of the process of generation of the cathode plasma in a vacuum arc (ion erosion, mean charge of ions) are considered in terms of the ecton model of the cathode spot of a vacuum arc. The estimates of the parameters of ions obtained for a unit cell of a cathode spot – an ecton – are in good qualitative and quantitative agreement with experimental data. The following mechanism for the generation of the cathode plasma of a vacuum arc is proposed. When a region of the cathode is destroyed in an explosive manner due to Joule heating, the material of the cathode sequentially goes over a series of states: the condensed state and the states of imperfect and ideal plasma. During this transition the charge state of the plasma is formed and the ions are accelerated under the action of the pressure gradient in the plasma whose density decreases by several orders of magnitude over distances of 10 m from the cathode surface. The increase in current results in an increase in number of cells, and the principal parameters of the ions are formed as a result of the operation of a unit cell of the spot.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.