Multichannel Discharge Research Articles

Mutual interaction of two shock waves with a different time delay

Generator of two successive shock waves focused to a common focal point has been developed. Two cylindrical pressure waves are generated by multichannel electrical discharges on separated electrodes with different diameters. Time delay between the discharges can be varied. Propagation of the waves has been studied by means of Schlieren photography and polyvinylidene fluoride (PVDF) shock gauges. Interaction of the shock waves takes place twice: inside the reflector volume and in the focal region. It was found that the mutual interaction of the waves inside the reflector volume (time delay up to 30 µs) influences propagation of the first wave, while propagation of the second wave is influenced at all time delays measured. Schlieren photos of the local region demonstrated that interaction of the two waves results in creation of a large number of secondary short wavelength shocks.

A pulsed power generator for x-pinch experiments

A ∼500 kV/400 kA/100 ns pulsed power generator (PPG-I) for x-pinch experiments was designed and constructed at Tsinghua University. It is composed of a Marx generator, a combined pulse forming line (PFL), a gas-filled V/N field distortion switch, a transfer line, and a copper-sulphate resistive load for testing. The PPG-I implements a novel design in lines that four pieces of waterline with impedance 5Ω in parallel constitute a combined PFL with 1.25Ω, and incorporate each other by a common self-break V/N switch on a matched 1.25Ω transfer line. At the peak charging voltage of the PFL, the V/N switch breaks down in multi-channel discharge mode, and electric energy is fed into the testing load through the 1.25Ω transfer line. This article presents the design and test of the PPG-I generator.

An unsteady multichannel discharge between a jet of electrolyte and a metal electrode at atmospheric pressure

An unsteady multichannel discharge between a jet of electrolyte and a metal electrode at atmospheric pressure

Generation of Two Successive Shock Waves Focused to a Common Focal Point

A generator of two successive shock waves focused on a common focal point has been developed. Cylindrical pressure waves created by multichannel electrical discharges on two cylindrical composite anodes are focused by a metallic parabolic reflector-cathode. Near the common focus, the waves are transformed into strong shock waves. The anodes are energized from separate power supplies. This allows us to vary the time interval between the discharges and stagger the waves' arrival to the focal point. Schlieren photographs of the focal region show that mutual interaction of the two waves results in generation of a large number of secondary short wavelength shocks. Measurements of the shock waveforms at the focus demonstrate that the second (i.e., later arriving) wave is strongly attenuate due to the medium inhomogeneity produced by the first wave. Localized injury of a rabbit's liver induced by the shock waves has been demonstrated by the method of magnetic resonance imaging. Histological examination of the liver samples taken from the injured region revealed a very sharp boundary between the injured and healthy tissues

A multichannel discharge between a stream electrolytic cathode and a metal anode at atmospheric pressure

An experimental investigation is performed of the structure and current-voltage characteristics (CVC) of a multichannel discharge (MD) between a stream electrolytic cathode (SEC) and a metal anode, as well as of the density of SEC and anode current in a wide range of values of current I = 0.02–10 A, interelectrode spacing l = 5–50 mm, and anode diameter d a = 5–40 mm. The development of cathode spots on the SEC surface is studied. It is found that a cathode spot of SEC may have various shapes and structures (regular circle, horseshoes, filamentary spots, and so on). The CVC of a multichannel discharge between SEC and metal anode depends significantly on I, l, and d a , as well as on the material of the anode and on the composition and concentration of electrolyte. The dependence of the density of SEC current on discharge current is nonmonotonic. It is for the first time that a bundle-like MD between SEC and metal anode and multichannel and diffuse discharges between the plasma region and stream electrolytic cathode are observed, and their characteristics are investigated.

Initiation techniques and the vacuum-ultraviolet-radiation efficiency of a stabilized multichannel surface discharge

Multichannel surface discharges possess a number of advantageous characteristics for the optical pumping of photochemically driven lasers. This work reports on a type of large-area low-inductance sources of intense UV-VUV (vacuum-ultraviolet) radiation based on spatially stabilized multichannel discharges in gases. The discharges consist of several dozens of closely located parallel plasma channels initiated synchronously along a dielectric surface on the area of several hundred square centimeters. In comparison with a large-area diffuse surface discharge, the plasma confinement within relatively thin channels greatly improves the efficiency of the discharge emission in the VUV spectral range. Several techniques are introduced, which allow a synchronous formation and homogeneous development of multiple spatially stabilized discharge channels. Technical efficiency of the discharge radiation within the spectral range of 120–200nm reaches 5% and an effective brightness temperature of the radiating plasma exceeds 20kK. Synchronous operation of a number of multichannel discharge modules producing high-intensity submicrosecond optical pulses is demonstrated, which is of importance for the pump source scaling and geometrical adaptation.

A Vapor-Air Discharge with a Porous Electrolytic Cathode at Atmospheric Pressure

An experimental investigation is performed for the first time of the structures and electrical and thermal characteristics of a vapor-air discharge between a porous (solid cylindrical body and hollow cylinder) electrolytic cathode and a solid anode in the range of current from 0.2 to 8 A with an interelectrode spacing of 2 to 200 mm for electrolytes of different compositions and concentrations with vertical and horizontal orientations of the plasma column in space at atmospheric pressure. An electrode of a new type is developed, namely, a porous electrolytic cathode (PEC), which makes it possible to produce cone-shaped, multichannel, and mixed discharges. The moist, boiling, and film modes of PEC operation are revealed. It is found that the heat loss on a PEC depends on the mode of its operation. The minimal heat loss is observed in the moist cathode mode, in which the electrolyte is delivered to the cathode working surface in the form of vapor only. In so doing, an almost complete regeneration occurs of heat delivered to the cathode from discharge plasma. It is found that the characteristics of a vapor-air discharge between a PEC and electrolytic anode depend significantly on the composition and concentration of the PEC electrolyte. The discharge voltage fluctuations and the nonuniform pattern of distribution of electric field intensity are revealed. The results of experimental investigation of a vapor-air discharge with a PEC are generalized in the form of an empirical formula.

A Vapor-Air Discharge between Electrolytic Anode and Metal Cathode at Atmospheric Pressure

An experimental investigation is performed of the structure, mechanism, and electrical and thermal characteristics of a vapor-air discharge between an electrolytic anode (EA) and a metal cathode (MC) in a wide range of parameters at pressure P = 8×104 to 105 Pa, current I = 0.02 to 60 A, interelectrode spacing l = 0.1 to 40 mm, and metal cathode diameter d c = 1 to 40 mm. The main types of vapor-air discharge with EA are identified. The possibility of burning of a multichannel discharge between a metal cathode and an electrolytic anode at atmospheric pressure is demonstrated for the first time, and a vapor-air discharge with a diffuse plasma column is investigated at high currents and large interelectrode spacings. It is determined that the electrical and thermal characteristics depend significantly on current, interelectrode spacing, electrolyte composition and concentration, geometric shape, diameter, and cooling of the MC. The significant effect of the vapor-air discharge on the electrolytic anode surface is revealed. Transverse waves are observed on the electrolyte surface. Significant turbulent mixing is observed for the first time on the electrolyte-plasma interface in the case of a vapor-air discharge with electrolytic anode at atmospheric pressure and high currents.

Experimental study of the discharge glow structure in the initial phase of a nanosecond breakdown of an insulating gap

The diffuse phase of high-voltage nanosecond atmospheric-pressure discharges in insulating gaps with a highly nonuniform electric field has been studied experimentally with the purpose of developing methods for protecting power supply equipment from natural and man-caused overvoltages. The structure of the discharge glow in 6- to 12-cm rod-plane gaps has been investigated for five configurations of the rod cathode. It is found that the discharge can exist in different forms and that the occurrence of one or another type of discharge is probabilistic in character. The electric characteristics of the discharge have been studied as functions of the electrode gap length for two types of cathode that provide the preferential development of volume and multichannel discharges. It is shown that the main factors governing the shape of the discharge glow are the distribution of the electric field near the cathode in the voltage growth phase and the dynamics of the accompanying discharge processes.

Localized Damage of Tissues Induced by Focused Shock Waves

A novel method for generation of focused shock waves has been developed. A cylindrical pressure wave, created by a high-voltage multichannel discharge in water with an increased electrical conductivity, is focused by a metallic parabolic reflector. The discharge is formed on a composite anode consisting of a cylindrical stainless steel electrode covered by a thin porous ceramic layer. In such an arrangement at the applied voltage of 30 kV, a large number of short discharge channels distributed homogeneously on the anode surface is initiated. Each discharge channel creates a semi-spherical pressure wave, and by superposition of all of the waves, a cylindrical pressure wave propagating from the anode is formed. The cylindrical pressure wave is focused by a metallic parabolic reflector (cathode) and near the focus it is transformed into a strong shock wave. The focal volume is 2.5 mm in diameter and 32 mm long. We have demonstrated that the focused shock waves destroy human red blood cells (erythrocytes) very efficiently. Interaction of the focused shock waves with fresh potatoes that have high water content have been used to assess the mechanical effects of the waves. The 6-cm-thick slab of potato was placed next to the focal region and exposed to 10 shocks at 30 kV. The experiment demonstrated that only the focal region inside the potato had been damaged and no damage was seen between the potato surface and the focal region. Local injury of animal liver tissues exposed to the shock waves were observed in vitro. Livers of three rabbits have been exposed in vivo. Histological analysis of the exposed tissues revealed injuries and hemorrhages at the focal region of the shock waves.

Determination of the Trigger-Electrode Position for the Field-Distortion Spark Gap

The optimal axial position of the trigger-electrode of the field-distortion spark gap is investigated through electric field calculation. The electric fields of the gap are calculated when locating trigger-electrode at different positions. It is shown that if the distance d between the trigger electride and the low electric potential electrode is greater than 1.3 mm, the maximum field intensity occurs at the edge of the trigger-electrode, which will facilitate the generation of multi-channel discharge. According to calculated results, the working characteristics of the gap is investigated and it shows that when d = 1.4 mm the field distortion gap can be triggered reliably.

Application of a ferroelectric plasma cathode as a high-current switch

In this paper the parameters of two types of high-current switches based on ferroelectric BaTiO3 ignition are presented. Both types of switches showed a reliable and controllable operation with a repetition rate of several Hz. The first type is a vacuum two-electrode switch ignited by the plasma which is generated by a BaTiCO3 cathode. This type of switch was tested in the voltage range of 3–25 kV and switched current amplitude of 2 –15 kA with either negative or positive polarity of the high-voltage electrode. The second type is a BaTi03 surface flashover strip-like switch ignited by a driving pulse which has an amplitude of several kV. It was shown that the application of the driving pulse (>10 kV) leads to the appearance of many non-complete surface discharges which transform further to a multi-channel discharge. This type of switch was tested in the voltage range of 1–25 kV and current amplitude of 0.5–15 kA. The design of the switches, their lifetime, the time jitter and the parameters of the switched current for different discharge conditions are presented.

Experimental Study of the Multi-Gap Multi-Channel Gas Spark Closing Switch

A coaxial multi-gap multi-channel spark switch with stainless-steel-spring ring gap electrodes is designed and investigated. The switch is triggered by a pulse applied to the cylindrical electrode outside the discharging channel through a parasitic capacitance coupling. The jitter of the switch is reduced by several short-distance gas gaps in series, and its inductance is reduced by a multi-channel discharge on account of the inductance isolation between the coils of the spring ring electrode. The experimental results indicate that the switch is of low inductance (15-30 nH), low jitter (∼3 ns), and stable breakdown performance.

Pulse electrical discharges in water and their applications

Partial electrical discharges in a water solution as a function of conductivity have been studied experimentally. Using needle-plate electrodes it has been demonstrated that the discharge evolves in two phases. During the first streamer-like phase, the discharge propagated with a velocity of 106 cm/s, while during the second arc-like phase the length of the discharge remained almost constant although the current still increased. Higher solution conductivity resulted in the generation of shorter channels, in larger discharge current, and in a higher plasma electron density. Degradation of phenol by the discharge has also been demonstrated. A special metallic electrode covered by a thin layer of porous ceramic has been developed and used for generation of a multichannel discharge. At comparable solution conductivity the ceramic-coated electrode produced plasma with very similar parameters as the needle-plate electrode configuration. Generation of strong focused shock waves by the multichannel discharge in a highly conductive solution has also been demonstrated.

Self-consistent development and fractal structure of leader discharges along a water surface

Ideas regarding the development of single-and multichannel leader discharges over water surfaces are generalized on the basis of experimental data. The Ohmic conductivity of water is found to be manifested in the self consistency of their dynamics and fractal structure. The fractal dimensionality of a single-channel leader discharge is found to be 0.96±0.05 and that of a multichannel discharge to be 1.85±0.05. Mechanisms are proposed for the branching of leader discharge channels and for the development of bifurcations from branches and channels.

Leader discharge over a water surface in a Lichtenberg figure geometry

A multichannel leader discharge over a water surface is investigated in a Lichtenberg figure geometry. It is established that the Ohmic conductivity of water causes nonlinearity of the R(t)C discharge circuit. A mutual one-to-one correspondence between the channel lengths and the currents flowing in them is established during the discharge, and the discharge has a selfconsistent character. A mechanism is proposed for the initiation of initial channels by maxima which arise in the charge structure of the planar double layer on the water surface during the development of Rayleigh-Benard instability in the layer after the pulsed corona from the anode reaches the water.

Diffusion-relaxed waveguide N2-lasers

A small-bore diffusion relaxed N2-laser (<i>P</i><200 Torr) with longitudinal and transverse excitation in single- and multichannel discharge configurations is developed. We show that the maximum repetition rate and specific average power of the laser are increased with the decrease of the characteristic transverse dimension of the discharge channel. An 8-kHz repetition rate and average power of 6.4 mW are obtained in very compact laser structures. A specific average power of about 160 mW/cm³ is achieved, that is a few times higher than that extracted from largebore plasma tubes. The laser with transverse excitation produces 30-kW pulses at 500 Hz. The waveguide effect in the laser channel and its influence on the spatial distribution and coherence of the laser radiation are also studied.

Atomic xenon recombination laser excited by thermal ionizing radiation from a magnetoplasma compressor and discharge

A description is given and the results are reported of the first photoionization-recombination laser using atomic xenon excited by thermal ionizing radiation from a plasma. The pump source was a multichannel plasmadynamic discharge in magnetoplasma compressors, which was ignited in the active medium of the laser. When the composition of the working mixture was optimal (Xe:Ar = 1:250) and the total pressure was 1 atm, the output energy was ~ 0.5 J in the form of pulses of ~ 10 μs duration and the maximum specific output energy represented by laser radiation was 1–2 J/liter. The unsaturated gain was 27 m − 1. A kinetic laser scheme was proposed and analyzed. It allowed for the processes of photoionization, ion conversion, dissociative recombination, interaction of excited states with electron and buffer gases, etc. An important role played by heating of the active medium during pumping was demonstrated and it explained the observed characteristics of the spatial and temporal structure of the lasing process, particularly bleaching of large volumes of the active medium. The potential output energy of the laser was considered and specific constructions were proposed to attain a lasing efficiency amounting to a few percent.

Antifriction coatings on aluminum and its alloys formed by electric spark alloying from powder materials

The effectiveness of the method of ESA with powder materials for the reinforcement of aluminum and its alloys may be linked with several factors. First, powder particles introduced into the electrode gap (EG) initiate discharges at a greater distance between the electrodes, and increasing the EG brings about a redistribution of the discharge energy in the anode-EG-cathode system such that the proportion of energy liberated on the electrodes decreases, while that liberated in the EG increases [5]. Secondly, the presence of powder particles in the EG results in the formation of a multichannel electric discharge structure [6], which divides a single erosion crater into a multitude of small craters. With the simultaneous decrease in energy liberation on the electrodes, the powder particles entering the discharge channel experience comminution with rapid melting and evaporation pulses, so that material transport to the basis is effected in the vapor, liquid, and solid phases. Relief forms mainly with the participation of the liquid phase because solid-phase particles seldom attach themselves firmly to the cathode surface. An electron microscopical examination of the microrelief on an aluminum cathode revealed long streaks of molten powder material and spreading of the liquid phase over the surface (Fig. i). Under these conditions the erosion of both electrodes was considerably reduced, with the anode being practically unconsumed and the volume erosion of the cathode being much smaller than the volume of the coating-forming powder material transported onto it. The effectiveness of the process of ESA of aluminum and its alloys was assessed in rubbing tests on coatings. A study was made of the effect of composition of the powder mixtures on the structure, thickness, hardness, and wear resistance of coatings. In these tests copper,

Static and dynamic characteristics of multichannel discharge in rare-gas-mercury mixtures

Analysis of multiarc formation in a concentric discharge tube is described. The effect of diffusion on static characteristics is also examined. The mechanism for current modulation by anode oscillation in a multianode tube is then simulated by an electric switching circuit. The simulated results agree well with the measurement results. It is found that the effect of diffusion on dynamic multiarc characteristics plays an important role in determining excess ionization patterns. When excess ionization takes place on one anode, discharge current increases, plasma density increases, and charged particles diffuse to adjacent channels. Consequently, the plasma density, hence the discharge current, in the adjacent channel also increases. This increased current causes excess ionization on the adjacent channel anode. The peculiar sequence patterns of excess ionization are explained by this effect of diffusion on dynamic characteristics.