Microstructure of the current channels in a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields

The microstructure of a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields is investigated. It is found that an 0.1-to 0.4-mm spark channel consists of a large number (from 100 to 1000) of 5-to 10-μm-diameter microchannels distributed nearly uniformly over the channel cross section. The current amplitude in the spark is 1.5–3 kA, and the current density in a microchannel is 10⁷ A/cm². It is shown that the formation of the microstructure cannot be attributed to ionization-heating instability. The instability of the ionization wave front is suggested as a mechanism for the microstructure formation.     

High-power discharge with a plasma cathode in dense gases

An attempt is made to create an electric-discharge source for pumping argon, krypton, and xenon dimer lasers. The device is based on a method proposed previously by the authors, wherein confinement of the discharge is achieved by removing the cathode spot from the main discharge region and closing the discharge to the spot along a narrow extended auxiliary plasma channel. The conditions for the formation of such a discharge are investigated. The high stability of the sparkless stage of the discharge permits the first-ever attainment of energy depositions at the level of 100 J/cm² at pressures ∼10 atm, a level several orders of magnitude higher than is attainable by conventional methods. A discharge cell and power supply system are designed for a multisectional discharge with an active length of 200 mm, and the reliability of the entire apparatus is demonstrated in long-term use. Zh. Tekh. Fiz. 67, 10–14 (November 1997)     

Influence of the cathode and anode jets on the properties of a high-current electric arc

A study is made of the effects related to the formation of electrode jets in discharges in hydrogen and air at a current of 10⁵–10⁶ A, a current growth rate of 10¹⁰ A/s, an initial pressure of 0.1–4.0 MPa, and a discharge gap length of 5–40 mm. After secondary breakdown, jets are observed in a semitransparent discharge channel expanding with a velocity of (4–7)×10² m/s. The formation of shock waves in the interaction of the jets with the ambient gas and the opposite electrode is observed by the shadowgraphy method. Seventy microseconds after the beginning of the discharge, the pressure of the metal vapor plasma near the end of the tungsten cathode amounts to 177 MPa. The brightness temperature in this case is T=59×10³ K, the average ion charge number is [`(m)] = 3.1\overline m = 3.1 , and the metal vapor density is n=5.3×10<sup>19</sup> cm<sup>−3</sup>. After 90 μs, the average ion charge number and the metal vapor density near the anode end are [`(m)] = 2.6\overline m = 2.6 and n=7.4×10¹⁹ cm⁻³, respectively. Based on the experimental data, possible reasons for the abnormally high values of the total voltage drop near the electrodes (up to ∼1 kV) are discussed.     

Investigation of the redistribution of the current in a nanosecond volume discharge after the appearance of a cathode spot

An investigation was made of the process of contraction of a nanosecond volume discharge in air with a homogeneous field distribution and a current density 500 A/cm² when the gap was 0.7 cm and the pressure 76 torr. It was found that the appearance of a cathode spot in this volume discharge resulted in an effective contraction of the current into a spot and formation of a diffuse channel. A high-conductivity channel emerging from the cathode spot appeared when the current density in the diffusion channel above this spot reached (1–3)·10⁵ A/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 16–18, January, 1981.     

Estimation of the pressure in a “slow” spark discharge in a cylindrical water-filled chamber

On the assumption that a spark discharge in water is quasi-steady, its pressure and channel radius are calculated. It is shown that the key discharge parameter is the action integral S=∫ ₀ ^t i ² dt, where i is the current in the channel. The conductivity only slightly affects the computational results and thus can be assumed to be constant. The formulas obtained can be applied to a discharge along the axis of a cylindrical water-filled chamber if the deformation of its walls over the discharge time is negligibly small and the pulse duration is several times greater than the time of sound propagation in water from the axis of the chamber to its wall. At relatively low pressures (P≤10⁸ Pa), P∼R ^−4/3, where R is the chamber radius.     

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%.     

Measurement of neutrons and X-rays from a vacuum spark

The Jülich DPF I machine was modified to operate as a vacuum spark. It was established that with an operating pressure varying from 10^-4 to 10^-5 torr a voltage within the range of 14–17 kV was necessary to ensure a dense plasma phase as indicated by soft X-ray pinhole photography and current measurements. Operating with a LiD-filled channel in the anode, occasional bursts of neutrons were detected with a time-of-flight scintillator system. The yield, however, was erratic and below 10⁷ per discharge.     

Model of pulsed electric discharge expansion in dense gas taking into account electron and radiative thermal conductivities. I. Expansion mechanism

Using an ionization sensor, it was found that weakly ionized plasma with an ionization degree larger than 10^?6 is formed under exposure to UV radiation of a high-current pulsed electric discharge in gas (air, nitrogen, xenon, and krypton) at atmospheric pressure at a distance of ～1.2–2.5 cm from the discharge boundary. It was shown that the structure of such discharge includes, in addition to the discharge channel, a dense shell and a shock wave, also a region of weakly ionized and excited gas before the shock wave front. The mechanism of discharge expansion in dense gas is ionization and heating of gas involved in the discharge due to absorption of the UV energy flux from the discharge channel and the flux of the thermal energy transferred from the discharge channel to the discharge shell due to electron thermal conductivity.     

Model of pulsed electric discharge expansion in dense gas taking into account electron and radiative thermal conductivities. II. Energy balance and equation

Using an ionization sensor, it was found that weakly ionized plasma with an ionization degree larger than 10⁻⁶ is formed under exposure to UV radiation of a high-current pulsed electric discharge in gas (air, nitrogen, xenon, and krypton) at atmospheric pressure at a distance of ∼1.2–2.5 cm from the discharge boundary. It was shown that the structure of such discharge includes, in addition to the discharge channel, a dense shell and a shock wave, also a region of weakly ionized and excited gas before the shock wave front. The mechanism of discharge expansion in dense gas is ionization and heating of gas involved in the discharge due to absorption of the UV energy flux from the discharge channel and the flux of the thermal energy transferred from the discharge channel to the discharge shell due to electron thermal conductivity.     

Method for modifying the structure and elemental composition of the solid surface during a high-voltage vacuum discharge

A new method for modifying the surface of a solid, which makes it possible to change effectively the structure and elemental composition of the surface with a high precision, is developed and tested experimentally. The method is based on the action of the plasma of a pulsed high-voltage vacuum discharge, the ion beam from the plasma, and the electron beam on a solid target. The emission and plasma parameters are observed in a pulsed electric field produced in the diode system to which a pulsed voltage with an amplitude of ～10³–10⁵ V and a duration from 10^?9 to 10^?5 s is applied.     

基于VHF辐射源短基线定位系统对闪电放电过程的研究

利用闪电VHF辐射源短基线定位系统2009年东北大兴安岭地区的观测资料,对正、负地闪和云闪放电过程中的VHF辐射源进行了定位研究.根据闪电VHF辐射源的二维位置(方位角和仰角)随时间的演变特征,结合同步观测的快、慢电场变化资料分析发现,持续时间较长的地闪预击穿过程在云中的放电通道呈双层结构,预击穿过程结束阶段的云内放电表现为反冲流光,预击穿过程的平均速度在10⁴m/s量级.预击穿过程为梯级先导的发展提供了必要的条件,梯级先导从预击穿起始位置开始并向下发展,产生较强的辐射,平均速度在10⁵m/s量级.K过程主要是流光沿之前已电离通道的传输.正、负地闪回击阶段前后的放电特征有明显不同,正地闪回击之后,连续电流期间的较长时间的云内放电产生较强的VHF辐射.对闪电在125---200MHz频段范围的VHF辐射频谱特征的统计分析发现,辐射能量呈现出幅值随频率增加而减小的趋势,在通带范围上基本遵循f^-2.9的衰减率递减.     

Spectral diagnostics of a unipolar high-frequency discharge excited in nitrogen and argon at pressures from 1 to 12 atm

By methods of spectral diagnostics, the temperature of neutral gas and the electron temperature and density have been determined in the channel of a unipolar high-frequency discharge excited at very high pressures. In nitrogen the h.f. discharge was excited at pressures of 1–5 atm, in argon at pressures of 1–12 atm. In the discharge excited in argon, the electron temperature does not change with increasing pressure and isT _e =(6–7)×10³ °K; the electron density increases with increasing pressure. It can be demonstrated that the electron velocity distribution is given by a Maxwellian distribution function although the plasma of a unipolar high-frequency discharge is non-isothermal (T _e –T _n 5×10³ °K).In conclusion, the author thanks Prof. Dr. V. Truneek for stimulating remarks and his kind interest in this work.     

Study of spatial characteristics of the incomplete surface discharge in atmospheric-pressure air

Spectra of plasma of the incomplete surface discharge in atmospheric-pressure air were measured. Bands of the systems 2⁺, 1⁺, and 1⁻ of nitrogen were identified. It was shown that periodic excitation at the C ³Π _u nitrogen level results in local equilibrium characterized by a vibrational temperature of 2250 K. For the 1⁺ band system of nitrogen, the structure of rotational transitions was resolved; it was found that the distribution in the 1⁺ band system is significantly nonequilibrium due to the A ³Σ _u ⁺ level metastability. Transverse and longitudinal sections of the plasma band were scanned. It was found that the transverse distribution of the emission intensity has a maximum at a distance of 1 mm from the electrode edge followed by an exponential decay. The plasma band width was ∼5 mm. The longitudinal discharge structure consisted of a system of microchannels (with a characteristic diameter of 0.4 mm) with diffuse overlapping. The average channel density was 10–15 cm⁻¹. It was shown that the spatial distribution of incomplete discharge plasma is characterized by high stability against wide-range variations of discharge electrical characteristics. For example, as the excitation voltage varies within 2–6 kV, the plasma band width changed by no more than 1 mm, and the vibrational temperature varied within 10–12%.     

Emission properties of apokamp discharge at atmospheric pressure in air,argon, and helium

Emission spectra of the plasma jet of apokamp discharge in air, helium, and argon are studied. Apokamp at atmospheric pressure is formed in the areas of strengthening of the electric field near the bends in the channel of the pulse-periodic discharge and is directed perpendicularly to the discharge channel. Apokamp consists of a bright narrow “appendage” connected with the discharge channel and with the diffuse jet emerging from the channel. It is shown that, in helium, the emission of the diffuse part of apokamp is dominated by N₂ and N₂ ⁺, while emission of the “appendage” display lines and bands of He, N₂, N₂ ⁺, O, and OH. In argon, emission spectra of the diffuse part of the plasma jets contain not only N₂ and N₂ ⁺, but also Ar lines. It is assumed that the surrounding air plays an important role in the formation of the diffuse part of apokamp in helium and argon.     

Kinetics of Oxygen Atom Production in R.F. Discharges of Moderate Pressures

The dissociation of molecular oxygen has been studied in a radiofrequency plasma at a pressure of 20 torr in a flow system. The results show that the main channel for dissociation is via electron excitation to the Herzberg and Schumann systems with negligible contribution of the channel involving the O₂(a¹Δg) metastable. While oxygen atom recombination can be followed outside the discharge region, kinetic data show that recombination within the discharge is effectively suppressed, probably as a consequence of a recombination assisted dissociation mechanism. This leads to high concentrations of O-atoms at the discharge exit.     

Sudden Jumps,Hysteresis, and Negative Resistance in an Argon Plasma Discharge. I. Discharges with No Magnetic Field

Jumps and hysteresis in discharge current are produced in a low-pressure (10^?5 – 10^?4 Torr). magnetic field free, thermionic argon plasma discharge by varying discharge voltage, neutral pressure, or filament temperature. This behavior can be attributed to a region of negative differential resistance in the voltage versus current characteristic of the discharge. A model is presented in order to understand the physical processes leading to this negative resistance.     

Population of 2p 55s levels of neon in He-Ne plasma: I. The evolution of mechanisms in the discharge and the afterglow

The spectroscopic study of population processes of neon 2p ⁵5s states was carried out in helium afterglow with small admixture of neon at PHe = 38.1 torr, [He]/[Ne]=10^?5 with pulsed discharge afterglow in helium with small admixture of neon (pressure equal to 38.1 mm Hg; ). It is established that the main mechanism of population of 3s ₂ level (in Paschen’s notation) in the discharge and the initial after-glow is the excitation transfer from metastable atoms of He(2¹ S ₀). The other three levels—3s ₃, 3s ₄, and 3s ₅—corresponding to 2p ⁵5s configuration are populated in the afterglow as a result of the dissociative recombination HeNe⁺ of ions with electrons. The same process is also the main channel of population of 3s ₂ level in the late afterglow phase, when the concentration of He(2¹ S ₀) atoms is small. The hypothesis of recombination mechanism is confirmed by observation of the response of line intensities to pulsed electron heating. The partial coefficients of dissociative recombination into 2p ⁵5s states are estimated.     

Measurement of Excited States Density and the VUV-Radiation in the Pulsed Xenon Medium Pressure Discharge

Time resolved absolute density measurements of the 1s5 (metastable) and 1s4 (resonance) level in the positive column of a pulsed xenon gas discharge for gas pressures between 1 and 40 Torr and currents between 100 and 300 mA are presented. The densities ranged from 1 × 10¹¹ to 5 × 10¹² cm^?3 and were found to be ten times larger in the afterglow than in the active part of the discharge. The enhanced radiation obtained in the afterglow in the near infrared regions as well as in the VUV region is caused by a dissociative recombination and formation of excimer states.     

一种多通道纳秒分幅相机

相机采用棱镜分光,薄片管增强象,电选通分幅。由四个薄片管组成四个通道,每一个通道代表一个单幅相机,分幅时序由专门电控电路进行控制,接触相机记录图象,最短曝光时间3ns,动态空间分辩率5～81lp/mm,最高分幅速率2×10⁸幅/秒,动态范围50°,在爆炸、激光、放电,光谱学等方面有广泛的应用。     

Radiation resistance of advanced GaAs MESFETs

The radiation resistance of GaAs MESFETs with a channel length of 30–80 nm is studied. It is shown that the resistance is controlled by quasi-ballistic effects in the transistor channel and amounts to 5 × 10¹⁴ −5 × 10¹⁵ fast neutrons per square centimeter.