A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed     

Plasma density decay of vacuum discharges after current zero

In vacuum circuit breakers the post-arc current caused by the remaining ions and electrons in the contact gap is an indication of the residual ionization and its decay. It coincides with the formation of a positive space charge sheath in front of the new cathode, which grows toward the new anode. In a vacuum test chamber an arc (1.5-15 kA RMS) is drawn between high current electrodes of the spiral type. At different times after current zero a transient recovery voltage is applied across a separate pair of high voltage electrodes. In contrast to real circuit breakers, where the transient recovery voltage reappears between the arcing contacts, this separation allows the study of residual plasma free from the thermal stress and melting on the contact surfaces. From the post-arc current across these electrodes, in comparison with a mathematical model of sheath growth, the density of the charge carriers can be evaluated. Such values and their temporal decay are presented     

Zur räumlichen Ausbreitung einer Entladung beim Generationen- und beim Kanalaufbau

The spatial expansion of a discharge was investigated under homogeneous field conditions, using an anode with a small central area separated from the remainder and wide band double beam oscillographic techniques. The discharge currents were initiated by electrons released along traces of single α-particles in the gas between the electrodes. The α-particles enter the gap volume through a small hole in the middle of the separated center of the anode. The measurements have shown that a secondary photon emission from the cathode causes a spreading over the total gap volume within the first few electron generations (N₂, air, CO₂ at lower pressures). If streamer mechanism occurs (O₂, CO₂ at higher pressures), the discharge remains localized to the trace of the α-particle, in which the avalanches yielded by the primary electrons have produced a high density of positive ions. — New details of streamer mechanism are revealed by analysing the rapid increase of the current during streamer formation.     

Theory of positive corona in SF6 due to a voltageimpulse

A theoretical examination is made of the mechanism of corona formation for a positive point-plane gap in SF₆ at 100 kPa. The impulse voltage applied has a rise time of 15 ns and peak value of 200 kV. Seed electrons are released 1 ns after the start of the voltage rise. For a 0.5-cm diameter positive sphere located 6.5 cm from a negative plane, the calculated circuit current initially consists of subnanosecond corona onset pulses, and then the current steadily rises to a maximum, as the voltage reaches a maximum, followed by a rapid fall in current. During the current rise a streamer moves out into the gap along a 100-μm channel, with the electric field in the streamer trail E>E*, where E* is the critical field where ionization equals attachment. The light output during the discharge is predicted to be a maximum at the anode with only a minor pulse of light at the streamer head, making it hard to detect. After the current maximum, recombination rapidly reduces the numbers of positive ions, negative ions, and electrons, but the net charge density remains constant and thus so does the electric field. The electric field is E~E* in the streamer trail, but has a sharp maximum, E≫E* at the head of the streamer trail. The origin of mid-gap precursors, observed when the streamer channel reilluminates after some 100 ns, is attributed to this field maximum in the remnant electric field. The evolution of positive ions, negative ions, and electrons is described by one-dimensional continuity equations, with the space-charge electric fields determined by the disk method. The effects of ionization, attachment, recombination, electron diffusion, and photoionization are all included. New numerical methods allow resolution of the streamer head and the anode fall region to be obtained with a 1-μm mesh, while following the streamer propagation for ~2 cm     

大气压管板结构纳秒脉冲放电中时域X射线研究

纳秒脉冲放电能在大气压下产生高电子能量、高功率密度的低温等离子体,由于经典放电理论无法很好地解释纳秒脉冲放电中的现象,近年来以高能逃逸电子为基础的纳秒脉冲气体放电理论受到广泛关注.纳秒脉冲放电会产生高能逃逸电子,伴随产生X射线,研究X射线的特性可以间接反映高能逃逸电子的特性.本文利用纳秒脉冲电源在大气压下激励空气放电,通过金刚石光导探测器测量放电产生的X射线,研究不同电极间隙、阳极厚度下和空间不同位置测量的X射线特性.实验结果表明,在大气压下纳秒脉冲放电能产生上升沿约1 ns,脉宽约2 ns的X射线脉冲,其产生时间与纳秒脉冲电压峰值对应,经计算探测到的X射线能量约为2.3×10-3J.当增大电极间隙时,探测到的X射线能量减弱,因为增大电极间隙会减小电场强度和逃逸电子数,从而减少阳极的轫致辐射.电极间距大于50 mm后加速减弱,同时放电模式从弥散过渡到电晕.随着阳极厚度增加,阳极后方和放电腔侧面观察窗测得的X射线能量均有所减弱,在阳极后面探测的X射线能量减弱趋势更加明显,这说明X射线主要产生在阳极内表面,因此增加阳极厚度会使穿透阳极薄膜的X射线能量减少.     

Spectra of electrons and X-ray photons in a diffusive nanosecond discharge in air under atmospheric pressure

The spectra of electrons and X-ray photons generated in nanosecond discharges in air under atmospheric pressure are investigated theoretically and experimentally. Data for the discharge formation dynamics in a nonuniform electric field are gathered. It is confirmed that voltage pulses with an amplitude of more than 100 kV and a rise time of 1 ns or less causing breakdown of an electrode gap with a small-radius cathode generate runaway electrons, which can be divided into three groups in energy (their energy varies from several kiloelectronvolts to several hundreds of kiloelectronvolts). It is also borne out that the formation of the space charge is due to electrons appearing in the gap at the cathode and a major contribution to the electron beam behind the foil comes from electrons of the second group, the maximal energy of which roughly corresponds to the voltage across the gap during electron beam generation. X-ray radiation from the gas-filled diode results from beam electron slowdown both in the anode and in the gap. It is shown that the amount of group-3 electrons with an energy above the energy gained by runaway electrons (in the absence of losses) at a maximal voltage across the gap is much smaller than the amount of group-2 electrons.     

Electron Hole Oscillations below the Plasma Frequency in a Planar Gap

The paper treats an initially cold, uniform, quasi-neutral plasma within a planar gap with walls that are perfectly absorbing but incapable of emission. In the idealized case where a voltage source is instantaneously applied to the gap, we find that at early times, while ions remain virtually motionless, some electrons are first expelled from the gap, after which the remaining electrons oscillate within the gap. The oscillation frequency lies below the electron plasma frequency and is shown to correspond to the plasma frequency of the expelled electrons if their density is averaged over the full gap.     

圆柱形阳极层霍尔等离子体加速器的质点网格方法模拟

对圆柱形阳极层霍尔加速器内的放电等离子体运用二维质点网格方法（particle in cell）进行数值模拟,用蒙特卡罗碰撞方法处理带电粒子与中性粒子之间的碰撞. 得到了放电通道内离子与电子的分布以及离子流的运动,并且对出口外侧的能量分布进行了统计. 结果发现圆柱形阳极层等离子体加速器的磁场对电子有明显的约束作用,电子集中于阳极附近很小的区域内. 由于电磁场的特殊分布,离子流呈现出双峰式的分布. 离子能量范围从放电电压的20%到接近放电电压,平均能量在放电电压的40%—50%之间. 关键词： 质点网格方法 蒙特卡罗碰撞 数值模拟 阳极层霍尔等离子体加速器     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

Triggered discharges with high arc voltages in a vacuum interrupter

Studies of nonsustained disruptive discharges (NSDDs), isolated cases of which can occur in vacuum interrupters, indicate lateral discharges between the cathode and shield, which can initiate a brief discharge between the contacts. To facilitate the study of such discharges, the sample discharges were triggered by a surface discharge induced by a spark gap, built into the side of the cathode, and observed with a high-speed film camera and image-converter camera. The tests showed a cathode spot after igniting. The emitted electrons first charge the shield negatively and then are directed toward the anode. The discharge burns at a high voltage, with current ranging from 10 to 100 A. After a period of up to 400 μs, the current demand increases abruptly; an arc discharge occurs between the contacts and discharges the capacitances near the switch. The contact gap undergoes a rapid dielectric recovery, and the restored voltage is maintained. These types of discharge were also observed with NSDDs; thus it can be assumed that the triggered discharges studied correspond to the NSDD type     

离子对rod-pinch二极管工作特性的影响

 利用PIC粒子模拟方法，结合对同轴结构二极管工作模式的半解析描述，研究了离子存在与否对rod-pinch二极管（RPD）工作特性的影响。模拟给出了二极管的伏安特性、不同时刻电子和离子的相空间分布以及空间电流分布等。结果表明随着电流逐渐增大，其工作模式从空间电荷限制流依次向弱箍缩流和磁绝缘流转变。只有存在离子的情况下，电子才能沿着阳极杆表面运动并在其末端发生箍缩，表明RPD中离子电流的存在对于RPD工作是必需的。     

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.     

Discharge initiation in a hollow-cathode plasma source of electrons

Experimental results on discharge initiation in a hollow-cathode plasma source of electrons are reported. The discharge is initiated by ions flowing out from the accelerating gap into the discharge region. The ions are generated by a high-voltage glowing discharge occurring in the accelerating gap at a pressure of more than 2 Pa and a voltage of no less than 2 kV. It is shown that an increase in the ion-electron emission coefficient, which can be raised by properly choosing the gas-metal pair, decreases the threshold ion current initiating the discharge at a fixed discharge voltage.     

Plasma characterization on carbon fiber cathode by spectroscopic diagnostics

This paper mainly investigates plasma characterization on carbon fiber cathodes with and without cesium iodide (CsI) coating powered by a ～300~ns, ～ 200~kV accelerating pulse. It was found that the CsI layers can not only improve the diode voltage, but also maintain a stable perveance. This indicates a slowly changed diode gap or a low cathode plasma expansion velocity. By spectroscopic diagnostics, in the vicinity of the cathode surface the average plasma density and temperature were found to be ～ 3× 10¹⁴~cm^-3 and ～ 5~eV, respectively, for an electron current density of ～ 40~A/cm². Furthermore, there exists a multicomponent plasma expansion toward the anode. The plasma expansion velocity, corresponding to the carbon and hydrogen ions, is estimated to be ～ 1.5~cm/μ s. Most notably, Cs spectroscopic line was obtained only at the distance ≤ 0.5~mm from the cathode surface. Carbon and hydrogen ions are obtained up to the distance of 2.5~mm from the cathode surface. Cs ions almost remain at the vicinity of the cathode surface. These results show that the addition of CsI enables a slow cathode plasma expansion toward the anode, providing a positive prospect for developing long-pulse electron beam sources.     

Discharge ignition phenomena in CO2 and N2O lasers with preliminary filling of the interelectrode gap by electrons

A comprehensive model is presented that describes preionization and discharge ignition processes in large-aperture molecular-gas lasers. By initially filling the interelectrode gap with electrons and establishing a space-charge screen at the cathode, electron avalanche begins adjacent to the anode. Cathode-directed ionization waves develop and ignite the discharge. Dynamic profiling of the electric-field distribution and Penning ionization of a readily ionizable additive combine to decrease both the ignition and quasi-steady-state voltages.     

The Boundary Layer between Electrodes and a Thermal Plasma

The electrical boundary layer between an isothermal, weakly ionized plasma and a plane electrode is discussed. Following a formulation of the complete problem, the governing equations are solved in the quasi-neutral continuum approximation to give explicit results for the ionization nonequilibrium layer. This allows three critical current densities to be identified: the first when a cathode must emit electrons; the second at which the anode potential fall changes sign from negative to positive; the third at which the anode must emit ions. The neglected space charge and inertia terms are evaluated to give conditions distinguishing collision-dominated and collisionless sheaths. Using an estimation for the location of the plasma-sheath boundary, the potential falls associated with the ionization nonequilibrium layer are evaluated. The results are illustrated for the case of combustion MHD plasmas.     

Electron beam formation in a gas diode at high pressures

Electron beam formation in krypton, neon, helium, and nitrogen at elevated pressures are experimentally investigated. It is shown that, when the krypton, neon, and helium pressures are varied, respectively, from 70 to 760 Torr, from 150 to 760 Torr, and from 300 to 4560 Torr, runaway electrons are beamed at the instant the plasma in the discharge gap approaches the anode and the nonlocal criterion for electron runaway is fulfilled. The fast-electron simulation of discharge gap preionization is performed. The simulation data demonstrate that preionization in the discharge gap is provided if the voltage pulse rise time is shorter than a nanosecond under atmospheric pressure.     

Velocity distributions in magnetron sputter

Results of the particle simulation of magnetron sputter are presented. Using a kinetic code, we obtain the spatial profiles of plasma density, potential, and velocity distribution function, along with the electron temperature, the ion density, the current density, and the deposition profiles at the anode surface. The result of simulation is compared with the Child-Langmuir law applied to the magnetron discharge and the global model. The velocity distribution function of electrons is Maxwellian, but that of ions is non-Maxwellian near the cathode with the majority in the energy range below 50 eV     

Ultrashort electron beam and volume high-current discharge in air under the atmospheric pressure

Conditions are studied under which an electron beam and a volume discharge with a subnanosecond rise time of a voltage pulse are produced in air under atmospheric pressure. It is shown that the electron beam appears in a gas-filled diode at the front of the voltage pulse in ∼0.5 ns, has a half-intensity duration of ≤0.4 ns and an average electron energy of ∼0.6 of the voltage across the gas-filled diode, and terminates when the voltage across the gap reaches its maximum value. The electron beam with an average electron energy of 60 to 80 keV and a current amplitude of ≥70 A is obtained. It is assumed that the electron beam is formed from electrons produced in the gap due to gas ionization by fast electrons when the intensity of the field between the front of the expanding plasma cloud and the anode reaches its critical value. A nanosecond volume discharge with a specific power input of ≥400 MW/cm³, a density of the discharge current at the anode of up to 3 kA/cm², and specific energy deposition of ∼1 J/cm³ over 3 to 5 ns is created.     

The Electrostatic Trapping of an Intense Relativistic Electron Beam in a Magnetic Mirror

Observations of rapid axial oscillations of an intense relativistic electron beam in a magnetic mirror are reported. The mirror field primarily provides radial confinement of the relativistic electrons. The axial confinement was achieved by placing thin aluminized mylar foils at the conjugate mirror field maxima. The region between these foils was filled with a few Torr air to provide a beam induced plasma for charge and current neutralization. The regions outside these foils were maintained at ~10-4 Torr. One foil formed the anode of a space-charge limited relativistic electron diode which launched the beam into the mirror. When the beam passed through the second foil it was no longer charge neutralized. In a manner quite similar to the anode foil oscillations observed by others, a space-charge limited electrostatic well was established which stopped the electrons and re-accelerated them through the foil-thereby reflecting the beam. When the reflected electrons re-entered the diode, they were once again "electrostatically" reflected. This process continued until the oscillating beam was either lost through the "virtual cathodes" outside the foils, dissipated in the drift region or quenched in the diode plasma after gap closure.