水中脉冲放电的电特性与声辐射特性研究

对水中脉冲放电等离子体通道电阻与放电参数之间的关系作了研究,得到了等离子体通道电阻与电容量、初始电压、电极间距离的关系,以及通道电阻随时间的变化规律.还对冲击波的峰值压力与放电参数间的关系作了研究,并对冲击波压力的功率谱作了分析,结果表明水中脉冲放电所产生的冲击波的声辐射频率在几十赫兹到几万赫兹之间,覆盖了所有水声设备的工作频率,且在低频段具有很强的声功率,是一种理想的水下声源 关键词： 水中脉冲放电 等离子体     

水下等离子体声源的冲击波负压特性

基于修正的Rayleigh气泡脉动方程对水下等离子体声源放电产生的 强声冲击波的传播过程进行了分析; 利用Euler方程作为控制方程组, 建立了水下等离子体声源的聚束声场模型, 通过仿真计算获得的传播云图对冲击波负压的形成机理进行了直观的理论分析. 结果表明: 经过聚能反射罩反射汇聚得到的聚束波在反射稀疏波和水的惯性作用下, 聚束波周围水域产生了拉伸, 形成负压区, 如果拉伸力大于水的抗拉上限, 就会使得水中形成不连续现象, 即出现空化气泡; 此外聚能罩边缘处产生的衍射波进一步加剧了负压的产生, 边缘衍射波最终与拉伸波叠加, 使冲击波负压达到最大值; 通过对比仿真波形和实验波形, 从而验证和进一步揭示了冲击波负压的形成原因. 研究结果对认识水下冲击波的传播规律和进一步改进等离子体声源的设计具有指导意义. 关键词： 等离子体声源 冲击波负压 聚束声场模型 气泡     

Study of shock waves interacting with Ar and N<Subscript>2</Subscript> low pressure dc discharges

Acoustical shock waves (Mach number <2) generated in situ by spark gap are propagated in weakly ionized dc discharges working at low pressure (399 Pa) and containing either Ar or N₂ gas. The electrical characterization and the laser deflection technique are used to measure the characteristics of dc discharge (such as voltage, resistance and power of discharge) and the structure and velocity of shock wave, respectively. The results stress the importance of atomic and molecular nature of the gases in affecting the power deposition and the shock wave properties.     

使用ZFK-500型高速摄影机对液中放电现象的摄影观察

液中放电是通过放电的方式将电能高速转化为热、光、力、声等其他形式的能量。它具有一系列独特的优点,使它广泛应用于许多领域。因此,关于液中放电的研究具有重要的理论与实践意义。我们使用ZFK-500型高速摄影机对液中放电的击穿与弧道过程、冲击波的形成与传播过程,进行了比较成功的观察。大量清晰的摄影,记录了击穿时微弱光线的光导及其在整个放电阶段的情况;记录了液中放电弧道膨胀的特点 此外,不发光现象-冲击波形成与传播过程的记录,都为液中放电的理论研究,提供了十分宝贵的科学依据。     

Modeling and optimization of the multichannel spark discharge

This paper reports a novel analytic model of this multichannel spark discharge, considering the delay time in the breakdown process, the electric transforming of the discharge channel from a capacitor to a resistor induced by the air breakdown, and the varying plasma resistance in the discharge process. The good agreement between the experimental and the simulated results validated the accuracy of this model. Based on this model, the influence of the circuit parameters on the maximum discharge channel number(MDCN) is investigated. Both the input voltage amplitude and the breakdown voltage threshold of each discharge channel play a critical role. With the increase of the input voltage and the decrease of the breakdown voltage, the MCDN increases almost linearly. With the increase of the discharge capacitance, the MDCN first rises and then remains almost constant. With the increase of the circuit inductance, the MDCN increases slowly but decreases quickly when the inductance increases over a certain value. There is an optimal value of the capacitor connected to the discharge channel corresponding to the MDCN. Finally, based on these results, to shorten the discharge time, a modified multichannel discharge circuit is developed and validated by the experiment. With only 6-kV input voltage, 31-channels discharge is achieved. The breakdown voltage of each electrode gap is larger than 3 kV. The modified discharge circuit is certain to be widely used in the PSJA flow control field.     

Air breakdown times of two-stage spark gaps and their applications

The air breakdown times of second spark formation, geometrically in series with trigatron switching, are studied and compared to that of a triggered spark gap. Despite a large breakdown jitter of ⩾1 μs in the trigatron, the latter is not observed to affect the electrical closure times of the series-connected second spark gap, which is triggered largely as a result of their direct electrical and optical coupling. While most of the multistage spark gaps are designed for fast and precision switching of high voltages, the air breakdown times of the second spark formation find a number of useful applications, particularly in achieving uniform glow discharge in pulsed gas lasers     

Der Einfluß einer zylindrisch konvergierenden Stoßwelle auf ein Funkenplasma

The influence of a cylindrically converging shock wave on a spark plasma is investigated. The shock wave is produced by a spark discharge in the axis of a cylindrical vessel. After the reflection at the wall the shock wave returns to the spark channel. The results of time resolved spectroscopic studies of the plasma state are compared with calculations on the basis of a gas-dynamic model. The influences of possible deviations from thermodynamic equilibrium are critically discussed.     

Geometrical study of spark ignition in two dimensions

Spark ignition, as the first step during the combustion in Otto engines, has a profound impact on the further development of the flame kernel. Over the last ten years growing concern for environment protection, including low emission of pollutants has increased the interest in the numerical simulation of ignition phenomena to guarantee successful flame kernel development even for lean mixtures.

However, the process of spark ignition in a combustible mixture is not yet fully understood. The use of detailed reaction mechanisms, combined with electrodynamical modelling of the spark, is necessary to optimize ignition of lean mixtures.

This work presents simulations of the coupling of flow, chemical reactions and transport with discharge processes in order to investigate the development of a stable flame kernel initiated by an electrical spark. A two-dimensional code to simulate the early stages of flame kernel formation, shortly after the breakdown discharge, has been developed. The model includes Joule heating. The spark plasma channel formed as a consequence of the breakdown is incorporated into the initial conditions. The computations include the initial phase (1–5 µs), which is governed by pressure wave formation, but also the transition to flame propagation. A thorough study of the influence of the electrodes' geometry, i.e. shape and size, and gap width, has been performed for air and a lean H₂–air mixture. Also a detailed methane-air mechanism was chosen as another example including combustion.

Due to the fast expansion of the plasma channel, together with the geometrical complexity of the electrodes, a complicated flow field develops after the emission of a pressure wave by the expanding channel. Special numerical methods, including artificial viscosity, are required to resolve the complicated flow field during these first 1–5 µs. The heat release through chemical reactions and transport processes is almost negligible during this short phase. The second phase, i.e. the development of a propagating flame and the flame kernel expansion, can last up to several milliseconds and is dominated by diffusive processes and chemical reactions. It has been found that the geometry greatly influences the developing flame kernel and the flow field. As soon as chemical reactions begin to contribute significantly to the heat release, the effect becomes smaller.     

液相放电等离子体特性与激波特性的数值分析

以能量平衡方程为基础,采用不同的电导率唯象模型描述了液相放电等离子体圆柱形通道特性,得到了通道内半径、温度、电阻、电流和耗散能量随时间的变化关系,还给出了距离放电间隙中心一定距离处的冲击波压力变化,并与前人利用等离子体通道球状模型计算得到的结果进行了比较。结果表明:把等离子体通道看成球状和看成圆柱状在描述通道压力和通道半径时差异显著,而在描述其他物理特性时差别不大;三种电导率模型在描述等离子体通道物理特性时,变化趋势大体相同,而在描述激波特性时,电导率模型σ₂更符合实际;通过对比电学参数与压力参数的变化,就可以在实验中根据实验数据以及具体的研究问题进行模型的适用性选择。     

Model of the spark discharge initiation of detonation in a mixture of hydrogen with oxygen

A model of the spark-discharge initiation of detonation was developed to integrally describe both spark channel processes and detonation initiation processes under the action of a spark-generated shock wave. The consumption of discharge energy for dissociation, ionization, and emission processes and conversion into thermal and kinetic energies with consideration for fuel combustion energy conversion was evaluated. The time history of the spark channel resistance was calculated. The time history of the fractions of energy inputted into a spark and lost for the heating of external circuits with respect to the total discharge energy was determined. Simulation was performed for capacitor discharge in a mixture of hydrogen with oxygen.     

Observation of enhanced prebreakdown electron beams in a vacuumspark with a hollow-cathode configuration

The generation of prebreakdown electron beams in a low-energy vacuum spark with a hollow-cathode configuration is observed under a range of experimental conditions. The vacuum spark studied is powered by either a 25-kV, 3.3-nF single capacitor discharge or a two-stage, 50-kV, 1.65-nF Marx. The electron beams are detected by observing the X-ray emission from the anode tip produced by electron impact. Results show that an electron beam is formed well before the onset of the electrical breakdown. This prebreakdown electron beam has an initial slow buildup phase followed by an exponential rise, leading to the breakdown of the discharge. This behavior of the electron beam evolution is in good qualitative agreement with the model simulation of the pseudospark phenomenon obtained for a transient hollow-cathode 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.     

Optical study of parameters of the passage of a shock wave from air to water

A study is made of the penetration of shock waves from air into water. The shock wave in air is generated as a result of dielectric breakdown induced by pulsed CO₂-laser radiation. A combination of the double-exposure shadow method and holographic interferometry is used to measure the shock-wave parameters. Density and pressure profiles behind the wave front are obtained at different times after onset of breakdown. It is shown experimentally that as the wave passes through the interface from the air to the water, there is a fourfold amplification of the pressure in the shock wave front. Estimates of the width of the shock wave front formed in the water are given in the context of studies of large-scale explosion processes. It is shown that simple empirical dependences, established in the course of studies of large-scale explosions, are also valid with certain corrections for microscopic laboratory experiments. Zh. Tekh. Fiz. 68, 39–43 (August 1998)     

氮气火花开关击穿机制的理论和数值研究

三电极气体火花开关带有触发极,相比两电极开关,其开关导通的可控性较高,工作电压较低且抖动小,所以气体火花开关中三电极开关的应用较为广泛.本文针对大气压氮气环境下的两电极开关和三电极开关的击穿机制进行了理论与数值模拟研究.通过理论和数值计算发现,对于平板-平板的两电极开关来说,低电压下(小于6.3 kV)无法产生流注击穿,高电压下(大于6.3 kV)会先形成由阴极到阳极的负流注,然后再形成由阳极向阴极的正流注.而在三电极开关的击穿过程中,首先会在触发极和绝缘体之间发生击穿,然后这个通道不断向阴阳极扩展,最终形成阴阳极之间的电弧通道.在本文的计算工况下,如果需要阴极-触发极、阳极-触发极同时击穿的话,其阴极-触发极之间的外加电压需要大于1.18 kV,而阳极-触发极之间的外加电压需要大于3 kV.当考虑触发极的场致发射后,该击穿阈值可以显著降低.     

水中针-板结构小能量脉冲火花放电特性北大核心CSCD

开展了J量级系统储能下电脉冲参数对水中火花放电特性影响研究。驱动源采用参数可调的固态重频纳秒脉冲电源,放电负载为水中针-板结构(间距1 mm),在低重频条件(约5 Hz)下进行实验。通过调节放电参数、拍摄高速阴影图像、光谱诊断以及声信号测量,研究水中脉冲放电的物理特性,得到不同放电参数下放电演化规律及其对声学、光谱特性影响。实验发现:在J量级储能下,放电通道连通两极后,回路电流在几百ns内快速上升至10 A左右,随后缓慢下降,持续50~60μs。发现预设脉宽对放电影响较大,短脉宽条件下放电会被电源固态开关强制截断出现反向放电,而长脉宽条件下放电通道在后期变得不稳定甚至熄弧中断,出现气泡中二次放电现象。辐射光谱揭示了更多等离子体信息,推断通道电子密度在1018 cm-3量级,随着脉宽增加,特征谱线强度增加,表明活性粒子数密度增加,但粒子种类不变。短脉冲(<150μs)作用下产生的脉冲声波的特征宽度在110~150μs,而当脉宽继续增大,声波脉宽并不继续增加而是保持不变,保持在150μs左右。研究结果对水中小能量火花放电的机理研究有一定参考价值,为水声学、液相等离子体等领域的应用提供思路。     

开关火花电阻对脉冲前沿的影响

 分析气体火花开关的电阻特性是研究开关能量损耗、电弧通道的热等离子过程、开关间隙绝缘恢复及输出脉冲特性的重要基础。基于气体开关的导通机理,建立了开关导通工作的电路模型,给出了与形成线、传输线联通的开关等效电路和开关电流表达式,分析了开关电感、电阻对电流增长（脉冲前沿）的影响。研究结果表明：对于大间距、高电压气体开关,火花电阻是影响开关输出脉冲前沿的主要因素。     

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.     

高速时空分辨光谱仪的构建及其在水中纳秒脉冲火花放电光谱诊断中的应用

近年来对水中高压脉冲放电等离子体特性的诊断研究越来越受到重视。测量单个放电脉冲放电等离子体的时间-空间分辨发射光谱,有助于研究水中脉冲放电等离子体的时空演化动力学特性和规律。在本研究中将四分幅超高速相机和单色仪结合,构建了一种跟踪单个放电脉冲的高速时空分辨光谱仪,开发了相应的光谱分析软件。用波长632.8 nm的He-Ne激光器,在1 200 g·mm-1刻线光栅条件下对光谱仪的性能进行了测试。结果表明：对应He-Ne氦氖激光632.8 nm谱线的像素分辨率为0.013 nm。在曝光时间20 ns时,单色仪狭缝宽度0.2 mm时632.8 nm谱线的仪器展宽为(0.150±0.009)nm,仪器展宽随着狭缝宽度的增加呈现增大趋势。曝光时间的变化不会引起仪器展宽的变化,能够确保在调节相机曝光时间的过程中不影响光谱仪性能。利用该高速分辨光谱仪对水中纳秒火花放电发射光谱进行了测量,单次曝光获得了单一脉冲放电等离子体时空演化光谱。今后进一步完善实验室的电路条件消除放电干扰,可以对单个放电脉冲进行更细致的测量,为研究单个放电脉冲等离子体参数的时空演化特性提供良好的技术手段。     

Experimental study of pulsed corona discharge in air

We report on the results of experimental investigation of a pulsed corona discharge in electric fields with different degrees of nonuniformity under the action of a standard thunderstorm pulse in a wide range of voltages from the origination threshold to the breakdown. A high-sensitivity video camera makes it possible to record microdischarge and streamer processes in air long before the spark breakdown. It is known that the size of the corona discharge sheath increases with the supplied voltage, and the shape of the corona sheath depends on the polarity of the active electrode [1, 2]. It was demonstrated for the first time by Peek [3] that the range of voltages from the initiation of a corona discharge to the spark breakdown increases with the degree of nonuniformity of the electric field. We show that an analogous pattern is observed for a pulsed corona also. Our results show that the form of a pulsed corona discharge considerably depends on the pulse polarity, and a spark breakdown becomes possible when most branches in the streamer corona cover the electrode gap.     

空气中激光支持爆轰波实验及理论分析

为了研究激光击穿空气产生的等离子体爆轰波形成机制和传播规律,利用高能量CO2激光器产生强激光,进行了空气中产生激光支持等离子体爆轰波实验。实验中:设置了诱导靶板,用于诱发和定位空气中的激光支持爆轰波;以激光器升压过程球隙放电产生的光信号作为触发源,触发高时间分辨率(纳秒级)的高速相机,记录了激光支持爆轰波的成长和传播全过程。分析了激光支持爆轰波的形成机理和传播规律。采用C-J爆轰理论,计算了激光支持爆轰波的压力和温度。研究结果表明:激光支持等离子体爆轰波形成初期,等离子体爆轰波发光体为球形;随着时间增加,等离子体爆轰波发光体的形状类似流星,且头部为等离子体前沿吸收层,亮度较高,而尾部等离子体温度较低,亮度较弱。等离子体爆轰波高速向激光源的方向移动,爆轰波速度高达18 km/s,温度约为107K。随着激光强度的减弱,爆轰波速度迅速按指数规律衰减,当爆轰波吸收的激光能量不能有效支持爆轰波传播时,爆轰波转变为冲击波。