介质阻挡放电点燃大气压直流均匀放电的光谱研究

由于大气压均匀放电等离子体在工业领域具有广泛的应用前景,为了获得大尺寸的大气压均匀等离子体,采用氩气作为工作气体,在大气压空气环境中利用同轴介质阻挡放电点燃了针-板电极间的大气隙(气隙宽度达到5 cm)直流均匀放电。研究发现,同轴介质阻挡放电能够有效降低针-板电极间的击穿电压。该均匀放电由等离子体柱、等离子体羽、阴极暗区和阴极辉区组成。其中等离子体柱和阴极辉区都是连续放电。而等离子体羽不同位置的放电是不同时的。事实上,等离子体羽放电是由从阴极向着等离子体柱移动的发光光层(即等离子体子弹)叠加而成。利用电学方法测量了放电的伏安特性曲线,发现其与低气压正常辉光放电类似,均具有负斜率。采集了放电的发射光谱,发现存在N₂第二正带系、氩原子和氧原子谱线。通过Boltzmann plot方法对放电等离子体电子激发温度进行了空间分辨测量,发现等离子体柱的电子激发温度比等离子体羽的电子激发温度低。通过分析放电机制,对以上现象进行了定性解释。这些研究结果对大气压均匀放电等离子体源的研制和工业应用具有重要意义。     

Optical characteristics of the plasma of a nanosecond atmospheric-pressure volume discharge in a nonuniform electric field

Optical characteristics of the plasma of nanosecond volume discharges in air, nitrogen, krypton, argon, neon, and Ar/N₂ and Ar/Xe mixtures at elevated pressures are investigated. The discharges are excited in a gap with a cathode of small curvature radius. The waveforms and spectra of plasma emission from discharges in different gases in the 230-to 600-nm spectral range are measured. Optical generation in an Ar/Xe mixture is achieved at an active length of 1.5 cm. A comparison is performed of the spectral characteristics of the emission from nitrogen, krypton, argon, and neon excited by a volume discharge in a nonuniform electric field, by a nanosecond electron beam, and by a pulsed volume discharge in a uniform electric field at a high initial voltage.     

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.     

Formation of the microscopic structure of high-voltage nanosecond diffuse discharges in sharply nonuniform geometry

The possibility of the development of ionization instability in the avalanche and plasma phases (of the long-wave and short-wave type, respectively) is demonstrated for high-voltage nanosecond discharges in sharply nonuniform geometry. Specific implementation is determined by the electric field distribution in the cathode region and by the emissivity of the cathode. The spatial structure of the discharge formed in both cases is self-similar.     

光谱学方法研究介质阻挡沿面放电辉光模式的电场分布

利用介质阻挡沿面放电装置,在低气压空气中实现了辉光放电模式。利用光电倍增管对放电发光信号进行检测,发现外加电压每半周期出现一个发光脉冲,并且正负半周期的光脉冲是不对称的。利用Photoshop软件处理放电的照片,研究发现平行于高压电极不同位置的发光强度基本相同,然而距离高压电极越远,发光强度减小。放电中总电场由外加电场和电介质积累的壁电荷电场共同决定,确定该电场具有重要意义。通过分析放电的发射光谱中N+₂(B 2Σ+_u→X 2Σ+_g)谱线391.4 nm和N₂的第二正带系(C 3Π_u→B 3Π_g)谱线337.1 nm的比值,可以定性地说明电场的分布。研究发现电场在高压电极附近较大而远离高压电极处较小。这些研究结果对沿面放电的数值模拟和工业应用具有重要的价值。     

大气中介质阻挡放电发光的时间特性

采用光学方法测量了大气中介质阻挡放电丝极模式的时间特性，揭示了介质阻挡放电动力学过程的时间规律。实验表明，在驱动电压的产周内的放电团簇是由多个放电脉冲构成的，放电脉冲的持续时间为30-50ns，相邻放电脉冲之间的间歇时间为几百ns。本工作的结果对介质阻挡放电的应用有重要意义。     

Numerical simulation of super-short pulsed discharge in Helium with particle-in-cell Monte--Carlo collisions technique

This paper reports that a simulation of glow discharge in pure helium gas at the pressure of 1.333×10³ Pa under a high-voltage nanosecond pulse is performed by using a one-dimensional particle-in-cell Monte Carlo collisions (PIC--MCC) model. Numerical modelling results show that the cathode sheath is much thicker than that of anode during the pulse discharge, and that there exists the phenomenon of field reversal at relative high pressures near the end of the pulse, which results from the cumulative positive charges due to their finite mobility during the cathode sheath expansion. Moreover, electron energy distribution function (EEDF) and ion energy distribution function (IEDF) have been also observed. In the early stage of the pulse, a large amount of electrons can be accelerated above the ionization threshold energy. However, in the second half of the pulse, as the field in bulk plasma decreases and thereafter the reverse field forms due to the excessive charges in cathode sheath, although the plasma density grows, the high energy part of EEDF decreases. It concludes that the large volume non-equilibrium plasmas can be obtained with high-voltage nanosecond pulse discharges.     

Experimental research of diffuse bi-directional pulsed dielectric barrier discharge plasma

A bi-directional nanosecond pulsed power supply is employed to generate diffuse dielectric barrier discharge in N₂ using needle-plate electrode configuration at atmospheric pressure. Both discharge images and optical emission spectra of diffuse bi-directional nanosecond high-voltage pulsed dielectric barrier discharge are successfully recorded under severe electromagnetic interference. The diffuse performance of the discharge at different electrode gap distances is observed. The effects of pulse peak voltage, pulse repetition rate, electrode gap distance, He addition, and O₂ addition on the optical emission spectra are investigated. The main physicochemical processes involved are discussed.     

The Poole–Frenkel effect in a dielectric under nanosecond irradiation by an electron beam with moderate or high current density

Processes of electron trapping and detrapping determine in many respects intense processes arising in dielectric and delayed by 1–100 ns from the irradiation pulse of a high-power electron beam, such as electron emission, electric discharge in the bulk of the dielectric, flashover, and electric breakdown. A model of charged donor center ionization in a dielectric exposed to a strong electric field is constructed. The model takes into account 1) the energy spectrum of the charged donor center in the dielectric, 2) the semiclassical state density in the donor center, 3) spontaneous emission of phonons by the electron localized in the donor center, 4) increase in the kinetic energy of the electron (heating) in the external electric field, 5) electron tunneling through a potential barrier and its reflection from the barrier depending on the external field intensity, and 6) thermal fluctuations of energy of the electron localized in the donor center. The probability of charged donor center ionization in the dielectric per unit time is calculated. In weak fields, the field dependence of the ionization probability almost coincides with that for the Poole–Frenkel theory. In strong fields, the contribution of electron heating to the external electric field is the deciding factor. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 10–16, December, 2008.     

Displacement of Field Emitters by Vacuum Discharges

The location of field emitting micro-points, produced by nanosecond discharges in UHV, has been investigated by field emission microscopy. Weak discharges (duration < 5 ns, current < 10 A) caused a displacement of the field emission over the cathode by a distance that corresponds to average crater diameters (4–6 μm). Thus new emitters are produced at the boundary of discharge craters. More intense discharges show sometimes a far higher displacement. This can be explained by the formation of micro-points by splashes of molten metal that fly out of the discharge craters. The results support the model of the development of micro-points, as it was published in [1]. They show furthermore that the motion of arc cathode spots can be related to the displacement of microscopic field emitters.     

介质阻挡放电中微放电周期特性的光学测量

采用特殊设计的双水电极介质阻挡放电装置,利用光学方法原位、无干扰地测量了大气压空气介质阻挡放电中微放电的单脉冲特性和周期特性,实验上首次发现在介质阻挡放电斑图模式中,微放电在驱动电压的正负半周的放电时刻并不是固定的,相邻两次放电的时间间隔是长短交替的。根据壁电荷对微放电通道相邻两次放电的不同作用,分析了相邻两次放电时刻的联系,很好地解释了相应的实验现象。     

介质阻挡均匀大气压辉光放电数值模拟研究

通过数值求解一维电子、离子连续性方程和动量方程，以及电流连续性方程，计算了氦气介 质阻挡大气压辉光放电电子、离子密度和电场在放电空间的时空分布，以及放电电流密度和 绝缘介质板充电电荷密度随时间的变化. 分析讨论所加电压频率、幅值及介质板性质等对均 匀大气压辉光放电性质的影响. 当外加电压频率足够高时，大量离子被俘获在放电空间，空 间电荷场又引起足够多的电子滞留在放电空间. 这些种子电子使得在大气压下发生汤森放电 ，放电空间结构类似于低气压辉光放电，即存在明显的阴极位降区、负辉区、法拉第暗区和 等离子体正柱 关键词： 大气压辉光放电 介质阻挡 数值模拟 等离子体     

Sheath formation study in Ne–Xe DBD discharge

The sheath is formed near the cathode immediately after applying the voltage across the electrodes. This formation is mainly due to the depopulation of this region by electrons. Using the one-dimensional model of the sheath region and the kinetic model coupled to the dielectric barrier discharge discharge electric circuit, we studied the formation of the cathode sheath and its evolution during the first pulse in Ne–Xe mixture for 10% and 20% of xenon at a total gas pressure of 400 Torr and for an applied voltage of 3 kV. The results illustrate the discharge behaviors as well as the evolution of the electric field and the charged particles in the cathode region. The effect of the xenon concentration is also studied and discussed.     

石英管对空气中锥-板结构纳秒脉冲放电的影响

报道了石英管对大气压下空气中锥-板电极结构纳秒脉冲放电的影响实验研究.实验过程中发现将石英管纵向移动位置,放电会出现四种明显不同的形貌状态,对其机理进行了初步的探讨.利用发射光谱诊断了其转动和振动温度(分别为295和2200 K),表明该等离子体具有高度非平衡度.还进一步提出大气压下空气中产生类辉光放电的有效途径. 关键词： 纳秒脉冲放电 空气 纵向介质 非平衡等离子体     

针-板DBD微流注与微辉光交替生成的机理研究

在介质阻挡放电体系中产生辉光放电可以有效的提高放电体系产生高能电子的性能, 为等离子体化学反应提供更加丰富的活性粒子.本文对针-板介质阻挡放电体系下的放电模式进行了研究,实验发现放电正负半周期表现出不同的放电模式, 激励电压为3 kV时放电正负半周期分别为微流注放电和电晕放电(或者Trichel脉冲放电),激励电压为6 kV时放电正负半周期分别为微流注放电和微辉光放电.微辉光放电形貌具有与典型辉光放电相同的分层次放电结构, 分析了激励电压6 kV时的放电过程,认为足够强的阴极电场强度和裸露针状电极形成的有效的二次电子发射过程是形成微辉光放电的主要因素,绝缘介质层的存在避免了微辉光放电向弧光放电过渡.     

Electrical characterization of dielectric barrier discharge driven by repetitive nanosecond pulses in atmospheric air

Dielectric barrier discharge (DBD) is an important method to produce non-thermal plasma, which has been widely used in many fields. In the paper, a repetitive nanosecond-pulse generator is used for the excitation of DBD. Output positive pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 30–40 ns, and pulse repetition frequency varies from single shot to 2 kHz. The purpose of this paper is to experiment the electrical characteristics of DBD driven by repetitive nanosecond pulses. The variables affecting discharge conditions, including air gap spacing, dielectric thickness, barrier fashion, and applied pulse repetition frequency, are investigated. The relationship between electric field, discharge current, instantaneous discharge power across air gap, and estimated electron density with the length of air gap, dielectric thickness, barrier fashion, and pulse repetition frequency is obtained respectively, and the experimental results are also discussed. In addition, two typical images exhibiting homogeneous and filamentary discharges are given with different experimental conditions.     

Aspects of the upstream region in a plasma jet with dielectric barrier discharge configurations

A plasma column with a length of about 65 cm is generated in the upstream region of a plasma jet using dielectric barrier discharge configurations. The effects of experimental parameters such as the amplitude of the applied voltage and the driving frequency are investigated in aspects of the plasma column by the optical method. Results show that both the plasma length and the propagating velocity, as well as the discharge current, increase with the increase in the applied voltage or its frequency. The discharge mechanism is analysed qualitatively based on streamer theory, where photo-ionization is important. Furthermore, optical emission spectroscopy is used to investigate the electric field intensity of the upstream region.     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

High-pressure runaway-electron-preionized diffuse discharges in a nonuniform electric field

High-pressure nanosecond diffuse (volume) discharges in a nonuniform electric field are studied experimentally using a recording system with a ?100-ps time resolution. As the voltage pulse shrinks to a width of ≈100 ps, the initiation of a diffuse discharge without a source of additional ionization is facilitated; specifically, a runaway-electron-preionized diffuse discharge is ignited in atmospheric-pressure air in the case of short interelectrode gaps. It is found that a major energy deposit into the plasma of this discharge is from an abnormal glow discharge following a maximum of the gap voltage.     

正极性纳秒脉冲电压下变压器油中流注放电仿真研究

建立了二维轴对称流体模型, 仿真研究了正极性纳秒脉冲电压下变压器油中针-板电极流注放电的起始与发展过程, 得到了不同的外施电压幅值、脉冲上升沿时间与电极间隙距离下油中流注放电的形貌、 电场强度与空间电荷密度分布等. 仿真结果表明: 空间电荷加强了流注头部前方电场, 使流注通道更易于向前推进, 形成"电离波"; 随着外施电压幅值升高, 流注发展的平均速度显著变大; 较陡的脉冲上升沿形成的放电半径较大, 对应的最大电场强度值变小; 随着电极间隙距离的增加, 流注发展平均速度变快. 仿真显示纳秒脉冲下放电中油温无明显升高, 表明此类放电过程没有明显的油气化现象. 我们认为, 场致电离是油中带电粒子产生的主导机制; 空间电荷效应增强流注前方电场使得电离进一步发展, 最终导致击穿. 本研究有助于加深对变压器油中放电起始、发展直至击穿过程的认识以及对液体电介质中电离机制的理解. 关键词： 变压器油 流体模型 流注放电 空间电荷效应