Use of a liquid-metal cathode in high-vacuum studies of electric breakdown

Studies of electric breakdown in a high vacuum involving liquid-metal (mercury and gallium) cathodes, whose surfaces are stabilized by centrifugal forces, have shown that an increase in the angular velocity at which the experimental apparatus rotates causes an increase in the breakdown field; the mechanism for the vacuum breakdown is found to be independent of the voltage across the electrodes. According to the Frenkel theory, vacuum breakdown results from a disruption of the steady state on the liquid-cathode surface in an electric field. Drops of liquid metal on the anode degrade the dielectric properties of the vacuum gap. Under these conditions, the breakdown mechanism becomes dependent on the voltage across the electrodes. Oxide films on the cathode surface also degrade the dielectric properties of the vacuum gap. It is suggested that the dielectric may be charged by positive ions emitted from the cathode.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 4, pp. 44–49, April, 1969.     

He-Ne激光器放电点火电压的实验研究

寻找能降低He—Ne激光器点火电压的外加催离剂．实验对He—Ne激光器的点火电压在阴、阳极被可见光照射、低放射源作用于放电管和阴极附近加强电场的条件下进行了测量．对不同外加条件作用下He—Ne激光器点火电压实验数据的误差进行了分析．实验结果指出，用可见光照射阴极或阳极不能降低He—Ne激光器的点火电压；能明显降低点火电压的因素可以是低放射源或外加强电场．     

The Role of the Field Emission Effect in the Breakdown Mechanism of Direct‐Current Helium Discharges in Micrometer Gaps

This paper contains results of experimental studies of the direct current breakdown voltage curves and volt‐ampere characteristics of discharges generated in a system consisting of two plane‐parallel tungsten and molybdenum electrodes at separations from 100 µ m to 1 µ m. The measurements were performed in the pressure range from 22.5 Torr to 738 Torr. The results are presented in the form of Paschen curves. Based on the measured breakdown voltage curves, the effective yields have been estimated in the case of different cathode materials. Differences between them are attributed to the influence of the work function of the cathode material on the current‐voltage characteristics due to field emission effect in small gaps and high pressures. At low‐pressures, however, vaporation of impurities from the electrodes material becomes significant. The present paper delivers new data on DC breakdown under these experimental conditions and conditions on the validity of the Paschen law in helium and provides better insight into the role of the field emission and the electrode materials on the breakdown voltage. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some Investigations of the Ignition and Development of Breakdown in Cylindrical Hollow Cathode Glow Discharges

The breakdown behaviour of a hollow cathode glow discharge is investigated in a cylindrical, hollow cathode structure having an internal diameter of 2 cm. The anode is a plane electrode across one end of the cathode cylinder. Pressures of argon between 20 and 107 Pa were used (0.15 to 0.80 torr), and applied voltages between 800 and 2500 V. It is shown that the statistical time lag for breakdown is in the range of ～ 1 ms and depends on the applied voltage, the gas pressure, and the history of operation of the discharge tube. The rise time of the discharge current ranges from about 10 ns at high pressure and voltage to about 200 ns at the lowest pressure and voltage used. The discharge propagates along the cathode axis at a speed of about 10⁸ cm s^?1. From the obtained data, a qualitative model of the first stage of the discharge is derived. Based on this model, a simple calculation gives values of Townsend modified first coefficient η at high values of E/N, 10⁴ < E/N < 8 · 10⁴ Td which fit well at the lowest E/N, where they approach the data of PENNING and KRUITHOF in argon. In contrast to the extremely short initial current-rise times, in the submicrosecond range, the discharge currents reach steady-state values only after about 300 μs.     

Zündspannung und normaler Katodenfall der Glimmentladung für Edelgase und reine Molybdänkatoden

Breakdown voltage near PASCHEN -minimum and normal cathode fall of the glow discharge were calculated for inert gases and molybdenum cathodes on the basis of a simple theory. The method used was similar to TAKEISHI 's one. The calculated breakdown voltages are in good agreement with measured values, if back scattering of the y_i -electrons and the distance which electrons travel from the cathode without ionization is taken into account. In the case of the normal cathode fall the values calculated with the same assumptions for inert gases and molybdenum cathodes are much higher (up to 50%) than the well known experimental values. It was concluded, that the y_i-process in the glow discharge ist not the only important one. Comparing the calculated and experimental values of the normal cathode fall we obtain the result, that photons and/or ions from the glow release about 40–60% of all electrons at the cathode.     

Stochastic and Relaxation Processes in Argon by Measurements of Dynamic Breakdown Voltages

Statistically based measurements of breakdown voltages U _b and breakdown delay times td and their variations in transient regimes of establishment and relaxation of discharges are a convenient method to study stochastic processes of electrical breakdown of gases, as well as relaxation kinetics in afterglow. In this paper the measurements and statistical analysis of the dynamic breakdown voltages U _b for linearly rising (ramp) pulses in argon at 1.33 mbar and the rates of voltage rise k up to 800 V s ^–1 are presented. It was found that electrical breakdowns by linearly rising (ramp) pulses is an inhomogeneous Poisson process caused by primary and secondary ionization coefficients α , γ and electron yield Y variations on the voltage (time). The experimental breakdown voltage distributions were fitted by theoretical distributions by applying approximate analytical and numerical models. The afterglow kinetics in argon was studied based on the dependence of the initial electron yield on the relaxation time Y ₀ (τ ) derived from fitting of distributions. The space charge decay was explained by the surface recombination of nitrogen atoms present as impurities. The afterglow kinetics and the surface recombination coefficients on the gas tube and cathode were determined from a gas‐phase model. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

金属表面镀高分子膜对真空击穿阈值的影响

探索提高金属表面真空击穿阈值的方法,对脉冲功率技术的发展和应用具有重要意义。在金属表面电子发射理论分析的基础上,采用有限元法计算阴极杆表面电场随二极管电压的变化规律,设计了实验系统,并开展了实验研究。实验对比了在脉宽约30 ns、阴极杆与阳极筒间隙12 mm时,钛合金TC4阴极杆在不同种类高分子膜（膜厚30～60 μm）下真空击穿阈值的变化情况。在表面粗糙度R_z（轮廓最大高度）为0.8 μm的TC4阴极杆表面分别镀环氧树脂膜和丙烯酸膜,实验结果表明,镀丙烯酸膜阴极杆的击穿阈值约505 kV/cm,相对于不镀膜阴极杆,击穿场强提高了约20.6%;在表面粗糙度R_z为0.2 μm的TC4阴极杆表面分别镀聚酰亚胺膜和聚醚醚酮膜,实验结果表明,镀聚酰亚胺膜阴极杆的击穿阈值为584 kV/cm,相对于不镀膜阴极杆,击穿场强提高了约28.1%。因此,在金属表面镀丙烯酸膜、聚酰亚胺膜可以有效提高金属表面的真空击穿阈值。     

金属基底对石墨烯薄膜阴极气体击穿稳定性影响

利用石墨烯二维材料极好的场发射能力和发射稳定性,提出了石墨烯阴极提高气体开关击穿稳定性的技术路线。采用化学气相沉积法和基底腐蚀转移法两种方法制备金属基底石墨烯薄膜阴极。利用扫描电子显微镜和拉曼光谱表征了石墨烯薄膜阴极质量,确认了石墨烯层数和均匀性。实验研究了两种石墨烯薄膜阴极气体开关,在微秒脉冲均匀电场作用下的击穿特性,获得了击穿电压幅值和分散性的变化规律。结果表明:当气体为0.6 MPa N2、电极间距为5 mm时,铜基底石墨烯薄膜阴极平均击穿电压为85.9 kV,相对标准差为3.2%;不锈钢基底石墨烯薄膜阴极平均击穿电压仅为59.8 kV,相对标准差为2.4%。当两种阴极击穿电压均为80 kV时,相对标准差比较,不锈钢基底仅为铜基底的44%。分析认为,不锈钢基底石墨烯薄膜质量优于铜基底,石墨烯薄膜导致阴极表面微观场增强因子更高,表面分布更均匀,在电场作用下场致发射产生均匀稳定的大量初始电子流,降低了气体开关击穿电压,有效提高了击穿稳定性。     

Application of Mn nano-flower sculptured thin films produced on interdigitated pattern as cathode and anode electrodes in field ionization gas sensor

The photolitography method was used for producing interdigitated configurations for cathode and anode electrodes of a field ionization gas sensor in which Mn helical nano-flowers with 3-fold symmetry were deposited using oblique angle deposition together with rotation of the substrate about its surface normal, with each rotation divided into six sections. These sections were alternately rotated at high and low speeds. Three different distances were chosen in the design between anode and cathode electrodes, namely 40, 100 and 200 μm. Physical structure and morphology of electrodes were studied by field emission scanning electron microscope and atomic force microscope analyses.The breakdown voltage of the system was studied for nitrogen, oxygen, argon, air and carbon mono-oxide gases. Investigations with these gases at different distances between anode and cathode and different gas pressures confirmed Paschen's Law. Results showed that at low pressures, decreasing the gap between electrodes increases the breakdown voltage. With fewer gas molecules between the electrodes the number of interactions between particles is reduced and higher energies are required for ionization of gas molecules. At high pressures, the breakdown voltage is decreased because of an increased number of molecular interactions. The sensor demonstrated good selectivity between the different gases and selectivity was enhanced with increasing gas pressure. A direct relationship was found at low pressures (e.g., 0.1 mbar) between the breakdown voltage and the gas ionization energy while at high pressures (e.g., 1000 mbar) this relationship was reversed.     

Theory of gas breakdown in an inhomogeneous field

Consideration is given to an independent electrical gas discharge in an inhomogeneous field, with photoionization of the gas as secondary mechanism. The condition for an independent discharge to exist is derived; this condition enables the breakdown voltage to be calculated, and also explains the short times required to form the discharge and the fact that the breakdown voltage is independent of the cathode material. The same condition may be considered as a criterion for the ignition of a self-sustaining corona in a long spark gap with nonuniform geometry.     

Growth of the spark current at the breakdown of short vacuum gaps at steady voltage

The growth time of the spark current at the time of breakdown in vacuum at the constant voltage and pressure existing during this period was investigated with high time resolution. It was shown that, as in the case of a pulsed breakdown: 1) the growth time t_c of the current increases in proportion to the length d of the gap and the ratio d/t_c is 2.5·10⁶ cm/sec; 2) the transition to a rapid rate of current growth (di/dt > 10⁸ A/sec) takes place in 1–3 nsec; 3) during the period of current growth, x-ray radiation appears and a transfer of anode material to the cathode is observed. These results serve as evidence that the origin of current growth is connected with the appearance of efficient electron sources on the cathode. These, as in the case of pulsed breakdowns, are evidently cathode flares, the formation of which has an explosive character and can be identified with the act of breakdown initiation. In essence this event does not change with the rate at which voltage is supplied across the gap and consists of the explosive disintegration of emitting microspikes on the cathode. The appearance of cathode flares indicates the start of an irreversible breakdown of vacuum insulation. The emergence of a burst of x-rays and the erosion of the anode are explained by the action on the anode of a powerful stream of electrons that are emitted from the cathode flares, providing the spark current.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 93–97, September, 1971.The authors thank G. A. Mesyats for his constant attention to the work.     

低阻抗大面积二极管的研制

采用高纯石墨环状阴极和有机玻璃绝缘子,研制了一套低阻抗大面积二极管系统。使用理论计算和数值模拟方法对二极管进行优化设计,在保证绝缘要求的同时,尽量优化二极管轴向长度和内外筒距离以减小二极管的回路电感。实验结果表明,优化后的二极管能在200 kV左右的电压上稳定工作,绝缘结构未发生击穿现象;实验中最高输出电压为213 kV,电流为221 kA,特性阻抗约为1 ,电流密度为8 kA/cm2,脉宽(FWHM)为50 ns。     

Measurements on electron beams in pulsed hollow-cathode discharges

Two different regimes of electron beams have been reported in the pulsed hollow cathode discharge-a low-current, high-energy beam, and a relatively high-current beam of low energy. The high-energy beam is related to the hollow cathode geometry and is found to be present even in the absence of subsequent gas breakdown, while the low-energy beam is always associated with voltage breakdown. Detailed measurements of the spatial and temporal distribution of the electron beam transported beyond a semitransparent anode associated with gas breakdown are reported. In particular, a high-energy component is observed after electrical breakdown. Low-energy electron beams are observed to be transported beyond the anode throughout the main discharge period     

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−10³ s⁻¹) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈10³ cm²). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface. Zh. Tekh. Fiz. 69, 20–24 (May 1999)     

微机电系统微尺度间隙的脉冲击穿特性

利用微机电系统(MEMS)工艺制备了5~40 m间隙的金属铝薄膜电极,测取了不同间隙下施加纳秒方波脉冲时的击穿电压,得到击穿电压和平均场强随电极间隙的变化规律,研究了脉宽、极性对击穿电压的影响,结合击穿后微电极扫描电镜观察结果讨论了其击穿机理,并与相同试样在直流下的结果进行了对比。研究表明:击穿电压随微间隙距离的增大而增大,击穿场强则随着间隙的增大迅速减小;与直流同间隙的击穿电压相比,脉冲作用下的击穿电压并不像宏观尺度下那样高出很多;试样击穿机理应为流注理论。在脉冲作用下,阳极出现了分层熔化现象,由于作用时间很短,阳极表面并未出现类似于直流击穿时留下的坑洞;阴极则出现了溅射沉积现象,只是沉积物质相对较少。     

电极表面光滑程度对水介质高电压击穿的影响

 采用水介质同轴实验装置,改变电极表面的光滑程度,在μs级充电时进行水介质击穿实验,并对实验结果进行了分析和解释。结果表明：抛光电极表面可有效提高水介质耐高电压击穿能力;表面粗糙度为0.4～0.8 μm的抛光电极表面的击穿场强比表面粗糙度为1.6～3.2 μm的粗糙抛光电极表面,更符合Martin公式。电极表面光滑程度的改善,使阴极场致发射电流减弱进而击穿延迟时间变长,气泡也更难以附着在光滑的电极表面,从而可以提高水介质耐高电压击穿能力。     

间隙长度对气体火花开关静态性能的影响

针对快放电直线脉冲变压器驱动源初级储能开关,调节开关气压,进行相同自击穿电压下不同间隙长度的开关自击穿实验。通过监测自击穿电压分布规律变化,研究开关间隙长度对开关自击穿特性的影响。结果表明,当开关间隙长度较大时,电极表面粗糙度较小,表明电极烧蚀程度较低。开关间隙长度为6 mm时,开关自击穿电压分散性达到最小,自击穿电压分布符合高斯函数,间隙长度大于6 mm时自击穿电压分布符合极限函数。适当增加开关间隙长度使自击穿电压分布失去对称性,有利于开关在低欠压比下获得更好的静态性能。针对电极结构,选择开关间隙长度为6~9 mm时能够获得最佳的静态性能。初步分析,引起开关自击穿电压分布发生改变的原因是电场强度的改变对阴极电子发射产生了影响。     

Der Einfluß der Wärmeleitung zur Kathode im elektrischen Feldbogen

For various cathode metalsKesaev has measured the arc lifetime as a function of current. RecentlyKingdon, assuming the field emission theory could not explain these experiments, suggested that the secondary electron emission produced by positive ions through a diffusion-controlled neutralization process determines the cathode mechanism. In this paper another assumption is made, based on the field emission theory: the cathode needs a minimal energy-supply for evaporation in order to maintain a higher vapourpressure and ion density near the cathode. For the mercury cathode this assumption seems to be valid. The temperature of the cathode spot and the voltage drop in front of it, for Bi, Pb, Zn, Tl, Cd, In, Sn, Cu, is not yet exactly known. Therefore the validity of the assumption made, concerning these materials can only be proved qualitatively.     

Abnormal polarity effect in nanosecond-pulse breakdown of SF6 and nitrogen

The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF₆, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF₆ and SF₆–N₂ pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF₆ pressure lower than 0.12 MPa.     

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.