Triggering of radial multichannel pseudospark switches by a pulsedhollow cathode discharge

We report on a special trigger discharge for pulsed high-power pseudospark switches. The switch used is a radial three-channel pseudospark switch. For triggering, a cylindrical trigger electrode is inserted into the hollow cathode of the main gap. This electrode acts as a hollow cathode for the dc preionization, while the hollow cathode of the main gap is the anode. A negative high-voltage pulse supplied to the trigger electrode ignites the main discharge. We report the temporal evolution of the trigger discharge observed with a fast camera. This trigger method gives an excellent current distribution among the discharge channels, as can be proven by fast photography. The switch has a delay of 220 ns and a jitter of 15 ns     

Investigations about triggering of coaxial multichannel pseudosparkswitches

A fundamental problem of pseudospark switches is erosion in the borehole area. One way to reduce erosion is to distribute the current to several discharge channels. Essential for multichannel operation is a reliable ignition of all these channels. The aim of this work was to find out the requirements for a trigger for multichannel pseudospark switches and to develop a suitable trigger device. The investigations were made with a three channel pseudospark switch. The developed trigger is a pulsed hollow cathode discharge with a 3 mA dc-preionization. A trigger voltage of 4 kV results in a current of about 6 A in the hollow cathode of the trigger-section. This hollow cathode discharge causes a trigger current into the hollow cathodes of the pseudospark chambers. The trigger current which is necessary to ignite an equally distributed discharge has to be at least 3 mA into each main switch hollow cathode. A jitter of 2 ns was achieved for the coaxial multichannel pseudospark switch     

Trigger devices for pseudospark switches

Effective triggering of pseudospark switches with long lifetime and low jitter remains an important problem. This paper presents results of investigations of trigger methods for pseudospark switches. based upon pulsed glow discharges in planar and hollow electrode geometry for charge injection. The influence of different wiring and geometries of the electrodes for preionization is investigated. The effect of additional blocking potentials in the hollow cathode to improve different trigger systems was measured. Calculations of the static potential in the hollow cathode with or without blocking potential are compared with parameters of the discharge     

赝火花开关放电的蒙特卡罗粒子模拟

采用粒子模拟和蒙特卡罗相结合的方法,应用静电求解模型,对赝火花开关初始放电过程进行了模拟。赝火花开关初始放电过程主要由汤森放电过程、等离子体形成、空心阴极效应和场致发射引发主放电组成;等离子体形成和空心阴极效应对赝火花开关的发展导通具有至关重要的作用。改变赝火花开关工作参数,如气压、电极孔径、阳极电压和阴极腔中初始粒子密度,研究其对赝火花开关电子峰值电流形成时间的影响。结果表明:随着气压、电极孔径、阳极电压和初始粒子密度的增大,赝火花开关电子峰值电流形成时间减小。     

Requirements for simultaneous ignition of all channels in ahigh-current radial multichannel pseudospark switch

In order to reduce erosion of electrodes and consequently achieve a long lifetime for high-power switches, a low-pressure switch, namely the pseudospark switch (PSS), is investigated as an alternative to the conventional high-power switches. The erosion can be reduced further along with an enhancement of the current and the rate of current rise, if more channels are introduced for the passage of current. An intense electron beam is known to be emitted by the hollow cathode during the starting phase in a pseudospark discharge. It has been reported that in a multichannel setup each installed channel can ignite only if it gets ionized by its respective beam. The results presented in this paper show how this multichannel operation can be accomplished. These results are based on a statistical study of a radial multichannel pseudospark switch and an observation of the discharge course by means of high-speed photography and show that a glow discharge trigger, with a high glow current (⩾1.2 mA) and slowly rising trigger pulses (⩾150 ns), allows a simultaneous ignition of up to 12 channels     

Silicon carbide as electrode material of a pseudospark switch

Through the last years, the pseudospark switch, a low-pressure gas discharge switch with hollow cathode geometry, became established as a promising element of pulsed power technology and a serious alternative to other high-power switches. The use of a novel electrode material silicon carbide yields performance improvements in two main areas. Quenching phenomena, a long-standing problem for several applications, are suppressed completely and the switch lifetime can be distinctly increased, approaching that of thyratrons for operation with high repetition rate. As a crow-bar switch, the lifetime is nearby unlimited due to cold electrode usage. Spatial and temporal resolved spectroscopy revealed new insight into the extraordinary discharge behavior of silicon carbide electrodes. The radial plasma expansion from the central bore hole to the outer electrode regions, forming vesicular shells of different ionization stages of Si and C, are described in detail. The remaining problem, a significant loss of deuterium gas during discharge, has been long-term tested and is assumed to be the outcome of absorption in the silicon carbide electrodes. An envisaged promising remedy is presented     

Corona-plasma triggered pseudospark discharges

This paper reports on a novel pseudospark triggering technique using a corona-plasma electrode to feed initial electrons into one of the pseudospark hollow electrodes. This corona-plasma trigger can be used either at the cathode or anode side of the pseudospark, and it can be connected as well to the high voltage electrode as to the grounded one. With this trigger scheme, a pseudospark switch working at pulse repetition frequencies (PRF) up to 100 Hz has been demonstrated in a continuous mode of operation, while PRF as high as 1 kHz has been achieved in a burst mode. The cumulative number of pseudospark discharges triggered with this technique is already above 5*10⁷ , and this corona-plasma triggered pseudospark is still in operation without performance degradation. The main advantage of this pseudospark triggering technique lies in its simultaneous simplicity and reliability allowing high repetition rate, long lifetime, and the complete absence of keep-alive electrode and standby power consumption     

Analysis of the hollow cathode emitted electron beams in a radialmultichannel pseudospark

A radial multichannel pseudospark switch seems advantageous for switching currents exceeding 100 kA. A single common hollow cathode guarantees the same trigger conditions for all channels. The electron beams emitted out of the hollow cathode of this switch are analyzed by means of Faraday-cups by varying the circuit as well as the trigger parameters. In addition to the cup measurements, the discharge is observed using a CCD camera. It is found that each channel of the switch must receive a respective beam, before the resistance of the switch turns low, if the discharge current is to run through that channel. In addition, most of the varied parameters do not influence the electron beam     

Experimental data on the initiation of a pseudospark discharge

In connection with the development of a pseudospark switch for copper vapor lasers, several long endurance tests at repetition rates between 50 Hz and 7 kHz have been performed during the past years. These tests give some clues to the basic phenomena that are involved in the current build-up of a pseudospark discharge, especially as concerns the interaction of gas and surface effects. According to these results, current flow is initiated by a glow discharge in such a way that the positive column penetrates into the hollow cathode; forming a virtual anode. When the current reaches a threshold value <100 A, the electric field produced in the cathode fall of the glow discharge is sufficient for the formation of cathode spots in the cathode backspace. The spots act as electron emitters and carry the discharge current up to approximately 1 kA, where large area emission processes at the front side of the cathode might become important     

阵列微孔阴极放电触发的纳秒脉冲开关

为了实现在大气压下低触发电压的多通道放电,以阵列微孔阴极结构作为触发装置设计了一种新型纳秒脉冲开关。以激光打孔的双面环氧板为阵列微孔阴极,研究了开关工作系数、微孔阴极放电电流、微孔阴极孔数及微孔阴极孔径对开关触发电压、延迟和抖动时间的影响。实验结果表明:更多的阵列微孔、100 m的微孔孔径能够降低开关的触发电压,同时高开关工作系数、大触发电流、多阵列微孔能够减少开关的延迟和抖动时间。因此,为了获得更高性能的纳秒脉冲开关,除了对开关结构的进一步改善,这几个影响开关性能的因素是设计开关时应主要考虑的。     

伪火花放电开关的陶瓷表面放电触发研究

 设计了伪火花开关的陶瓷材料表面放电触发器，该触发器体积小，结构简单，可焙烧。经大量实验表明，较传统的表面放电触发器可靠性提高，寿命延长，用它触发的伪火花开关（放电电压30~2kV）具有稳定的放电时延（50~340ns）和时延抖动（15~40ns）。由于触发电流大，因此可在极低的开关电压下触发开关，对耐受电压30kV的伪火花开关，其最低可触发开关电压可至600V。     

Influence of the hollow cathode dimensions on the electron beamcurrent in a pseudospark discharge

Investigations on the electron beam current of a free running pseudospark discharge are presented. A single gap system with hollow cathodes of different dimensions is used. The filling gases are argon and helium. The electron beam signal consists of a first pulse with currents of several hundreds of mA and a main pulse with currents up to 20 A. A variation of length and diameter of the hollow cathode significantly influences the pressure range in which a free running pseudospark discharge exists and the peak current of both electron beam pulses. Plasma formation and electron beam generation are studied by high speed photography. The experimental results give some information on the discharge mechanisms that is integrated in a qualitative model of the pseudospark discharge     

Development of a high current pseudospark switch and measurement ofelectron density

The pseudospark, a low-pressure gas discharge in a special geometry, is suitable for high-current switching. A single-channel prototype is tested with a 3.3-μF capacitor bank, voltages up to 30 kV, and peak currents up to 120 kA. The electrical circuit, not comprising any load resistor causes weakly damped sinusoidal pulses of 5-μs duration at 90% current reversal. For lifetime tests, a switch with an alumina insulator and copper seals is used. Hydrogen is the working gas. Several electrode materials like molybdenum, tungsten, graphite and chromium-copper are tested. Optical investigations of the discharge and of plasma parameters are done with an O-ring sealed pseudospark switch. The light of the discharge is observed spectrally integrated with a streak camera. Spectral resolution is obtained by using a high-speed shutter in combination with a monochromator. The radial electron density is determined by measuring the Stark broadening of the Balmer H_β-line. At 60 kA a maximum electron density of about 2×10¹⁷cm^-3 is calculated     

新型电子束源—虚火花放电室设计

提出一种新型的脉冲线加速器驱动的高功率，强流密度，低发射率，高亮度电子束源－虚火花放电室的初步设计。基于空心阴极效应和虚火花放电经验公式，确定了空心阴极，多隙阴－阳极，取及工作气压范围，最后提出关于虚火花产生高亮度电子束源的总体实验方案。     

Generation of a pulsed high-current low-energy beam in a plasma electron source with a self-heated cathode

The transition of a low-current discharge with a self-heated hollow cathode to a high-current discharge is studied, and stability conditions for the latter in the pulsed–periodic mode with a current of 0.1–1.0 kA, pulse width of 0.1–1.0 ms, and a pulse repetition rate of 0.1–1.0 kHz are determined. The thermal conditions of the hollow cathode are analyzed, and the conclusion is drawn that the emission current high density is due to pulsed self-heating of the cathode’s surface layer. Conditions for stable emission from a plasma cathode with a grid acting as a plasma boundary using such a discharge are found at low accelerating voltage (100–200 eV) and a gas pressure of 0.1–0.4 Pa. The density of the ion current from a plasma generated by a pulsed beam with a current of 100 A is found to reach 0.1 A/cm². Probe diagnostics data for the emitting and beam plasmas in the electron source are presented, and a mechanism behind the instability of electron emission from the plasma is suggested on their basis.     

Determination of electron density and energy influx in a hollow cathode glow discharge used for powder modification

The electron density and energy influx in an argon hollow cathode glow discharge were determined to obtain adequate parameters for subsequent surface modification of low density polyethylene (LDPE) powder to change the wettability. The electron density was studied by Langmuir probe measurement in dependence on process gas pressure and hollow cathode material. Besides the determination of the rate of increasing electron density with input power an optimal experimental pressure was determined. The energy influx was studied by thermal probe measurements in dependence on process gas pressure, bias voltage, axial position and hollow cathode material. Inside the hollow cathode the energy influx is nearly constant along the whole cathode length. With increasing pressure the energy influx decreased. At biased thermal probe the energy influx was observed to decrease up to the floating potential and beyond it increases with increasing voltage. Using different hollow cathode materials the electron density as well as the energy influx reach higher values for aluminum than for copper and stainless steel.     

The spatial and temporal development of pseudospark switch plasmas

Optical spectroscopy is used to investigate the spatial and temporal development of high-current pseudospark switch plasmas. At a peak discharge current of 12 kA in amplitude and a current reversal of 20%, the electron density is measured from Stark width broadening of the hydrogen Balmer beta line. The peak electron density of ~4×10¹⁵ cm^-3 is measured briefly after the current maximum. The discharge initially starts on the symmetry axis of the cathode hole. A cylindrical plasma column is observed, which is produced mainly by ionizing collisions of beam electrons formed in the hollow cathode during the early part of the discharge. This plasma column rapidly expands in the radial direction, until it contacts the edge of the cathode hole. The same behavior is found when the Balmer beta line intensity is evaluated rather than the line shape. Although statistically distributed, localized bursts of light are found occasionally, an axially symmetric, homogeneous light intensity distribution is always predominant, and the local arcing is merely superimposed on it. These results confirm that the discharge remains diffuse during most of the current pulse     

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     

辉光放电触发赝火花开关实验研究

设计了一种辉光放电触发赝火花开关,进行了He气介质下的放电实验研究。测得了耐受电压和气压关系,结果表明:随着气压的升高,开关耐受电压减小;开关电极孔径为3mm、电极间距为4mm时的耐受电压约是电极孔径和电极间距均为3mm时的85%。在电极孔径和电极间距均为3mm时,研究了辉光放电电流、气压、触发电压等参数对开关时延、抖动的影响。结果显示:当辉光放电电流大于0.45mA、气压为7～30Pa、触发电压达到一定值,开关就能比较稳定地触发,时延短、抖动小。在辉光放电触发下,实现了开关耐受30.5kV、时延223.2ns的输出,并实现了开关抖动小于1ns的稳定输出。     

辉光放电触发赝火花开关的时延及抖动特性

 设计了一种辉光放电触发赝火花开关,对其时延和抖动特性进行系统研究。研究了开关时延、抖动与辉光放电电流、气压、触发电压及阳极电压的关系。当辉光放电电流小于0.30 mA时,开关无法触发导通;当电流为0.35~0.60 mA时,随辉光放电电流的增大,开关时延、抖动减小;当辉光放电电流为0.60 mA时,开关时延、抖动基本不变,出现饱和。当氦气气压低于6 Pa,开关难以触通,与理论计算值6.95 Pa吻合;当氦气气压为6~12 Pa时,开关的时延、抖动随气压的升高而减小;气压为12~30 Pa时,开关工作在比较稳定的状态。当触发电压小于3 kV,开关难以触通;随着触发电压的增大,开关时延、抖动减小;当触发电压大于5.3 kV,开关时延、抖动基本保持不变。开关在稳定工作条件下,阳极电压在8~25 kV范围内变化时;开关时延基本不变。