Factors affecting and methods of improving the pulse repetitionfrequency of pulse-charged and DC-charged high-pressure gasswitches

The results of this paper describe some of the factors which affect the repetitive operation of high-pressure gas switches (spark gaps) for both pulse-charged and DC-charged operation. Also discussed are methods which may be employed to improve the pulse repetition frequency (PRF) of spark gaps operating under such conditions. Under pulse-charged conditions, the voltage recovery process of the spark gap has been shown to be restricted following partial density recovery by the residual ion population. This restriction may be minimized by applying a suitable bias voltage across the gap to remove the ion influence. It is also possible to manipulate the voltage-pressure (V-p) breakdown characteristic of a spark gap in order to improve the rate of rise of recovery voltage by reducing the recovery voltage dependence upon gas pressure. The combination of these effects has been shown to reduce the voltage recovery time of pulse-charged spark gaps from several hundred milliseconds to several milliseconds. Under DC-charged conditions, where no “dead time” is available for voltage recovery, it is possible to employ corona discharge effects, which occur in highly nonuniform fields, to stabilize and control the breakdown process. The use of corona stabilization has enabled the operation of a self-closing spark gap at a PRF of more than 5 kHz, without employing gas flow techniques. A triggered version of a corona-stabilized spark gap has also been developed which has demonstrated single run capabilities of 10⁷ (4 h continuous operation at 700 pps) and a lifetime of ~10 shots (maintenance free, sealed switch). The triggered corona switch has also demonstrated controlled switching up to a PRF of 1.2 kHz     

气体火花开关重复频率特性的实验研究

气体火花开关在重复频率脉冲功率技术中具有广泛的应用,影响气体火花开关绝缘恢复与重复频率工作性能的因素众多。实验研究了气体火花开关的能量沉积和气压对开关重复频率指标的影响,给出了气体火花开关绝缘恢复时间与气压及开关导通功率的关系式,初步分析了气体开关击穿通道在自然散热(不吹气)条件下,重复频率稳定工作的最高功率与重复频率指标、气压的关系。     

吹气对氮气火花隙开关重复频率运行性能的影响

 基于紧凑重复频率Tesla变压器实验平台,运用设计加工的紧凑型内部气流循环式火花隙开关,对吹气提高火花隙开关重复频率运行性能进行系统的实验研究,结果表明：重复频率小于50 Hz时,火花隙开关击穿电压不稳定度(RMS)小于5%,不需要吹气;在给定吹气速度下,火花隙开关重复频率运行存在临界重复频率;火花隙开关重复频率运行存在最佳吹气速度,且最佳吹气速度与重复频率之间满足线性关系,并可由实验确定;在优化吹气速度条件下,当气压处于0.7~1.5 MPa范围时,该火花隙开关击穿电压不稳定度(RMS)小于3%;在2.0 MPa的压强范围内,该吹气式火花隙开关可稳定工作至重复频率300 Hz,击穿电压400 kV。     

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     

亚纳秒气体开关工作特性

 设计了3种结构的同轴Peaking-Chopping组合型亚纳秒气体开关,以半导体开路开关脉冲源为实验平台,分别对它们的击穿特性进行了实验研究。结果表明：亚纳秒气体开关采用环形组合电极Ⅱ时,可以在1～500 Hz稳定工作,输出前沿400 ps、后沿320 ps、脉冲宽度460 ps和电压129.2 kV的脉冲。开关输出脉冲的前后沿、脉冲宽度和电压幅度与开关间隙、气压和重复频率等因素有关,亚纳秒气体开关在小间隙（1～2 mm）、高气压（约10 MPa）时具有良好的重频特性。在开关气压和输入脉冲幅度不变时,输入脉冲上升沿越快,开关的击穿时延越小,击穿电压越高。     

可调间隙亚纳秒气体开关的研制

 设计了一种高工作电压、高重复频率的亚纳秒气体开关,该开关由Peaking间隙和Chopping间隙组成,可以将纳秒信号转化为亚纳秒脉冲。开关腔外有两组调节旋钮,分别用来调节Peaking间隙和Chopping间隙,使输入脉冲的前后沿能同时得到锐化。对设计的开关进行的实验研究结果表明：在系统重频5 Hz运行时,开关能稳定输出电压278 kV、脉宽620 ps的脉冲;在系统重频100 Hz运行时,开关能稳定输出电压270 kV、脉宽700 ps的脉冲。     

高功率大电荷量重复频率气体开关实验研究

 在自行研制的纵吹式场畸变型气体开关上，开展了大电流、大电荷传递量重复频率实验研究，已实现的开关最大工作参数为：电压100kV，电流85kA，电荷传递量0.6C/脉冲,重复频率10Hz，单脉冲能量5kJ。结果表明，气流流速、电极烧蚀及绝缘筒污染对开关重复频率和工作寿命有较大影响。     

Power scaling of printed-circuit-board preionized TEA CO2 laser up to 3.6 kW

A high average power, high-repetition-rate TEA CO₂ laser employing printed-circuit-board preionizer is described. The power scaling of this preionization configuration has been demonstrated. The average output power reaches 3.6 kW at pulse repetition rate of 180 pps. Furthermore, the scaling-up ability of the rotating spark gap as high-voltage switch for high-average-power gas laser has been validated.     

喷射等离子体触发气体开关导通特性

利用内嵌微孔火花放电产生喷射等离子体、作用于两电极开关,研究了间隙距离、气压、气体种类、开关工作系数和电压极性配合等因素对等离子体喷射控制开关导通特性的影响。实验结果表明,等离子体喷射触发开关可在工作系数为10%的条件下可靠快速导通,当开关采用0.5 MPa_N₂作为绝缘介质、间隙距离5 mm时,触发导通时延为11.7 μs,抖动为1.42 μs;当间隙距离增大到18 mm时,触发导通时延增大至19.7 μs,触发可靠性降低;当工作系数由10%增大到60%时,触发导通时延由11.7 μs降低至1.1 μs。在确保开关自击穿电压一致的前提下,短间隙、高气压、负触发脉冲电压、正工作电压更有利于减小开关触发导通时延。     

Switching characteristics of the triggered vacuum gap for ahigh-repetition-rate pulse-power source

Switching characteristics of sealed-off triggered vacuum gaps (TVGs) were examined from the viewpoint of a high repetition rate at high current discharge. With a triggered energy of 11 J, it was possible to fire the main gap in both the anode and cathode modes of operation. In repetitive discharge experiments, the TVG-tube was put in the center of the cylindrical conductor and the switching tube inductance was about 27 nH. The TVG-tube was confirmed to have capabilities for 1000 shots with a pulse current of 120 kA and a high repetition rate of 5 Hz. These experimental results indicated that the TVG-tube is a potential repetitive closing switch for the plasma X-ray source     

High-voltage microwave-triggered switches

The operation of three types of experimental high voltage microwave-triggered switches developed by the University of St. Andrews and DRA Malvern is described. The POLOTRON and the microwave-triggered three-gap-switch (MTGS) are closing switches and the microwave-triggered tacitron, (MITTON) is a closing and opening switch. The switches are robust hydrogen thyratron type devices, with an annular geometry, where closure is achieved using microwave fields to ionize the low pressure gas within the switch. In the case of the MITTON, the switch is opened by the application of a negative bias voltage to a large area metal grid. The POLOTRON is designed for fast switching applications. Its annular geometry results in an inherent low inductance and anode fall-times of less than 3 ns have been measured for a cold-cathode POLOTRON at charging voltages up to 30 kV. The MTGS, also a cold cathode switch, was developed for use in the DUOTRON transient generator (a voltage doubler). Output voltages of up to 16 kV have been measured at repetition rates of 90 Hz. The MITTON has closing characteristics similar to the POLOTRON and anode voltage fall-times of less than 10 ns have been measured at voltages up to 16 kV. Opening times of 0.5 μs have been achieved with an anode voltage of 10 kV and current of 10 A     

Compact 1000 pps high-voltage nanosecond pulse generator

A compact high-voltage nanosecond generator is described with pulse repetition rate of up to 1000 pps. The generator includes a 30-Ω coaxial forming line charged by a built-in Tesla transformer with high coupling coefficient, and a high voltage (N₂) gas gap switch with gas circulating between the electrodes. The maximum forming line charge voltage is 450 kV, the pulse duration is ~4 ns, and its amplitude for a matched load is up to 200 kV. The generator has been applied to create powerful sources of ultrawide-band electromagnetic radiation and nanosecond microwave pulses     

基于磁开关的全固态、高功率、长脉冲发生器的初步研究

大多数电容储能型脉冲功率装置的共同特点是采用了气体开关, 由于气体绝缘恢复的限制和电极的烧蚀, 使得其重复频率都不太高, 而且寿命有限. 而目前广泛研究的磁脉冲压缩技术在脉冲功率系统的长寿命、高平均功率和高重复频率运行方面具有很大的应用前景. 通过对BOOST电路、LC谐振电路和磁脉冲压缩电路详细的理论分析和计算机模拟, 设计了由BOOST电路, LC谐振电路, 脉冲变压器, 磁压缩系统, 负载五大部分组成的脉冲发生器电路系统模型, 使用Pspice软件对系统进行了模拟、优化, 得到了电压幅值为55kV, 脉宽为500ns的输出脉冲.     

Fluid dynamics analysis of a gas attenuator for X‐ray FELs under high‐repetition‐rate operation

Newtonian fluid dynamics simulations were performed using the Navier–Stokes–Fourier formulations to elucidate the short time‐scale (µs and longer) evolution of the density and temperature distributions in an argon‐gas‐filled attenuator for an X‐ray free‐electron laser under high‐repetition‐rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite‐volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X‐ray laser–gas interaction region, where large pressure and temperature gradients have been built upon the initial energy deposition via X‐ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter‐pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. By performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time‐averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous‐wave input of the equivalent power.     

Influence of gas flow rate and reactor length on NO removal usingpulsed power

A short duration of 100-ns pulsed power has been used to remove nitric oxide (NO) in a mixture of nitrogen, oxygen, water vapor, and NO, simulating flue gases from a power station. The effects of the gas flow rate, the reactor length, and the pulse repetition rate on the percentage of NO removal and its energy efficiency are reported. The percentage of NO removal at a fixed gas flow rate increased with increasing pulse repetition rate due to the increased energy into the discharge. At a fixed pulse rate, the removal of NO increased with decreasing gas flow rate due to the increased residence time of the gas in the discharge reactor, thus facilitating the creation of increased radicals of O and N which then decreased NO. The energy removal efficiency of NO (in mol/kWh) decreased with increasing gas flow rate and increasing removal ratio of NO. The removal of NO increased with increasing energy density (J/I), input into the discharge at different reactor length     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

紧凑Tesla变压器型纳秒脉冲源

采用带有开路磁芯的Tesla变压器与单筒脉冲形成线一体化结构,研制了一台基于Tesla变压器的紧凑GW级纳秒脉冲源,该源包括一个40Ω脉冲形成线、内置Tesla变压器、初级电路及高压吹气主开关等,具有变比高、结构紧凑、能量传输效率高、便于重复频率运行等特点。给出了脉冲形成线、Tesla变压器和主开关等的工作原理、设计方法和模拟计算。实验结果表明,该脉冲源输出电压大于200kV,脉冲宽度约8ns,可以在重复频率100Hz、平均输出功率1GW情况下稳定运行,实验结果与理论设计相符。     

新型等离子体喷射触发气体开关

为了提高气体开关的同步性,降低自放电概率和抖动,提出并设计了一种新型等离子体喷射触发气体开关。开关整体结构与传统的三电极场畸变开关相似,最大特点是在触发电极内部嵌入一个微型激励腔,用于产生等离子体喷射。研究了触发电压和气压对等离子体的喷射特性的影响,对比场畸变和等离子体喷射两种触发方式下开关触发特性的差异,并研究触发电压对开关性能的影响。实验结果表明:等离子体喷射高度随触发电压的升高而增加,随气压的升高而降低;采用等离子体喷射触发能够显著降低过压间隙的延时和抖动;提高触发电压不但降低开关的抖动,而且还扩展开关稳定触发的工作范围;当触发电压80kV,工作系数27.4%,开关仍然能够可靠触发,延时和抖动分别为97.1ns和3.5ns。     

用于LTD模块的多间隙气体开关

针对800 kA,0.1 Hz重复频率LTD模块设计新的多间隙气体开关,要求在较高工作电压(高于80 kV)和较低工作系数(低于70%)下抖动优于2 ns。通过电场仿真分析,优化了气体开关电极形状。串联间隙从4个增加为6个,总间隙长度为36 mm,充电100 kV、触发100 kV时触发间隙场畸变系数为3.98。实验表明,新气体开关静态自击穿性能稳定,动态触发性能可靠。充电90 kV、工作系数60%时击穿延时40.1 ns,抖动1.3 ns。     

Continuous-flow optical pumping NMR in a closed circuit system

In a typical continuous-flow optical pumping setup, the chemical shift of xenon in the adsorbed phase depends on the gas flow rate due to warming of the sample surface by the gas stream. Calibration of the system using the (207)Pb resonance of solid lead nitrate is necessary to determine the actual sample temperature. Optimum pulse repetition rates are strongly affected by gas flow and spin-lattice relaxation rates. The interplay of flow and pulse repetition rate alters signal intensity ratios and may lead to the complete suppression of signals.