真空中铝单丝电爆炸的实验研究

开展了铝单丝在负极性电流脉冲作用下电爆炸特性的研究.利用皮秒激光探针,搭建了阴影、纹影和干涉的光学诊断平台,得到了不镀膜铝丝典型的能量沉积过程,在电压崩溃时刻其沉积能量为2.4 eV/atom.为了增加金属丝内的沉积能量,开展了相同电参数及金属丝尺寸下的镀膜铝丝电爆炸实验,其沉积能量可达到5 eV/atom,实现了在电压崩溃之前铝丝完全气化(完全气化所需能量为4 eV/atom).阴影图像展示了高密度丝核区域的膨胀过程,不镀膜铝丝平均膨胀速度为2.2 km/s,而镀膜铝丝因为沉积能量大,其膨胀速度约为不镀膜铝丝的2.3倍,高密度区域膨胀速度为5 km/s.由于阴影不能反映低密度等离子体的膨胀,开展了平行双丝实验,通过测量自发光辐射,估算了低密度等离子体的膨胀速度.利用条纹相机拍摄了不镀膜铝丝电爆炸过程中自发光区域的图像.纹影图像清晰地展示了不镀膜铝丝在电爆炸过程中形成的核冕结构,而镀膜铝丝电爆炸过程中核冕结构得到了一定程度的抑制.从干涉图像计算了相移,在轴对称假设下对相移进行阿贝尔逆变换,重构了三维的铝原子数密度分布.     

并联金属丝提高电爆炸丝沉积能量的数值模拟

对于低气压或真空环境中的电爆炸丝,因丝沿面击穿会过早终止能量沉积过程,使丝中沉积能量(Ed)大大低于金属丝完全汽化时所需能量(Es).本文提出并联金属丝法延缓沿面击穿时刻以提高电爆炸丝沉积能量.对电流上升时间为几十纳秒、幅值约为1 kA级作用下的金属丝电爆炸过程进行了数值模拟.结果表明,在电爆炸丝两端并联一定尺寸的金属丝可降低爆炸丝端电压上升率,从而推迟电压上升过程中沿面击穿时刻,显著提高丝中沉积能量和过热系数.     

真空中金属丝电爆炸数值模拟研究

利用金属丝电爆炸物理数学模型对电爆炸物理过程开展了数值模拟,研究了不同直径铝丝电爆炸特性,进一步分析了金属丝内沉积能量、电压击穿时刻、电压峰值随金属丝直径的变化规律,并与相关实验数据作了对比。     

铝单丝Z箍缩形成无核冕结构的物理分析

针对铝单丝Z箍缩负载,计算其可形成金属蒸气而不形成核冕等离子体的电路和负载参数范围。提出了铝丝电爆炸形成金属蒸气的能量沉积判据和击穿电压判据;建立了热动力学模型,选取电路参数使得金属丝气化时放电回路电流恰好迅速下降,从而避免发生电压击穿。计算了典型电路下的负载电流、电压、电阻及沉积能量的变化曲线,并分析了回路总电感、充电电压以及负载丝长度、直径对其的影响规律。计算结果表明:当储能电容为150pF、充电电压为65kV、回路电感为300nH时,可驱动直径20μm、长2cm的铝丝电爆炸形成铝丝蒸气。快电流前沿、小丝直径和较短的丝长度有助于提高负载中的单位质量沉积能量,容易电爆炸形成金属蒸气负载。     

快前沿电流产生气化铝单丝Z箍缩负载的研究

为了抑制丝阵Z箍缩单丝电爆炸过程产生的核冕结构,分析了激光探针诊断的物理内涵,并基于约30 ps激光探针研究了负极性快前沿脉冲(90—170 A/ns)下铝丝的电爆炸特性.直径15μm,长2 cm的铝丝,阻性电压峰值为35—50 kV,电压击穿前金属丝电阻率增加至30—40μΩ·cm.电压峰值时刻沉积能量为1.5—2.5 eV/atom,欧姆加热功率下降至峰值一半时的沉积能量为2.5—4.0 eV/atom,接近铝丝从室温加热至完全气化所需的能量约4.0 eV/atom.快前沿脉冲可增加金属中的欧姆能量沉积速度,提高负载击穿电压.激光纹影图像可以观察到气体通道和等离子体通道,得到冕等离子体的平均电离度约为0.3.由于极性效应,电极附近区域的能量沉积超过负载中部区域,电极附近负载基本完全气化,而负载中部区域仍存在液态或团簇状颗粒.一些发次中,实现了轴向均匀且完全气化的铝蒸气,在电压击穿后的约127 ns,70%的初始质量分布在直径1 mm的区域内,100%的初始质量分布在直径2 mm的区域内.     

铜丝水中电爆炸能量沉积特性

对s脉冲电压作用下铜丝水中电爆炸的能量沉积过程进行了实验研究,利用自积分Rogowski线圈和电阻分压器分别测量铜丝电爆炸时的电流和电压。利用测量电压波形确定了熔融起始、熔融结束、汽化起始和击穿时刻点,将铜丝电爆炸划分成熔融、液态和汽化3个阶段。通过数学方法计算了3个阶段和击穿前的沉积总能量。通过实验和计算,分析了电路参数,包括放电电压和回路电感,以及铜丝特性,包括铜丝长度和直径,对铜丝电爆炸过程中3个阶段和击穿前沉积总能量的影响。结果表明:在s脉冲电压作用下,放电电压、回路电感、铜丝长度和直径对熔融阶段能量沉积影响较小,但对液态和汽化阶段能量沉积影响较大,通过调节电路参数提高电流上升速率,可以显著提高汽化和击穿前的沉积能量。     

介质环境对铜丝电爆炸制备纳米粉体的影响

为了探究介质环境对电爆炸制备纳米粉体的影响, 搭建了相应的电爆炸实验平台, 以铜丝为例分别在水和不同空气压力下开展了电爆炸制备纳米粉体实验.通过Rogoswki线圈和高压探头分别测试了电爆炸过程中的电流和电压波形图.通过电压、电流及能量沉积特征分析了电爆炸的基本过程以及介质环境在电爆炸过程中的作用.运用透射电子显微镜对爆炸产物进行了粒度分析.研究发现, 介质环境对于电爆炸过程的影响主要表现在汽化化阶段以后, 包括介质对蒸汽膨胀的抑制作用, 介质的电离对于铜丝表面击穿的影响以及其对高温金属蒸汽及等离子体的冷却作用.水中铜丝电爆炸能够制备局部均匀的小尺寸纳米粉体, 粒度多数集中在10–20 nm之间, 但粉体易积聚, 且整体粒度跨越较大.空气中制备的粉体分散良好, 符合对数正态分布, 基本上分布于20–100 nm之间, 平均粒度约为40 nm.     

氩气中铝丝电爆炸沉积能量对制备铝纳米粉体特性的影响

研制了基于脉冲电容器放电回路的亚微秒金属丝电爆炸纳米粉体制备实验平台,包括电爆炸过程电流和电压测量系统。利用透射电子显微镜(TEM)观察纳米粉体的形态与结构,并通过电镜统计观察法分析TEM图像得到纳米粉体的粒度大小及其分布。在氩气中电爆炸铝丝制备铝纳米粉体,通过改变电容器充电电压,即初始储能,实验研究沉积能量对铝纳米粉体特性的影响规律。结果表明:铝纳米粉体颗粒形态与结构主要由氩气气压的高低决定,与沉积能量基本无关。增大丝爆过程的沉积能量可显著缩小铝纳米粉体粒度分布范围,减小颗粒平均粒径,并有效地抑制纳米粉体中亚微米颗粒的形成。随着沉积能量E与氩气气压p比值(Ep-1)增大,铝纳米粉体颗粒平均粒径、最大粒径和粒径大于100 nm颗粒所占比例均呈指数函数单调减小。     

铝丝电爆炸过程的光学诊断

采用100 m和40 m两种直径的铝丝,在不同放电电压下,通过分幅成像技术和光谱诊断方法,对铝丝电爆炸过程放电特性及放电等离子参数进行了诊断。实验研究表明:铝丝电爆炸过程中金属蒸气的二次击穿分为内部击穿和沿面击穿两种类型,较细的铝丝更容易发生内部击穿,发生内部击穿时产生的等离子体具有更好的空间均匀性和对称性,其放电过程具有更高的稳定性和可重复性。通过光谱诊断可知铝丝电爆炸等离子体电子温度在104 K量级,电子密度在1018 cm-3量级。     

带载丝电爆炸提高沉积能量的研究

在丝电爆过程中,金属丝的沉积能量是决定爆炸效果的关键参数。在研发连续送丝电爆装置的基础上,提出带载丝电爆炸提高金属丝沉积能量的方法。根据金属丝在电爆过程中的相变理论及旁路并联电阻的非线性时变性,建立了金属丝负载的电阻-能量分段模型。使用带载丝和裸丝分别开展电爆炸实验,同步采集丝电爆过程中的放电波形并分析计算,探究带载丝电爆炸相关机理以及沉积能量的变化规律。结果表明,电爆炸前期,由于载丝带具有绝缘性,其旁路并联电阻大于裸丝,从而使得带载丝电阻大于裸丝;随着欧姆加热的进行,带载丝中液态金属沿轴向由两端向中间聚集,加快了电爆炸相变过程,等效电阻减小,延缓了沿面击穿过程,从而获得更多的能量。     

Formation of extended directional breakdown channels produced by a copper wire exploding in the atmosphere

Experimental data for switching initiated by the electrical breakdown of air gaps up to 1.9 m long with an arbitrary geometry that are produced by an exploding copper wire 90 μm in diameter are presented. At an initial voltage of 11 kV, the stored energy equals 100–2100 J. Two channel formation conditions are possible: explosion of a wire without electrical breakdown and electrical breakdown in a channel produced by an exploding wire with a delay (current pause) no longer than 250 μs. Current and voltage waveforms across the discharge gap, as well as the resistivity values, under the electrical breakdown conditions are shown. Mechanisms and conditions for streamer initiation at a mean electric field strength in the discharge gap of 5.3–17.0 kV/m are discussed. The geometrical dimensions of plasma objects in the forming channel, the run of the electrical current under breakdown, and the formation mechanism of wire explosion products are found from color microphotographs. The formation mechanism of large aerosols in the form of tiny spherical copper and copper oxide (CuO, Cu₂O) particles under wire explosion conditions is discussed.     

Homogeneous electrical explosion of tungsten wire in vacuum

Experimental results on Joule energy deposition upon initiation of a fast electrical explosion of 16-μm tungsten wire in vacuum at current densities of more than 10⁸ A/cm² are reported. We have found that explosion with a fast current rise time (～170 A/ns into a short) results in homogeneous and enhanced deposition of electrical energy into the tungsten before surface flashover. The maximum tungsten wire resistivity reaches a value of up to ～185 μΩ cm before surface flashover that significantly exceeds the melting boundary and corresponds to a temperature of ～1 eV. The highest values for light radiation and expansion velocity of wire ～1 km/s were observed for the fast explosion. For the explosion mode with a slower current rise time (～22 A/ns into a short), we observed the existence of an “energy deposition barrier” for tungsten wire. In the slow explosion mode, the current is reconnected to the surface shunting discharge before melting. The maximum tungsten wire resistivity in this case reaches the value of ～120 μΩ cm, which is less than indicative of melting. Also, the energy deposition along the wire is strongly inhomogeneous, and wire is disintegrated into parts. We attribute the early reconnection of the current to the surface discharge for the slow explosion to high electron emission from the wire surface, which starts before melting.     

Numerical calculation of the current specific action integral at the electrical explosion of wires

The electrical explosion of aluminum wires is numerically simulated in the magnetohydrodynamic approximation for the current density ranging from 10⁷ to 10¹⁰ A/cm² and times to explosion varying from 10^?10 to 10^?6 s. It is shown that, at current densities of 10⁸?10⁹ A/cm², low-temperature explosion conditions change to high-temperature ones, when inertial forces preventing the wire dispersion play a decisive role. This transition is accompanied by a sharp change in the thermodynamic parameters (the temperature and the energy deposited into the wire by the instant of explosion increase by several times), and the action integral for this transition increases smoothly approximately threefold as the explosion characteristics (current density and time to explosion) change by two orders of magnitude. The instant of transition from the low-temperature explosion to the high-temperature one depends on the radial dimensions of an exploding wire and does not depend on the properties of the environment.     

Development of exploding wire ion source for intense pulsed heavy ion beam accelerator

A Novel exploding wire type ion source device is proposed as a metallic ion source of intense pulsed heavy ion beam (PHIB) accelerator. In the device, multiple shot operations are realized without breaking the vacuum. The basic characteristics of the device are evaluated experimentally with an aluminum wire of diameter 0.2 mm and length 25 mm. A capacitor bank of capacitance 3 μF and a charging voltage of 30 kV was used, and the wire was successfully exploded by a discharge current of 15 kA with a rise time of 5.3 μs. Plasma flux of ion current density around 70 A/cm² was obtained at 150 mm downstream from the device. The drift velocity of ions evaluated by a time-of-flight method was 2.7×10⁴ m/ s, which corresponds to the kinetic energy of 100 eV for aluminum ions. From the measurement of the ion current density distribution, the ion flow is found to be concentrated toward the direction where the ion acceleration gap is placed. From the experiment, the device is found to be acceptable for applying the PHIB accelerator.     

Effects of circuit parameters on the electrical explosion of wires

The numerous papers [1–4] on exploding wires give mainly qualitative explanations of some of the processes and inadequate quantitive evidence, in spite of the practical importance of the technique. The present paper reports the effects of source parameters (voltage, capacity, inductance, stored energy) and of load parameters (specific resistance, diameter, resistance) on the energy deposited in the wire before rupture, and on the time and rate of deposition.We are indebted to Professor A. A. Vorob'ev for his interest in this work.