Research on early stages of wire array Z-pinch
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摘要: 利用X箍缩等离子体产生的μm级、亚ns脉冲X射线点源对双丝电爆炸过程进行了X射线背光照相，结果表明：真空环境下爆炸丝通常形成“核冕”结构，即高密度丝核表面围绕着低密度冕等离子体；随后在全局磁场驱动下，冕层将被连续剥离并向轴线汇聚，形成“先驱等离子体”，大大降低丝阵的内爆品质。针对上述问题，进一步对实现“无核丝爆”(提高丝核沉积能量实现金属丝的均匀汽化)的方法进行了研究。实验结果表明：提高驱动电流上升率以及在金属丝表面构造正向径向电场均有利于丝核沉积能量的提高。结合上述两种方法，提出了阴极串联闪络开关的电极构型，大幅度提高了丝核沉积能量：正负极性驱动电流下比能量分别提高到原来的2倍(从5.7 eV/atom到13 eV/atom)和3.5倍(3.4 eV/atom到12 eV/atom)，均超过了钨丝汽化能(8.8 eV/atom)，且激光干涉图像表明爆炸产物具有很高的汽化率，即实现了“无核丝爆”。Abstract: Electrical explosion of wire (EEW) is the first stage of wire array Z-pinch, which has great influence on the following dynamics and the final X-ray emission.In this paper, the physical processes of two-wire Z-pinch was observed by X-ray backlighting.The sub-nanosecond, micrometer-scale X-ray source was generated with X-pinches connected in series with the Zpinch.Experimental results show that electrically exploded wires in vacuum usually develop into "core-corona"structure (highdensity core surrounded by low-density corona plasma).The corona plasma is then driven to the axis by global magnetic field to form a precursor plasma column, adding to the instabilities of the implosion stage.Therefore single wire EEW experiments were carried out to increase the energy deposition of wire core so as to suppress the formation of"core-corona"structure.Experimental results show that the performance of EEW can be improved by higher rising-rate of driving current and positive radial electric field along wire surface.In order to combine the two favorable methods, a flashover switch was inserted between wire-end and cathode.As a result, the specific energy of EEW was increased by 2 times (5.7 eV/atomvs 13 eV/atom) and 3.5 times (3.4 eV/atomvs12 eV/atom) under positive and negative driving current, exceeding the atomization enthalpy of tungsten (8.8 eV/atom) by a factor of 1.4.The laser interferograms show no high-density wire core; in other words, fully vaporized (core-free) wire explosion of bare tungsten wire was achieved.
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图 2 PPG-1负载腔结构以及利用X-pinch对Z箍缩过程拍照的实验布置
Figure 2. Structure of load cavity of PPG-1 and the configuration of using X-pinch as backlighting source for wire array Z-pinch
图 3 输出电流波形和X-pinch1、X-pinch2辐射波形
Figure 3. Waveforms of output current and radiation of X-pinch1 and X-pinch2
图 5 储能电容充电电压60 kV时短路放电输出电流波形
Figure 5. Waveform of short-circuit current with 60 kV charging voltage on the energy storage capacitor
图 8 直径50 μm双钼丝Z箍缩背光图像
Figure 8. Backlighting image of two 50 μm-diameter molybdenum wires
图 9 钨丝电爆炸比能量随电容充电电压的变化
Figure 9. Statistical relationship between specific energy of tungsten wires and charging voltage on the capacitor
图 11 金属丝表面径向电场及比能量随直径屏蔽板的变化情况
Figure 11. Radial electric field along the wire surface, specific energy vs different diameters of shield plate
图 12 串联闪络开关实验中使用的三种电极构型
Figure 12. Three types of load configuration used in the "flashover switch related" experiments
图 13 负极性驱动电流下三种电极构型丝爆的阴影照片和干涉照片
Figure 13. Laser shadowgrams and interferograms of three load configurations, taken at ~320 ns after initiation of current
图 14 负极性驱动电流下三种电极构型丝爆的电信号波形
Figure 14. Electrical waveforms of the three load configurations driven by negative current pulse
图 15 普通构型和阴极绝缘子构型径向电场计算结果
Figure 15. Distribution of radial electric field along wire surface of normal load and cathode-insulated load
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