人工引发闪电上行负先导的发展传输特征

在地面大气电场为正极性的条件下,成功实现12次人工引发闪电,对其放电特征、初始阶段上行负先导的传输特征与机理进行了研究.引发闪电时地面大气电场强度均值约5 kV/m,最高超过13 kV/m.除一次个例的放电发生了正、极性反转并产生多次负回击以外,其他11次引发闪电均未产生继后回击过程,闪电放电电流总体上在几百安培量级.引发闪电起始后,其向上传输的负梯级先导平均二维速度为1.85 × 10~5 m/s,获得132次梯级的长度范围为0.8—8.7 m,平均3.9 m.先导起始阶段的电流和电磁场呈现显著的脉冲特征,其脉冲间隔、电流峰值、转移电荷量、半峰值宽度、电流上升时间T~(10%—90%)平均值分别为17.9 μs,81 A,364 μC,3.1 μs和0.9 μs,单次梯级的等效线电荷密度为118.5 μC/m.先导通道的分叉一般伴随梯级过程发生,存在两种方式:1)先导头部前方成簇的空间茎/空间先导在同一梯级周期内先后与先导头部发生连接,对应的电流脉冲表现为多峰结构,峰值点时间间隔约2—3 μs,最长6—7 μs;2)曾熄灭的空间茎/空间先导重燃后侧向连接至先导通道.     

人工引发雷电先导-回击过程的电场变化特征

 2005年夏季在山东滨州进行了以人工引发雷电为主的综合观测实验，在负电场环境下，成功引发雷电5次，其中传统引发方式3次，空中引发方式2次。详细分析了双向先导-小回击过程和箭式先导-回击过程的垂直电场变化特征。结果表明：2次空中引发雷电的双向先导发展过程在地面60 m处产生的电场变化为4.70~14.60 kV/m，呈负向变化；在550 m处为0.11~0.16 kV/m，呈正向变化。利用60 m处的先导电场变化，估算双向先导电荷线密度分布的斜率为(1.2~3.8)×10^-7 C·m^-2。箭式先导-回击电场变化波形呈不对称V形，V形的底部对应先导的结束和回击的开始。60和550 m处的箭式先导电场几何平均值分别为17.80和1.20 kV·m^-1。随水平距离的增加，箭式先导电场以水平距离的-1.18次方衰减。     

基于VHF辐射源短基线定位系统对闪电放电过程的研究

利用闪电VHF辐射源短基线定位系统2009年东北大兴安岭地区的观测资料,对正、负地闪和云闪放电过程中的VHF辐射源进行了定位研究.根据闪电VHF辐射源的二维位置(方位角和仰角)随时间的演变特征,结合同步观测的快、慢电场变化资料分析发现,持续时间较长的地闪预击穿过程在云中的放电通道呈双层结构,预击穿过程结束阶段的云内放电表现为反冲流光,预击穿过程的平均速度在10⁴m/s量级.预击穿过程为梯级先导的发展提供了必要的条件,梯级先导从预击穿起始位置开始并向下发展,产生较强的辐射,平均速度在10⁵m/s量级.K过程主要是流光沿之前已电离通道的传输.正、负地闪回击阶段前后的放电特征有明显不同,正地闪回击之后,连续电流期间的较长时间的云内放电产生较强的VHF辐射.对闪电在125---200MHz频段范围的VHF辐射频谱特征的统计分析发现,辐射能量呈现出幅值随频率增加而减小的趋势,在通带范围上基本遵循f^-2.9的衰减率递减.     

形变对10.6MeV/u 84Kr+27Al反应裂变前粒子发射的影响

研究了形变效应对10.6MeV/u ⁸⁴Kr+²⁷Al反应中断前粒子发射的影响.发现通过测量的裂变前粒子多重性提取的裂变延迟时间将因此从20×10^－21s减少到5×10^－21s,表明增强的断前粒子发射有一部分来自形变效应的贡献.     

用激光雷达探测合肥高空钠层的变化

 介绍了自行设计的钠共振荧光激光雷达系统，并对钠层密度、钠层柱密度、中心高度及均方根宽度进行了计算和分析。计算结果表明:从2005年12月到2006年3月合肥上空钠层变化显著，钠层柱密度从12月平均值5.4×10⁹cm^-2变化到3月平均值2.4×10⁹cm^-2，下降了50%以上。均方根宽度也下降，中心高度略有波动。合肥钠层的变化趋势与中纬度其它地区具有可比性。     

深圳城市气溶胶物理光学特性的观测研究

气溶胶是影响气候变化和空气污染的重要因子.在深圳地区展开气溶胶观测实验,可以获得可靠的光学物理特征,进而有助于准确评估气溶胶在新型超极城市区域的气像和环境效应.本文利用2010年12月至2011年8月太阳光度计、黑碳和浊度计等气溶胶观测资料,分析了新型超级城市深圳地区的气溶胶物理光学特性.深圳地区气溶胶呈明显季节变化,冬、春季由于城市污染性气溶胶的影响,气溶胶光学厚度和Angstrom波长指数都较大,夏季受海盐气溶胶的影响,光学厚度较小,Angstrom波长指数也较小.光学厚度与Angstrom 波长指数对比表明城市综合性污染是引起深圳气溶胶高光学厚度的主要原因.深圳地区气溶胶的散射系数、吸收系数的平均值(标准偏差)分别为178.7×10^-6 m^-1(126.6×10^-6 m^-1)和32.5×10^-6 m^-1(18.1×10^-6 m^-1),均低于珠三角腹地多年观测平均值的二分之一和国内其他大型城市观测值.而单次散射反照率为0.81,与珠三角其他地区得到的结果接近.此外,气溶胶吸收、散射和单次散射反照率呈明显日变化,可能主要受大气边界层变化的影响.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

金刚石薄膜中二次电子输运的蒙特卡罗模拟

本文通过编写的二维MATLAB蒙特卡罗程序, 对倍增的二次电子在金刚石薄膜中的输运特性进行了初步模拟. 研究表明: 二次电子的迁移率对温度和外加电场的大小很敏感, 在杂质浓度比较低时(<10¹⁷/cm^-3)受杂质浓度的影响不大. 模拟得到的 二次电子的饱和速度为1.88×10⁷ cm/s, 无外加电场时的迁移率为3732 cm² /V.s. 同时, 通过对二次电子束团在金刚石薄膜中的整体输运特性的模拟, 证明了束团电荷密度在应用要求的范围内时, 空间电荷力的影响可以忽略不计.     

电子通量对ZnO/K2SiO3热控涂层光学性能的影响

 研究了电子通量对ZnO/K₂SiO₃热控涂层光学性能的影响。分别采用通量为5×10¹¹/cm²·s，8×10¹¹/cm²·s，1×10¹²/cm²·s 和5×10¹²/cm²·s的电子对试样进行辐照。电子辐照下涂层的光学性能发生了退化，并且发现了退化涂层在空气中的“漂白”现象。分析了ZnO/K₂SiO₃热控涂层光学性能的退化机制，同时讨论了电子通量对太阳光谱吸收系数的影响。实验结果发现，在5×10¹¹～1×10¹²/cm²·s的电子通量范围内，电子通量对ZnO/K₂SiO³热控涂层光学性能的影响相同。因此在这个电子通量范围内，采用加速地面试验来模拟空间的电子辐照效应是有效的。     

高Tc超导薄膜约瑟夫逊结光探测研究

我们研制了具有约瑟夫逊效应的高Tc GdBa₂Cu₃O₇-薄膜双晶晶界结,对其交直流约瑟夫逊效应进行了观测,并用其进行光探测,用波长为0.6328μm的He-Ne激光器辐照双晶结结区,系统观测了双晶晶界结的光响应特性,得到的最好结果如下噪音等效功率NEP=1.9×10^-13W,归一化探测率D=53×109cmHz^1/2W^-1,响应率Rv=4.2×10⁷V/W,响应时间τ=4.35×10^-7s.     

Modeling the development of the stepped leader of a lightning discharge

A stochastic-deterministic model is presented for the propagation of a downward-moving leader. Lightning formation is described by a stochastic growth of branching discharge channels which is determined by the electrostatic field. The dynamics of the electric field and of the charge distribution over the lightning structure are calculated deterministically. The model includes the initiation of lightning, a preliminary discharge in a cloud, the propagation of a downwardmoving stepped leader toward the earth, and the initiation and upward motion of a return stroke from the earth’s surface. Numerical execution of the model yields a dynamic picture of the development of the downward-moving leader and of the intracloud discharge structure. The effect of the charge density in the cloud and the parameters of the developing channels on the spatial-temporal, current, and charge characteristics of the stepped leader’s propagation are studied. The effect of free-standing structures on the distribution of points on the earth’s surface where lightning strikes is examined. Zh. Tekh. Fiz. 69, 48–53 (April 1999)     

Identification of lightning vulnerability points on complex grounded structures

The identification of the most vulnerable points on a given structure to be struck by lightning is an important issue on the design of a reliable lightning protection system. Traditionally, these lightning strike points are identified using the rolling sphere method, through an empirical correlation with the prospective peak return stroke current. However, field observations in Kuala Lumpur and Singapore have shown that the points where lightning flashes strike buildings also depend on the height and geometry of the structure. Since a lightning strike point is believed to be the place on a grounded structure where a propagating upward leader is first initiated, a physical leader inception model is used here to estimate the background electric field required to initiate a stable upward leader from the corners of some complex buildings. The computed location of the points from where leaders are incepted are compared with the damaged points on buildings struck by lightning. The observed lightning strike points coincide rather well with the corners of the buildings which are characterized by lower leader inception electric fields. Furthermore, it is found that the geometry of the buildings significantly influences the conditions necessary to initiate upward leaders and, therefore, the location of the most likely strike points.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.     

飞秒激光引导闪电的模拟实验研究

采用能量40 mJ，脉冲宽度50 fs的超短脉激光脉冲形成的等离子体通道诱发和引导了3—23 cm长间隙的静态高压放电.实验观测显示，等离子体通道整体上能使空气间隙的击穿阈值降低到自然击穿阈值的40％.实验中通过对放电电弧发出的白光信号与激光信号的时间延迟进行分析得到激光引导下梯级先导的发展速度约为10⁷ cm/s. 关键词： 等离子体通道 高压放电     

闪电双向先导理论及观测:极性不对称、不稳定及间歇性

闪电双向先导原理的提出及观测验证是闪电物理研究近几十年最重要的进展之一,而正、负先导及流光的极性不对称性及传输的持续、间歇性,是理解闪电各种过程物理机制的关键.本文对闪电双向先导的概念及进展进行了总结和讨论,重点强调了正、负先导流光传输机制特别是门限电场的不对称性,阐述了正先导传输的连续性及其在闪电始发、负先导的空间先导形成、不稳定先导通道中的反冲先导建立等过程中独特的启动作用.     

Recoil leader formation and development

The existing interpretation in the lightning literature, based on field measurements, defines recoil leaders as negative leaders. However recoil leaders are floating conductors, and, based on this physical assumption, they should be defined as bipolar and bidirectional leaders. This physics-based assumption has never previously been verified experimentally. Such verification, reported in this paper, has been obtained from observations of branched upward positive leaders from a tall tower using a high-speed video system synchronized with electric and magnetic field change and luminosity measurements on the ground. The analysis of these observations clearly reveals the nature of recoil and dart leaders as bidirectional and bipolar electrodeless discharges that develop from a small region along a path of the decayed channels of a previous positive leader, or a positively charged return stroke of negative CG flashes.     

Leader propagation speed and the final jump distance of 8 m long laboratory sparks

Vertical profile of the leader in long laboratory sparks was analyzed using high speed video photographs. Sparks were generated in an 8 m gap of sphere – plane electrodes with positive impulse voltage of 250/2500 μs wave form. The spatial resolution of the high speed frame camera was 19.6 mm per pixel with a time resolution of 50 μs. It was found that the average final jump distance of five sparks was about 4 m and the average electric field in the final jump region is about 5 × 10⁵ V/m the leader speed varies from 1.3 × 10⁴ m/s to 3.1 × 10⁴ m/s as it propagates toward the ground. The average leader speed of the leader was in the order of 1.8 × 10⁴ m/s. The observations show that there is a significant increase in the leader speed just before the final jump.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. II. Modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.     

Numerical simulations of thunderstorm-induced corona processes near lightning rods installed on grounded structures

For the purpose of estimating the efficacy of a lightning rod, the current of a corona was numerically calculated from a rod placed (i) centrally on the roof of a vertical grounded cylindrical structure, a model of a man-made object, and (ii) at the top of a grounded hemispherical structure of large radius, such as a hill or mountain. The calculations were carried out for a slowly varying thundercloud electric field and when this field was enhanced by the charge of an approaching downward leader. In case (i), variation of the ratio between the height and radius of the cylindrical structure leads to a variation in the (a) corona current from the tip of the rod over a wide range and (b) distance from which lightning is attached to the rod. In case (ii), it is shown that, contrary to the case of rods installed on the ground surface, a slowly rising thundercloud electric field can be sufficient to initiate streamers and upward leaders from rods tens of meters in height installed on the top of tall, grounded, hemispherical structures. When the thundercloud electric field is enhanced by the charge of an approaching downward leader, the discharge processes near the rod depend on its length and the height of the tip of the downward leader relative to the top of the hemispherical structure, but are almost independent of the hemisphere radius.