高空核爆炸瞬发辐射电离效应的数值模拟

采用蒙特卡罗方法模拟了高空核爆炸瞬发辐射中子、γ射线、X射线电离大气的过程,给出了几种爆炸场景下瞬发辐射产生的附加电离电子密度空间分布.针对大气密度随高度非均匀连续变化的特性,采用质量距离抽样方法取代常用的步长抽样方法,无需根据大气密度随高度的变化进行分层处理,提高了计算效率.结果表明:对于不同的爆高,瞬发辐射电离分布存在显著的差异;随着爆高的增加,瞬发辐射附加电离区范围增大,但电子密度的峰值减小. 关键词： 高空核爆炸 瞬发辐射 大气电离 蒙特卡罗方法     

瞬态电子碰撞激发类氖锗19.6nm X射线激光的理论研究

近来，许多实验室仅采用一个脉宽为几个ps的短脉冲就得到了强放大的软X射线激光.这表明瞬态是来得及电离的.为此，在类氖锗的瞬态电子碰撞激发19.6nm 波长X射线激光研究中，提出了采用低预脉冲产生低电离度的预等离子体，后续一个ps级短脉冲既电离又加热等离子体的驱动方式.并用系列程序进行了模拟.模拟结果表明，这种驱动方式也可以得到高增益. 关键词： X射线激光 等离子体 增益系数 瞬态电子碰撞激发     

自触发紫外预电离开关脉冲击穿时延抖动影响因素及改进方法

研究了一种自触发紫外预电离开关击穿时延抖动特性的影响因素,结果表明:触发间隙电容放电阶段起预电离作用时,预电离注入时刻开关电场是开关时延抖动的决定性因素,提高工作系数和采用逸出功更低的电极材料对降低开关在脉冲峰值附近击穿时的时延抖动效果有限。提出的改进方法为:减小开关均压电阻阻值,显著延长触发间隙的有效燃弧时间,消除预电离注入时间及抖动的影响。采用改进方法时可以使开关在工作电压300～800 kV、前沿100 ns、180 ns的脉冲峰值附近击穿时的时延抖动分别小于1.3 ns、2.8 ns。     

横向电容自耦式紫外预电离开关

对紫外预电离技术在气体开关方面的影响进行原理研究,分析并实验验证电容并联的横向辅助电极对主开关进行紫外预电离的可行性。辅助电极击穿释放紫外光,通过光电效应在主开关表面产生初始电子,以减小主开关的击穿电压离散度。预电离效果与辅助电极击穿导通电流的峰值强度、辅助电极两端的电压以及预电离储能电容有关,优化这些参数可以提高光照强度来增强预电离效果。实验结果表明:当主开关内部充满氮气、分压电容为pF量级时,距离主电极15 cm远的横向辅助电极可以产生预电离效应。在此基础上,提出一种新型的横向型电容自耦式紫外预电离开关的设计。     

TEA CO2激光器预电离结构电极系统的电场研究

 高功率气体激光器的非对称复杂电极系统中,电极参数的变化以及预电离结构的引入将会使所设计的均匀电场发生具体的改变。采用高精度有限元方法,对这类复杂电极系统进行建模并且计算其电场分布,根据分析结果更合理地指导激光器设计。对一种典型的非对称复杂电极系统TEA CO₂激光器,应用该方法并结合了预电离过程,研究了引入预电离器导体和变化电极形状对电极表面的电场强度起伏和放电空间的均匀电场面积产生的影响。结果表明:通过这种预先模拟,合理选择电极构型参数和预电离结构安装位置,可以产生性质较好的均匀电场以及效率较高的激光输出。     

高功率横流CO2激光器脉冲预电离过程研究

本文给出了对高功率横流CO_2激光器脉冲预电离过程的理论和实验研究.表明了预电离过程中光电离的重要作用.实验结果表明,脉冲预电离可增大高功率横流CO_2.激光器的pd值(p为放电气压、d为放电间隔),增大放电区注入功率密度.对于提高此类型激光器的放电稳定性和输出激光功率是一种技术简单而有效的手段.     

管状预电离可调谐TEA CO2激光器.

研制了一种新型的预电离结构--管状预电离器,并将其应用于可调谐TEA CO2激光器.利用光栅选线方案,采用光栅谐振腔,实现了激光的调谐输出.在输出耦合率为50%、气压为40 kPa的条件下,10P(20),10R(20),9P(20),9R(20)四条谱线获得的最高电光转换效率,分别为9.5%,9.7%,9.8%,9....     

XeCl激光器的增益特性

本文根据稳腔均匀加宽激光器的模型,按照激光器的输出功率与小信号增益系数g,恒定的吸收损耗系数a_o,饱和强度I_s,腔长L之间的近似关系式,利用变换输出耦合的方法,对采用紫外预电离的Blumlein快放电激励XeCl激光器的增益进行测量.并讨论了器件的最佳透过率.发现最佳透过率是泵浦功率的函数.     

强激光能源系统预电离实验研究

 新型能源系统预电离实验研究证实：采用预电离技术可以减少闪光灯的导通延时,使电流脉冲前沿变陡,这对提高泵浦效率和闪光寿命无疑是十分有益的。     

高功率横流CO2激光器大体积放电研究

详细讨论了脉冲预电离高功横流ＣＯ２激光器的工作特性，研究了一套新型的电极结构，对其放电，注入功率，输出功率等待进行了讨论。实验结果表明，脉冲预电离可以提高高功率横率流ＣＯ２激光器的注入和输出功率，并有效地改善了放电的稳定性和均匀性。     

High-pressure pulsed avalanche discharges: formulas for requiredpreionization density and rate for homogeneity

The requirements on preionization for the formation of spatially homogeneous pulsed avalanche discharges are examined. We derive two formulas which apply in the case of a slowly rising electric field, one which gives the required preionization density at breakdown, and one which gives the required preionization rate. These quantities are expressed as functions of the electrochemical properties of the gas, the neutral density, and the electric field rise time. We also treat the statistical effect that the electrons tend to form groups, in contrast to being randomly distributed in space, during the prebreakdown phase. This process is found to increase the required preionization rate significantly, typically by a factor of five for a discharge at atmospheric pressure     

Characteristics of a wire preionized nitrogen laser with helium as buffer gas

The operation of a small N₂ laser using wire preionization and He as a buffer gas is discussed. The preionization is found to increase the output power and energy by 50% while the addition of helium can increase fourfold the output power.     

Densities and Life Time of Electrons in KrF* Laser Gas Mixtures During the X-Ray Preionization

The homogeneity and stability of TEA-Excimer laser discharges strongly depend on the preionization. A homogeneous discharge can only develope if it is ensured that the electron density exceeds a certain level at the moment of the ignition. The kinetic of these electrons during the preionization is analyzed theoretically by the use of the Boltzmann equation for x-ray preionized KrF* laser systems. The time dependent velocity distribution of the electrons is calculated for different buffer gases and gas mixtures. The resulting electron densities and life times are discussed and compared with experimental investigations. It is quantitatively shown at the first time that the kind of the buffer gas has a decisive influence on the velocity distribution, density and life time of the preionization electrons.     

Preionization and discharge stability study of long optical pulse duration UV-preionized XeCl lasers

This paper presents a complete study of the temporal and spatial characteristics of the preionization of a XeCl rare-gas halide laser. The detailed study was made possible using the technique of laser-induced preionization which utilizes the uv radiation from a KrF laser to preionize a second rare-gas halide laser. In addition to the preionization study, high spatial and temporal resolution framing camera photographs have been used to investigate the growth of discharge instabilities which can lead to the premature termination of the XeCl optical pulse. The roles played by HCl, Xe, the buffer gas as well as the discharge energy loading in the development of discharge instabilities have been determined experimentally.     

Evaluation of diffraction errors in precise pulse-echo measurements of ultrasound velocity in chambers with waveguide

Ultrasound velocity measurements in medicine and biology usually are performed using relatively small measurement chambers. When the pulse-echo method is used, the presence of the reflector close to the transducer can cause essential diffraction errors. These errors may be reduced using an additional buffer rod as a waveguide between the transducer and the measurement chamber. The objective of the presented work was analysis of diffraction errors in measurement chambers with a buffer rod. The work was performed in two steps. In the first stage propagation of transient ultrasonic waves in a buffer rod was analysed using an axisymmetric finite element model. This approach enables all dimensions of the measurement chamber and the waveguide to be taken into account, but is less accurate in the time domain. In the second step the absolute values of diffraction errors were evaluated using a mixed analytic-numeric disk shaped transducer diffraction model. In this case only the dimensions of the waveguide and measurement chamber along the wave propagation direction were taken into account. Diffraction errors were calculated by simulating small changes of ultrasound velocity in the liquid under investigation. The simulation performed allowed optimisation of the dimensions of the measurement chamber and a buffer rod thus minimising measurement errors.     

Radiative characteristics of nitrogen upon excitation by volume discharge initiated by runaway electron beam

The amplitude, time, spectral, and energetic characteristics of a volume (diffuse) discharge at an elevated pressure (up to 5 atm) in a gap with an inhomogeneous electric field without an additional preionization source are studied. The concentration and temperature of electrons in the discharge plasma are mea-sured by spectral methods. The effective lifetime of the C ³Π _u state of the nitrogen molecule at the trailing edge of the radiation pulse is shown to be determined by radiative and collisional quenchings. For transitions of the second positive system of nitrogen, a plasma discharge radiation power into the complete solid angle is obtained to be ～120 kW, with a specific radiation power of up to ～50 kW/cm³.     

A laser ion source for on-line mass separation

A laser ion source based on resonance photo ionization in a gas cell is proposed. The gas cell, filled with helium, consists of a target chamber in which the recoil products are stopped and neutralized, and an ionization chamber where the atoms of interest are selectively ionized by the laser light. The extraction of the ions from the ionization chamber through the exit hole and skimmer is similar to the ion-guide system. The conditions to obtain an optimal system are given. The results of a two-step one-laser resonance photo ionization of nickel, and the first results of laser ionization in a helium buffer gas cell are presented.