Triggering of high voltage discharge by femtosecond laser filaments on different wavelengths

The triggering and guiding of high voltage discharge by plasma filaments generated by 400 nm and 800 nm femtosecond laser were studied experimentally. The detailed diagnostics for characteristics of filaments, such as amount of free electrons, diameter and electron density, were performed using sonographic method, fluorescence imaging and resistivity measurement. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filaments was observed. It is shown that the 400 nm laser pulses demonstrated stronger triggering ability than 800 nm laser pulse under same pulse energy. This behavior of 400 nm femtosecond pulse is connected to the rich population of highly excited particles in filaments.     

Triggering and guiding high-voltage discharge in air by single and multiple femtosecond filaments

The abilities to trigger and guide high-voltage discharge by using single and multiple filaments (MFs) are experimentally studied. It is shown that the discharge voltage threshold can be reduced significantly in both regimes of single and MF; however, the MF does not gain a larger reduction than a single filament. This behavior of the MF is attributed to the single discharge path rather than simultaneous multiple ones as one might expect during the discharge process.     

飞秒激光等离子体通道传导能量特性的研究进展

基于强飞秒激光诱导等离子体通道的基本性质,分析了等离子体通道在传导电能和射频电磁能时的一些基本问题。综述了飞秒激光等离子体通道传输电能和电磁能方面的新概念和新技术,重点描述了该领域近几年所取得的一些理论及应用的最新成果。最后,展望了飞秒激光等离子体通道在传导能量技术领域中的发展趋势。     

强激光对典型激光制导光电探测器的干扰效应研究

为了达到对激光制导武器的有效干扰目的,需要分析强激光对典型激光制导光电探测器的干扰效应。理论分析和数值计算表明四象限光电二极管和成像型CCD的光学饱和阈值较低,很容易达到光学饱和。在功率密度10MW/cm2,脉冲宽度5ns的单脉冲激光辐照下即可产生300K的温升,在多脉冲温升的累积作用下,光电探测器的基底材料很容易达到熔点,从而造成不可逆的永久损伤。     

Influence of negative leader propagation on the triggering and guiding of high voltage discharges by laser filaments

The triggering and guiding of negative discharges using filaments induced by a femtosecond-terawatt laser pulse have been studied in sphere-plane gaps up to 4.5 meters. Fast-frame camera pictures allow the evaluation of the influence of the negative leader propagation on the triggering and guiding process. We show that the plasma channel can either trigger a space-leader discharge or act as a guiding path for the negative leader head. For the latter case the results suggest a linear dependence of the guided lengths up to 2.4 m, while the formation of a space-leader reduces this guided length by up to 50%. This effect is explained by the limited plasma lifetime of the filament that is measured to be about 1 μs.     

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     

Characterization of relativistic electrons generated by a cone guiding laser pulse

We demonstrated the interaction of a gold cone target with a femto second(fs) laser pulse above the relativistic intensity of 1.37×10 18 μm 2 W/cm 2.Relativistic electrons with energy above 2 MeV were observed.A 25%-40% increase of the electron temperature is achieved compared to the case when a plane gold target is used.The electron temperature increase results from the guiding of the laser beam at the tip and the intense quasistatic magnetic field in the cone geometry.The behavior of the relativistic electrons is verified in our 2D-PIC simulations.     

Triggering and guiding megavolt discharges by use of laser-induced ionized filaments

We have demonstrated the ability to trigger and guide high-voltage discharges with ionized filaments generated by femtosecond terawatt laser pulses. The plasma filaments extended over the whole gap, providing a direct ohmic connection between the electrodes. Laser-guided straight discharges have been observed for gaps of as much as 3.8 m at a high voltage reduced to 68% of the natural breakdown voltage. The triggering efficiency was found to depend critically on the spatial connection of the laser filaments to the electrode as well as on the temporal coincidence of the laser with the peak of the high voltage.     

空气中飞秒激光等离子体荧光辐射光谱研究

系统地研究了不同聚焦条件下飞秒激光空气等离子体的荧光辐射特性以及空间演化情况.在紧聚焦情况下,由于焦点附近比较高的激光强度以及比较高的电子密度,辐射光谱表现为连续谱和线状原子光谱的叠加.在弱聚焦情况下,辐射光谱主要由很多分子线谱组成,而没有观测到连续谱的产生.还研究了光谱谱线强度随激光传输距离的演化情况,结果显示,光谱谱线的强度变化在一定程度上间接反映了等离子体细丝的演化情况.     

介质阻挡放电中放电丝结构相变过程研究

在氩气和空气混合气体介质阻挡放电中,研究了放电丝结构随外加电压及气体压强的变化,并从二维体系相变的角度进行了分析.随着电压的增加,放电丝结构的演变过程为:稀疏的随机放电丝—稠密的随机放电丝—六边形结构—超六边形结构—混沌态,此过程相应于二维体系的气相—液相—简单晶体—超点阵晶体—液相的相变过程.实验还研究了相变过程中超六边形形成中晶格常数及相邻格点间距离的变化、超六边形结构中大点的形成过程以及超六边形结构的Penta-Hepta缺陷.     

纵向脉冲放电的Ne-CuBr紫外激光参量的数值研究

建立了一个描述纵向脉冲放电的Ne-CuBr紫外激光的动力学模型，数值计算了实验中主要参量对激光性能的影响.给出了激光输出特性随缓冲气压、工作温度和激光管口径之间的变化关系，定量解释了已有的实验结果，为进一步提高激光输出性能提供了依据. 关键词： Ne-CuBr紫外激光 脉冲放电 参量研究     

Comparative investigation of resistance and ability to trigger high voltage discharge for single and multiple femtosecond filaments in air

A comparative investigation of resistance and ability to trigger high voltage (HV) discharge for single filament (SF) and multiple filaments (MF) has been carried out. The experimental results show that the trend of the breakdown threshold of the SF exactly follows that of its resistance, but this is not the case for the MF. The MF's resistance is much smaller than SF's. However, the MF shows a bit higher HV breakdown threshold than the SF. The underlying physics is that the measured resistance of the MF is collectively contributed by every filament in the MF while the HV breakdown threshold is determined by only one single discharging path.     

飞秒光丝阵列对10 GHz电磁波的吸收特性

为了研究飞秒光丝阵列对10 GHz电磁波的吸收特性,建立了飞秒光丝阵列吸收电磁波的有限元模型,研究了光丝内电子温度、电子数密度、光丝直径和电磁波的极化等参数对吸收系数的影响。研究结果表明：当电磁波偏振方向与光丝轴向垂直时,阵列对电磁波是透明的;增加光丝内电子数密度或提高电子温度,吸收系数先增大后减小;当光丝直径与电磁波趋肤深度相等时,吸收系数达到最大值。对于S极化电磁波,当光丝直径为50 μm时,吸收系数随入射角的增大而变大;当光丝直径为100~200 μm时,在入射角较小时,吸收系数随入射角的增大而变大;在入射角较大时会出现吸收峰值,最高可达0.45,且光丝直径越大,吸收峰值对应的入射角就越小;对于P极化电磁波,吸收系数随入射角增大而降低。     

高能激光对四象限制导器件的影响分析

为了有效地实施激光干扰,需要确定高能激光对四象限制导器件的影响。分析了由高能激光诱导的四象限制导器件的温升分布。理论分析和数值计算表明,在持续的强激光作用下,会造成光电材料的硬损伤。利用典型的比例导引跟踪系统的系统函数,验证了高能激光对四象限制导器件实施激光干扰的有效性。     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

放电参数对毛细管放电泵浦的软X射线激光的影响

 以哈尔滨工业大学可调谐激光技术国家重点实验室中的Marx发生器的放电脉冲波形为基础,理论上模拟计算了在不同放电参数下充氩气的毛细管放电产生的等离子体状态和类氖氩离子3p_3s跃迁线的增益系数的时空演变过程。中心模型中,选取内径为3.1 mm 的陶瓷毛细管并充入初始密度为1.07×10^-6g·cm^-3的氩气,电流脉冲峰值为27.81 kA,脉冲宽度为61.4 ns。改变放电参数进行模拟,结果表明：上升前沿越陡,则增益系数越大,电流脉冲上升时间在20~40 ns,电流峰值在25~40 kA,电流脉冲宽度在50~70 ns范围内,毛细管放电产生的等离子体状态比较理想,可获得较高的增益系数。     

放电参数对毛细管放电泵浦的软X射线激光的影响

以哈尔滨工业大学可调谐激光技术国家重点实验室中的Marx发生器的放电脉冲波形为基础,理论上模拟计算了在不同放电参数下充氩气的毛细管放电产生的等离子体状态和类氖氩离子3p_3s跃迁线的增益系数的时空演变过程。中心模型中,选取内径为3.1 mm 的陶瓷毛细管并充入初始密度为1.07×10-6g·cm-3的氩气,电流脉冲峰值为27.81 kA,脉冲宽度为61.4 ns。改变放电参数进行模拟,结果表明：上升前沿越陡,则增益系数越大,电流脉冲上升时间在20~40 ns,电流峰值在25~40 kA,电流脉冲宽度在50~70 ns范围内,毛细管放电产生的等离子体状态比较理想,可获得较高的增益系数。     

Comparative investigation of the resistance and ability to trigger high voltage discharge for single and multiple femtosecond filaments in air

A comparative investigation of the resistance and ability to trigger high voltage(HV) discharge for a single filament(SF) and multiple filaments(MFs) has been carried out.The experimental results show that the trend of the breakdown threshold of the SF exactly follows that of its resistance,but this is not the case for the MF.The MF’s resistance is much smaller than the SF’s.However,the MF shows a slightly higher HV breakdown threshold than the SF.The underlying physics is that the measured resistance of the MF is collectively contributed by every filament in the MF while the HV breakdown threshold is determined by only one single discharging path.     

高压耦合高功率脉冲磁控溅射的增强放电效应

等离子体源离子注入与沉积技术作为一种可生产高结合力、高致密度涂层的真空镀膜技术,具有广阔的应用前景,尤其适用于高载荷工况下服役的功能涂层制备.该技术中金属等离子体源是关键,而现有的脉冲阴极弧源结构复杂,且由于伴随"金属液滴"而需要增加过滤装置.本文研究了另一种简单结构的金属等离子体源备选一高功率脉冲磁控溅射源(HPPMS)的放电特性,采用等离子体发射光谱仪探索了不同的耦合高压对HPPMS放电靶电流特性和等离子体特性的作用.发现耦合高压对HPPMS放电有明显的促进作用,相同靶电压下的放电强度大幅增加,相对于金属放电,耦合高压对气体放电的促进作用更加明显,但在自溅射为主的高压放电阶段对金属放电的促进作用明显增强.讨论了耦合高压对HPPMS放电的增强机制,发现耦合高压自辉光放电、耦合高压和HPPMS电压构成双向负压形成的空心阴极效应,以及耦合高压鞘层改善的双极扩散效应都对HPPMS放电的增强有明显作用.