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Characterization of nanosecond, femtosecond and dual pulse laser energy deposition in air for flow control and diagnostic applications.

机译：空气中纳秒，飞秒和双脉冲激光能量沉积的特征，用于流量控制和诊断应用。

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摘要

The non-resonant heating of gases by laser irradiation and plasma formation has been under investigation since the development of 100 megawatt peak power, Q-switched, nanosecond pulse duration lasers and the commensurate discovery of laser air sparks. More recently, advances in mode-locking and chirped pulse amplification have led to commercially available 100 gigawatt peak power, femtosecond pulse duration lasers with a rapidly increasing number of applications including remote sensing, laser spectroscopy, aerodynamic flow control, and molecular tagging velocimetry and thermometry diagnostics. This work investigates local energy deposition and gas heating produced by focused, non-resonant, nanosecond and femtosecond laser pulses in the context of flow control and laser diagnostic applications.;Three types of pulse configurations were examined: single nanosecond pulses, single femtosecond pulses and a dual pulse approach whereby a femtosecond pre-ionizing pulse is followed by a nanosecond pulse. For each pulse configuration, optical and laser diagnostic techniques were applied in order to qualitatively and quantitatively measure the plasmadynamic and hydrodynamic processes accompanying laser energy deposition. Time resolved imaging of optical emission from the plasma and excited species was used to qualitatively examine the morphology and decay of the excited gas. Additionally, Thomson scattering and Rayleigh scattering diagnostics were applied towards measurements of electron temperature, electron density, gas temperature and gas density.;Gas heating by nanosecond and dual pulse laser plasmas was found to be considerably more intense than femtosecond plasmas, irrespective of pressure, while the dual pulse approach provided substantially more controllability than nanosecond pulses alone. In comparison, measurements of femtosecond laser heating showed a strong and nonlinearly dependence on focusing strength. With comparable pulse energy, measurements of maximum temperature rise ranged from 50K to 2000K for 500mm and 175mm focal length lenses, respectively. Experiments with various lens and pulse energy combinations indicated an important connection between gas heating and the phenomena of intensity clamping and self-guiding. The long-term behavior of the heated region varied considerably among pulse configurations. However, in each case, the formation of a toroidal vortex could be suppressed or enhanced depending on the variables of pressure, focusing and pulse energy.
机译：自从开发出100兆瓦峰值功率，调Q开关，纳秒脉冲持续时间激光器并相应地发现了激光空气火花以来，就一直在研究通过激光辐射和等离子体形成对气体进行非共振加热。最近，锁模和chi脉冲放大技术的进步导致了市场上可买到的100吉瓦峰值功率，飞秒脉冲持续时间激光器，其应用范围迅速增加，包括遥感，激光光谱，空气动力学流量控制以及分子标记测速和测温诊断。这项工作研究了在流量控制和激光诊断应用中聚焦，非共振，纳秒和飞秒激光脉冲产生的局部能量沉积和气体加热。;研究了三种脉冲配置：单个纳秒脉冲，单个飞秒脉冲和双脉冲方法，飞秒预电离脉冲后跟一个纳秒脉冲。对于每种脉冲配置，应用光学和激光诊断技术以定性和定量地测量伴随激光能量沉积的等离子体动力学和流体动力学过程。时间分辨成像的等离子体和受激物种的光发射用于定性检查受激气体的形态和衰减。此外，汤姆森散射和瑞利散射诊断技术还用于电子温度，电子密度，气体温度和气体密度的测量。纳秒级和双脉冲激光等离子体的气体加热强度远大于飞秒等离子体，而与压力无关。而双脉冲方法比单独的纳秒脉冲提供了更多的可控性。相比之下，飞秒激光加热的测量结果显示出对聚焦强度的强烈非线性依赖性。使用可比较的脉冲能量，对于500mm和175mm焦距镜头，最大温升的测量范围分别为50K至2000K。用各种透镜和脉冲能量组合进行的实验表明，气体加热与强度钳制和自导现象之间有着重要的联系。加热区域的长期行为在脉冲配置之间有很大差异。然而，在每种情况下，取决于压力，聚焦和脉冲能量的变量，可以抑制或增强环形涡流的形成。

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  • 作者

    Limbach, Christopher M.;

  • 作者单位

    Princeton University.;

  • 授予单位 Princeton University.;
  • 学科 Aerospace engineering.;Optics.;Plasma physics.
  • 学位 Ph.D.
  • 年度 2015
  • 页码 280 p.
  • 总页数 280
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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