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Title
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Optical diagnostics of spatiotemporal evolution characteristics of nanosecond laser-induced plasma in gases
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Author
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Abstract
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| Laser-induced plasmas (LIP) in gases are of great importance in various fields, such as for elemental analysis, aerospace, electrical equipment, medical devices, and semiconductor manufacturing. However, the physical and chemical processes during the eneration, expansion, and decay of LIP in gases are complex. Understanding these processes in LIPs is beneficial for the further optimization and development of different applications. In this PhD thesis, LIPs in gases were generated using a 1064 nm nanosecond Nd: YAG laser, and different optical diagnostic methods were combined to investigate the interaction between the plasma and laser at the early stage and the subsequent plasma evolution. In previous experimental research, some passive diagnosis methods were used, such as optical emission spectroscopy (OES). The OES method has strict requirements for the selection of emission lines and should be based on several assumptions, like local thermodynamic equilibrium. The measured results cannot well describe the complete evolution process of LIPs in gases. In Chapter 2 of this PhD thesis, efforts were paid to reduce the stray light and achieve a high signal-to-noise ratio, when the laser Thomson scattering (LTS) method was used in low-temperature plasma diagnosis. Using the LTS method, a high temporally and spatially resolved electron number density (ne) and electron temperature (Te) measurement was achieved. The perturbation of the probe laser to the plasma was also studied. For the LIP at atmospheric or low pressure, the maximum error of Te caused by the probe laser should be less than 10% with a fluence of less than 20 J/cm2. Regarding the interaction between the laser and gas, previous studies have focused on the measurement of the critical breakdown energy. There was a lack of research on the absorption capacity and the expansion characteristics of LIPs under different conditions. However, such parameters can provide quantitative guidance for the precise control of laser deposition energy and ignition position in applications such as laser triggered switches and laser spark plugs. In Chapter 3 of this PhD thesis, the absorption rates were measured for different laser energies, gas pressures and gas mediums. The shock wave is the main energy dissipation mechanism in LIPs. The evolution of the shock wave is an important topic in the research of LIPs. Previous studies have mainly focused on the dynamic characteristics of the shock wave at atmospheric pressure. The energy utilization rate of the shock wave at different pressures is not clear, which can be important to improve the performance of applications such as laser propulsion. The characteristics of the shock wave were studied using laser shadowgraph and laser Rayleigh scattering. Initially, the shock wave is an asymmetric ellipsoid. Then, the asymmetry weakens with time. A few microseconds after the breakdown, the shock wave quickly decays to a moderate intensity explosion wave. The Taylor-Sedov model cannot well describe the early evolution of the shock wave. The energy of the shock wave accounts for 40%~65% of the total absorbed energy of LIPs. The higher the pressure and the lower the laser energy, the higher the proportion of the shock wave to the total absorbed energy. The characteristics of the decay phase of the plasma were studied in Chapter 4 of this PhD thesis. Special attention was paid to the evolution of the ‘torus structure’ in the late stage of the decay phase (about tens of microseconds after breakdown). The previous studies were limited to qualitatively describing the time when the torus structure appeared but lacked quantitative measurement of its internal physical parameters. For the most widely used LIP in atmospheric pressure air, the results based on the LTS method showed that the decaying speeds of ne and Te near the axis accelerate, becoming valleys in the radial distribution. Combining with the 2D distribution of density measured by the Mach-Zehnder interferometric system, it is clarified that in the later stage of the decay phase, a cold gas flow with a high density appears near the axis of LIPs. Such cold gas flow passes through the plasma centre from bottom to top and squeezes the low-density area radially outward, and then the torus structure appears. The correlation between the generation of torus structure and the accelerated attenuation of plasma radiation intensity was also found. For laser-induced breakdown spectroscopy and other applications that use the emission intensity of spectral lines for measurement, this result can provide data support for improving their accuracy. Due to the limitation of diagnosis methods, it was difficult to measure the microscopic parameters of LIPs in gases under different conditions in previous studies. In the last chapter of this PhD thesis, the influence of different factors on the dynamic characteristics of LIPs during the decay phase was studied. The spatial size of the plasma, the peak intensity of the plasma emission, ne and Te were measured separately. Special attention was given to the decay rate of ne and Te during the early stage of the decay phase. Near atmospheric pressure (0.6~2atm, air), the higher the pressure and the lower the laser energy, the faster the decay rates of ne and Te. For different gases, the electron number density decays exponentially with time, and the decay index is between -1.5~-0.9. Among the measured gases, SF6 plasma has the fastest decaying rate. Based on the results of this experiment, the decay phase of LIPs in gases can be used to simulate the current-zero period of gaseous arc in the circuit breaker without introducing contamination from the electrodes. |
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Language
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English, Chinese
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Publication
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Xi’an Jiaotong University
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2020
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Volume/pages
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117 p.
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Note
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Wu, Yi [Supervisor]
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Bogaerts, Annemie [Supervisor]
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Full text (publisher's version - intranet only)
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