Diagnostic measurements of optically thick and inhomogeneous laser induced plasmas with emphasis on self-absorption effects.

摘要

Laser induced plasmas (LIP) have increased in popularity and the number of applications throughout the fields of science and technology. With this hike in usage, it is pivotal that there are trustworthy methods to describe the plasma dynamics. One of the main problems with determining various LIP parameters (temperature, number densities, etc.) is that all emission lines suffer from self-absorption to some degree; therefore, a reliable method of determining the degree of self-absorption that a spectral line experiences is necessary.;Temperature is the most important plasma parameter, and many plasma parameters are dependent on the plasma on it. Therefore, the temperature must be known accurately. The most popular spectroscopic method to determine the plasma temperature is the Boltzmann plot method. It is well known for its simplicity and easy use. However, the Boltzmann plot method cannot handle self-absorption. Under optically thick plasma conditions where self-absorption can not be ignored, the Boltzmann plot can produce inaccurate temperature values. For cases where self-absorption can not be neglected, alternative methods of calculating temperatures must be used. A 2-line method based on Bartels' model is one such method designed to produce accurate temperature measurements under optically-thick conditions.;This research presents a fundamental comparison of the 2-line Bartels' method and the Boltzmann plot method to calculate the time-resolved temperatures of a LIP created on different Ba-containing targets. Emission lines from the same type of plasma were used to determine the temperatures. For the temperature calculations with the Boltzmann plot method, the requirement of "only optically-thin lines" was purposely violated in order to view the effects of self-absorption. It was found that early in the plasma lifetime before 2 micros, that the Boltzmann plot method predicted higher temperature values than the Bartels' method. This is due to the self-absorption experienced by each individual spectral line in the Boltzmann plot.;In addition, this research introduces the Duplication factor (D) as a way to measure the degree of self-absorption experienced by a spectral line emitted from the LIP and as an avenue to calculate time resolved ionic number densities of different regions of the plasma. The ionic number density was determined by taking calculated D's and fitting them to theoretical Gouy (or duplication) curves. Several Barium (Ba) ion emission lines were observed from a pure Ba target with a Laser Induced Breakdown set-up containing a Nd:YAG laser, Acton 0.5 m spectrometer, and gated ICCD, and a spherical mirror was used for the duplication.