On Laser-induced Plasma Containing Hydrogen

Laser-induced micro-plasma dynamics are investigated in laboratory air, ultra-high-pure hydrogen gas, and hydrogen-nitrogen gas mixtures. The dissertation focuses on atomic spectroscopy of hydrogen in the visible region.

Line-of-sight measurements are analyzed to obtain spatial distributions of electron densities and excitation temperatures. The studies include evaluation of self-absorption phenomena. The plasma dynamics occur initially well above re-entry speeds and diminish to hypersonic and then supersonic expansions. Expansion velocities are measured that are above three hundred times the speed of sound in standard atmosphere. Optical breakdown is induced by using pulsed laser radiation. Emission spectra are collected by employing a spectrometer equipped with an intensified charge-coupled device. Atomic emission profiles for hydrogen alpha, hydrogen beta and hydrogen gamma lines are utilized to determine plasma characteristics such as electron density and excitation temperature for specific time delays. The duplicating mirror approach is applied for the evaluation of self-absorption. The extent of self-absorption is investigated for various time delays from plasma generation. The electron density is also determined from singly ionized nitrogen lines and compared with values obtained from the hydrogen lines to further evaluate self-absorption.

Experimental records are modeled using computational physics including inversions of integral equations to infer radial distributions from line-of-sight measurements. The presented work contributes to the fundamental understanding of laser-induced breakdown spectroscopy widely utilized for analytical diagnostics of gases, liquids or solids.