M.S. Thesis Presentation by Brian K. Stewart
Thursday , December 2, 2004

(Dr. S. Mostafa Ghiaasiaan, Chair)

"Development of Thin-Film Evaporative Cooling System for a High-Energy ThHo:LuLiF Solid State Laser Oscillator Crystal"

Abstract

In this study, the feasibility and the critical design parameters for the development of a thin-film evaporative cooling concept for a high energy, pulsed solid-state laser oscillator crystal rod were investigated. The scope of the investigation was broad, and could be divided into distinct analytical segments. The first segment is an engineering analysis aimed at identifying and quantifying the trends of key design parameters. The next three segments targeted three topical areas in need of further development and refinement; bulk flow hydrodynamic analysis, TmHo:LuLiF4 crystal surface energy characterization, and the design of a rugged crystal-metallic hermetic seal.

The engineering analysis supports the fundamental feasibility of the concept, but reveals a need for better understanding of the parameters that suppress the onset of nucleate boiling (ONB). In particular, the ONB analysis reveals that ultra-thin liquid films may be necessary to ensure the suppression of nucleate boiling. The engineering analysis also develops guidance to the three areas that are studied in greater detail.

The hydrodynamic bulk flow analysis makes use of a one-dimensional two-fluid flow model. The analysis indicates that reasonable fluid velocities and pressure drops are possible with the flow morphologies that were studied. Additional analytical work is required to study system performance with the ultra-thin film flows indicated by the ONB suppression analysis. This additional work may be accomplished by modification of the equations and code developed to solve the conservation equations that govern the one-dimensional flow model.

The study of interfacial phenomena between the fluid and the solid surface is hindered by the lack of data regarding the magnitude and the nature of the surface energy of the crystallineTmHo:LuLiF4 rod material. This study outlines the key parameters to be investigated, and offers arguments that support the need to close key knowledge gaps. The treatment then concludes with experimental values of TmHo:LuLiF4 crystal surface energy using the sessile drop method and computed using numerous surface energy models. The potential of electrostatic enhancement of surface energy is also discussed.
The design of a strong hermetic seal between the rod and a metallic support structure is proposed, with particular emphasis on the effects of thermal expansion mismatch between the crystal rod and a metallic sleeve support structure. The problem is complicated by anisotropic thermal expansion of the TmHo:LuLiF4 crystal rod. Various seal types are discussed, and the technique of anodic bonding is proposed to produce the required hermetic seal. Although current literature is rich with studies of anodically bonded metal-ceramic joints and silicon-glass joints for MEMS technologies, analytical and experimental techniques for the materials and geometry proposed do not yet exist. This segment develops the analytically expressions required for a closed-form solution to estimate the thermally-induced stresses in the metal-ceramic joint. The results show that the proposed measures selected to reduce thermally induced stresses will be effective, and that structural performance of the joint will not be limited by thermally-induced stresses using the proposed design rules. Finally, a system for assembling and then testing actual crystal-metal components was designed and fabricated to help with on-going studies of the joint assembly and sealing problem.