Experimental and theoretical study of cw HF chemical laser overtone performance
Carroll, David Leon
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https://hdl.handle.net/2142/22286
Description
Title
Experimental and theoretical study of cw HF chemical laser overtone performance
Author(s)
Carroll, David Leon
Issue Date
1992
Doctoral Committee Chair(s)
Sentman, Lee H.
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Aerospace
Physics, Optics
Physics, Fluid and Plasma
Language
eng
Abstract
The overtone performance of the UIUC supersonic cw HF chemical laser was characterized as a function of flow rates, cavity pressure, mode volume, mirror reflectivity and method of He injection. The overtone performance was optimized by the same set of flow rates that optimized the fundamental performance. When the absorption/scattering losses of the mirrors were taken into account, an overtone efficiency of 70-90% was achieved, which is the highest reported to date. The overtone efficiency was a strong function of media saturation. There was no significant change in overtone power and efficiency as the mode volume increased. However, there were an increase in the number of lasing lines and a shift to higher J lines. Overtone performance was as sensitive to cavity pressure as fundamental performance. There was no significant change in overtone efficiency when the method of He injection was changed. Base region absorption by ground state HF has a significant effect on fundamental and overtone performance. Even with very high degrees of saturation, approximately 10% of the total power was lost when the He purge was turned off.
For a laser having an extensive data base with well characterized mirrors to firmly establish the Rigrod parameters g$\sb0$ and I$\sb{\rm sat}$, the accuracy to which the mirror reflectivities can be deduced using measured mirror transmissivities, measured outcoupled power and Rigrod theory is approximately $\pm$0.07% for high reflectivity overtone mirrors. Rigrod theory shows that a higher media saturation yields a higher overall overtone efficiency, but does not necessarily yield a higher measureable power (power in the bucket). For low absorption/scattering loss overtone mirrors and a 5% penalty in outcoupled power, the intracavity flux and hence the mirror loading can be reduced by more than a factor of two when the gain length is long enough to well saturate the media.
The inclusion of HF-H$\sb2$ multi-quantum VV transfer reactions in the ORNECL model significantly improved the prediction of the zero power gains for the H$\sb2$ rich UI supersonic laser. The inclusion of absorption by SF$\sb{\rm x}$ molecules in the model significantly improved the prediction of the Fabry-Perot spectra.
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