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A Feasibility Analysis of a Directly Sun-Pumped Carbon Dioxide Laser in Space

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A Feasibility Analysis of a Directly Sun-Pumped Carbon Dioxide Laser in Space Portland State University PDXScholar Dissertations and Theses Dissertations and Theses 1979 A feasibility analysis of a directly sun-pumped carbon dioxide laser in space Seiichi Morimoto Portland State University Follow this and additional works at: https://pdxscholar.library.pdx.edu/open_access_etds Part of the Materials Science and Engineering Commons Let us know how access to this document benefits ou.y Recommended Citation Morimoto, Seiichi, "A feasibility analysis of a directly sun-pumped carbon dioxide laser in space" (1979). Dissertations and Theses. Paper 2798. https://doi.org/10.15760/etd.2794 This Thesis is brought to you for free and open access. It has been accepted for inclusion in Dissertations and Theses by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. I AN ABSTRACT OF THE THESIS OF Seiichi Morimoto for the Mast.er of Science in Applied Science presented May 17, 1979. Title: A Feasibility Analysis of a Directly Sun-Pumped Carbon Dioxide Laser in Space. APPROVED BY MEMBERS OF THE THESIS COMMITTEE: GeOrgeTSODgaS , ~M. Heneghan Selmo Taub The possibility of using sunlight to pump a CW carbon dioxide laser has been analyzed. Such a laser could be of interest for such applica- tions as space communication and power transmission. In order to optically pump co2 using sunlight, the intense vibrational-rotational absorption bands of in the 4.3 micron spectral region would have co2 to be utilized. The total pumping power from sunlight can be calculated 2 from the known data of the solar spectral irradiance outside the atmosphere and the infrared absorption by carbon dioxide at 4.3 microns. The pumping power is proportional to the collector area of the sunlight and is also dependent on the characteristics of the absorbing gas mixture, such as the gas composition, the gas tempera- ture, the total pressure of the mixture, the partial pressure of co , and the. absorption path length of the sunlight in the gas. 2 To analyze the carbon dioxide laser system, a thermodynamic approach was used with a simplified co chemical kinetic model. The 2 gain and saturation intensity were obtained by solving a set of energy balance equations which describe the processes among the various vibrational modes. From those results, along with the estimated cavity losses, the output power was found. Initially, the computation was carried out for a co -He mixture 2 for the total pressure range 0.01-0.05 atm with various concentrations of co at temperatures of 200°K and 300°K, and with the following 2 system parameters specified: a tube of 100 cm length and 1.7 cm 2 diameter, pumping from one end, 1% residual losses, and a 10 m col- lector. All the collected sunlight was assumed to enter the cavity without losses. Then the parameters were varied to examine their 2 influence on the results. The 10 m collector and a 2 m absorption path length were found to be necessary to achieve sufficient gain and high output power. While the performance was found to be quite sensi- tive to a number of the individual parameters, the net effect was that the optimum output was obtained under conditions which are close to those initially set. The optimum performance at 200°K was obtained 3 at 0.02 atm total pressure with 50% co ; then the gain was about 5%/m 2 and the output power was about 250 milliwatts. The corresponding efficiency of converting absorbed pump light into laser output was about 4%. At 300°K the gain was found to be unacceptably low because the lower laser level was more highly populated. Thus, the gas must be cooled to achieve sufficient gain. At total pressures above about 0.05 atm the··small signal gain was too low, while at pressures below 0.01 atm the gain and output power decreased on account of the low input power and diffusion e_ffects. Therefore, this particular type of co2 laser would not appear to be capable of operation at pressures above atmospheric where continuous tunability can be achieved. This preliminary analysis indicates that it should be feasible to directly sun-pump a carbon dioxide laser in space. The laser would be a modest power, low gain system compared with the demonstrated sun-pumped Nd:YAG laser. If co output is required or desirable for 2 any of a number of reasons, then this study indicates that direct sun­ pumping is possible. A FEASIBILITY ANALYSIS OF A DIRECTLY SUN-PUMPED CARBON DIOXIDE LASER IN SPACE ·. by SEIICHI MORIMOTO A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in APPLIED SCIENCE Portland State University 1979 TO THE OFFICE OF GRADUATE STUDIES AND RESEARCH: The members of the Committee approve the thesis of Seiichi Morimoto presentad May -17, 1979. Selmo Tauber APPROVED: Terrence Wil~s; Mechanieal Engineering Section Head, Department of Engineering and Applied Science Graduate StudieS and Research ACKNOWLEDGEMENTS The author wishes t? express his sincere appreciation to Dr. George Tsongas for his invaluable guidance and unending assistance throughout the course of this study. In addition, his kind and patient contribution of so much time and effort to improve my writing is gratefully appreciated. It is also a pleasure to acknowledge Dr. Gail Massey of the Oregon Graduate Center and Dr. Walter Christiansen and Dr. Oktay Yesil of the University of Washington for fruitful discussions and helpful suggestions. TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS · iii LIST OF FIGURES vi NOMENCLATURE viii 1. INTRODUCTION 1 2. FEASIBILITY ANALYSIS · 4 2.1 SOLAR POWER OUTSIDE THE ATMOSPHERE AND ITS ABSORPTION BY co2 MOLECULES 4 2.2 THEORETICAL MODEL AND CHEMICAL KINETICS OF CARBON DIOXIDE 10 2.2.1 Vibrational Energy Levels · · · · · · · · 10 2.2.2 Rotational Energy Levels 14 2.2.3. Vibrational Relaxation· 16 2. 3 PREDICTION OF GAIN AND POWER OUTPUT · 22 2.3.1 Minimum Pumping Power Required for Population Inversion 22 2.3.2 Gain Coefficient 23 2.3.3 Energy Balance Equations 28 2.3.4 Saturation Intensity and the Maximum Available Power ... 30 2.3.5 Cavity Losses and Output Coupling 31 2.3.6 Threshold Gain and Output Power 32 _J v . .... 34 3. RESULTS AND DISCUSSION ...... 3.1 PROPOSED LASER SYSTEM MODEL 35 3.1.1 Description of the System ..... 35 Computed Results for Proposed Laser . 36 3.1. 2 3.2 EFFECTS OF PARAMETER VARIATION ON THE SMALL SIGNAL. 40 GAIN AND· THE OUTPUT POWER. 3.3. PRACTICAL DESIGN CONSIDERATIONS .• 47 4. CON CL US IONS . 49 REFERENCES· . · . · . · . 51 LIST OF FIGURES FIGURE PAGE 1. The Infrared Absorptivity of co in the 4.3 Micron 2 Region at 200°K for Various Total Pressures and co Partial Pressures for a 2 Meter Absorpt-ion 2 Path Length . 6 2. Segmentation of the Absorptivity Curve for Calculation of Total Pumping Power 7 2 3. Total Pumping Power at 200°K for a 10 m Collector and a 2 Meter Absorption Path Length. 8 2 4. Total Pumping Power at 300°K for a 10 m Collector and a 2 Meter Absorption Path Length ... 9 5. Partial Energy Level Diagram for Carbon Dioxide 11 6. Temperature. Dependence of Relaxation Lifetime of (010) Level . 20 7. Temperature Dependence of Relaxation Lifetime of (001) Level . 21 8. Simplified Energy Level Diagram 28 9. Net Generated Power per Unit Volume Versus Intensity . 31 10. Variation of Small Signal Gain with Total and Partial Pressures .................. 37 vii LIST OF FIGURES (Cont'd) FIGURE PAGE 11. Variation of Output Power with Total and Partial Pressures . 39 12. Output Power Versus Residual Loss at 200°K . 42 13. Output Power and Small Signal Gain Versus Sun Collector Area· at 200°K ... 43 14. Output Power and Small Signal Gain Versus Beam Radius ................ ... 45 15. Output Power and Small Signal Gain Versus Absorption 0 Path Length at 2qo K ...... .. 46 16. Schematic Diagram of a Sun-pumped co Laser with End- 2 pumping . ...... 47 NOMENCLATURE English: -1 A Einstein A coefficient, sec 2 A Cross-sectional area of the beam, cm m 2 A Area of the sun collector, cm s -1 B Rotational constant, cm -1 c Velocity of light, cm•sec The mean relative velocity of co and the species 2 -1 k (C for co and H for He), m•sec 2 D Di. ff usion. constant, cm 2 •torr•sec-1 -3 E Vibrational energy per unit volume, J•cm v -3 E Vibrational energy in equilibrium, J•cm v -3 Vibrational energy of symmetric mode, J•cm -3 Vibrational energy of bending mode, J•cm -3 Vibrational energy of asymmetric mode, J•cm -3 Combined vibrational energy of E and E , J•cm 1 2 Lineshape function, sec Degeneracy of J-th rotational level Degeneracy of lower vibrational level Degeneracy of upper vibrational level 34 Planck's constant = 6.626 x l0- J•sec -2 I Intensity of light, watts•cm -2 Intensity of incident light, watts•cm ix NOMENCLATURE (Cont'd) ' English: -2 I Saturation intensity, watts•cm s i Vibrational quantum number J Rotational quantum number of the upper level J' Rotational quantum number of the lower level 23 k Boltzmann constant = 1.38 x l0- J•oK-1 L Total intensity loss per pass, % L. Residual intensity loss per pass, % l. Length of the laser tube, cm Absorption path length of the sunlight, cm m The mass of a co molecule, kg c 2 The mass of a molecule of species k, kg -3 N Population density of co , molecules•cm 2 Population dnesity of species k (C for co , H for 2 -3 He), molecules•cm N.
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