thermal laser excitation by supersonic mixing in a gas dynamic nozzle

8/13/2019 THERMAL LASER EXCITATION BY SUPERSONIC MIXING IN A GAS DYNAMIC NOZZLE

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UNCLASSIFIEDDOCUMENT CONTROL DATA . R&D

(Scurity Classificatlonof Ml1e. body of ebstAcl end indexing atin ficn muat be entlmed when the overall report toleeaifIed i1. ORIGINATING ACTIVITY (Coatore 80or; I20. REPORT SECURITY CLASSIFICATIONPhysical Sciences Directorate UNCLASSIFIEDDirectorate for Res, Dev, Eng, & Msl Sys Lab bUS Army Missile Conmand NA.RoupRedstone Arsenal, Alabama 358093. REPORT TITLE

THERMAL LASER EXCITATION BY SuPERSONIC MIXING IN A GAS DYNAMIC NOZZLE4, OESCRIPTIVE NOTES (2y of report and in lusive dofte)

Technical ReportS. AUTHORNS, First NSe, idS. Initieal leet nasmo)

H. Lee Pratta REPORT DATE 78. TOTAL NO. OF PAGES 1b NO. OF REFS1 April 1971 88 39

e. CONTRACT 1 R GRANT NO. S9. ORIGINATOR'S REPORT NUMUVERIS

b. PROJECT NO. (DA) 1T262303A308 RR-TR-71-2c. MC Management Structure Code No. Ob. OTHER REPORT NOMSIAny othe nushere Mat iAY be assled522C. 11.47300 thi repor.SAD_10. DISTRIOUTION STATEMENT

Distribution of this documAnt has been approved for public release and sale;its distribution is unlimited.SO. SUPPLEMENTARY NOTES f12. SPONSORING MILITARY ACTIVITY

Nonej Same as No. 1IS. ATRACTAn arc-driven laser suitable for gas-dynamic and similar modes of operation is

described. Inversion is obtained by rapid expansion of hot gas mixtures through asupersonic aerodynamic nozzle. Upper vibrational lasing levels are "frozen" at pre-expansion conditions while the lower levels are characterized by conditions in thesupersonic flow downstream of the nozzle. A population inversion exists for somedistance downstream. Inversion is enhanced by the injection of additives to relax thelower levels at a faster rate. The system is so designed that the gases used, themixture ratios, and the mass flow rates, as well as the specific enthalpy of thesystem, may be varied nearly independently. In addition, gases may be injected atvarious points along the flow, and the results of this feature is stressed.The laser has been operated under marginal gas-dynamic conditions with all com-ponent gases of a C02 system premixed prior to expansion. However, most of the workpresented deals with the injection of cold CO2 into the hot supersonic flow mixtureimmediately downstream of the nozzle throat.The variation of the 10.6 micron output power, both in magnitude and in space, ispresented as a func .-n of the mass flow rate of each component gas. Additionalfactors, such as input power and upstream pressures and temperatures, are alsoconsidered.It was found that the range of plenum conditions for which lasing may occur isconsiderably extended in the supersonic mixing mode. In addition, powers more thanan order of magnitude higher were obtained with supersonic mixing for the range ofavailable operating conditions. (Continued)

DD 14 7A 3 DOPL ..o_04. 1 JA 04. W-ICH IS,DISwv 0914 o7SOL6TE FOR ARMY U9s. UNCLASSIFIED79 Scurity Clasification


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UINCLAS IFIEDSecurity Cltssification

14 LINK A LINK 0 LINK COROLE WY ROI SI T ROLE WT

Arc-driven laserSupersonic flowSupersonic mixingOutput power

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Abstract (Continued)Th report illustrates how parametric

studies may be made of various aerodynamicsystems. In particular, the effects ofcontrolled amounts of impurities, suci asthose formed in various combustion processes,may be investigated.

UNCLASSIFIED80 StcuIty Classification


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1 April 1971 Report No. RR-TR-71-2

THERMAL LASER EXCITATION BY SUPERSONIC MIXINGIN A GAS DYNAMIC NOZZLE

byH. Lee Pratt

DA Project No. IT262303A308AMC Management Structure Code No. 522C.11.47300

DISTRIBUTION OF THIS DOCUMENT HAS BEEN APPROVED FO RPUBLIC RELEASE AND SALE; ITS DISTRIBUTION IS UNLIMITED.

Physical Sciences DirectorateDirectorate for Research, Development, Engineeringand Missile Systems Laboratory

US Army Missile CommandRedstone Arsenal, Alabama 35809


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ABSTRACT

An arc-driven laser suitable for gas-dynamic and similar modesof operation is described. Inversion is obtained by rapid expansionof hot gas mixtures through a supersonic aerodynamic nozzle. Uppervibrational lasing levels are "frozen" at pre-expansion conditionswhile the lower levels are characterized by conditions in thesupersonic flow downstream of the nozzle. A population inversionexists for some distance downstream. Inversion is enhanced by theinjection of additives to relax the lower levels at a faster rate.The system is so designed that the gases used, the mixture ratios,and the mass flow rates, as well as the specific enthalpy of thesystem, may b-' aried nearly independently. In addition, gases maybe injected st various points along the flow, and the results ofthis feature is stressed.

The later has been operated under marginal gas-dynamic condi-tions with ll component gases of a CO2 system premixed prior toexpansion. However, most of the work presented deals with theinjection of cold CO2 into the hot supersonic flow mixture immedi-ately downstream of the nozzle throat.

The variation of the 10.6 micron output power, both inmagnitude and in space, is presented as a function of the mass flowrate of each component gas. Additional factors, such as input powerand upstream pressurzes and temperatures, are also considered.

It was found that the range of plenum conditions for whichlasing may occur is considerably extended in the supersonic mixingmode. In addition, powers more than an order of magnitude higherwere obtained with supersonic mixing for the range of availableoperating conditions.

This report illustraites how parametric studies may be made ofvarious aerodynamic systems. In particular, the effects of controlledamounts of impurities, such as those formed in various combustionprocesses, may be investigated.

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AC KNOWL E DGME NT

The author is grateful for the advice and assistanceof personnei of the Chemical Physics Functions, PhysicalSciences Directorate, who helped install and operate thehardware and also participated in the collection of thedata. Dr. Thomas G. Roberts provided much of the informa-tion concerning the special configuration laser as wellas invaluable assistance in organizing and writing thisreport. Information concerning mass flow rates, as wellas information concerning the Army Missile Command plasmafacility was obtained in part from Dr. Thomas A. Barr, Jr .Mr. W. LaVaughn Hales and Mr. Charles M. Cason gaveassistance in the form of computational aid. Appreciationis especially due Mr. Billie 0. Rogers, Mr. Herbert C. Ruge,and Mr. Charles N. Rust who operated and maintained thelaser facility.

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s 4

CONTENTS

PageLIST OF ILLUSTRATIONS ......................................... viiCHAPTER 1. INTRODUCTION ..................................... 1

1.1 Background ................................. 21.2 Scope of this Work .......................... 4

CHAPTER 2. THEORETICAL CONSIDERATIONS ........................ 62.1 N2-CO2 Molecular Laser Scheme .............. 62.2 Effects of Additives ....................... 82.3 Gas-Dynamic Laser Scheme and Its

Advantages ................................. 102.4 Thermal Mixing Scheme ...................... 13

CHAPTER 3. EXPERIMENTAL FACILITY ............................ 153.1 Power Supply, Control System, and Arc ...... 173.2 Gas Supply and Vacuum Facility ............. 233.3 Laser Components ........................... 26

3.3.1 The Plenum ......................... 263.3.2 The Nozzle ......................... 323.3.3 The Cavity ......................... 32

3.4 The Assembled Special Configuration Laser... 37CHAPTER 4. INSTRUMENTATION AND CALIBFATION .................. 42

4.1 Measurement of GaF Flow Rates ............... 424.1.1 Calibration Procedures ............. 434.1.2 Caizulation of Mass and Molar

Fractions .......................... 464.2 Plenum Temperatures T and T474.3 Input Power ................................ 48

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Page4.4 Output Power ............................... 9

4.4.1 Thermopile Detector System ......... 494.4.2 Laser Burn Patterns ................ 50

4.5 Problems Encountered ....................... 51CHAPTER 5. EXPERIMENTAL RESULTS OF GAS-DYNAMIC LASER

(MODE I) OPERATION ............................... 545.1 Maximum Power versus N2 Flow Rate .......... 545.2 Power versus H20 Flow Rate ................. 565 3 Summary of Mode I Operation ................ 56

CHAPTER 6. EXPERIMENTAL RESULTS OF SUPERSONIC MIXING(MODE II) OPERATION .............................. 586.1 Effects Due to T ....................... 58wall6.2 Puwer Variation Due to the N, Flow Rate .... 606.3 Power Variation Due to the CO2 Flow Rate ... 636.4 Power Variation Due to the He Flow Rate. 636.5 Power Variation Due to the H2 0 Flow Rate ... 69

6.6 Additional Factors Affecting Performance ... 696.7 Summary of Mode II Operation ............... 71

CHAPTER 7. DISCUSSION AND RECOMMENDATIONS ................... 727.1 Comparison of Modes I and II ............... 727.2 Recommendations and Plans for FutureWork .................................. ..... 73

aREFERENCES .................................................... 75

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LIST OF ILLUSTRATIONS

Figure Page1 Diagram of Energy Levels of N2 -CO2 Laser ............ 72 Diagram of Gas-Dynamic Laser and Associated

Temperatures ....................................... 113 Diagram of Special Configuration Laser ............... 164 Photograph of dc Power Generators ................... 185 Diagram of Special Configuration Laser Power

System ............................................ 196 Diagram of Entire Power and Control System .......... 207 Photograph of Control Console ....................... 218 Diagram of Electric Arc Plasma Generator ............ 229 Diagram of Gas Flow System .......................... 24

10 Photograph of N2 Supply ............................. 2711 Photograph of Vacuum Tank and Plasma Facility ....... 2812 Diagram of Nozzle, Cavity, and Transition Sections 2913 Diagram of Plenum, Viewed from Above ................ 3014 Diagram of Plenum, Viewed from Side ................ 3115 Diagram of Nozzle, Viewed from Downstream ........... 3316 Diagram of Nozzle, Cutaway View ..................... 3417 Diagram of Cavity, Viewed Along Optical Path ........ 3518 Diagram of Output Mirror ............................ 3619 Photograph of Special Configuration Laser,

Exploded, View 1 .................................... 3820 Photograph of Special Configuration Laser,

Exploded, View 2 .................................... 3921 Photograph of Special Configuration Laser, View 3 ... 4022 Photograph of Special Configuration Laser, View 4 ... 41

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Figure Page23 Gr,?h of Output Energy Versus Burn Volume ............ 522. Graph of Output Poi:er Versus N2 Flow Rate (Mode T) ... 5525 Photograph of Burn PaLterns, H2 0 Variation (Mode I) .. 5726 Graph of Output Power Versus Twall (Mode II) ......... 5927 Graph of Output Powcr Versus X2 Flow Rate (Mode II) .. 6128 Spatial Burn Patterns with N2 Variation (Mode Ii) .... 6229 Graph of Output Power Versus CO2 Flow Rate (Mode II) . 6430 Spatial Burn PatteLns with CO2 Variation (Mode II) A . 65

23i Spatia Burn Patterns with CO2 Variation (Mode II) B . 6632 Graph of Output Power Versus He Flow Rate (Mode II) .. 6733 Spatial Burn Patterns with He Variation (Mode II) .... 682 4 Spaial Burn Patterns with H 0 Variation (Mode II) .. 70

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VCHAPTER 1

INTRODUCTION

Since the development of the first working lasers barely adecade ago, considerable time and effort has been expended in thesearch for higher output laser systems. The possible applications of

power lasers are unlimited in the fields of communication, manufactur-ing, construction, medicine, space exploration, and defense.

Unfortunately many obstacles exist in the attainment of high

power systems. Most lasers have a low efficiency. To obtain highpower or high energy outputs, considerably more energy must be fur-nished to the system. If this energy is electrical, then the systemcannot have a large average power and still be portable, as is desired

in some cases. The relative size, weight, and availability of materialshave introduced more obstacles, and the problem of handling a high powerbeam once it is obtained must also be considered. Then too, many laserschemes are limited in power because of the inherent Tharacteristics ofthe lasing materials and the reactions taking place.

Despite these obstacles, continuous wave output power hascontinued to increase from milliwatts, watts, kilowatts, and morerecently tens of kilowatts. New lasing materials, methods of producingpopulation inversions, and coupling procedures have been developed.Detectors, mirrors, windows, and other auxiliary equipment have beenadapted for the new systems.

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One of the more promising areas of work now being studied isthe field of gas-dynamic lasing (GDL) systems. Once fully developed,such GDL systems may be able to operate independent of electrical orother external power sources. Bulky and heavy equipment such asvacuum chambers, pumps, and compressors may be eliminated. A portable,high-power, and relatively simple laser may result.

1.1 BackgroundIn 1962-1963 Basov [1] and Hertzberg and Hurle [2,3] reported

the possibility of producing population inversion by the heating andcooling of various systems. Inversion is obtained by noting differ-ences in relaxation times for various energy levels during the timethermodynamic equilibrium is being established. Inversion through thismeans can quite easily be obtained in N2-CO2 systems, which have pre-viously produced several kilowatts of laser power [4,51 from glow dis-charge excitation.

In a typical N2 -CO2 laser the upper lasing level of the CO2 isvery near the first vibrational level 3f nitrogen. Energy is pumpedinto the nitrogen (usually through electron bombardment) to populatethis level. By near resonant collisional transfer of energy the excitednitrogen molecules fill the upper CO2 lasing level, producing a popu-lation invarsion in the CO2 molecules.

Gas-dynamic techniques [1-3, 6-22] may also be used topreferentially populate the upper CO2 level. If the component lasergases are heated and rapidly cooled, the temperature, Tvib, associatedwith the vibrational mode of N2 remains almost as high as before coolingfor an appreciable length of time. Powever, the time for temperaturesassociated with the rotational and translational modes to drop is very

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short by comparison. Thus a relatively high density of the (v = 1 N2state occurs, which selectively excites the upper CO2 level incollisions.

I Several types of rapid, adiabatic cooling systems may beused. Rarefaction waves in shock tubes [10, 17, 23] and chemical shocktubes [10, 24] may be employed. Perhaps the most effective method isthe expansion of the gas through a narrow slit [6, 10] or anozzle [6, 10, 15, 25]. The latter methods are Dreferred because theyprovide shorter cooling times and can be operated continuously ratherthan pulsed.

Several CO2 GDL systems are now being studied in researchprograms by AVCO-Everett Research Laboratory [20-22], UnitedAircraft [16, 19], Ames Research Center [17], and the University ofIllinois [12, 14].

The techniques employed in a GDL are not peculiar to a COlaser. Other systems have been treated [6, 10, 13, 191 and much workremains to be done in these fields.

Inversion can be more readily obtained if cold CO2 is addedto the heatea N2 [16, 19]. Selective excitation of the upper lasingr2level by the excited nitrogen molecules will occur, but initially thelower lasing level will not be populated appreciably by this process.The problem encountered here is that the N2 and CO2 must be well mixedto obtain maximur. population of the upper lasing level by collisionaltransfer, but the gases must not be mixed long enough for the lowerlasing level to be thermally populated. Systems operating on this

principle have been called "thermal mixing" lasers. If the nczzle in a GDL system is used only to "freeze" the

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vibrational levels of K2 , and cold CO2 is iniected into the N2 flowat various positions relative to the nozzle throat, then a gas-dynamic-thermal mixing laser results. For example supersooac mixing can takeplace when the CO2 is injected anywhere downstream of the nozzle threac.Near sonic mixing can we accomplished by injecting cold CO just up -tream of the nozzle threat. When operated in this manner, the laser is

not a GDL in the ordinary sense and may more appropriately be called afast flow thermal laser or, as above, a gas-dynamic-thermal mixing laser.Such a laser extends the ordinary operating range of a gas-dynamic laser,as will be shown by the work reported here.1.2 Scope of This Work

The purpose of this paper is to report the results of operat-ing the same laser system both as a gas-dynamic laser and as a super-sonic thermal mixing laser. These experiments were performed utilizingan existing laser facility at the Physical Sciences Laboratory, U. S.Army Missile Command, Redstone Arsenal, Alabama. The facility wasdesigned so that it could be operated in these forms as well as in otherforms which include those of some chemical lasers. The system has beennamed the Special Configuration Laser (SCL) facility.*

An introduction to the theory of gas laserr which exemplifiesthe characteristics of both GDL and thermal mixing lasers is given in

*Further information concerning the SCL may be found inreports of the following meetings:

Roberts, T. G., Pratt, H. L., Hutcheson, G. J., and Barr,T. A., Jr., A Versatile Experimental Laser," to be presented in TheLaser in Science and Technology conference, University of Washington,Seattle, Washington, date unannounced.

Roberts, T. G., Pratt, H. L., Hutcheson, G. J., and Barr,T. A., Jr., "The Performance of a CO2 Laser with Supersonic Mixing,"Bull. Arm.Phys. Soc., 15, 1970, p. 1324.


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Chapter 2. The SCL facility is described in Chapter 3 for the con-figurations used in these experiments. The details of instrumentationand calibration procedures are included in Chapter 4. The resultsobtained in the GDL mode are presented in Chapter 5, while Chapter 6includes results obtained in supersonic mixing moees. Finally, compari-son of the results obtained in these two modes of operation andrecommendations for further work are included in the last chapter.

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CHAPTER 2THEORETICAL CONSIDERATIONS

In this chapter various laser schemes that are relevant tothe present work are considered. In particular the N2-CO2 laser isdiscussed, with the laser operating in gas-dvnamic, thermal mixing, andsimilar modes. The effects of additives are considered, with specialinterest being given to the effects produced by helium and water vapor.

2.1 N2 -CO2 Molecular Laser Scheme

Figure I is a simplified energy level diagram of the CO., andN2 vibrational levels most important in laser operation. CO2 as theworking gas has three vibrational modes corresponding to symmetr:.cstretch, asymmetric stretch, and bending modes. The laser transitiontakes place between the first excited asymmetric level (001) -r d thefirst excited symmetric level (1000). Characteristic radiatior isemitted near 10.6 microns in the infrared spectrum. The exact fre-quencies that are emitted depend upon which sublevels (i.e., rotationalenergy levels within the vibrational level) are lasing. Transitionsfollow the selection rule of AJ - 1, where J is the total angularmomentum or rotational quantum number. Generally the line with maxi-mum gain is at or near the P20 transition between the 00*l-I0*0 levels.

Nitrogen serves as the carrier or pumping gas. Since N2 is ahomonuclear diatomic molecule, it has no dipole moment, and relaxation

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ENERGY

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of an excited state rannot occur by dipole radiation. The metastableN2 therefore has its lifetime controlled by de-activation throughcollisions with other molecules and the container wall. Furthermore,the first vibrational level (v = 1) of N2 is very near the upper lasinglevel of CO2. The two levels differ in energy only by 18 cm as com-pared to an average thermal erergy at room temperature of over-i200 cm In other words, IAE'


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but that too much water would virtually stop the lasing action.Moeller and Rigden [32] found that a N2 -CO2 -He mixture was far superiorto an N2 -CO2 system. Although Howe [30] had observed that H2 added topure CO2 had no marked effect, Rosenberger [33] obtained improved per-formance with a N2-CO2-H2 system over a N2-CO2 system.

These and certain other additives all texid to produce improvedlaser performance by creating a greater population inversion among theupper and lower CO2 lasing levels. The methods of accomplishing thisinversion are not the same for each gas and are not fully understood.

it is believed that helium contributes to the inversionin at least three ways: lowering the rotational temperature of theCO2 and shifting the gain to the lower P branch transitions, increasingthe cross relaxation rate so that the populations of adjacent J levelsfeed the lasing level, and shortening the vibrational lifetime of thelower lasing level. All three factors tend to produce a largerinversion density.

Witteman [34] has noted that the lower vibrational lasinglevel (1000) of CO2 is nearly in resonance with the vibrational levelassociated with the angle between the O-H bonds in a water molecule.Because of this, only a few collisions with a water molecule are neededto de-excite the CO2 lower lasing level. In turn the vibrationallyexcited water molecule relaxes to translational motion when collidingwith other water molecules. Perhaps the dominant effect of H20 is todepopulate the lower lasing level. It also has some effect on the upperCO2 or N2 levels, since too much water decreases lasing action. (Mostadditives do have an optimum mixing ratio, which if exceeded eitherdecrease or do not change the laser performance.) H2 has an effect

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similar to that of H2 0, perhaps due to the water created in the system,as well as other influences.

In a system where power is extracted over a certain distancealong the flow, additives may prove to be a hindrance. For example,in the SCL, where power is extracted over a limited portion of the flow,helium decreases the total output power which can be obtained. Thiseffect will be discussed later.

2.3 Gas-Dynamic Laser Scheme and Its AdvantagesSeveral factors must be considered in deciding which laser

:vstem is best suited for a particular purpose. A laser of high effi-ciency may be desired, but the same system may have an output level thatis too low. Power is not the only important factor, since most sys-tems may be made larger to produce more power. An important parametermay be energy (or power) per cavity length or volume. In the case ofa flowing system, a useful figure of merit may be output power permass flow rate. Cost, simplicity, reliability, size, mobility, andpower requirements must also be considered.

A gas-dynamic laser shows promise of meeting several of thesecriteria perhaps better than other systems. Figure 2 is a diagram of aGDL CO2 system under typical conditions. The top portion is a cross-section of the plenum, nozzle, and cavity sections. Upstream or

stagnation conditions are denoted by the zero subscript. An asteriskdenotes conditions at the nozzle throat. Conditions downstream in thecavity region are indicated by the symbol for infinity. The lowerportion of the figure is an idealized plot of temperatures as a func-tion of distance in the flow direction.

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In the upstream region the gas mixture is in thermal equili-briuu at a high temperature and pressure. The gas is then rapidlyexpanded through a supersonic nozzle. A high Mach number and lowpressure results. Temperatures associated with translational androtational energies rapidly drop to the free stream temperature. Thecoo.-ati-n of the 1000 and 0100 levels of CO2 also drop quite rapidly tothe freestrea, value. The upper lasing level (0001) of CO2 and theexcited- 2 levels have a slower vibrational relaxation rate and theteaverature associated with the upper lasing level remains nearly ashigh as before exnpansion. That is, the population of this level remainsabout the same. The level is said to be "frozen" at this temperatureand dc-nsity. Since the population of a level is proportional to itsvibgatrional temperature, an inversion exists when the difference in thevibrational temperatures is large enough. Once achieved, a populationinversion may exist for a considerable distance downstream. Dependingupon the configuration and flow parameters, inversion may exist for adistance of less than a centimeter from the throat to perhaps a meterdownstream. A portion or all of this region way be equipped withmirrors and a metnud of coupling out part of the beam.

An important feature of the GDL is the fact that the gasmixture is in equilibrium at the plenum. It does not matter how thesecor,.itions were formed. An arc heater, chemical combustion, or othersource of heat may be utilized. Ideally, this 6ource would be inde-pendent of electrical power. Then too, the gaL low may be exhaustedeither into a vacuum chamber or into the atmosphere when the device isfitted with a proper diffusor. Continuous wave powc up to 60 kilo-watts nave been reported from these devices [21, 22). The GDL shows

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promise of being quite useful as a source of high power laser energy.

2.4 Thermal Mixing SchemeFor proper gas-dynamic operation, the nozzle configuration

must be fairly well optimized. Gas properties such as mixture ratios,pressures, and temperatures must also lie within a certain range. Theupper level of N2 must be frozen, but the lower CO2 level must beallowed to relax during expansion.

Inversion can be more readily obtained if some of theserequirements are removed. For example, if cold CO is used, then the2lower CO2 level need not be relaxed and it is necessary only to freezethe upper level of N The CO2 molecules will be selectively excited tothe upper level as previously mentioned. This can be accomplished ifthe CO2 is injected into the hot N2 at a point further downstream. If*cold CO2 is-injected downstream of the throat, it will be mixing withthe N2 supersonically. The CO2 may also be mixed immediately upstreamof the throat in the near-sonic region.

Such thermal mixing schemes all have as their basic objectiveto produce maximum inversion of the CO2 states. The upper CO2 levelwill become most populated by mixing upstream as far as possible. Thelower lasing level will remain deplenished only if mixing occurs verynear to the optical cavity. Therefore, to produce optimum inversion,the point at which mixing occurs must be a compromise between these twoincompatible conditions.

In the work reported here, both GDL and supersonic mixingschemes were employed. A -very short period of time was devoted to thenear-sonic mixing scheme. Promising results were obtained, but the

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aerodynamics of the nozzle were such that the results cannot be com-pared to the other modes of operation.

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CHAPPTER 3.XPE..IYONTAL FACILITY

The SCL (Figure 3) utilizes an optical cavity with insidedimensione approximetely 1.5 x 6 x 12 inches. Through it the componentgases of the carbon dioxide laser flow at hypersonic velocities.Hot nitrogen, obtained from an electric arc plasma generator, is

S1swepthrough a 1.5 x 2 x 12 inch nozzle into the cavity. Carbondioxide and other gases are either premixed through sonic orifices ormixed upon injection into the plenum upstream from the nozzle. Injec-tion ports enable additional gases to be injected into the flow down-stream of the nozzle throat. After being used in the optical cavity

the gases are exhausted into a 1500 cubic foot vacuum tank. Power forthe arc generator is obtained from two dc generators, each rated at575 volts and 2250 amperes.

Power, control, and vacuum components for the SCL areobtained by utilizing portions of the Army Missile Command 8000 kilo-watt Plasma Facility [35, 36). This facility is a general-purpose lowtemperature plasma device (T 5000* to 10,0000 K) and has been operatedprimarily as a hyperthermal blowdown wind tunnel for reentry simulationof IRBM's and ICBM's in certain altitude regions. Two of thefacility's six dc generators, one arc identical to those of the testfacility and a portion of the cont -1 system and vacuum chamber havebeen adapted for the SCL work.

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Ia3.1 Power Supply, Control System, and Arc

Power is obtained froa the local high voltage three phase acdistribution syste. 44,000 volts of three phase ac is stepped downby a local substation to 4160 volts of three phase ac (nominal 8000 kw).This voltage is distributed to three synchronous drive motors with anominal power rating of 1500 horsepower. Each motor drives two

1 575 volt, 2250 ampere dc generators on a common shaft (Figure 4). Forthe SCL, two generators are wired in series (Figure 5) to double therange of the applied voltage to the arc. This allows required voltageand current levels to be reacbed without pushing one generator near itslimits. The typical operating level is near 1 megawatt of power to thearc but lower and higher power levels have been used.rl

The power installation also includes rotary and solid-stateexciters for the field windings of the motors and generators. Thegenerators are driven at a constant speed and the output is regulatedby varying the field excitation. Six 65 kilowatt, three phase, phase-controlled thyristor amplifiers regulate the field currents of thegenerators (Figure 6). Control and digital multiplexing components,together with overload protection complete the power supply. Allfunctions are monitored and operated from a single console (Figure 7).The power control system may be used in a feedback circuit with theplenum pressure controlling the arc current, or with an external pro-grammed reference current providing feedback, or with no feedback.(So far the SCL has been operated with no feedback from the laserproper, although feedback may be included if desired.)

The electric arc plasma generator, designed by Mayo, Wallio,and Wells [361 is shown in Figure 8. When operated on the SCL, the

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arc is used to heat N2 instead of air. The generator is so designedthat the strong solenoidal magnetic field (0.07 v/amp) causes the arcto rotate rapidly around the anode in the gap. This design tends to

reduce overheating and evaporation of the electrode surfaces, increasethe voltage of the arc for a given current, improve electricalstability, atd decrease electrical noise and pressure fluctuation. Th earc is ignited by a spark plug system [371. The spark plug arc pro-duces a small tongue of plasma which initiates the main arc in theelectrode gap. Water at 300 to 500 psig is used to cool the arc and ashunt fr measuring current. Conductors running from the dc generatorsto the arc heater are made of 500-MCM copper cables. Suitable con-tactors (Square D Class 8502, Type GG 1) are used to switch the mainpower leads to the arc.

3.2 Gas Supply and Vacuum FacilityFigure 9 is a diagram of the SCL's gas flow system. All

hand regulators and manual valves are preset before the laser is oper-

ated. Air actuated valves and solenoid valves may be controlled manuallyat the SCL for test purposes. During an actual power run, however,they are controlled at the main power supply console (Figure 7). Theentire sy-tem is operated remotely because of safety precautions.

Only a portion of the N2 supply is passed through the arc.The rest of the N2 and other gases are injected into the plenum as

22iluents. The diluents are heated by the hot N2 from the arc and this

produces the equilibrium conditions in the plenum prior to expansionthrough the nozzle. The mass flow rates of all the gases are con-trolled by the use of sonic orifices. When the'gases are premixed

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before injection into the plenum as diluents for GDL type..operation,the system is said to be operating in Mode I. If cold CO2 is injecteddownstream of the nozzle throat for supersonic mixing, then the systemis said to be operating in Mode K.

Water may also be injected into the plenum through one ofthe plenum lines while operating in cither mode. The water flow rateis controlled by setting the nitrogen back pressure and upon reachingthe plenum the liquid state is changed to v..por and mixed with theother cowponent gases.

N2 , C02 , He, and H20 are the gases most often used, but anyone or several gases may be eliminated from the system at any time, and

additional additives may be included. The SCL lends itself quitereadily to parametric studies of the effects of additives on the opera-tion of gas dynamic or similar laser systems.

Again referring to Figure 9, standard Lomponents for gashandling are employed. The dome regulators are manufactured by GroveValve and Regulator Company, and Models 94 W and BX 205 N2 are used.The solenoid valves are Marotta Models MV 74 and MV 140. The airvalves are Worthington Controls Model 3620. For the hand regulators,Tescom multiple range Flo-tron regulators, Model 26-1025-34 are used.Various manual valves are user depending upon required flow rates.

Figure 9 shows the SCL flow chart for Mode I operation. Inthis mode, N2 , C02 , and He are mixed in a cross fitted with sonicorifices for each component gas. The sizes of the orifices togetherwith gas pressures determine the flow rate of each gas. (This isdiscussed furthe.. in Chapter 4.) The premixeti gases are then fed intothe plenum through six lines or five lines with H20 being injected

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through the sixth line. In iode II operation the CO2 line is dis-connected from the cross and connected to the downstream side of thenozzle.

Commercial grade gases are used with He and CO2 supplied fromstandard size gas bottles. Because of the much larger requirements forN2 this gas is obtained from a special tank (Figure 10).

The vacuum facility includes a 13,550 cubic foot vacuumchamber (Figure 11 . However, only a portion, 1550 cubc feet, of thevacuun chamber is normally used for the SCL. Pumping is obtained by a300 cfm Stokes Microvac pump, Model 412 H-1O, with a Roots boosterpump, type RGS-SP-AVM, size 615. A Texas Instruments fused quartz pre-cision pressure gauge, Model 141, monitors the vacuum tank pressure.

3.3 Laser ComponentsIn addition to the arc and vacuum system, the SCL has three

primary parts: plenum, nozzle, and cavity (Figures 3 and 12). Transi-tion sections from the plenum to the nozzle and from the cavity to thevacuum tank valve enable the three sections to be coupled together andconnected to the arc and the vacuum tank. Thc l1ardware is made ofstainless steel, fitted with appropriate gaskets for bolting together.

3.3.1 The PlenumLocated between the arc and the nozzle, the plenum (Figures

13 and 14) seives as a mixing chamber for the component gases. Hot N2from the arc and the premixed cold gases are combined and allowed to

reach thermodynamic equilibrium in this section before expansion throughthe nozzle and into the cavity. H20 may also be injected into thesystem at this point. Thermocouples and pressure transducers measure

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U)I ze

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00

0'-I

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II27

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E-4

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z

0.

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28


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zw o

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z 0

-I-

zz

zza.U

-C-ILN

29N


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GAS INLET

I~II

FROM ARC TO NOZZLEIIi

II I I

FIGURE 13. DIAGRAM OF PLENUM, VIEWED FROM ABOVE

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NN010

0zI--

0 uz0H I--

w >z-J

CIL o

CL,

r44

0

IL

31


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the temperature and pressure just before the gas mixture reaches thenozzle.

3.3.2 The NozzleFigures 15 and 16 show the nozzle. The actual working area

of the nozzle is approximately 12 inches wide and 1.5 inches high, thenozzle being 2 inches long. The distance from the throat to the nozzleexit is 1.26 inches. The throat height is 0.032 inch. As seen in thediagrams there arc three narrow slits for gas flow. Six rows of0.030-inch holes, two rows in each slit, are located downstream of thethroat and enable CO to be injected into the system for Mode II opera-2tion. Each of the six rows contains sixty holes along the width of thenozzle. The CO2 enters the nozzle through two ports in the top cap.Other nozzle configurations have been used, but only this nozzle wasemployed in the work reported here. Information for the nozzle contourwas obtained from the AVCO-Everett Research Laboratory.

3.3.3 The CavityFigures 12 and 17 show the optical cavity. Figure 12 views

the cavity along the flow path with the plenum and nozzle in place.Figure 17 views the cavity perpendicular to the flow, along the opticalpath. The distance between the mirrors is approximately 12 inches,with the mirrors forming part of the flow channel. The mirrors are goldplated optically flat pieces of copper, aluminum, or stainless steel.181 holes (0.039 inch in diameter) in one of the mirrors enablepower to be extracted from the cavity at a distance of more than5 inches along the flow (Figure 18). The upstream edge of the nirror is2.3 inches from the throat, while the first output holes are 2.9 inches

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IZ0 I

ILlI.j -jNN0

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ENTRANCE EXITi

/I

L ii

CO2 INJECTION(MODE II)

FIGURE 16. DIAGRAM OF NOZZLE, CUTAWAY VIEW34


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0 0

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LU 6 LU

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|from the throat. NaCi windows are mounted at the sides of the cavity,approximately 2 inches behind each mirror. Each window serves as avacuum seal, and the beam is extracted through one of these windows.Pressure may be monitored through a port in the top of the cavity.Various dry gases such as A or N2 may be used to purge the windows toprotect them from moisture or harmful gases in the flow.

3.4 The Assembled Special Configuration LaserThis section presents photographs of the SCL, taken in

partially and completely assembled views. Figures 19 and 20 view theSCL from opposite sides and ends. Starting from the upstream end,the center electrode and a special water flow connector of the plasmagenerator are shown. Next the generator with its field winding andhousing are shown, supported on the generator base. This is followedby the plenum, nozzle, cavity, and end transition sections. On eachside of the cavity are the mirrors and NaCi windows. The output windowis the largest. The other window is suitable for gain measurementswith the rnirrors removed and for alignment purposes when the mirrorsare used.

Figure 21 views the SCL from above and opposite the outputside of the cavity. The large vacuum tank may be partially seen on theright. Vacuum pumps are at the top of the picture. Several of the Heand CO2 supply bottles are seen at the bottom. Most of the other com-ponents of the SCL may also be seen. Figure 22 is the same, exceptviewed from the output side of the cavity.

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E-4

P41~z

4 4o

38x


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FIGURE 21. PHOTOGPkPH OF SPECIAL CONFIGURATION LASER, VIEW 340


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II

i Ti

FIGURE 22. PHOTOGRAPH OF SPECIAL CONFIGURATION LASER, VIEW 441

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CHAPTER 4INSTRUMENTATION AND CALIBRATION

This chapter discusses the various parameters that aremeasured when the laser is operated. The associated equipment is alsocovered. Calibration of the various instruments and reduction of datais discussed. In general, the most important parameters are input andoutput powers, gas flow rates, and temperatures of the gases. Each ofthese are considered in this section.

4.1 Measurement of Gas Flow RatesPressure transducers in the gas lines (Figure 9) and the SCL

itself monitor the component gases, both individually and aftermixing. For the most part these are slide wire transducers, Model842-A, and manufactured by Sparton Southwest. In operation the N2pressure to the arc and the plenum is recorded as well as the CO2 andHe pressures in their respective lines. The plenum pressure is moni-tored immediately upstream from the nozzle. The N2 back pressure onthe H2 0 reservoir is also measured, since it controls the flow rateof the water.

A 5 volt dc signal is applied across each slide wire trans-

ducer. Acting like a potentiometer, the transducers emit a 0 to 5volt signal, corresponding to the percentage of the maximum ratedpressure that is applied to the transducer. This signal is amplified

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If the ratio of the cross-sectional areas S /S* is 25 or0more, then the mass flow rate is given [38] by

k+l]~kg M 2 ~-S Po[T 0cR \k +41

Here C is a coefficient of velocity and is near unity. M is th evm)lecular weight, gc is a gravitational constant numerically equal tothe acceleration of gravity, and R is the gas constant per mole.

This equation was used to determine orifice sizes S* fo r

each gas, given the required flow rates and upstream pressures. Withthe orifices in place, more accurate plots of P versus i were obtained0experimentally. With the pumps shut off, gases were injected throughthe orifices and into the vacuum tank for a measured length of time. Aknowledge of the tank volume and tank pressure before and after th egas flow allowed the total mass flow rates to be determined.

In equation form:in M --RT At

were V is the tank volume. (This equation is derived by differentiat-ing the-ideal gas law with respect to time, t.) Then a graph of m as afunction of the upstream pressure P may be plotted. During an actuallaser run the transducer pressure readings of P are recorded and th eomass flow rates are subsequently determined from the experimentallydetelmined graph of iii versus P0

The tank volume was again measured by connecting a gascylinder of known volume to the tank and allowing some gas to flow intothe tank. if the cylinder and tank pressures are recorded before andafter the gas f 'r, the number of moles, n = m/M, lost by the cylinder

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being measured in the vacuum tank, the water is caught and measuredwhile still a liquid.

4.1.2 Calculation of Mass and Molar FractionsIt is also convenient to know the percentage of each gas in

the flow. Gas percentages or fractions may be given in either of twoways: mass fractions or molar fractions.

By definition, the mass fraction is given bym. m.nn

Here m. is the mass of the ith gas, and mt is the total mass of all thegases. In this case mass flow rates may be substituted for the masses,and the mass fraction easily determined.

In addition to mass fractions, gas components are often givenin terms of molar fractions. By definition, the molar fraction isgiven by

Nif, - N

nHere Ni is the number of moles, mi./i, M. is the molecular weight ofththe i gas, and the index n is summed over all constituent gases. Toobtain molar fractions from mass fractions:

m. m.N. M. m1 1 t

nM I

or

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fm.]f. = I3- fmM. fin~ nn

Conversely, to obtain mass fractions from molar fractions, solve thelast equation for 1.m.

1f

f =f M y nm

f = ffMimi n

f.jX M

f f M.[ii

M ii f Mnnn

fm. 1sed.

Therefore, given the mass flow rates and molecular weights, either massor molar fractions may be determined. Gas percentages may then be com-pared to flow percentages given in the literature, regardless of whichform is used.

i 4.2 Plenum Temperatures T and T

An important parameter in gas-dynamic studies is the gastemperature T, measureO just prior to expansion through the nozzle.In the SCL there is a port for this purpose located on top of theplenum hardware, immediately upstream from the center of the nozzle.Standard platinum versus platinum 10 percent rhodium thermocouples are

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used to mcnitor the gas temperature. The output signal of a fewmillivolts in magnitude is amplified by a Preston 8300 amplifier andthen displayed on the Brush recorder. Conversion charts [39] thatrelate output voltages to temperatures are later used to determine thetemperature of the gas flow.

The gas temperatures depend upon how much power is given tothe plasma arc and also the gas flow rate. Recorded temperaturesranged from 13000 to 1700*K for the input powers and flow rates thatwere used.

One problem in experimentation was overheating of the hardwareafter a run. Conduction from the arc soon heated the plenum, asdid the hot gas in the plenum. This heat was conducted downstream intothe nozzle and cavity. (The problem will be further discussed insection 4.5.) It was found necessary to monitor the temperature of theplenum hardware and allow time between runs for the hardware to cool.

Designated as Twall' this temperature was measured by a Temco PortablePyrometer and Millivoltmeter, Model PM-1K17. Essentially this is athermocouple in thermal contact with the top oi the expansion or transi-tion section of the plenum.

Readings were taken from the millivoltmeter before and aftereach run. At various times the effect of purposefully overheating thehardware was studied; the results of these tests will be discussedin Chapter 6.

4.3 Input PowerPower delivered to the arr is usually on the order of 106

watts. Typical current and voltage levels are 2600 amperes at 390 volts.

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However, the input power has been varied from 0.6 to 1.2 megawatts.Although each of the six generators is capable of producing this power,it is easier to obtain desired power levels over a wide ran.e of gasflows with two generators in series driving one arc.

Again referring to Figure 5, the arc voltage is measured bya shunt across the arc and one contactor. The current is measured

between the ground side of the arc and the field winding by using awater-cooled high current shunt in series. The voltage drop across this

element determines the arc current. Voltage and current levels aredisplayed on a Sanborn Model 850 strip recorder.

From the differential form of rhe heat equation (powerhcAT), an estimated 20 to 25 percent of the input energy is coupled

into the gas.

4.4 Output PowerThe 10.6 micron laser output was recorded in three ways. For

a quantitative power measurement, a thermopile detector system wa sused. To determine which section of the cavity is lasing, i.e., atwhat distance from the throat .asing occurs, another method must beused. This type of information was obtained from patterns burned intothermofay paper or polyurethane foam packing material.

4.4.1 Thermopile Detector SystemPower output measurements (in watts) were obtained by using a

Coherent Radiation Laboratories Model 203 Power Meter. Radiation wasgathered by a 6-inch concave mirror with a 24-inch focal length. Thebeam was then focused ino the array of thermocouples in the detector.Instead of using the manufacturer'. amplifler and meter, a Preston 8300

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1ifter fed the signal into the strip recorder.

4.4.2 Laser Burn PatternsIt was desired to know at whar pugitioL. In the cav~ty -hat

lasing occurs. Depending upon the gas flow mixtures and the effeits ofadditives, the output beam may vary in several ways as the di3tancefrom the throat varies. For some gas mixtures mosc cf the lasing occurs

- -'..-treom side of the mirror, with very little radi;tion emittedfurtier downstream. In oL..z. , m:.: t fai


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Additional burn pattvrns were produced with a co~wentional100 watt laser, in order to ceipare energy and power levels wi:h burndapths ard volumes. Figure 23 plots the total laser energy as a func-tion of the volzee of the burn cavities. The threshold for damage ofthe polyurethane may be noted by the positive y-intercept. Althoughnot shown, iL is also possible to plot the energy density as a funcrion

* of the depth of the burt,. The result is a graph very similar to theone in Figure 23 .

Quite a bit of experimental data was taken in pairs, theLt.ermopie detector used in ,% e ru" a-t: &he 3ol',.-thane j .% the Othcr,all other parau .. e... i*--.Lble to cor.-elate thesize and shape of a burn pattern with a pow-r reading. Rowever, inoperation the output power increases slightly foi 4 or 5 seconds an dthen drops because of heating of the hardware. Power readings given ipthis report are the maximum obtained during the run. It is not possibleto correlate this maximum power with the burn volumes. Instead aneffective or average power must be compared to the corresponding burnvolume. Although not shown, this can be obtained by integrating thearea under the curve of the thermopile output signal on the striprecorder.

4.5 Problems EncounteredWith virtually any new or different piece of equipment, the

early models tend to possess inherent design or construction defectsand inconveniences. This is especially true with state of the artexperimental equipment whose optimized theoretical design may be worthless than last week's newspaper by tomorrow. The SCL is no

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0

U9U,

0 ON

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S3,inor 'A9H3N3 1IVJOJ.

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ex:eption. Although it did produce valid, worthwhile results, it doeshave some shortc- o

To 'i. .4sadvantages kept the ZCL from operatingas well as desli First, the hardware was designed to be used withsix arc heaters. izy one arc was made available by the PlasmaFacility. So opt.L.Am pressures, temperatures, and gas flow rates couldnot be reacthad, aupecially for the GDL or Mode I experiments. Second,the plenim and other hardware overheated after a few seconds and mustthen be allowed to cool before it was operated again.

Since thcse difficulties did exist, a new system wasdesigned and is now being assembled. The new system will correct theseand other minor -c -ms. This has allowed only a minimum amount oftime to be stent on the SCL, as work had to be stopped prematurely tomfe sei equipment available to its successor. As a consequence,much less information was obtained than was desired.

The remainder of this report deals with the results obtainedduring the period the SCL was available for experimentation.

5

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CHAPTER 5EXPERIMENTAL RESULTS OF GAS-DYNAMIC LASER (MODE I) OPERATION

As mentioned earlier, desired plenum pressures and tempera-tures could not be reached with the single arc system. Typically thepressure P should be 15 to 20 atmospheres at a temperature of 14000to 2000*K. The SCL as described in Chapter 3 could obtain 4 atmosphteresof stagnation pressure with temperatures up to 1700'K but not at thesame time. Typically P was near 2 or 3 atmospheres and T was near1400*K. A limited amount of data was taken in the gas--dynamic mode,since most of the available time was spent operating in the supersonicmixing mode. Several conclusions may be drawn, however, and are

presented below.5.1 Maximum Power Versus N2 Flow Rate

Figure 24 is a graph of the output power as a function ofthe nitrogen flow rate at constant CO2 and H 0 flow rates. The outputincreases up to a point, at whicb more N2 slightly decreases the beamintensity. Maximum power obtained was only 15 watts and the ordinate isnormalized to this value. Since N2 alone is heated in the arc, thereare lower limits at which the N2 flow rate may be set and still keep asteady plasma arc. For this reason no data is shown for lower N2 gasflows.

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5,2 ?,: er Vers-us H20 Flow Rate

For this data patterns were burned into thermofax paper(Figure 25). N2 and CO2 flow rates werp held constant as indicated andthe molar fraction of 1120 was varied from 1.8 to 5.8 percent. No burnwas obtained for the lowest percentage of water. The other data isshown in the figure. The heavy lines on each side indicate the posi-tions of the first and last r,- 3f output coupling holes. These areseparated by a distance of about 5.13 inches.

As seen in the photograph, the cross-sectional area of thebeam' increases with the output power. The peak intensity occursslightly upstream of the center of the optical cavity. No power wasextracted from either end of the cavity. Maximum output was with3 percent water, with additional water decreasing the beam intensity.

5.3 Summary of Mode I Operation

From the two previous sections and other data not presentedhere, several observations may be made concerning the SCL operating inthe gas-dynamic mode with margiiial plenum conditions. Mixture ratiosaffect the output beam quite strongly, especially the ratio of watervapor. No output was obtained without using water, but maximum inten-sity occurred with only 3 percent water. N2 and CO2 composed 86-and 11-percent respectively, of the gas flow. Operating under themarginal conditions described above, maximIum Dower was only 15 watts.(A fev earlier atttp,, were also made using helium as an additive, bu tno output powet was obtained.)

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AH2O(FHo

2.2 gm se(3%)

3.3 girvdsec(4.4%)

4.0 gm/sec(5.3%)

4.4 gm/sec(5.8%)

I N2 100 gm/se-c' rMC 2 =19 gm seFIGURE 25. PHOTOGRAPH OF BURN PATTERNS, H20 VARIATION (MODE I)

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U ,U

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________ ________ 0O

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mirrors may be changing as the cavity heats, oi the hot walls may beaffecting the rate at which lower levels are depopulated. In any case,the problem was eliminated by separating each 5-second run by a periodof two or more hours. During this time the hardware was allowed to coolto near room temperature.

6.2 Power Variation Due to the N2 Flow Rate

Figure 27 is a graph of normalized output power as a functionof the N2 flow rate. In this and succeeding data, the power outputhas been normalized to the maximum value obtained with supersonic mix-ing. Two curves are shown, corresponding to two different CO massflow rates. In general, output increases kintil a point is reachedwhere further addition of N2 has little or no effect. If extended tolower N2 flow levels, the curves would drop rather sharply.

Figure 28 is a spatial representation of the oeam patternobtained by burning polyurethane. The horizontal axis plots the dis-tance from the firit row of output coupling holes in the mirror. Th evertical axis represents the depth of the burn and is a function ofthe energy deposited by the beam. This coordinate then gives arepresentation of the intensity of the beam. The axis out of the pagerepresents different N2 flow rates.

Corresponding to the previous figure, not much change inpower is observed while varying the N2 flow. The most important featureis the dominant peak in the curve near the upstream end of the cavity orat low values of z. Note that no helium was used in this set of data.Thiis feature will be further discussed in section 6.4.

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0

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1-4

-44

zz

62-


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6.3 Power Variation Due to the CO2 Flow Rate

Figure 29 is a graph of normalized output power as a functionof the CO2 flow rate. Two curves are given, one with and one withouthelium. In addition the mass flow rates of N2 are slightly different

I for the two curves. Over the regions covered, the output power v~riesalmost linearly with the CO2 flow rate. Note also the run with heliumproduces lower power.

Figures 30 and 31 are spatial representations of burnpatterns, taken while varying the CO2 flow. The N2 flow rates are notthe same for the two figures but the important difference is thatFigure 30 was obtained with helium, while Figure 31 was obtained with-

.out helium. Note that with helium there is not too large a peak atlow values ofZhile there is a peak in Figure 31, at leasi forhigher CO2 flows. These peaks tend to occur in runs not containinghelium and at higher CO2 flow rates.

6.4 Power Variation Due to the He Flow RateFigure 32 is a graph of the normalized output power as a

function of the helium flow rate. Note that the power decreases forine-easing gas flow, with a sharper drop at the higher He flow rate.Maximum power occurs when no helium is used.

Then compare this information with that given in Figure 33.Note here that increased amounts of helium decreases the initial peakand tends to flatten the burn pattern. The total burn volume issmaller but is deeper for higher values of Z than for the data with nohelium. (Although the lower He flow rates produce a longer burnpattern, this may be due to edge effects and the mirror configuration.)

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0

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_ _ _ _ _ N~ EIfl

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0N

From these figures and those in the two previous sections, itmay be concluded that hel um does not increase total. power from the SCLin Mode II operation. However, it does tend to produce a smoother,more uniform beam over the cavity length. It is quite likely that witha longer cavity the lasing conditions might remain the same for somedistance. In this case the addition of helium might be advantageous.

6.5 Power Variation Due to the H2 0 Flow Rate

A limited amount of data was taken using water to depopulatethe lower levels. In the supersonic mixing mode, there was not asdrastic a change as in the gas-dynamic mode. Figure 34 shows the burnpattern changes due to H2 0 for two sets of data. In both graphs, theaddition of water lengthens the burn pattern and increases the totalintensity over all but the upstream edge of the mirror. In that regignthe intensity is not appreciably affected.

6.6 Additional Factors Affecting PerformanceSeveral factors have not been considered so far. Certainly

the length of time the laser is operated will affect the amount ofenergy obtained. For the data presented here, run times have been heldconstant to within 0.1 second in order to minimize this effect. Asmentioned earlier, power readings increase for several seconds of oper-ation and then slightly drop. (This point varies with the gas flowrates and associated temperature.) Therefore, much of the data pre-sented might be different if the hardware remained at a constant tem-perature.

It was found that varying the electrical input power producesan effect that would be expected. With lower input power, lower

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U

thermal laser excitation by supersonic mixing in a gas dynamic nozzle

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