AD/A-002 010

EXPERIMENTAL STUDY 'jF A LASER SUSTAINEDAIR PLASMA

Dennis R. Keefer, et al

Ballistic Research LaboratoriesAberdeen Proving Ground, Maryland

October 1974 I

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4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

IXPERIMENTAL STUDY OF A LASER SUSTAINED AIR PLASMA Final

6 PERFORMING ORG. REPORT NUMBER

7. AUTHOR(e) 0. CONTL. CT OR GRANT NUMBER(e)

)ennis R. Keefer, Bruce B. Henriksen, William F.Brae rman

E ERFORMING ORGANIZATION NAME AND ADDRESS 10 rROGRAM ELLMENT. PROJECT. TASKAREA A WORK UNIT NUMBERS

U.S. Army Ballistic Research Laboratories 61101\Aberdeen Proving Ground, Maryland 21005 IT161101A91A

00 041AJII CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

U.S. Army Nia'ceriel Command OCTOBER 19745001 Eisenhower Avenue 13. NUMBER OF PAGES

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II. SUPPLEMENTARY NOTES

19 KEY WORDS (Continue on re tere side if neceosary and Identify by block number)

Opf ical plasmatronLaser supported absorption wave

20. ANSTRACT (Continue on r-eree, ald. if necessary ad Identify by block number) (i dk)An experiment was performed to determine the temperature distribution

withip a stationary plasma in atmospheric air, produced by a continuously oper-ating CO,, laser. A peak temperature of 17,000°K was found to exist at I point1.1 cm ahead of the focal point. The radiation surrounding the hot plasma corewas found to consist, primarily, of the first negative system of nitrogen.

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TABLE OF CONTENTS

Page

LIS'T OF ILLUSTRATIONS .

1. INTRODUCTION ..... ..... ........................ 7

I I. DESCRIPTION OF EXPERIMIEN' . ................ 7

Ill. EXPERIMENTAL RESULTS ..... ...................... 9

I. DISCUSSION OF RLSUIS ..... ................... is

PISTRIBUTInN LISI. .... ......................... .17

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LIST OF ILLUSTRATIONS

Figure Page

1. Schematic of the Experimental Arrangement . . . . . .. 8

2. Photograph of the Plasma Taken Through a Narrow BandpassFilter Centered at 5125 A . ............. . . 10

3. Continuum Emission of Air at Atmospheric Pressure at512s A . . . . . . . . . . . . . . . . . . . . . . . .. . 12

4. Temperature Isotherms for the Plasma as Determined fromthe Continuum Emission ...... ................. ... 13

5. Compariso)n of Radial Temperature Profiles Determined byContinuum and Discrete Line Emission ..... .......... 14

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I. INTRODUCTION IExperiments with high power, continuously operating (CW) carbon

dioxide lasers have shown that a plasma is often formed.1 This plasmacan be formed by interaction of the beam with a solid target or by ini-tiation with a spark gap. The plasma either propagates toward the las.ver,in the direction of the laser, or becomes stationary, depending on thedegree to which the beam is focused and the output power of the laser.When formed, the plasma absorbs a substantial portion of the laserpower, significantly reducing the power transmitted throagh the plasma.

An experiment has been peiformed to determine, by spectroscopictechniques, the temperature profiles within a stationary laser-maintainedplasma. The plasma is produced by a carbon dioxide laser naving a nomi-nal continuous power of 6 kW.

The experiment is described in Section II. The results are presentedin Section III and discussed in Section IV.

II. DESCRIPTION OF EXPERIMENT

The plasma was maintained in ambient laboratory air by the laserbeam which had been brought to a focus along a vertical axis by a saltlens of 15-cm focal length. The laser was a flowing, electric dischargecarbon dioxide laser operating in the TEMt mode and had a nominal out-

put power of 6 kW. The plasma was initiad near the focal volume by aspark gap which was removed after the plasma ignited. The laser andplasma initiation techniques were essentially those described by Smithand Fowler 1 except that the beam was focused along a vertical axis toinsure the axial symmetry of the plasma in the presence of the thermallyinduced free-convection flow. This synm.!try was essential to allow forAbel inversion of the optical spectroscopic uata.

Spectroscopic data were obtained by foc using the image of the plasma,rotated through 900, on the slit of a 1-meter McPherson spectrograph.Data were recorded on Kodak Type 103-F glass spectioscopic plates. Along rectangular field stop was used with the external optiL:, to limitthe field of view to a narrow slit perpendicular to the plasma axis. Atungsten ribbon filament lamp, calibrated by Eppley Laboratories, wasused to provide a calibration source. A schematic of the experimentalarrangement is shown in Figure 1.

1Smith, D.C. and Fowler, M.C. , "Ignition and Maintenance of a CW Plasmain \tmospheric-Pressure Air with CO 2 Laser Radiation," App]. Phys. Lett.,

1-p 500-502 (1973).

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To provide for the determination of the two-dimensional temperatureprofiles the plasma was photographed through a narrow band interferencefilter of 100 A bandpass centered at 5125 A. This wavelength was chosento coincide with a region where the continuum emission has minimum per-turbations due to atomic or ionic line radiation.

Ill. EXPERIMENTAL RESULTS

The plasma was initiated by the discharge across a spark gap placed,ear the focal point. When the plasma formed, the spark gap was removed.The visual appearance was that of a stationary, bright, arc-like regionabout one centimeter in diameter and two centimeters in length, surroundedby a luminous blue region of lower intensity about one centimeter inthickness. A plume of still lower intensity extended some thirty centi-meters above the bright region beyond the focal point anu moved underthe influence of the free-convective flow in a manner similar to that ofa candle flame. Visual observation through neutral density filters re-vealed a narrow elongated central core with a blunt upstream end (towardthe laser) which narrowed toward the downstream end.

Spectra of the stationary plasma were obtained in several selectedwavelength regions as well as spectral scans in the region from 2500 Ato 6000 A. The plates were pre-fogged to a density of approximately 0.6to place them in the linear region of the 11-D curve, and they were cali-brated by exposing them through a calibrated Kodak photographic steptablet to determine the value of y. Radial scans were made of selectedspectral regions of the plasma image using a Joyce-Loebl microdensitometer.The densitometer output was fed into an PAI hybrid computer where thedata were digitized ,and stored on paper tape for further computer process-ing.

In order to obtain the emission coefficient as a function of radiallocation, it was necessary to perform an Abel inversion of the radialscans.2 These Abel inversions were obtained with the aid of a digitalcomputer which performed a least squares spline fit to the data by usingthird degree polynomials. This made it possible to evaluate the Abelintegral in closed form.

A photograph of the plasma taken through the narrow bandpass filteris reproduced in Figure 2. The exposure was 1/200 sec at f/45 on KodakPlus-X film. Radial densitometer scans of this photograph were made atvarious axial locations and were Abel inverted to provide relative emis-sion coefficients for two spatial dimensions of the plasma.

2 Greim, Hl.R., Plasma Spectroscopy, Mc-Graw-Ilill (1964).

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Morris and Yos3 have computed the continuum emission of an airplasma in local thermodynamic equilibrium (LTE), including the contribu-tion of free-free and free-bound processes for both ions and neutralatoms. The emission at 5125 A for air at one atmosphere is shown inFigure 3. It is seen from the figure that the emission passes througha maximum at a temperature of approximately 16,2000 K. This feature ofthe emission proved useful in establishing an absolute value for thetemperature in the plasma. These maxima were associated with the16,200 0K isotherm in a variation of the Fowler-Milne technique which al-lowed the entire temperature field to be determined. The resulting two-dimensional isotherms are shown in Figure 4. The peak temperature ofapproximately 17,0000 K and the general shape of the isotherms are simi-lar to those determined by Fowler, et al, 4 using a two wavelength holo-graphic interferogram. Our results, however, show a relatively long andnarrow temperature region extending along the plasma axis in qualitativeagreement with visual observation.

To obtain an independent check of the temperatures predicted by thecontinuum meaasurement, radial scans of the 4935 A NI and 5045 A NIl linesof nitrogen were made at an axial location approximately 4mm from theleading edge of the plasma. Like the continuum, the emission coeffi-cient of the atomic line 4935 A NI passes through a maximum at a tem-perature of 15,500*K for air at one atmosphere. Thus, the Fowler-Milnetechnique could be used with this line. The ionic line 5045 A NIl, how-ever, does not pass through a maximum in che temperature range of inter-est and the temperature was determined from its absolute intensity. Thetransition probability of 4.18 x 107 sec -I was taken from Morris5 andthe values for the species population and partition function were thosegiven by Drellishak et al. 6 The temperature obtained from the 4935 ANI and 5045 A NII lines are compared with those obtained from the con-tinuum measurement in Figure S. The results are in good agreement exceptnear the axis where numerical errors involved in the Abel inversion aremost serious.

3 i orris, J.C. and Yos, J.M., "Radiation Studies of Arc Heated Plasmas,"ARL 71-0317, December 1971.

4Fowler, M.C., Smith, D.C., Brown, C.O. and Radlcy, R.J., Jr., "LaserSupported Absorption Waves," Final Report N921716-9, United AircraftResearch Laboratories, March 1974.

5Morris, J.C., Krey, R.U. and Garrison, R.L., "Radiation Studies of ArcA tHeated Nitrogen, Oxygen and Argon Plasmas," ARL 68-0103, May 1968.

6 Drellishak, K.S., Aeschliman, D.P. and Cambel, A.B., "Tables of Thermo-dynamic Properties of Argon, Nitrogen and Oxygen Plasmas," AEDC-TDR-64-12, Arnold Engineering Development Center, January 1964.

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Figure 3. Continuum Emission of Air at Atmospheric Pressureat 5125 A (Reference 3)

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Figure F. Comp~arison of Radial Tremiperature Profiles Determinedby Continuum, and Discrete Line Emission

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The spectral scans reveal that near the plasma axis the emissionis dominated by the continuum emission and discrete spectral lines, pri-marily due to the atoms and first ions of nitrogen. In the cooler reg-ions surrounding the core, the dominant radiation comes from the firstnegative system of the molecular ion N2 . In the outermost regions theCN violet bands at 3883 A appear. The CN is presumably formed when car-bon from the graphite block, used to absorb the laser beam beyond theplasma, is brought into the heated region by entrainment in the free-convective flow. The second positive system of N , normally present inhigh temperature air, is conspicuously absent in ihe luminous sheathsurrounding the plasma. The second positive system of N appears onlyin the afterglow region well downstream of the high temperature regionwhere its intensity is comparable to the intensity of the first negativesystem of N

IV. DISCUSSION OF RESULTS

The measured plasma isotherms indicate a maximum temperature near17,0O"0 K. This is in substantial agreement with the results of Fowler,et al and very nearly coincides with the temperature at which the ab-sorption coefficient is a maximum for atmospheric air.7 At the axiallocation where the temperature was a maximum, the beam diameter was ap-proximately 2mm and the beam intensity was approximately 120 kW/cm2 .This is in substantial agreement with the threshold intensity found byFowler et a] 4 and calculated by Batteh and Keefer 8 for a laser supportedplasma in a cstannel. The length of the plasma, as determined by theIO,000'K isotherm, was approximately 1.5 cm and the plasma had a narrowelongated high temperature core similar to that calculated by Batteh andKeefer.8 The highest temperature region did not occur at the focus ofthe laser beam, but some 1.5 cm forward of the focal point. This indi-cates that the plasma, after initiation, propagateg into the laser beamas a result of inhomogeneous absorption of energy. The motion of thetemperatire maximum was arrested when the inhomogeneous absorption wasbalanced by thermal conduction and the induced free-convective flow.

It was pointed out in the previous section that, in the luminoussheath surrounding the bright central core, the radiation was predom-inantly that of the first negative system of the molecular ion N2 +.

7Hall, R.B., Maher, W.E. and Wei, P.S.P., "An Investigation of LaserSupported Detonation Waves," AFWL-TR-73-28, June 1973.

Batteh, J.H. and Keefer, D.R., "Two Dimensional Ge,.2ralization of Raizer'sAnalysis for the Subsonic Propagation of Laser Sparks," mEiir Trans.Plasm, Sci., to be published.

Maecker H.H., "Principles of Arc Motion and Displacement," IEEE Proc.59, pp. 439-449 (1971).

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Closer examination of the (0,0) band, with the band head at 3914 A, re-vealed a highly excited rotational structure with the peak intensity ofthe P-branch t-r:urring at rotational quantum numbers near 40. The rota-tional structure of the R-branch could be observed to rotetional quantumnumbers greater than 90. This highly developed rotational structure per-sisted even out to a plasma radius of 7mm where, presunably, the tempera-tures are decreasing. Venable and Shumaker10 in an experimental studyof a nitrogen arc found that the intensity of the radiation from the(0,0) band of the second positive system of nitrogen at 3371 A was 23%of the intensity+from the (0,0) band of the first negative system of themolecular ion N2 at a temperature of 80000 K. They found that this ratioincreases at lower temperatures. The absence of radiation front the sec-ond positive system in our experiment suggests that some highly nonequi-librium process, such as photoionization, may be responsible for theanomalous radiation from the first negative system of N2

10 Venable, W. ., Jr., and Shumaker, J.B., Jr., "Observations of Depar-

tures from Equilibrium in a Nitrogen Arc," J. Quant. Spectrosc. Radiat.Transfer, 9, pp. 1215-1226 (1969).

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