Contains reports on two research projects.National Aeronautics and Space Administration (Grant NGR-22-009-131)National Aeronautics and Space Administration (Grant NGR-22-009-114)National Aeronautics and Space Administration (Contract NAS 12-436

DeWolf, J. B.

Fiocco, G.

Koons, H. C.

DSpace@MIT

V. GEOPHYSICAL RESEARCH*Academic and Research StaffProf. G. FioccoDr. G. W. GramsGraduate StudentsS. J. Bless D. F. KitrosserJ. B. DeWolf H. C. KoonsA. EXPERIMENTS TO DETERMINE THE AEROSOL CONTENT OFAIR FROM SPECTRAL ANALYSIS OF LASER ECHOESSpectral analysis of the echoes obtained from atmospheric constituents in opticalradar experiments yields information related to wind motion, temperature, and com-position.We have performed some preliminary laboratory experiments, in which the spec-trum of the scattered radiation is analyzed in order to provide a measurement of theaerosol component.The laser is a Spectra Physics Model 130 with a nominal output of 0. 7 mW at 6328 A,with a hemispherical resonator configuration producing a single transverse mode withseveral longitudinal modes separated approximately by 0. 0067 A (since the resonatorlength is 30 cm). Although we have evidence of thermal drift of these modes, we haveassumed that, by integrating several scans and allowing each scan to be sufficientlyslow, the laser spectral output could be considered to be independent of time.The laser beam was passed through an aperture to limit the amount of fluorescenceradiation emitted, and then passed through a synchronized attenuator the function ofwhich will be described below. Light scattered in a volume approximately representedby a cylinder of 0. 5-cm diameter and 15-cm length was observed with a receiving tele-scope located 3. 6 m from the scattering volume at a scattering angle of 177. 2 . Thecollected light was collimated and then passed through a pressure-scanned Fabry-Perotinterferometer.The Fabry-Perot interferometer has plates of 3. 75-cm diameter and is flat to anominal surface accuracy of 1/100 %, coated with a broadband, dielectric coating ofreflectivity 94% at 6328 A, and separated by INVAR spacers of different thicknesses.In the particular experiments described here the spacer thicknesses were 1 and 2 cm.The chamber was filled with N2 , and the pressure inside was controlled by passing N2through a controlled leak and a flowmeter. The free spectral range of the interferometerThis work was supported principally by the National Aeronautics and Space Admin-istration (Grants NGR-22-009-131 and NGR-22-009-114, and Contract NAS 12-436).QPR No. 87(V. GEOPHYSICAL RESEARCH)with a 1-cm spacer is 0. 20 A at 6328 A, and the finesse achieved by passing lightthrough a circular area -2 cm in diameter was estimated to be approximately 20. Thefocal length of the fringe-forming lens was 17. 2 cm, and the scanning aperture was acircular hole, 0. 56 mm in diameter. The size of the scanning aperture was matched tothe focused image of the scattering volume.The photomultiplier was an EMI9558A cooled with methanol previously refrigeratedto dry-ice temperature. The amplified photoelectron pulses in a 1-sec time intervalwere then counted with a TSI Model 385-R counter, and then printed.To allow for the possibility that the laser spectrum or instrumental response mightchange during the course of the measurements, the following scheme was used to affordcontinual comparison of the radiated and scattered spectra. A rotating wheel with por-tions of the surface cut away was placed at the center of the scattering volume so thatthe telescope was presented at alternate count intervals with scattering from themedium, and scattering from the flat wheel surface. In order to present the detectorwith comparable signal levels, the synchronous attenuator mentioned above was usedto attenuate the beam by several orders of magnitude with a neutral density filter when-ever the wheel surface was in position. Admittedly, an instrument requiring such adevice at the location of the scattering volume would not be very suitable for remotesensing. Its presence is not essential, however, and it is not a very difficult problemto provide an internal rather than an external reference.The preliminary experiments carried out consisted of measuring the spectrum oflight scattered from air in the laboratory containing natural aerosol particles, and aircontaining an artificially produced dense fog.The laboratory air contains an unknown amount of aerosols, presumably small,since such air is kept circulating through air conditioning filters and the room haslimited access and no windows on the outside. The artificially produced fog was obtainedby blowing moist air through dry ice, and filled approximately the same scattering vol-ume as was observed in the case of ambient air. The cross section per unit volume ofthe fog is many times that of the air; hence, the spectrum of the fog should exhibitnegligible broadening, because of the high concentration of aerosol particles and theresponse from the fog may be considered a measure of the instrumental response.To a first approximation, the spectrum of the light scattered from the fog should beidentical to the spectrum of the light scattered from the wheel surface. Since the scat-tering volume for the fog is extended in space, however, the instrumental response inthe two cases is not quite identical. For one thing, the spatial distribution of illumina-tion in the plane in which the fringes are formed is different and may produce a modu-lation of the output. For another, the finesse of the Fabry-Perot depends on the amountand portion of the etalon utilized, and this also differs in the two cases. Thus, althoughthe spectrum of light scattered from the wheel is useful for comparative purposes, itQPR No. 87@ - AIR(1 -.- TARGETI GHzo l1000zn 900>- 800Cr" 800Cr- 600CCOr 500400- 300U)Z 200z 1000Fig. V-I. Spectrum of light scattered from laboratory air and a referencetarget as observed with a Fabry-Perot interferometer having a2-cm spacer. The ordinate distance is proportional to the photo-electron counts obtained in a 1-sec interval while scanning.S- AIR@ +---+--- TARGET[-i-. 1WAVELENGTHFig. V-2. Spectrum of light scattered from laboratory air and a referencetarget as observed with an FPI having a 1-cm spacer.10009008007006005004003002001000(- .-.. AEROSOL+---+---+ TARGETWAVELENGTHFig. V-3. Spectrum of light scattered from an artificially produced fogas observed with an FPI having a 1-cm spacer.QPR No. 87+ +WAVELENGTH10009008007006005004003002001000(V. GEOPHYSICAL RESEARCH)should not be assumed to be identical to the instrumental response.The data taken are shown in Figs. V-l, V-2, and V-3. Figure V-l (curve a) rep-resents an average of 3 scans obtained by using ambient air and the 2-cm spacer inthe Fabry-Perot. Figure V-2 (curve a) represents an average of 10 scans obtained byusing ambient air and the 1-cm spacer in the Fabry-Perot. Figure V-3 (curve a) rep-resents a single scan obtained from the fog by using the 2-cm spacer. Curves b inFigs. V-1 through V-3 represent the simultaneous spectral scans obtained from thewheel surface (target). The noisiness in such curves is due to the nonstationary char-acter of the cross section of the aerosol particles, the thermal fluctuations of the laserspectrum, and shot noise introduced by the photodevice. The signal level was substan-tially higher in Fig. V-3, and consequently the shot-noise fluctuations are greatlyreduced. In spite of the noisiness of the data, the broadening effect is evident inFigs. V-1 and V-2. The increased resolution used in Fig. V-1 makes the effect evenmore noticeable. An analysis of the curve indicates that the amounts of light scatteredby dust and by air were approximately equal.Experiments with improved resolution and the use of two laser frequencies are inpreparation. It is hoped that information on the aerosol size distribution may beobtained in this manner.J. B. DeWolf, G. FioccoB. MEASUREMENT OF THE DENSITY AND TEMPERATURE OF THEELECTRONS IN A LOW-DENSITY REFLEX DISCHARGE BYSCATTERING OF CONTINUOUS-WAVE A+ LASER LIGHTAn experiment to measure the electron temperature and density in a laboratorydischarge by scattering of continuous-wave A laser radiation at 4880 A has been suc-cessfully carried out.The power now available from cw lasers operating in the visible region ofthe spectrum is several orders of magnitude lower than the pulsed output madepossible by the ruby laser. In plasma scattering experiments, then, the levelof the scattered laser radiation will be small compared with the level of thebackground plasma radiation; thus, in general, long integration times are requiredbefore statistical evidence of detection can be achieved. The characteristics ofthe discharge under consideration may prevent the use of high radiation levels.Changes in the relative population of excited states, or an increase in ionizationmay be effected by high-intensity radiation. Some evidence for these effects wasfound in previous experiments by Thompson and Fiocco with a hollow cathodeQPR No. 87ROTATABLE INTERFERENCE FILTERPOLAROID FILTERINTERFERENCE FILTERFig. V-4. Schematic diagram of the experimental apparatus.(V. GEOPHYSICAL RESEARCH)discharge. Such effects would be most likely to occur in under-dense plasmas. Signif-icantly, much of the work by other investigators has been directed toward the investiga-tion of highly ionized, hot plasmas.In some experiments, a continuous-wave laser source may have several advantagesover pulsed lasers. They allow the use of counting or synchronous detector systems;the output power is, at present, considerably more stable than the power per pulse ina ruby laser; the available wavelength stability and high degree of coherence offer con-siderable advantages for the narrow-band spectral analysis that is necessary for mea-suring ion temperatures and collective effects; they are also free of thermal driftswhich simplifies electron density and temperature measurements. A schematic diagramof the experiment is shown in Fig. V-4. The light from an A+ laser was focused intothe plasma, and the scattered radiation was observed at a scattering angle of approxi-mately 80. Both plasma current and laser light were modulated in order to permitsynchronous detection of the scattered radiation and rejection of the background. Thespectrum of the scattered light was obtained by rotating a narrow-band interferencefilter.The plasma was formed by a reflex discharge in helium, at a pressure of-32. 5 X 10-3 Torr. The value of DC current through the discharge was I = 1. 8 amps, withan axial magnetic field of B = 0. 1 Wb/m 2 . The gas was then approximately 5% ionizedat the center of the discharge. The ion density at the center obtained from Langmuir18 -3probe curves was approximately 5 X 10 m . The electron temperature was also mea-sured by a probe. The value obtained by averaging for probe determinations was1. 30 ± 0. 42 eV.The A+ laser, built by Spacerays, Inc. has a total power output at several wave-lengths of approximately 2 watts. The output power available at 4880 A was approxi-mately 0. 5 watt.For the particular values of the various parameters at which the experiment wascarried out, the scattered spectrum has a Gaussian shape centered about the wave-length of the incident radiation with a width determined by the electron temperature.The output of the lock-in amplifier as a function of the angle of the rotation filteris shown in Fig. V-5. By using the value for the width obtained from the scatteringmeasurements, the electron temperature obtained is 1. 2 eV. This is in good agreementwith the temperature determined by the probe data.The electron density may be determined from the total number of photons scatteredinto the solid angle of the detector if the efficiency of the optical detection system andelectronics is known. Using values for all parameters which were either measured inour laboratory or provided as calibrations by the manufacturers, we obtained18 -34. 8 X 10 m as the value for electron density from the scattering data, which is alsoin good agreement with the probe measurements.QPR No. 87(V. GEOPHYSICAL RESEARCH)Ii353 354 355 356 357 358ROTATION ANGLE OF INTERFERENCE FILTER (DEG)Fig. V-5. Scattering intensity vs the rotation angle of the inter-ference filter. The angle indicated in the figure istaken with respect to an arbitrary zero.We plan to add a Fabry-Perot interferometer to the detector system at a smallerscattering angle. This will allow narrow-band spectral measurements to be madefrom which we might be able to determine the ion density and observe collectiveeffects in the plasma.The experimental arrangement will also be used to measure scattering crosssections of air for a better understanding of Brillouin scattering effects and of therole of aerosols in atmospheric scattering experiments.H. C. Koons, G. FioccoQPR No. 87E40a-D0a 30wU0S20zO 10oi

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