Contains reports on three research projects

Breeding, R. J.

Fiocco, G.

Grams, G. W.

Halverson, W. D.

Urbanek, K.

Contains reports on two research projects

DSpace@MIT

VI. GEOPHYSICSProf. F. Bitter R. J. Breeding G. W. GramsProf. G. Fiocco J. C. Chapman W. D. HalversonDr. T. Fohl A. J. Cohen H. C. KoonsDr. J. F. Waymouth J. B. DeWolf K. UrbanekR. J. DiGraziaA. PLASMA DIFFUSION IN A MAGNETIC FIELDComplete results of experiments on the diffusion of plasma of low-pressure arcdischarges across a homogeneous magnetic field are now available. 2Further experiments are planned. Argon and Helium-Mercury discharges in homo-geneous longitudinal fields to 25 kilogauss will be studied to obtain data on turbulentplasmas. Initial measurements will include determination of axial electric field andradial potential drop as a function of magnetic field. Spectral analysis and correlationsof the RF noise and light fluctuations of the turbulent discharge will be performed bylater experiments.W. D. HalversonReferences1. W. D. Halverson, Plasma diffusion in a magnetic field, Quarterly ProgressReport No. 75, Research Laboratory of Electronics, M. I. T., October 15, 1964, p. 31.2. W. D. Halverson, Study of Plasma Columns in a Longitudinal Magnetic Field,Sc. D. Thesis, Department of Geology and Geophysics, M. I. T., January 1965.B. OBSERVATIONS OF THE UPPER ATMOSPHERE BY OPTICAL RADARIN ALASKA AND SWEDEN DURING THE SUMMER 1964 (Part II)By invitation of the Geophysical Institute of the University of Alaska, and the Instituteof Meteorology of the University of Stockholm, two optical radar devices were taken toAlaska and Sweden during the summer of 1964 to perform observations of the upperatmosphere at latitudes where noctilucent clouds might be visible. Some results werereported in Quarterly Progress Report No. 76 (pages 43-46).Because optical radar observations require a dark sky background in order to maxi-mize the signal-to-noise ratio, the unit in Sweden was mobile so that observations couldbe started at lower latitudes where there are longer periods of darkness. It was also ofspecial interest to make observations during those nights when rocket experiments werebeing carried out at Kronogard, Sweden. These experiments, designed to measure tem-perature and wind velocity at the mesopause and to sample the particulate content of thenoctilucent clouds, were carried out jointly by scientists of the Swedish Space Commit-tee, the National Aeronautics and Space Administration, and the U. S. Air ForceCambridge Research Laboratories.QPR No. 77(VI. GEOPHYSICS)Visibility conditions were less than satisfactory much of the time in Alaska, as wellas in Sweden. Precipitation was not uncommon and cirrus clouds or lower clouds wereoften present. On July 15, 1964, in Sweden, a very strong noctilucent cloud display wasvisible overhead and to the South, but the apparatus was not operational. On some otheroccasions, noctilucent clouds were seen low on the horizon, that is, at a distance of sev-eral hundred kilometers. All of our measurements were obtained when noctilucent cloudswere not visible overhead. At our locations, during that period of time, we visuallyobserved noctilucent clouds on the following nights:5-6 August, Torsta6-7 August, Torsta (strong display)7 August, College7-8 August, Torsta (strong display)9-10 August, Torsta10-11 August, Torsta15-16 August, Torsta16-17 August, TorstaIt is possible that on occasions noctilucent clouds may have been present, but were notobservable from our location because of local visibility conditions.The intensity of the echoes for a thin layer of homogeneously distributed scatterers,expressed as n R , the expected number of photoelectrons per transmitted pulse emittedat the photodetector cathode, can be computed with the aid of the following expression:2 HNCnR = WtA k ktk2 HNR he t rr a 4rR 2where W t is the transmitted energy per pulse, Ar is the collecting area of the receivingtelescope, kr and kt are the efficiencies of the receiving and transmitting systems, ka isthe atmospheric transmission, H is the geometrical depth of the layer, NG is the aver-age radar cross section of the scatterers per unit volume, R is the distance, X is thewavelength, -1 is the quantum efficiency of the photocathode, h is Planck's constant, andc is the speed of light. The product HNo is defined as the optical thickness of the layer.Because of the difficulty in measuring some of the parameters in the equation, we havecalibrated the sensitivity of the apparatus by assuming that the echoes from heights of40-60 km were produced by a model molecular atmosphere. This calibration was usedto estimate the optical thickness of the layers above 60 km. It should be pointed out thatthe assumption of a dust-free atmosphere in the 40-60 km interval may lead to an under-estimate of the optical thickness of these higher layers. On the other hand, our photo-electron counting procedure may be subject to a reduced counting efficiency at lowerQPR No. 77(VI. GEOPHYSICS)heights, which would result in an overestimate of the optical thickness of the upperlayers. While these and other systematic instrumental uncertainties may modify theresults to a certain extent, the system does provide us with the capability of obtaininga quantitative measure of the optical thickness of scattering layers in the upper atmos-phe re.The important source of noise is the sky background which varies throughout thenight and from night to night. In order to illustrate the nature of this variability,Fig. VI-1 has been constructed to show, for a typical night, the solar depression angleand the intensity of the sky background as a function of time. The intensity of the back-ground is displayed as the average number of photoelectrons emitted in a 66-Lsec inter-val (equivalent to a resolution in distance of 10 km). The scatter in the points is duepresumably to the occasional presence of thin cirrus clouds that increase the brightnessof the background. Since the noise background increases very rapidly when the solardepression angle is less than 7°-8', a very short interval of time is available forobservations at high latitudes.(I")I-z 0.1D00z00ILUJ-0F-w__Joo 0.01I21 22 23 24 01LOCAL TIME02 03-15 zo-o )a-JoFig. VI-1. Time variation of (a) solar depression angle, and (b) intensityof sky background for 16-17 August 1964 at Torsta, Sweden.QPR No. 7720ISD20-OI ISD02010 ISD020KIO ISD30 ISD20-S60 80 100kmOLAND, SWEDEN25-24 JULY 19642117 - 0010 UT309 PULSESOLAND.SWEDEN26 -27 JULY 19642138-2321 UT293 PULSESOLAND.SWEDEN28 - 29 JULY 19642221-0050 UT254 PULSESOLAND,SWEDEN29- 30 JULY 19642103- 0029 UT877 PULSESOLAND, SWEDEN30-31 JULY 19642119-2354 UT676 PULSESCOLLEGE,ALASKA6 AUGUST 19640810 - 0836 UT56 PULSESIISD80 ISD70- .- -"i60504030-20I003020 ISO105040-30 ISOD2010060 s0 100kmTORSTA,SWEDEN6-7 AUGUST 19642224-0041UT1027 PULSESCOLLEGE, ALASKA7 AUGUST 19641148-1207 UT90 PULSESTORSTA,SWEDEN7-8 AUGUST 19642131- 0014 UT851 PULSESCOLLEGE, ALASKA8 AUGUST 19640850 -1034 UT359 PULSESTORSTA,SWEDE N8-9 AUGUST 19642143-2216 UT208 PULSESI0 f200TORSTA, SWEDEN9-10 AUGUST 19642103 -0005 UT799 PULSESCOLLEGE,ALASKA10 AUGUST 19640915-1152 UTISD 223 PULSESCOLLEGE, ALASKAISD II AUGUST19640955-1056 UT259 PULSESTORSTA ,SWEDEN13-14 AUGUST 1964ISD 2036-2353 UT731 PULSESTORSTA,SWEDEN16-17 AUGUST 1964ISD 2017-0147 UT1216 PULSESCOLLEGE, ALASKAIO _ 17 AUGUST 1964,0 0900-0924 UT0 60 80 100 130 PULSES20 COLLEGE. ALASKAI ISO 21 AUGUST 19640715 -0956 UT0 60 80 100 722 PULSES40- COLLEGEALASKA30 22 AUGUST 196420 ISO 0825-1225 UTIO- 851 PULSES60 80 100TORSTA,SWEDEN40 22-23 AUGUST 196430 2040-0052 UT20 589 PULSES10 ISDO60 80 100kmFig. VI-2. Summary of observation for summer 1964.300290280260250240230m 220u 210-z0LrH 200-.U 3SDo 190O -oI 2SDa- 180 -170 ISD160-150 -140130-120110100 i 60 80 100kmFig. VI-3. Summary of observations obtained on 7, 7-8, 8, and 8-9 August 1964.QPR No. 77(VI. GEOPHYSICS)Figure VI-2 is a summary of all observations performed. For each night, the totalnumber of photoelectrons accumulated in intervals of 4-km width from 52 km to 100 kmis indicated, as well as the total number of pulses radiated and the Universal Time whenthe observations were made. The average noise level, n, is taken as the average of themeasurements between 88 km and 100 km. A measure of the statistical fluctuations, f-n,is also indicated and is taken to be equivalent to the standard deviation of the measure-ment. Although we have a great number of traces with records from 100 km to 200 km,these will not be presented at this time. We point out, however, that these records donot show echoes of intensity comparable to those obtained in the summer of 1963 at thoseheights. The data presented in Fig. VI-2 indicate the presence of relatively strongechoes from an altitude of 68-72 km at both the Swedish and Alaskan locations on thenights of 7, 7-8, 8, 8-9 August, 1964. Note that we visually observed noctilucent cloudsat both locations on the nights of 7 and 7-8 August. In order to gain statistical evidencethat a scattering layer was present at that altitude, the observations for those four con-secutive nights were averaged and the optical cross sections computed. The result ispresented in Fig. VI-3. The optical thickness of this layer is estimated at 10 - 5 ZX 10 - 6Some measurements were also obtained on the night of 16-17 August in Torsta during anoctilucent cloud display. The partial count obtained from 0057 to 0147 U. T. has alreadybeen reported. The data obtained during this time interval suggest the presence ofscattering layers at altitudes of 60-64 km and 80-88 km. No comparable echoes werereceived during the earlier part of the night. This thickness of these layers is estimatedto be 2 X 10 - 5 with a standard deviation of 1 X 10 - 5 for the 60-64 km layer, and 4 X 10 - 5-5with a standard deviation of 2 X 10-5 for the 80-88 km layer.Although these measurements are admittedly of a preliminary type, they indicate avalue of the optical thickness for the noctilucent clouds substantially larger than thatreported by Ludlam 2 and closer to the value suggested by Deirmendijan. 3G. Fiocco, G. W. Grams, K. Urbanek, R. J. BreedingReferences1. G. Fiocco, G. Grams, K. Urbanek, and R. J. Breeding, Quarterly ProgressReport No. 77, Research Laboratory of Electronics, M. I. T., January 15, 1965, p. 45(Fig. VI-3).2. F. H. Ludlam, Noctilucent clouds, Tellus 9, 341-364 (1957).3. D. Deirmendijan, Detection of Mesospheric Clouds from a Satellite, Proc. FirstInternational Symposium on Rocket and Satellite Meteorology, edited by H. Wexler andJ. E. Caskey, Jr. (North-Holland Publishing Co., Amsterdam), pp. 406-411.QPR No. 77

Geophysics

Koons, H. C.

Prof. F. BitterProf. G. FioccoDr. T. FohlDr. W. D. HalversonV. GEOPHYSICSDr. J. F. WaymouthR. J. BreedingJ. C. ChapmanA. J. CohenJ. B. DeWolfR. J. DiGraziaG. W. GramsH. C. KoonsK. UrbanekA. PLASMA DIFFUSION IN A MAGNETIC FIELDExperiments on the transverse diffusion of plasma in low-pressure arc dischargeshave continued since the last report.1 The discharge tube shown schematically inFig. V-1 was filled with argon at 0. 5 torr pressure and operated in longitudinalmagnetic fields up to 1. 8 Wb/m 2 . The magnetic field is homogeneous to 1. 3 per centover a 140-cm length of the tube that includes all of the plasma probes.1. Two-Mode OperationThe occurrence of the two-mode operation found previously I is strongly influencedby the alignment of the discharge tube with respect to the magnetic lines of force. Forfields approximately 0. 25-0. 8 Wb/m , the axial electric field E can be varied by asmuch as a factor of 10 by tilting the tube one or two degrees with respect to the magneticlines. The onset of the high electric field ("turbulent mode") is sudden and is accompa-nied by increased light and electrical noise output. In this range of magnetic fieldstrength, the discharge is channeled by the converging magnetic lines at the solenoid coilnearest the cathode end of the tube. The visible plasma is concentrated in a column ofapproximately 2-cm diameter which expands to refill the tube at a distance downstreamthat is determined by the magnetic field and the degree of turbulence of the discharge.The first center probe (Probe 2 in Fig. V-1) is approximately 25 cm from the center2.76 cm DIAMETERANLATHODEFig. V-1. Schematic diagram of the discharge tube. All probes made of40-mil tungsten wire. Center probes encased in 3-mm glasssheaths, cut flush at the end to expose the probe.QPR No. 78(V. GEOPHYSICS)line of the end coil of the solenoid, and is in a region of high plasma density. The probecauses a plasma perturbation at these magnetic fields which sets up a bright glow aroundthe probe, extending approximately 10 cm on the anode side and 1 cm on the cathode side.A bright beam of plasma, approximately 2 mm in diameter, also extends from the probefor a distance of 20-30 cm toward the cathode.It is likely that the two-mode operation is associated with the plasma disturbancecaused by Probe 2. The turbulence appears to set in just when the compressed dischargecolumn no longer intercepts the probe. The perturbation may tend to smooth out theradial plasma density gradients that, according to Kadomtsev,2 are necessary for theonset of spiral instabilities in the discharge column. Further evidence supporting thishypothesis is the fact that the strength of turbulence is a direct function of the dischargecurrent. When the current is increased from 0. 5 amp to 0. 7 amp, the discharge will nolonger remain in the low-voltage mode. The density gradients are almost directly pro-portional to the discharge current (under the assumption of constant longitudinal electrondrift velocity), and it seems likely that the smoothing effect of the probe disturbance isovershadowed by the destabilizing tendency of the increased plasma density gradients.2. Longitudinal InhomogeneityThe longitudinal plasma inhomogeneity, observed visually, is also evident from elec-trical measurements. Figure V-2 shows the axial electric field E calculated from thezfloating potential drop between Probes 6 and 1 compared with E from Probes 6 and 5.zThe radial potential drop V r, defined asV =V -Vr wall center'is also shown for measurements at opposite ends of the discharge. The curves indicatethat the electron flux to the tube walls is considerably greater at the anode end than atthe cathode end when the discharge is in the turbulent state. The fact that the strongdeparture from longitudinal homogeneity occurs at the onset of turbulence indicates thatthe turbulence itself is not uniformly distributed over the length of the tube, tending tobe more concentrated toward the anode.As the magnetic field is increased above 1. 0 Wb/m 2 , the discharge becomes morelongitudinally homogeneous, both visually and from electrical measurements. At thispoint the visible discharge does not completely fill the tube throughout the length of thesolenoid. As the field is increased to 1. 75 Wb/m 2 , the discharge becomes somewhatmore compressed at the cathode end of the solenoid, and the expansion angle downstreamsteadily decreases. At maximum magnetic field, the visible discharge is concentratedin a diameter of approximately 1 cm at the cathode end and 2-3 cm at the anode end ofthe solenoid. The E and V measurements in different parts of the tube also tendz rQPR No. 78(V. GEOPHYSICS)22ARGON P = 0.5 TORRDISCHARGE CURRENT I z = 0.7 AMPz PROBES 0-0--- - -A PROBES 0-0AVU-U--p.PROBES -0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8Bz ( weber / meter 2 )Fig. V-2. Axial electric field and radial potential drop measured by probesindicated in Fig. V-i.toward uniform values at this magnetic field.Tests with the anode or cathode in the homogeneous magnetic field region indicatethat the axial position of the discharge electrodes does not influence the qualitativebehavior of the plasma column in the magnetic field. The results are also independentof the polarity of the solenoid electromagnet.W. D. HalversonReferences1. W. D. Halverson, "Plasma Diffusion in a Magnetic Field, " Quarterly ProgressReport No. 75, Research Laboratory of Electronics, M. I. T., October 15, 1964, p. 31.2. B. B. Kadomtsev, "Convection of the Plasma of a Positive Column in a MagneticField," Soviet Phys. - Tech. Phys. 6, 927-933 (1962).B. RECOMBINATION COEFFICIENT MEASUREMENT BY LANGMUIR PLASMA PROBEA simple technique is being used for determination of recombination coefficients innoble gases plasmas. A plasma is generated by running a DC discharge at the centerQPR No. 78(V. GEOPHYSICS)of a large glass sphere (radius, 25 cm) filled with noble gas. Radial distribution ofelectron temperature and density is then measured by means of a movable Langmuirprobe. Under the assumptions that the electron temperature is much greater than theion temperature, ions are positive and singly ionized, and the plasma is neutral, theequation governing the diffusion of ions and electrons radially towards the walls of theglass sphere isk dr e i dr [(TTi)] (1)whereFr = net flow of electrons or ions per unit area perpendicular to ri = ion mobilityn = number density of electronsT , T. = ion and electron temperatures.e 1If all of the ions and electrons are produced at the center of the tube (which is also theorigin of the coordinate system) and there is no loss of ions or electrons, then the flowFr at a given radius r isZ.r 1 __ (2)r 4Trwhere Z. is the rate of production of ions at the origin. Equating (1) and (2) and inte-grating, we obtain the expression for n (T +T i ) as a function of radius reZ.n (Te+T) = 1 . + C. (3)ee i 4Trkp. rThe constant C can be determined from the boundary conditions. In a simple casewe neglect the presence of the glass sphere; therefore, we can set n e(T e+T i ) as zeroat infinity. The constant C is then zero. Plotting the quantity n (T e+Ti) against 1/r,10x10148 Fig. V-3.E6 ne(Te +Ti) as a function of 1/r for Argon plasma.+ Dotted line represents diffusion without recombi-nation; solid line (actual values) indicates recom-2 - bination.00 0.1 0.2 0.3I/r (cm - I)QPR No. 78(V. GEOPHYSICS)we shall obtain a straight line connecting the origin with the value of ne(Te+Ti ) at theparticular radius (dotted line in Fig. V-3). The slope of this line will beeZ.14nkLiIf, however, recombination takes place in the diffusing plasma, Zi will vary with theradius and we no longer have a straight line. In this case we can interpret the quantityZ. = Z.(r) as "apparent" or "net" rate of ion production inside the particular radius, and1 1the slope of the curve will decrease with decreasing 1/r (Fig. V-3).Electron temperature and density at various radii can be measured by means of amovable Langmuir probe. From the measurements, the "apparent" production ratescan be determined for different values of r. Calculating the decrease in "apparent" pro-duction rate between two radii, we can determine how many ions are lost because ofrecombination. Dividing this value by the difference of volume of the two correspondingspheres, we obtain the number of recombination events per unit of volume per second, R.The recombination coefficient a is then defined by the relationR = an.n . (4)leWe have determined the recombination coefficient for an Argon plasma with electron11 -3 -10 3 -1density 1 X 1011 cm and temperature 9, 000K as 5 X 10 - 0 cm3 sec-K. UrbanekReferences1. E. W. McDaniel, Collision Phenomena in Ionized Gases (John Wiley and Sons, Inc.,New York, 1964).2. S. C. Brown, Basic Data of Plasma Physics (The Technology Press of the Massa-chusetts Institute of Technology, Cambridge, Mass., and John Wiley and Sons, Inc.,New York, 1959).C. PROTON FLOW INTO THE MAGNETOSPHERERecent measurements of the proton flow in the transition region and in interplanetaryspace from the Imp- satellite have been reported by Olbert and Moreno, Lyon et al.,and Egidi et al. 3 The data covering the energy range 45-5400 ev give evidence forordered flow in the transition region.On the basis of these measurements, we have carried out a simple continuity calcu-lation that may be interpreted as suggesting a transfer of protons between the transitionregion and the magnetosphere. The geometry is shown in Fig. V-4. The boundariesof the transition region that give the best paraboloidal fit to the position of the observeddiscontinuities were rotated about the earth-sun line through a small angle, 6, above andQPR No. 78(V. GEOPHYSICS)below the ecliptic plane. The proton flow per second per degree of latitude, 0,crossing a plane perpendicular to the flow at an hour angle, 4, as calculated from the262422 -20TRANSITION18 - REGION16Vo 141412 -0 - SOLAR" 8- WIND64 MAGNETOSPHERE Fig. V-42 - EARTH (a) Boundaries of the transition region in the0 -TO SUN N ecliptic plane.(b) Dawn meridian cross section showing the(a) transition region and a magnetic field18 line resulting from a centered dipole.16-14 -12- \ TRANSITIONS10 \ REGIONH B8 MAGNETOSPHEREu 642 - EARTHO 2 4 6 8 10 12 14 16 18 20 22 24EARTH RADII(b)Imp-1 data, is illustrated in Fig. V-5 (curve a). One point representing almost a three-fold increase in the proton flow in the transition region at 4 = 85 0 was obtained on Feb-ruary 12, 1964, during a period following a sudden-commencement magnetic storm.Although inserting this point in the data would not contradict the results of this calcu-lation, it has been omitted from curve a. The density and flow velocity are averagesacross the transition region. Curve b is the flow expected, under the assumption thatall particles from the solar wind enter and remain in the transition region and that thereis no flow across the inner boundary. The curve is drawn for a solar wind flux of 2 X8 -2 -110 protons cm secThe data indicate that the total number of particles crossing a plane at a given hourangle is generally greater than can be accounted for by the solar wind alone.If we assume that there is no ordered flow of protons in the 0 direction in the tran-sition region, we can account for the additional protons by allowing flow between theQPR No. 78x 102650 x 10244030 zI20 Z0 0LO0- -10d0 20 40 60 80 100 120 140DEGREES12 00 10.00 8'00 6.00 4:00 2 00HOURS (A.M.)LONGITUDE FROM SUB SOLAR POINTFig. V-5. (a) Observed proton flow in the transition region (particles per second perdegree, 0) crossing a plane perpendicular to the flow at an hour angle, 4.(b) Expected proton flow in the transition region for a solar wind flux of 2 X8 -2 -110 protons cm sec(c) Protons flowing into the magnetosphere per second per degree, 0, perdegree, p, required to account for the measured number of protons in thetransition region in excess of those coming directly from the solar wind.(d) Curve (c) normalized by 1/r 3 to represent a flow density.QPR No. 78(V. GEOPHYSICS)magnetosphere and the transition region. The number of protons flowing into themagnetosphere per second per degree, 0, per degree, 4, required to account for themeasured number of protons in the transition region in excess of those coming directlyfrom the solar wind is illustrated in curve c. It is found that flow is required from themagnetosphere to the transition region during the hours from 6 a. m. to 10 a. m., andfrom the transition region to the magnetosphere from 3 a. m. to 6 a. m. There is quali-tative agreement between this behavior and the flow patterns in the model of the magneto-4sphere as postulated by Hines. It would seem unlikely that recombination could causethe particle loss observed in the transition region before 6 a. m.If this influx is further normalized by a factor 1/r 3 , where r is the distance fromthe center of the earth to the inner boundary of the magnetosphere, to take into accountthe volume of the flux tube exposed to the boundary, it is found that the maximum inflowdensity occurs around 5 a. m., as shown in curve d. This finding corresponds approxi-mately with the time of peak occurrence of the VLF emission known as chorus (Laaspere,Morgan and Johnson ). Several mechanisms have been proposed for the generation ofVLF emissions. Two of these involve protons in the magnetosphere. MacArthur 6 hassuggested Doppler-shifted cyclotron radiation from protons, and Kimura 7 has suggesteda transverse resonance plasma instability for a beam of protons.H. C. Koons, G. FioccoReferences1. S. Olbert and G. Moreno, "Analysis of the solar wind in the transition region,"Trans. Am. Geophys. Union 45 (4), 588 (1964).2. E. Lyon, H. Bridge, A. Egidi, and B. Rossi, "Imp-i plasma summary data," Trans.Am. Geophys. Union 45 (4), 588 (1964).3. A. Egidi, H. Bridge, E. Lyon, and B. Rossi, "Spatial properties of the interplane-tary plasma and its interaction with the Earth's field as observed on Imp-l," Trans.Am. Geophys. Union 45 (4), 588 (1964).4. C. O. Hines, "Hydromagnetic motions in the magnetosphere," Space Sci. Revs. 3,342-379 (1964).5. T. Laaspere, M. G. Morgan, and W. C. Johnston, "Chorus, Hiss and other audio-frequency emissions at stations of the whistler-east network," Proc. IEEE 52, 1331-1349 (1964).6. J. R. MacArthur, "Theory of the origin of the very-low frequency radio emissionsfrom the earth's exosphere," Phys. Rev. Letters 2 (12), 491-492 (1959).7. I. Kimura, "Amplification of the VLF electromagnetic wave by a proton beam throughthe exosphere, an origin of the VLF emissions," Rep. Ionosphere Space Res. Japan,15 (2), 171-191 (1961).QPR No. 78

Contains reports on three research projects.Joint Services Electronics Program (Contract DA36-039-AMC-03200(E)

DeWolf, J. B.

Prof. F. BitterProf. G. FioccoDr. T. FohlDr. W. D. HalversonV. GEOPHYSICSDr. J. F. WaymouthR. J. BreedingJ. C. ChapmanA. J. CohenB. DeWolfW. GramsC. KoonsUrbanekA. ABSORPTION OF X = 4880 A LASER BEAM BY ARGON IONSThe absorption of the output beam of a steady-state argon ion laser (X= 4880 A) byexcited argon ions in a low-pressure arc discharge has been observed. The argon ions,excited to the 4s2 P 3/Z state, strongly absorb the laser beam that is produced by the Ar IItransition 4p 2 D5/ 3/2 '5 o/2 3/The ultimate purpose of this work is to develop a technique to measure the tempera-ture of ions in moderate temperature plasmas by determining the Doppler width of anion absorption line. The steady-state ion laser provides a series of extremely sharpemission lines corresponding to the optical resonant cavity modes of the laser. Theratios of the intensities of the laser modes, determined with and without the absorbingions, can be related to the Doppler width of the ion absorption line. For excited argonions absorbing energy corresponding to the 4s2p 3 /2  4p2Do transition, the relationbetween the Doppler width of the absorption line and the ion temperature is shown inFig. V-1. The density of the excited ions can also be determined if the total absorbedpower from the beam and the oscillator strength of the absorption line are known. The10 x 109987604n3Ix 109100 200 400 600 800 1000 2000 4000 6000 8000 10000ION TEMPERATURE T. ( OK )Fig. V-l. Absorption linewidth resulting from Doppler4s 2 3/2 - 4p 2 D5/ 2 (X= 4880 A).This(Contractbroadening of Ar II transitionwork was supported principally by the Joint Services Electronics ProgramDA36-039-AMC- 03200(E).QPR No. 79(V. GEOPHYSICS)ARGON ION ARGON ARCLASER DISCHARGE400 cps MONOCHROMETERCHOPPERPHOTOMULTIPLIERELECTROMETERUHFRADIORECEIVERLOCK-INAMPLIFIER400 cpsSTRIPCHARTRECORDERFig. V-2. Experimental arrangement.DISCHARGE ARGON PRESSURE = 0.11 torr1 2 3 4 5DISCHARGE CURRENT ( amperes )Fig. V-3. Laser beam intensity transmitted through argonradiation at X = 4880 A vs discharge current.discharge and backgroundoscillator strengths of important argon ion lines have been measured by Olsen. 1For preliminary evaluation of this technique, the experimental arrangement ofFig. V-2 was set up. A steady-state argon ion laser (X= 4880 A) sends a beam througha second argon discharge in a capillary, 2-mm in diameter and 49 cm long. The beamis then sent into a Jarrell-Ash monochromator and the X = 4880 A component is detectedby a photomultiplier. The transmitted laser beam intensity and the argon dischargebackground radiation at 4880 A, monitored directly at the photomultiplier output by ahigh-impedance electrometer, are shown in Fig. V-3 as a function of discharge current.The beam intensity decreases by 24 per cent as the absorbing discharge current isQPR No. 79S0.4Z 0.2000.04 E0.03 - Dz0.02 -0.01o<a-- 1 1(V. GEOPHYSICS)increased from 0 to 5 amps.In order to have a preliminary idea of the absorption of the individual laser modes,the radiofrequency beats between the modes were detected by a UHF radio receiver. Thesignal-to-noise ratio of the receiver output is increased by chopping the transmitted-1laser beam at 400 sec and amplifying the audio output with a Princeton Applied-1Research lock-in amplifier tuned to 400 sec-. The laser modes are separated in fre-quency by approximately 150 Mc/sec.Preliminary measurements of the laser beat frequencies up to 1 Gc/sec have beenmade, but problems in obtaining reproducible data have been encountered because thelaser output power had not been adequately stabilized. Modifications of the experimentalarrangement are now under way which should correct this problem and extend the fre-quency spectrum.G. Fiocco, W. D. HalversonReferences1. H. N. Olsen, J. Quant. Spectroscopy and Rad. Transfer 3 59-76 (1963).B. OPTICAL DOPPLER RADAR IPreliminary experiments to demonstrate the feasibility of developing an optical radarcapable of measuring the velocity of moving reflectors are being carried out. Our aimMICHELSON INTERFEROMETERREFERENCE ARMDOPPLERWHEELFig. V-4. Detection system for an optical Doppler radar.QPR No. 79(V. GEOPHYSICS)is to construct a device capable of measuring velocity distributions of atmos-pheric gases at a distance, by the detection of the broadening and shifting ofa laser-emitted line.For this purpose, we have assembled in the laboratory the device illustrated inFig. V-4. This is basically a Michelson interferometer in which the signal in one of thearms is reflected from a moving surface. The moving surface simulates the motion ofa real target; in the present case it consists of a wheel, 25 cm in diameter, with180 teeth that can be rotated at various speeds. The wheel is contained in an enclosurethat can be evacuated in order to achieve high speeds. At present, however, the wheelrotates at 1800 rpm which results in a peripheral speed of 24 m/sec- . Taking intoaccount the geometry of the encounter, the frequency shift is calculated to be approxi-mately 45 Mc.The signal from the wheel is mixed with the signal from the reference armof the interferometer and both are detected by a photomultiplier. Frequency anal-ysis is carried out with a radio receiver with a bandwidth of 13 kc. To improvethe signal-to-noise ratio, synchronous detection methods in which a lock-in ampli-fier is used in conjunction with a beam chopper placed in the wheel arm havebeen employed.z0zm-40 50FREQUENCY ( MEGACYCLES )Fig. V-5. Power spectrum after detection.Figure V-5 is a reproduction of the power spectrum after detection showingthe beat signal at 46. 6 Mc. The line has a width of approximately 1 Mc whichis caused by the fact that the reflection occurs over the surface of a tooth whichmoves faster on the inside edge than on the outside edge. In obtaining this beatsignal, a number of the conditions necessary for successful detection have beenclarified.G. Fiocco, J. B. DeWolfQPR No. 79(V. GEOPHYSICS)C. INVESTIGATION OF A REFLEX DISCHARGEElectron gyro frequency or cyclotron harmonic radiation has been observed from ahot-cathode reflex or Penning discharge. Probe measurements have been made and thedensity determined from the ion saturation currents. No change in radiometer output18 -3maxima near the harmonics was observed for densities between 2 and 20 X 10 mThe discharge cavity was a water-cooled, stainless-steel cylinder, 19. 7 cm in diam-eter and 15. 2 cm high. The anodes, also water-cooled, were hollow copper annuli,10. 2 cm apart. Spirals of 25-mil tungsten wire were used for cathodes (see Fig. V-6).These cathodes emitted tungsten copiously and soon developed hot spots that resulted incopious, but localized, emission of tungsten and electrons, and a bright pencil in thedischarge.Magnetic fields of up to 0. 15 Wb/m 2 were used. The field was uniform 1 per centwithin 3 cm of the axis of the cavity. The X-band radiometer had no filter before themixer and so was sensitive to two 20 Mc/sec bands 120 Mc/sec apart. The IF ampli-fier detected output was fed through a P. A. R. lock-in amplifier to an X-Y recorder. AWINDOWANODETOTU.S RADIOMETERETO PUMPS Q TO RADIOMETERSPIRALANODECATHODESPIRALWINDOW(a) (b)Fig. V-6. (a) Cavity seen from above.(b) Cavity seen from the North.QPR No. 79(V. GEOPHYSICS)signal proportional to the currentthrough the magnet coils was fed tothe other axis of the recorder.Densities were calculated fromthe ion saturation current by usingthe small-sheath approximation. Sheathradii calculated from the V3/2 lawwere found to be between 1. 2 and 1. 4times the probe radius which was3 mil. The effect of the magnetic fieldon these results should be negligiblebecause the probe radius was an orderof magnitude smaller than the mini-mum ion gyro radius. The densitycould be changed by varying the neu-tral pressure in the cavity and/or thedischarge current. Densities between18 -32! and 20 X 10 m were obtained.The electron thermal energy was foundto be approximately 3 ev.Figure V-7 shows some typicalresults for three different pressures.Peaks near the gyro-frequency har-monics were usually observed for har-monic numbers 3-10 and occasionallyas high as 25. Within the accuracyof the system, the peaks were locatedright at the harmonics and no changeFig. V-7. with density could be found. PeaksRadiometer output as a function of mag- not near the harmonics were alsonetic field; IDC = 0. 8 amp. The verticalpresent, but their occurrence andscale is arbitrary, except that the lowestcurve has been amplified by a factor of 100 intensity were directly related to thewith respect to the others, prominence of the pencil in the plasma.When the plasma was uniform thesemaxima were either greatly diminished or absent. The intensity of the peaks at the har-monics was also dependent on the strength of the pencil.In our future plans we envision improvement of the discharge by the use of othercathodes that will provide a uniform plasma. A filter and an IF amplifier with a nar-rower bandwidth will give narrower peaks and improved accuracy. Improvements inQPR No. 79_ h-L rC(V. GEOPHYSICS)probe design should extend the range and increase the accuracy of the density determi-nations.R. J. BreedingQPR No. 79

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