Contains reports on four research projects.United States Air Force (Contract AF19(628)-500)Lincoln Laboratory (Purchase Order DDL BB-107

Fiocco, G.

Grams, G. W.

Halverson, W. D.

Redi, O.

Waymouth, J. F., Jr.

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V. GEOPHYSICSProf. F. Bitter R. J. Breeding R. G. LittleProf. G. Fiocco J. C. Chapman O. RediDr. T. Fohl G. W. Grams C. J. Schuler, Jr.Dr. J. F. Waymouth W. D. Halverson K. UrbanekH. C. KoonsA. PERTURBATION OF A PLASMA BY A PROBEMaterial previously reported in Technical Report 406, Research Laboratory of Elec-tronics, M. I. T., December 26, 1962, has now been revised with additional experimentalevidence supplied by R. Bullis of the United Aircraft Corporation Research Laboratoryin private communications (also published in part in Advanced Energy Conversion 2,523, 1962) and prepared for publication. A short summary follows.The theory of the electrostatic probe immersed in a plasma is discussed under theassumptions that particle mean-free paths are comparable to or smaller than probeXc/rp < 10, rp/100 XD > 1, Xc/XD > 10, where rp is probe radius, Xc is particle mean-free path, and XD is the Debye length. In the limit as X c/rp approaches infinity, theresults converge toward the Langmuir result for the same probe. The probe-currentvs voltage characteristic obtained when mean-free paths are short is distorted becausethe degree of perturbation of plasma density and potential vary with probe voltage. Anunexpected result is that the shape of the probe characteristic in the vicinity of the "knee"is sensitive to ion temperature, and may possibly be used to determine the ion temper-ature.J. F. WaymouthB. PLASMA DIFFUSION IN A MAGNETIC FIELDThere has been considerable interest in low-pressure gas discharges in magneticfields since the time when it was found that instabilities develop which increase therate of charged-particle diffusion across the magnetic-field lines. (A good review oftheory and experiment has been given by Hoh. ) The instabilities are only beginningto be understood theoretically; new experiments, for example, the recent work ofD. L. Morse, 2 show that a great variety of instabilities can arise.Three discharge tubes have been built to study the processes of plasma diffusion ina homogeneous magnetic field. The tubes are approximately 2. 5 meters long with insideradii of 1.1, 2.76, and 4.76 cm, respectively. Seven probes, arranged as in Fig. V-l,are used to measure axial and radial potential drop; three probes can be operated asLangmuir probes.The tubes are placed in one of the National Magnet Laboratory's water-cooled sole-noids 3; this provides an axial magnetic field that is homogeneous within 2.5 per cent overQPR No. 73(V. GEOPHYSICS)15 CM 15 CM LANGMUIR PROBES30 CM WALL PROBES90 CM220 CMFig. V- 1. Probe arrangement.a 140-cm length of the discharge. At the present time, the magnet is powered by a 70-kwpower supply and is limited to fields below 3465 gauss. Experiments at higher fieldscan be conducted by using the large National Magnet Laboratory power supplies.Preliminary measurements of axial electric field vs magnetic field for argon dis-charges in the 1. 1-cm and 2. 76-cm radii tubes are shown in Fig. V-2 for pressures of0. 1 and 1 mm Hg of argon.The "normal" theoryl of magnetoambipolar diffusion indicates that the axial electricARGON GASDISCHARGE CURRENT, I z = 0.5 AMPERES r R - 2.76 CMP= 0.1 mm Hg{R= 1.1CMP 1 mm HgI{R = 2.76 CMP = 1 mm HgR = 1.1 CMP 0.1 mm HgBz (KILOGAUSS)Fig. V- 2. Axial electric field vs axial magnetic field.(Argon gas; discharge current, Iz = 0.5 amp.)zQPR No. 73(V. GEOPHYSICS)field should decrease monotonically with increasing magnetic field. This is apparentlythe case for the 1. 1-cm radius tube up to B = 3465 gauss, but the 2. 76-cm tube showsz 1behavior similar to that originally studied by Lehnert. It appears that the initial upturnof the E curve for R = 2. 76 cm, p = 0. 1 mm Hg is associated with a local instability atzthe end field coil nearest the anode of the discharge tube. The large discontinuity at2600 gauss, on the other hand, involves the entire length of the plasma column withinthe magnetic field.In the future, helium, mercury-helium, and mercury-argon discharges will be inves-tigated as a function of discharge parameters and magnetic field, and the electromagneticradiation emanating from the tube will be studied.W. D. HalversonReferences1. F. C. Hoh, Low-temperature plasma diffusion in a magnetic field, Revs. ModernPhys. 34, 267 (1962).2. D. L. Morse, Rotating plasma instabilities, Quarterly Progress Report No. 7 1,Research Laboratory of Electronics, M. I. T., October 15, 1963, pp. 116-122.3. F. Bitter and W. D. Halverson, High-power solenoid electromagnet, QuarterlyProgress Report No. 66, Research Laboratory of Electronics, M. I. T., July 15, 1962,pp. 12- 14.C. LEVEL CROSSING IN Hgl9 9The level-crossing method 1 of atomic spectroscopy which was used in this experi-ment involves the observation of the decrease of intensity of the resonance fluorescenceRF AMPLIFIER OANDHARVEY-WELLS DETECTOR OSCILLOSCOPE12" MAGNET2" GAPHEWLETT - PACKARD524 BPROTON ELECTRONIC COUNTERCELL RESONANCEPROBE30~ FIELD GERTSCH RFMODULATIONCOILS FM-6AUDIO AMPLIFIER OAND1P28 LOWPASS FILTER OSCILLOSCOPEPHOTOMULTIPLIERFig. V-3. Proton resonance and level-crossing detection systems.QPR No. 73(V. GEOPHYSICS)at a 90' scattering angle when two Zeeman levels of the 6s6p 3P 1 state of Hg 1 9 9 arebrought into coincidence ("crossed") through the application of a magnetic field. Theprevious precision measurements 2 ' 3 of this type gave results for the magnetic fieldat which the Hg 1 9 9 crossing occurs which do not agree within their limits of error, andhence a remeasurement was desirable.TIME -Ho+1.4HoOH -1.4 -Fig. V-4. Level-crossing signal with magnetic field modulated at 30 cps. H isthe level-crossing field. Scattering angle adjusted to 90". Cell at roomtemperature.The magnets and optical system used in this experiment were the same as thoseused by Smith.3 New apparatus that should make possible more accurate measurementsis described. Precision measurements of the magnetic field at the position of the 1-cmcubical cell containing natural mercury were made by using a proton resonance probethat could be moved to the location of the cell after the magnetic field has been adjustedQPR No. 73\\,J Z2 1 :(V. GEOPHYSICS)to the level-crossing value. With the magnet power supply used (Harvey-Wells 1365B),the stability of current was such that the magnetic field changes during a measurement(-5 sec interval) were less than one part in 106. In order to eliminate errors caused bychanges of frequency of the rf source that was used for proton resonance, a system(Fig. V-3) having frequency stability better than one part in 107 over the time of a meas-urement was used. The level-crossing and proton-resonance signals were observedsimultaneously on two oscilloscopes. See Fig. V-4 and V-5.TIMEFig. V-5. Level-crossing signal with scattering angle approximately 100*.Due to a change of line shape from pure Lorentzian to a mixture of dispersion andLorentzian line shapes upon a deviation of scattering angle from 90*, the minimumpoint of the line shape shifts away from the level-crossing point. The adjustment ofthe effective scattering angle to 90* was made by observing asymmetry of line shapewith oscilloscope display. See Fig. V-5.Results:Proton resonance frequency at crossing field of Hgl9930198. 14 + . 15 kc (Protons in 0. 01 molal H2 0 solution of FeC13)This value can be compared with the previous precision results:Reference Proton Resonance Frequency at Hg 1 9 9 Level Crossing30197. 37 ± . 15 kc30197. 95 ± . 36O. RediQPR No. 73(V. GEOPHYSICS)References1. F. D. Colegrove, P. A. Franken, R. R. Lewis, and R. H. Sands, Phys. Rev.Letters 1, 420 (1959); H. R. Hirsch, Bull. Am. Phys. Soc. 5, 274 (1960).2. R. D. Kaul, Ph. D. Thesis, Department of Physics, Case Institute of Technology,1963.3. W. W. Smith, Ph. D. Thesis, Department of Physics, M. I. T., 1963.D. OBSERVATIONS OF THE AEROSOL LAYER AT 20 kmThe existence of an aerosol layer at approximately 20 km altitude has been wellestablished by optical techniques1-3 and by direct sampling from balloons and aircraft23014 15 16 17 18 19 20 21 22 23 24 25HEIGHT (K m) -Fig. V-6. Intensity of echoes: curve a, experimental results; curve b,computed echoes from standard molecular atmosphere; curve c,ratio of curve a to curve b, interpreted as the ratio of the totalbackscattering cross section a- to the molecular backscatteringcross section aR'This work was supported in part by Purchase Order DDL BB-107 with LincolnLaboratory, a center for research operated by Massachusetts Institute of Technology,with the support of the U. S. Air Force under Contract AF 19(628)-500.QPR No. 73(V. GEOPHYSICS)(see, for example, Junge and Manson4). Preliminary results of observations of thislayer with an optical radar are reported here.The optical radar system, which has a pulsed ruby laser as the source of radiation,presents some changes and improvements over the instrumentation previously utilized.5'6The receiving telescope is of the Dahl-Kirkham type with 40-cm aperture. The detectorincorporates a mechanical shutter to prevent exposing the photomultiplier to the strongechoes from short distances. The wavelength is 0. 694 micron.At the present time, the technique gives profiles of the backscattering radar crosssection of the atmospheric constituents as a function of height; since the componentattributable to molecular scattering can be separately evaluated on the basis of knownmodel atmospheres, anomalous contributions caused by the presence of aerosols, in somecases, can be isolated. A profile of the relative intensity of the echoes from 14-15 kmaltitudes is represented by curve a in Fig. V-6. This profile was obtained by averaging20 successive traces to eliminate fluctuations; the traces were taken during a sixteen-minute interval (2247-2303 E. S. T. ), 14 February 1964, at Lexington, Massachusetts.Curve b in Fig. V-6 is a theoretical curve of the echo intensity for a model molecularatmosphere. Because an absolute calibration for the apparatus is not available at thistime, the relative position of the two curves is subject to uncertainty; in particular,curve b was constructed to agree with the experimental curve a at the extreme ranges.The signal at -20 km is much greater than is expected in the molecular atmosphere,indicating the presence of a large aerosol component. The ratio of the total backscat-tering cross section T to the molecular backscattering cross section a-R was evaluatedfrom the previous curve, and it is shown as curve c in Fig. V-6. This curve is in fairagreement with published profiles of scattering cross sections for this layer. 3G. Fiocco, G. W. GramsReferences1. P. Gruner, Handbuch der Geophysik, Vol. VIII, Physik der Atmosphere I(Borntraeger, Berlin, 1942), pp. 432-526.2. E. K. Bigg, J. Meteorol. 13, 262 (1956).3. F. E. Volz and R. M. Goody, J. Atmos. Sci. 19, 385 (1962).4. C. E. Junge and J. E. Manson, J. Geophys. Research 66, 2163 (1961).5. G. Fiocco and L. D. Smullin, Nature 199, 1275 (1963).6. G. Fiocco and G. Colombo, Optical radar results and meteoric fragmentation,J. Geophys. Research (in press).QPR No. 73

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