Contains reports on six research projects

Chu, L. J.

Dyall, W. T.

Eckhardt, D. L.

Harris, L. A.

Henry, J. H.

Hung, C. S.

Mahlman, G. W.

Marcinkowski, C. J.

Martin, S. T.

Mayper, V., Jr.

Moats, R. R.

Mutter, W. E.

Nottingham, W. B.

Rotman, Walter

Sprague, L. E.

Wilkinson, M. K.

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I. MICROWAVE AND PHYSICAL ELECTRONICSA. HIGH-POWER MAGNETRON RESEARCHStaff: Professor S. T. MartinD. L. EckhardtJ. H. HenryV. MayperR. R. MoatsW. RotmanIntroduction. The construction of a 10-Mw pulsed magnetron at 10.7 cmremains the focus of a group of related researches. These are:(1) The fabrication of a suitable output window for waveguidecoupling of the magnetron, capable of sustained operation atan average power of about 10-20 kw.(2) The design and fabrication of a cathode free from the spark-ing or arcing troubles encountered with oxide cathodes athigh voltages and high currents.(3) A study of the mechanism of mode jumping observed as thepower output of a magnetron is increased.(4) Collateral with Item (31 a study of the buildup of oscil-lations in a given mode from the random noise generated bythe space charge.(5) A study of the noise generated by the space charge in acavity magnetron at voltages and currents below whichcoherence in the space-charge structure may be observed.The production of a suitably evacuated high-power magnetron scaledfrom the AX-9 has not yet been achieved. The first tube failed on thepumps because of the development of a split in the kovar cup which supportedthe output window. Assembly of a second tube has been delayed pending theresolution of minor troubles with the hydrogen supply which has preventedfabrication of a leakless ceramic window for this tube. The nroblem is toreduce the concentration of oxygen and water vapor to the point where thetitanium hydride brazing process, developed by the General Electric Company,is effective.Progress on the items enumerated above is detailed in the sectionsbelow. In summary, all of these problems are in an intermediate stage ofdevelopment and not yet advanced to the point where decisive conclusionsmay be reached.1. Ceramic WindowsStaff: D. L. EckhardtCircular ceramic windows having a diameter of about 11 in. havebeen successfully brazed into both chrome iron and kovar. Eutectic silver-1-solder was used. Wetting of the ceramic was obtained by the use oftitanium hydride. The seals were made with the ceramic in compression.They were tested over several cycles of heating to 4500C and 5000C, followedby cooling to room temperature. The vacuum continuously improved, finallygiving no registration on an ionization gauge nominally calibrated to-8indicate a minimum pressure of 10- 8 mm of Hg.Windows 31 in. in diameter have been made without cracking buteither the solder failed to wet a small portion of the seal, leaving a leak,or the metal surrounding it failed when stressed during evacuation. Thelatter defect is attributable to the flimsiness of the chrome iron obtain-able, not to the seal. It has been observed that even though extreme pre-cautions are observed to purify the hydrogen (passing it through calciumoxide, phosphorous pentoxide, hot magnesium, and hot copper, in succession),uniform results are not obtained in the resulting flow of solder on theceramic. Usually a black, sooty surface is evident on the exposed part ofthe solder. The composition of this is unknown. It would appear from thisexperience that vacuum firing is preferable, in spite of the inconvenienceof this procedure as compared to hydrogen furnace operation.2. Thoria CathodesStaff: J. H. HenryWork beyond that detailed in the last progress report has been heldin abeyance since September. It will be resumed in January.3. Mode SelectionStaff: R. R. MoatsA c-w rising-sun magnetron, designed as a voltage-sealed equivalentof the high-power 10.7-cm magnetron under construction, has been completed.Preliminary tests show the presence of at least two modes of operation atvoltages lower than the threshold voltage for the w-mode, and that the vol-tage ranges for these two modes overlap. Both are surprisingly strong. Inorder to obtain stable operation throughout the range of currents requiredto cause the shift in mode of operation, pulsing will be required. Becauseof the requirements a 50-70 microsecond pulse has been determined the mostsatisfactory for the experiment and present effort is to assemble thenecessary equipment.4. R-F Buildup TimeStaff: Walter RotmanAs discussed in the last progress report, the transient behavior of-2-B --L -L---- It III---~ ~-IYI-_IYL-~Xl~lli_---~-lt_^l ~I.^.- _I~-I^-_4_-1111 --~--~I1 -...-i.ll~--~*~-- ~I ----~.~LlllsYPII~-a QK-61 magnetron during the buildup of oscillations has been under investi-gation. A high-speed oscilloscope, the "Winterscopeh, allowing resolutionof time intervals approximating 10- 9 seconds has been employed to photographvoltage versus time of both the applied plate voltage pulse and the r-fvoltage appearing across a suitable load. Figure 1 shows typical experi-mental results. Curve (a) is the rise of the plate voltage as a function ofwo>40z-ILTIMEFigure 1. Voltage as a function of time. Curve (a) is theapplied plate voltage. Curves (b), (c), (d) and(e) are the peak r-f voltage on the load forseveral different values of loaded Q.time, and curves (b), (c), (d) and (e) are the rise of peak r-f voltageacross the load as a function of time for several different loaded Q's.All are plotted in arbitrary voltage units.A characteristic feature always observed is the apparently finitetime of several hundredths of a microsecond before any appreciable voltageis detected at the load. The exact cause of this has not been determinedexperimentally. The different apparent starting times for the different-3-loaded Q's are not known to be significant -- they may merely represent theuncertainty of time measurement from one experiment to the other.From such measurements, phenomenological quantities such as theelectronic conductance of the space charge as a function of r-f voltageacross, and current to, the magnetron vanes may be determined under tran-sient conditions. These data are considered essential to an adequateunderstanding of the mechanism of mode selection and the starting character-istics of the magnetron.5. Noise Generation in the Pre-oscillating MagnetronStaff: V. MayperFurther measurements have been made on the noise properties of themagnetron at voltages below oscillation at constant magnetic field with theeventual objective of a more complete understanding of the behavior of thespace charge. All measurements have been made on a type QK61 10-cm,16-cavity strapped magnetron.It has been noted that there is a sharp break in the noise behavior(as well as in the V-I plot) with increasing voltage, at a value well belowthe Hartree voltage of the lowest discoverable mode (in this case, then-mode). See Figs. 2 and 3. It is at the current that corresponds to this1000800600O 5 10 15 20 25SmaFigure 2. Principal features of magnetron plate voltage -plate current curve at constant magnetic field, inregion below the onset of definite oscillation.-4-QK61 PLATE VOLTAGE v& PLATE CURRENTCALCULATED HARTREE VOLTAGEBAND WIDTH OFOBSERVED NOISESPECTRUM BECOMES VERY SMALLCHANGE OF Ib WITH LOADBEGINNING OF NARROWING OF BAND WIDTHOF OBSERVED NOISE SPECTRUM~ -------L e~ -~I I--- -- -u~r~-~ _.......x.... . .-.~--. _~___ _ --r -U--YI~Us~ ---rY ..-F-F-0WzX, cmFigure 3. Noise power as a function of frequency in a QK61.Noise power reference level is that of a klystron,estimated to be about 20 db above a temperaturelimited diode at the same current.voltage thatat constant magnetic field, changes in plate current with loadbecome noticeable. Below this current (or voltage), the noise outputincreases exponentially with increasing plate current (an unknown fractionof this current is leakage to the end caps of the magnetron) and itsfrequency dependence may be represented by a constant-current noisegenerator in parallel with the equivalent parallel-tuned circuit of theloaded cold magnetron (see Fig. 4); above this current (or voltage) thenoise output becomes less and less dependent on plate current (or voltage),and it becomes more and more peaked in frequency as if the equivalent tunedcircuit were increasing in Q. This behavior continues up to and throughthe point at which definite oscillations can be detected; this latter pointdoes not seem to be well-defined, but rather might be represented as thepoint at which the gradually narrowing bandwidth approaches infinity (orstrictly, becomes limited by the non-linearity of the electronic conduct-ance). The suggestion has been made that below the point of transition(in noise properties) the magnetron is acting purely as a d-c magnetron.Above this point phase focusing starts to produce the familiar spokes inthe space-charge cloud. If one regards the magnetron as a form of tunedtraveling-wave amplifier with its output run directly back into its input,then the bunches become merely a form of feedback mechanism, and the-5-n*0WCl)azIbmaFigure 4. Noise power output as a function of plate current.decrease in bandwidth is similar to that in a low-frequency regenerativefeedback amplifier when the feedback voltage is increased.The noise behavior with varying magnetic field is more complex, andshows peculiarities not as yet completely understood.The actual mechanism of the noise production is uncertain. Adefect of the experiment is the presence of end-cap leakage in the commer-cial magnetron used for the measurements; this factor is apparently quitevariable in a given tube over a moderately long time, thus makingmeasurements inconsistent and difficult to interpret. At present a tube isunder construction which will eliminate this difficulty by having its endcaps insulated from the oscillating cavity structure. It is expected thatsuch a tube will be more consistent in its behavior, and that plate currentreadings will have more fundamental meaning.-6-~_111^ .-~~---II._ _  I~IIC~XIPIII ^ _I.. .~-CXIIPICL~-P*III. B. CATHODE RESEARCH1. Conductivity and Work Functions of Oxide CathodesStaff: G. W. MahlmanSeveral experimental tubes have now been constructed which aredesigned to measure the photo-electric and thermionic work functions andthe coating conductivity of an oxide cathode simultaneously. Only one ofthese, recently off the pumps, appears to be satisfactory in all respects.Several difficulties have been encountered in the construction and opera-tion of these tubes. Cathodes having a coating weight exceeding 30 mg/cm 2crack, in some cases so badly that the nickel sleeve beneath is readilyvisible. Badly cracked cathodes are rejected as unsuitable for coatingconductivity measurements. Coatings of insufficient thickness result eitherin the shorting of the embedded probe to the nickel sleeve or in the probe'sbreaking through the coating, since the oxide coating shrinks about 30 percent in the process of activation. Coating weights of 20-30 mg/cm 2 are themost satisfactory for the single probe measurementsMeasurements were made on one tube of currents in retarding andaccelerating fields. Schottky plots were straight with a slope of abouttwo times the theoretical Schottky slope, over the range of temperatures385-571 K, provided the power into the anode did not become too great. Toolarge currents or voltages caused an increase in emission with time overthe straight-line portion of the Schottky plot. Reproducible retardingpotential data yielded curves (log I vs. V/T, where I is tube current, V iscollector voltage, and T is cathode temperature) whose slopes approachedasymptotically the theoretical slope corresponding to the cathode tempera-ture as measured by a thermocouple. The interpretation of these curves isotherwise doubtful at the present time. Tube leakage was excessive, andthe probe was found to be open, so that no further work was done on this tube.Results on the newest tube indicate an anomalous behavior in plotsof probe voltage (with respect to the cathode) vs. probe current, but probevoltage vs. tube current plots are linear, and thus enable measurements ofcoating resistance to be made. It is planned to measure the variation ofoxide-coating conductivity with temperature and work function of the oxidecoating.2. Spectral Emissivity of TungstenStaff: Professor W. B. NottinghamW. E. MutterA number of measurements have now been made in the low-temperaturerange 11000 to 16000K. It has been found necessary to regulate the outputvoltage of the d-c generator used to operate the tungsten source. This has-7-been done by controlling about 10 per cent of the field excitation currentwith a 6L6 tube. The generator voltage is bucked out by a battery and devia-tions from balance are indicated on a galvanometer. The light beam of thegalvanometer plays on a phototube which supplies correction signals to thegrid of the 6L6. The generator voltage may now be held constant to 0.02 percent during the course of a measurement (about 15 minutes).Figure 1 shows a typical curve of plate current of the 931A detectoragainst distance of travel of the 0.005-in. square aperture of the mono-chrometer along the axis of the image of the source. The peak correspondsto the passage of the aperture over the 0.0175-in. hole in the tubularlight source. The current value "An measures the power per unit area perunit range in wavelength from the miniature black-body cavity, while "B"measures the same quantity for the tungsten surface. The spectral emis-sivity for the wavelength for which the monochrometer is set is then simplythe ratio B/A.The values of emissivity obtained are about 5 per cent high comparedto Worthing and Forsythe's1 data at N = 6650 and 4670, the only wavelengthsat which data are available. Since all expected errors would lead to highemissivities, the design of the experiment has to be carefully considered..25Co A0I-zw.150Ba-I-.150S.vv .vI .U6U .U0U JiODISTANCE OF TRAVEL ALONG AXIS OF SOURCE IMAGE (INCHES)Figure 1.1. A. G. Worthing and W. E. Forsythe, Astrophysical J. 61, 146 (1925).-9-0A - IDT u AIoBANDWIDTH IAI5o Io,/A- 4865o B/A =.486~- -*1IPI1 -"~ L-l~---- L---- X- -^ -)I-I-~I Y-~--I*Li ~ ---sieLI~ IY ~ ~-(-aslil^ y ~~ slUP*IYIPEICY'"-I^;-i~Cu~x^(rc~ilrarrr~- .Ilr-.r----r-. t. .-r-rrl..-r._-^.--l-- - ---- - ^.~ll x-~-m~- *rrc- - -~---~LI~I~- ~L~I~.. .Measurements with different size holes opening into the black-body cavityof the present source yield the same results. This indicates that theapproximation to a true black-body radiator achieved in the source design issatisfactory. Emissivity values obtained by the standard pyrometric method(at X = 6650A) correspond to those of Worthing and Forsythe within I to 1per cent. Thus the surface state of the tungsten is probably comparable tothat of their source. It appears that the high values obtained are causedby a background illumination arising from the scattering of light from thewalls of the glass envelope containing the source. This difficulty isbeing eliminated by improvement of the optics of the system as well as bya redesign of the envelope.3. Electron Emission in Accelerating and Retarding FieldsStaff: Professor W. B. NottinghamC. S. HungAll experimental equipment including well-regulated power suppliesand an electrometer setup has been completed. Minor changes and repairs ofthe experimental tube, however, have only now been made and experimentalwork will be undertaken soon.C. IONIZATION GAUGE RESEARCHStaff: Professor W. B. NottinghamL. SpragueDuring the past quarterly period, the ionization gauge that wasdescribed previously, has been tested and shortly after the first testswere begun, the gauge became inoperative, because of the loss of a connec-tion lead. Prior to this, a study of the phenomenon of gas absorption on afilament within the tube showed that the gauge design was probably workingsuccessfully and really measuring the ionization as distinct from some ofthe spurious currents that make accurate readings in the neighborhood of108 mm pressure impossible with conventionally designed ionization gauges.This phenomenon of gas absorption and subsequent measurement was one pointedout to us by Dr. Apker of the General Electric Company. More detailsconcerning the usefulness of this test will be made available in the nextreport.-9-D. PROPERTIES OF CATHODE-RAY TUBE SCREENSStaff: Professor W. B. NottinghamW. T. DyallInvestigations of persistence characteristics of cathode-ray tubephosphor screens has continued through this quarterly period. A technicalreport covering tubes having P7, P14, and P4 type screens is now beingwritten.E. DETERMINATION OF EMISSION PROPERTIES OF SINGLE CRYSTALSStaff: Professor W. B. NottinghamC. J. MarcinkowakiM. K. WilkinsonAccording to present plans, the thermionic emission will be measuredby a photometric method, using the light produced by electron bombardmentof a fluorescent screen. A promising method of depositing uniform coatingsof phosphor inside a glass tube has been developed. Tube components,including several polished filaments of tungsten specially processed forlarge crystal growth, are being prepared.F. TRAVELING-WAVE AMPLIFIER TUBESStaff: Professor L. J. ChuL. A. HarrisThe last quarterly period has been devoted to a study of theproblem of producing and maintaining a narrow high current density electronbeam. This phase of the work has been completed, and it is felt thatadequate electron beams for the traveling-wave tube can be obtained withthe Pierce electron guns that were studied. Work is now going forward onthe actual construction of two complete tubes. Many of the components havebeen completed and the remainder are under construction.The electron-gun study indicated that a strong longitudinal magneticfield should be applied over the entire length of the tube. Accordingly,a large air-core electromagnet has been designed for this purpose and isnow being built.The auxiliary waveguides and tube supports to fit inside the magnetand to provide for accurate alignment of the tube are being designed,In addition, numerical calculations, based on previous theory, havebeen carried out. These provide design data for obtaining the helix dimen-sions for maximum gain at various wavelengths.1. L. J. Chu and J. D. Jackson, "Field Theory of Traveling-Wave Tubes", RLETechnical Report No. 38, April 28, 1947.-10-_~_L _~~_I II __ 1_1___4 ~_~~ I_~L____C_^ ~ _ Y .- ^-LI- ^lll ~-_~_*----r-.--^L-_ .~l^~C111-  -^I~--~UIX_^ __L --ill&l~- - ..__..... Illllll~i-I.L-l- I~I-~^X-P~I~C

Microwave and Physical Electronics

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Microwave and Physical Electronics

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