During the summer of 1957, The Martin Company, Denver Division, expressed a need for experimental data which would enable more exact design of surface- mounted antennas on missiles which would avoid corona and voltage breakdown in the higher atmosphere. Inasmuch as some experimental data were then avail* able, a request was also made for a literature search on voltage breakdown is air as a function of pressure, frequency, pulse length, electrode spacing, and degree of ionization. Contract No. DEN 57-5652, under Prime Contract AF 04 (645) 56, was initiated with the Purdue Research Foundation in January, 1958, to obtain (l) a literature search and (2) experimental data covering a range of the parameters listed above. The literature search was delivered to The Martin Company on November 25, 1958. The experimental data are the subject of this report

George, R. H.

Hayt, W. H., Jr.

Heim, H. J.

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Technical ReportsDepartment of Electrical and ComputerEngineering4-30-1959VOLTAGE BREAKDOWN IN IONIZED AIRAT FREQUENCIES FROM ZERO TO 10,000MEGACYCLESW. H. Hayt Jr.Purdue UniversityH. J. HeimPurdue UniversityR. H. GeorgePurdue UniversityFollow this and additional works at: https://docs.lib.purdue.edu/ecetrThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Hayt, W. H. Jr.; Heim, H. J.; and George, R. H., "VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZEROTO 10,000 MEGACYCLES" (1959). Department of Electrical and Computer Engineering Technical Reports. Paper 501.https://docs.lib.purdue.edu/ecetr/501TR-EE61-11VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZERO TO 10,000 MEGACYCLESW. H. Hayt, Jr.H. J. Heim R. H. GeorgePurdue University School of Electrical EngineeringTHE PURDUE RESEARCH FOUNDATION Lafayette, IndianaforApril 30, 1959VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZERO TO 10,000 MEGACYCLESSponsors The Martin Company Denver Division Denver 1, ColoradoContracts DEN 57-5652Under Prime ContractAF 04 (645) 56Report PreparedbyW. H. Hayt, Jr. H. J. HeimR. H. GeorgeEngineering Experiment Station, Electrical Division Purdue University Lafayette, IndianaforThe Purdue Research Foundation Lafayette, Indiana Funds 1785-61-780April 30, 1959VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZERO TO 10,000 MEGACYCLES1. Introductiona. BackgroundDuring the summer of 1957, The Martin Company, Denver Division, expressed a need for experimental data which would enable more exact design of surface- mounted antennas on missiles which would avoid corona and voltage breakdown in the higher atmosphere. Inasmuch as some experimental data were then avail* able, a request was also made for a literature search on voltage breakdown is air as a function of pressure, frequency, pulse length, electrode spacing, and degree of ionization.Contract No, BEN 57-5652, under Prime Contract AF 04 (645) 56, was initi­ated with the Purdue Research Foundation in January, 1958, to obtain (l) a literature search and (2) experimental data covering a range of the parameters listed above. The literature search was delivered to The Martin Company on November 25, 195&, The experimental data are the subject of this report,b, Experimental Data RequiredThe experimental data requested in the contract are shown as curves of. electric field intensity at voltage breakdown plotted as a function of pressure in air. The breakdown is to oecur in an essentially uniform field, and measure­ments are tp be made at the frequencies, pulse lengths, and electrode spacings listed: ;Zero frequency:pulse length: continuouselectrode spacing; 1 mm and 4 mm200, 500, and 1000 mcs;pulse lengths 10 /user, or longerelectrode spacing : - - ' ' 4 mm: 3000 and 10,000 mcs- ;pulse length: 0.25 and 2.0 /usec.electrode spacing: 1 and 2 mmFor each frequency*pulse length, and electrode spacing combination,  three values of ionization density are to be  used: the ambient ionization present inthe laboratoryenvironment,artificial ionization sufficient to reduce the breakdown field strength by at least a factor of two, and one intermediate value. -Ionization Sourcesa. Cobalt 60fen milllcuries of the artificial radioactive isotope cobalt 60 were pro- cured and installed In a specially designed, lead shielded container.. The con­struction of this container was such as to produce a reasonably collimated beam of gamma rays vertically downward from the bottom of the vessel. -Provisions were also made to allow for two different intensities by the movement of the isotope within the sample holder.Following the methods used on pages 232-233 ©f "Microwave Transmission Circuits", vol, 9, Eadiation laboratory Series, a crude estimate of the ion density may be made. Ten millicuries of cobalt 60 gives 3.7 x 10® disinte- grations per second; perhaps one per cent, or 3.7 x 10w gamma rays, areemitted in the desired direction; of these, some three per cent are effective in producing secondary electrons from the brass cavity walls and, therefore, about 1©3 ionizing events per second result, Each secondary electron produces perhaps 100 ion pairs per centimeter of path length, and, if an effective cross section of 3 cm^ is assumed, the ionization intensity is roughly 3x1©^ ion pairs per second per cubic centimeter.Since the generation and recombination of ion pairs is governed by thesteady state relationship,% -fi»2where % is the rate of generation (ion pairs per second per cm3), n is the ion density (ion pairs per m^), and 0 is the recombination coefficient, about 1©~^ in air at atmospheric pressure, a value of 3 x 10^ for nQ leads to about 2 x 106 ion pairs per ew?9 This is only a crude estimate and is most applicable for the 10,00© mes equipment at atmospheric pressure. The lower frequency experiments employed physically larger devices and consequently had somewhat smaller ionization rates and densities. The ionization rate also drops with the pressure.Without any artificial means of creating an appreciably ion density, it is estimated by the Radiation laboratory that natural sources, such as cosmic rays and natural radioactivity, produce a rate of generation of 10“^ ion pairs per second per cm? and a resultant ion density of 3 x 1©^ ion pairs per cm?.Judging from the long periods of time that it was necessary to spend waiting for breakdown to occur when no ionization was used, this estimate appears to be on the high side, b« Point BischargeIonization of better known and more easily controlled intensity was also-4-obtained from a point discharge, A negative voltage of a few thousand volts was applied through !# megohms to a fipely sharpenedplatinum-iridium point.A grounded ring surrounding and just ahead of the point produced a high elec— trio field- intensity at the point and a resultant electron current, A small, controlled, air leak behind the point assisted the electric wind effeet in moving the electrons into the region of the-gap*....Above a pressure of about1 mm Hg, an air flow of approximately 10© cm^/minute was used, both to help propel the electrons and to insure renoval from the gapofrdtrogen compounds produced daring the breakdown.In order to estimate the electron density produced by this point discharge, a special vacuum chamber was constructed in which the electron current could be measured in a relatively small portion of the cross section of the beam. Pro­visions were xaade to allow this current density to be determined at various distances from the point. Knowing the current density J, then the relationshipwas used to determine the charge density ^ (coul/cm?) or electron density (electrons/cm?) in terms of the mean velocity v. This velocity was obtained by using an electroBtatic voltmeter tp measure the potential of the collecting screen. Prom the potential, the velocity was easily found byv - 5.93 x 10? /V. (cm/sec)where ? is in volts.Breakdown data were taken for nominal currents of 10 and 5© microamperes from the high voltage supply. The electron densities varied with the; pressure and cavity geometry between l©2 and 3 x 10^ electrons/ciP, Figures 1 to 4 show the logarithm of the electron density in electrons/cm? as a function of the logarithm of the pressure in mm Mg for the five different resonant sections.-5-(e**40/ Su°J+09/z) U ot6o7S7 i-6-(e uto/suojp9jaJ u 0,Bo-j-7-—8—The starves for 945 mcs and 937© mcs are identical.3. Experimental Methods a. 211. 443. and 945 mcsThe frequencies, 211, 443, and 945 mcs, correspond to thethreefrequenciesgiven in the specifications, 200, 500, and 1000 mcs, and were determined by the exact dimensions of the resonant cavities described below*Figure 5 is a diagram of the experimental equipment and Fig, 6 shows a representative resonantcavity inmore detail» The dimensions applicable to each of the three frequencies aretabulated on the figure.The Q of such a cavity is high and therefore a relatively small power in­put at the resonant frequency produces a large electric field intensity across the gap.A crystal detector and wide-band oscilloscope, together with suitable co­axial attenuators, were used to monitor the field strength in the gap by samp­ling the cavity fields at the output connection. After converting the oscillo­scope deflection to voltage, applying the experimentally determined crystal de­tector and attenuator calibrations, and using the measured ratio of cavity in­put power to output power, the peak power input to the cavity was obtained.The gap field strength was related to the input power by using Slater‘s per­turbation theorem for a cavity with a matched input,where Q « cavity Q with input matched Pim * peak input power (watts)" rgf - = shift in resonant wavelength (cm) per cubiccentimenter of inserted volume of perturbing plunger.-10-PointdischDrierFlowMeterAir Leak InFig. 5Equipment block diagram for voltage breakdown measurements at 211, 443, and 945 mes.Purdue University, April, 1959| ax/s JIZ/i mcsDetails of the 2 //, and ^45 mcs ecu/it its-12-Cylindrical plungers were installed in the long center conductor and thechange in resonant frequency determined as the plunger was advanced l/S© inch■ \into the gap and withdrawn am equal distance from the flush position. The Q and resonant frequency were measured* hy conventional methods.Peak power inputs of the order of 2Q0 watts were required to produce volt- age breakdown at atmospheric pressure with some ionization in the l#m gap.Since the pulse repetition rate for the 10 microsecond pulses was 20© ppsee, the average power required was only about one-half watt, b, 3300 and 9370 mcsAt each of the twomicrowave frequencies, the resonant cavities consisted of short-circuited waveguide sections which tapered down to a shorter section, approximately one-half guide-wavelength long and having a Mbwdimension equal to the desired electrode spacing. Sketches of the experimental equipment arrangement for 330© mes and 937Q mcs are shown in Figs, 7 and 8, respectively. Each evacuated section contains the point discharge, an air drying chamber, a metered air leak, provisions for manometer, thermocouple, and Pirani vacuum gages, and the exhaust line to the pump. The Sob© container is placed immed­iately above the gap when it is being used.The field strength in the gap at breakdown is determined by using a directional coupler to measure the circulating power in the resonant waveguide section, wbich extends fromthe short circuit to an iris following the E-I tuner. The relationship between the peak transmitted power and the peak elec­tric field intensity in the center of a matched guide excited in the TE^q mode is given by£ j-? jflZ7J ■ y-■$•$0 It ^ 2^' (wattsAIRFLOWMETERDRYING—.CHAMBERr ELECTRON II W INJECTOR (XI DIRECTIONALr*L MIIDIFP \«J(r—f \ VAC. SEAL—I fVALVE 1TTLIONIZINGCURRENTSUPPLYRESONATORCOAX. ATTN. PLUG-IN CONN./T~ COAX. ATTN.^TO SCOPE CRYSTAL.------- 1 POWER“V METER.THERMtSTO/t-----"-----§) EH £-HTUNERoITHERMO­COUPLE VAC. GAGE'1'TO PUMPATTENUATOR ROUND IRIS-D!A. 1.34 " 3"*/.5* WAVEGUIDEp—-MANOMETERGAPAPS-2 Transmitter 3300 MC.RESONATOR-SIDE VIEWEquipment Set-up for l/o/tage Breakdown in Air at 3300 Me. per sec. Figure 7PUR.DUE UNtVeHStTY APRIL !9£9/ * 0.5 WAVEGUIDE AIR. FLOW METER IONIZINGCURRENTSUPPLYDRYING—^ CHAMBERTO VACUUM GAGESSPLITTER ------ ------1 TUNERTO PUMPELECTRON ♦ fVAC. SEAL INJECTOR | | IATTENUATORVALVERESONATOR-TO MANOMETER VAC. SEAL y-GAiROUND IRIS-DIA. 0.4DIRECTIONALCOUPLER RESONATOR-SIDE VIEWTO SCOPETUNERCRYSTALATTENUATORCOAX. ATTN.TUNERTHERMISTOR.Equipment Set-up for Voltage Breakdown /n Ait at 9370 Me. per sec.I__POWERPURDUE UNIVERSITY APRIL 1959-15where,E = maximum peak eleetrie field intensity (v/cm)a = wave-guide width (larger dimension) (cm)b * wave-guide height (cm)0 = free space wavelength (cm)In a short-circuited section, the maximum peak electric field strength is twice Ey0, and, therefore,Pave 3-s determined by a thermistor and thermistor bridge connected to the directional coupler through calibrated attenuators or other directional couplers. For breakdown at atmospheric pressure, an average power input of about 4© watts was necessary,Neither the available 3300 mes or 9370 mes radar transmitters were designed for pulse widths less than a nominal one-half microsecond, which was actually measured as being closer to 0,6 microseconds between 90$ amplitude points. Shorter pulse lengths lead to greater possible field strengths before break­down,e„ Zero FrequencyFigure 9 is a diagrammatic sketch of the equipment assembled for the tests) te ra b \h~ (K/2sjzic/zo n(v/cm)where,Pave “ fp Y P “ average power (watts) fp = pulse repetition frequency (pps) T “pulse length (sec)Eg * gap field, strength (v/cm)m-16-Supply 15000 Vair leak valvebell jarmetal cylindert© scopeadjustable lower electroc*table to manometerto pumpEquipment Set-up forin Air at Zero FrequencyRirdue UniversityApril 1959-17-at zero frequency. The electrode assembly consists of a flat upper plate mounted above the adjustable mushroom-shaped lower electrode which rests on a brass plate, Both electrode surfaces'in the gap are silver-plated and polished, and the entire assembly is surrounded by a conventional glass bell jar, A second plate containing the electrical connection to the upper elec­trode and an air leak valve seals off the upper opening of the bell-jar, while the lower opening is covered by the brass table containing the pump, manometer, and vacuum gage connections. Whenever ©06© was used, the container was ©enter­ed on the top of the upper electrode, and when the isotope was not being used, it was replaced by a metal cylinder of the same dimensions.The d-e power supply, continuously adjustable from zero to 15,000 volts,’ ' Iis connected as shown in Fig, 9. The four megohm resistor serves to limit the current and the ten thousand ohm resistor is used to provide an indication of incipient breakdown on an oscilloscope.Data for the curves in Figs, 1© to 12 were obtained over the full range of air pressure, from about 75© Him Hg to a few microns, for the three electrode spacimgs of 1, 2, and 4 mm. Ionization of the gap, when used, was achieved by means of the cobalt 6© only and was always the maximum obtainable from the available 1© millicurie source. For each observation made, the voltage was raised slowly and noted at the point of incipient or of complete breakdown. The incipient value was always recorded when observable. The field strength was easily obtained by dividing the recorded voltage by the known electrode spacing.4. The Experimental CurvesThe curves appearing on Figs. 1© to 23 show breakdown electric field in-OOO'OO!Pressure, mm HgBreakdown field strenqih $ V/cM 1111111111111111! Hill Hill Hill Hill Hill Hill mui iiiiiiini ling mu mu iiiii iiiii iiiiiBreakdown field StrengthV/cm100,00010,000mo0.0/ / PressureJ mm HgBreakdown field strength v/cmO) Nl co <o-i 000!I OOO'Q!OQO'OO!Breakdown -field strenjihv/cmfoo.ooo* O10,000Pressuret mm Hg moBreakdown field •strength M/cm y®!!ll!!m!Lti!!!nM!ii;3!Pressure , mm MqBreakdown field strengthv/cmmm HgPressurePressure^ mm Hgooo'ootBreakdown field strengthPressure, mm Hg12By unu ‘sjnssojytoooooooiPressure, mmBreakdown field strengthv/cm 1PressureBreakdown field strength &v/cm %Pressurebreakdown field strength §v/cm §imiiiimiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiiiiiiiiimiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii00 <0obvious and statements maytensity in volts/cm as a function of pressure in mm of mercury for the various frequencies, pulse lengths, electrode spacings, and ion densities given on the figures. The curves are arranged by increasing frequency first, then by in­creasing electrode spacing for each frequency, and finally by increasing pulse length.Several general (and, in some cases, be made which apply to all of the curves?a. Breakdown at atmospheric pressure occurs at about 30,000 v/em.b. The minimum breakdown field strength occurs within the approximate pressure range of 1 to 10 mm Hg.c. The effect of cobalt 60 and of b tion is about the same.d. Smoother Curves with less scattering of the points are obtained with some ionization. Such ionizatio ionization or radiation,e. Without ionization, the field st than that needed with some stabi value may be reduced somewhat ifoth values of point discharge ioniza-n has been aptly termed ,,stabilizingMrength required for breakdown is larger lizing ionization. However, this larger the field is applied for a longer period of time. Stated another way, it it apparent that, in a given interval of time, the probability of breakdown decreases as the voltage is reduced. The breakdown field intensity must, of course, be larger than that required with ionization. When stabilizing ionization is em­ployed, breakdown occurs simultaneously with the application of the field or of the ionization.Some of the data at pressures below the point of minimum breakdown field intensity are not directly applicable to the electrode spacings indicated-33-because the breakdown moves away from the center of the gap and occurs across a longer path, This will also occur in any system in practice, such as in the region surrounding a slot antenna. This effect was readily observed at zero frequency because the gap was visible inside the bell jar at all times.The curves at zero frequency are drawn with a solid line in the pressure range for which the discharge occurred in the gap, and with a broken line for the lower pressures where the discharge moved out across a longer path.Ail of the curves at the other frequencies show the field strength in the gap at all pressures, but breakdown is probably taking place across a longer path for pressures below the curve minimum. Thus, if the breakdown were re­strained to oceur in the gap, the curve obtained would lie above the one shown for that gap by an amount dependent on the geometry of the cavity. Safe de­signs are achieved by designing for the minimum breakdown field strength.A comparison of the several figures indicates these general conclusionssa. Higher field strengths are possible with smaller electrode spacings«b. The safe may-irmnn field strength before breakdown for large electrode spacings is somewhat less than that indicated for the largest spacing used, 4 mm,e. Greater field strengths are permissible with shorter pulse lengths,d. Greater field strengths arepermissible with lower pulse repetition rates.e. The breakdown field strength shows little change when-the product of pulse width and repetition rate is maintained constant*f. Breakdown at atmospheric pressure Is relatively independent of fre­quency.g. The maximum safefield intensity in the neighborhood of 1 mm Hg ‘is smallest at 200 mes and has a value of about 160 ?/cm for the 4 mm gap.

VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZERO TO 10,000 MEGACYCLES

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VOLTAGE BREAKDOWN IN IONIZED AIR AT FREQUENCIES FROM ZERO TO 10,000 MEGACYCLES

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