Contains reports on two research projects

Baghdady, Elie J.

Barrett, A. G.

Berk, A. D.

Boekelheide, A. W.

Epsztein, B.

Fried, C.

Haus, Hermann A.

Lax, B.

Martin, S. T.

Muehe, C. E., Jr.

Rowe, H. E.

Russell, C. R.

Smullin, Louis D.

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VIII. TUBE RESEARCH AND DEVELOPMENTA. MAGNETRON DEVELOPMENTDr. S. T. MartinA. G. Barrett1. Testing and Design of High-Power 10. 7-Cm MagnetronsAssembly of MF-13B magnetron was discontinued after a leak developed at thewindow during a second attempt to braze the window assembly to the output section.Magnetron MF-14B was assembled successfully with the structural and cathodechanges described in the last Quarterly Progress Report and placed on the pumps.These structural and cathode changes included oversize cathode end shields, new endplate rings, and the addition of carbonyl process nickel powder to the cathode coatingmixture in equal proportions with the carbonates. During the preliminary stages ofbakeout a leak occurred in a cathode lead metal-ceramic seal while the temperature was450 0 C. Efforts to patch the leak failed, and since the pressure was 1. 2 x 10-5mm Hgat 425 0 C, it was decided to continue baking by raising the temperature to 700'C in threesteps, as an external atmosphere of very pure nitrogen was surrounding the tube. Hencethe leak was not likely to affect the cathode.Upon cooling the tube down after it baked for 19 hours at 700 0 C, we found it to be1300 GAUSSXo= 10.68CMCONSTANT VOLTAGE90" IN VOLTAGEFig.Rieke diagram forVIII-1magnetron MF-14B.-24-(VIII. TUBE RESEARCH AND DEVELOPMENT)50a l-Lsec pulse.The pulse transformer was changed to the 3:1 ratio, 20-Mw unit built especially forthe project, and the following maximum performance data were observed:Peak output 5. 0 MwPeak input 10. 6 MwEfficiency 47. 5 percentPulse length 1. 1 LsecRepetition rate 120 ppsPeak voltage 47. 1 kvPeak current 225 ampMagnetic field 2420 gauss.Operation at this power level was accompanied by severe internal sparking withcurrent flowing as long as 10 psec for many successive individual pulses. The sparkingcontinued to increase in intensity after reaching this maximum power level, and subse-quent operation of the tube was marked by a sharp decline in output, a shift in the mag-netic field requirements, and a change in dynamic impedance as indicated in theincomplete performance chart shown in Fig. VIII-2. Only a few points were secured-25-r45coI-io 400o,04--, 350W3025ICMFMagin g3:1vacuum tight. The dc primary emission fromthe cathode was very low (approximatelyS56soa 7 ma/cm 2 at 600 volts and normal heater power2420 5.MW of 200 watts). Such poor cathode performance471%is believed primarily due to processing condi-2210 4.35 MW473% tions rather than to the cooling effect of the large58R end shields, since the current density increased2 95MW/ 2. 1910 to only 20 ma/cmZ at 600 volts and a heater2.33MW29 7 1810 power of 275 watts.- 2.73MW1600 2?- .7o Despite its low-emission cathode, magnetron- 2 3. MF-14B performed well on the test bench. By34.1%using a 2. 25:1 ratio, 10-Mw pulse transformer* I.IS 2809 MC/SEC BEFORE CHANGE- l.3LS 2806MC/SEC AFTER CHANGE originally designed for the HP-10V magnetron,peak power outputs of 2. 5 Mw to more than 4 Mw0 150 200 250 300 350 were obtained at 40 percent efficiency with aPEAK CURRENT AMPERES 1-psec pulse. With a 5-pLsec pulse the tubeFig. VIII-2 delivered 1-Mw peak and 650 watts average-14B performance chart, power at 50 percent efficiency. Output and effi-netic-field values shown are ciency were limited by what appeared to be anauss. Repetition rate 120 ppsauss. Repetition rate 120 pps internal parasitic oscillation involving the cathoderatio pulse transformer.and cathode lead structure. Figure VIII-1 showsa Rieke diagram taken at a low power level withcI0(VIII. TUBE RESEARCH AND DEVELOPMENT)Fig. VIII-3MF-14B magnetron.because of the sudden change in the magnetron. In additionto sparking, the tendency to misfire and become unstablealso increased as the power output decreased.Accordingly, magnetron MF-14B was removed fromthe test bench after a total operating time of 32. 6 hours.Figure VIII-3 is a photograph of the tube with its highvoltage connectors attached. A resonance curve was takenwhich disclosed a large decrease in the unloaded Q of themagnetron while there was no significant change in theloaded Q.In order to account for these changes in performanceand characteristics, the magnetron was cut apart for exam-ination. The cathode was severely pitted, and powderednickel on the surface was vaporized by sparking at the5 - Mw level. Figure VIII -4 shows part of the surface areawhich was opposite the output slot upon which copper wassputtered from hot spots on the adjacent vane tips. Thebrighter spots in the picture are due to reflections fromthe nickel mesh exposed by the sparking.The interior surfaces of the anode block were discol-ored by sparking and vaporization of the nickel powder.The end surfaces of the laminations showed signs ofheavy sparking. The output circuit surfaces remainedFig. VIII-4Photomicrograph of cathode surfacemagnified ten times.-26-(VIII. TUBE RESEARCH AND DEVELOPMENT)uncontaminated except for a dark band across the inner surface of the ceramic windowparallel to and opposite the output slot. No conductance could be measured from endto end of the band even though it appeared to be composed of nickel.The results obtained from magnetron MF-14B indicate that its performance was lim-ited by the cathode. Hence, it is entirely conceivable that a magnetron provided with amore rugged cathode, such as the sintered molybdenum-thoria type, and designed toeliminate spurious resonances will produce substantially more pulse power at S-band.Research Laboratory of Electronics Technical Report No. 242 will summarize thenow complete experimental work on this project.-27-(VIII. TUBE RESEARCH AND DEVELOPMENT)B. MICROWAVE TUBESL. D. Smullin A. W. Boekelheide C. E. Muehe, Jr.Prof. L. J. Chu B. Epsztein H. E. RoweE. J. Baghdady C. Fried C. R. RussellA. D. Berk H. A. Haus L. Stark1. Noise and Space-Charge Wavesa. A multivelocity electron beam in a cylindrical drift tubeAn attempt has been made to explain the experimentally observed variation of thestanding-wave ratio of noise waves in a drift tube with a longitudinal magnetic field. Thetheoretical formula for the standing-wave ratio in the presence of a velocity spread asreported on page 42 of the Quarterly Progress Report, April 15, 1952, has been applied.The following assumptions are made:(a) The beam is completely neutralized.(b) The electrons entering the magnetic field obtain a transverse velocity accordingto Busch's theorem at the expense of their longitudinal velocity. Thus, a velocity spreadis introduced which is a function of the magnetic field.(c) The effect of the velocity spread can be treated by an infinite parallel-planeanalysis by introducing the usual correction factor for the plasma frequency.The equations for the fractional velocity spread a/W and the standing-wave ratio are:a l(rlB)2W - 16V0SWR = i PwhereTr is the charge-to-mass ratio of the electronr 1 is the maximum radius of the beamB is the magnetic fieldp /w is the ratio of plasma frequency to operating frequencyV is the voltage of the beam.In plotting the "theoretical curve" in Fig. VIII-5, the experimental values for theplasma wavelength, and F 2 are inserted into the theoretical formula. The space-chargesmoothing factor F 2 expresses the ratio of the maximum mean-square noise currentfluctuations 2eI AfF 2 to pure shot noise fluctuations 2eI Af.It can be seen from the figure which represents a typical plot of one of several exper-iments that there is no agreement between this simple theory and experiment in the-28-(VIII. TUBE RESEARCH AND DEVELOPMENT)IO= 4 MAV =IO00VOLTSr, 0.028 INCHESr2 = 0.050f = 2735 MC\SWR (FIRST MINIMUM TOFIRST MAXIMUM)SWR (FIRST MAXIMUM TOSECOND MINIMUM )180 360 540 720MAGNETIC FIELD IN GAUSSFig. VIII-5Standing-wave ratio vs magnetic field.region of low magnetic fields. The agreement becomes quite good at large magneticfields. It seems reasonable to consider the velocity spread due to the magnetic fieldto be responsible for the reduction of the standing-wave ratio at values of the magneticfield approximately two and a half to three times Brillouin field.A similar experiment in which the gun was situated in the magnetic field showed noinfluence of the magnetic field upon the standing-wave ratio. In this case the electronsall make similar spirals about their individual orbits, so that no velocity spread isintroduced by the magnetic field.The experimental work of C. E. Muehe, Jr. has been written as a Master's Thesisfor the Department of Electrical Engineering and will appear shortly as a ResearchLaboratory of Electronics technical report.C. E. Muehe, Jr., H. A. Hausb. Calculation of noise in a finite diameter beamThis work is being prepared as a Doctoral thesis for the Department of ElectricalEngineering.H. E. Rowe-29-I i I I(VIII. TUBE RESEARCH AND DEVELOPMENT)2. Traveling-Wave Amplifiersa. Double helicesA bifilar helix has been built for the pulsed, 3-cm, 10-kv, traveling-wave ampli-fier. No difficulty was experienced in matching it to the coupling waveguides.A demountable system is being assembled for testing pulsed tubes of this type.E. J. Baghdady, C. R. Russell3. l-Mev Pulsed Electron SourceThis work has been summarized in a Master's Thesis for the Department of Elec-trical Engineering, and in a Research Laboratory of Electronics technical report whichwill appear shortly.A. W. Boekelheide4. Internally Coated CathodesA series of preliminary experiments has been conducted on a new type of cathode.In the present experiments two different configurations have been used. In Fig. VIII-6athe inside of the cathode cup was all oxide-coated, and current was drawn through a0.0135-inch diameter hole to an anode 0. 1-inch away. One theory of the operation ofthis cathode is that the interior of the cup contains a neutral electron gas, produced byvirtue of the positive ions known to be emitted from oxide cathodes. Another theoryassumes that current is only drawn from the ring of coating immediately adjacent to thehole. The cathode shown in Fig. VIII-6b should not produce an electron gas, since onlythe top is coated.Figure VIII-7 shows the current-voltage characteristics as a function of brightnessANODE -- HOLE.-OXIDECOATINGCATHODESLEEVE-- HEATERFig. VIII-6Internally coated cathodes.temperature for the cathode shown in Fig. VIII-6a.Currents up to 60 ma were drawn, with densities up to 60amp/cm 2 . The tube never obeys Child's Law (I- V3/2.There is never a true temperature-limited region nor aspace-charge-limited region of operation. Figure VIII-8shows the noise-reducing properties (" 2 ) of the cathode,measured at about 2 Mc/sec and at various cathode tem-peratures. The minima were real and reproducible.We note that the least value of r 2 is lower than iscommonly reported for conventional cathodes.In order to check the electron gas theory, a strongaxial magnetic field was applied to the cathode. Withlarge anode voltages, no change in current was caused-30-1_I V '7 [T= 988.C]Is VI'. [T 947GC]I VI 04[T 912 GC]I0 100VOLTSFig. VIII-7Voltage current characteristics.T 850*G947OGT= 988oC10 100VOLTSFig. VIII-8Ratio of noise power to shot noise vsvoltage. (Frequency = 2. 45 Mc/sec.)n.u Yv(VIII. TUBE RESEARCH AND DEVELOPMENT)by the field. At low voltages (< 10 volts), reductions of about 20 percent in current wereobserved. It was postulated that the field would prevent current from the side wallsfrom reaching the hole if the cathode emission occurred as an electron gas. The nega-tive results of this test led to the construction of the cathode shown in Fig. VIII-6b.Here, only the inside of the top surface was coated and the heater was inside the cathodeproper. This tube gave substantially the same current as the first so that it now appearsthat emission is from the edge of the hole rather than from a gas. The very largecurrent densities are tentatively explained by the fact that the coating is shielded fromthe anode and therefore escapes contamination from that source.A number of hysteresis phenomena have been observed, both long and short time-constant. Further tests will be made on multiple-hole cathodes and radially emittingcylindrical cathodes.L. D. Smullin, B. Epsztein, C. Fried5. The Use of Ferrites at Microwave Frequenciesa. Impedance of a cavity containing a generalized mediumThe investigation of the input and transfer impedances of a two-input cavity con-taining a generalized medium has been extended to include the case for which both oand [. are tensor quantities. (See Quarterly Progress Report, July 15, 1952, pp. 60-63,for a precise formulation of the problem.) The method followed is that given by Slater(ref. 1). It has been necessary, however, to extend it by introducing magnetic currentsin order to include the effect of inhomogeneity in the magnetic susceptibility within thecavity. In this general case the impedance matrixZ 11 Z12Z 1 Z22is given bykSK(1 + J )- AJ pZl l = V a (1)kK J - A (1 + J~ppZ 12 = vavL 2 A (2)-32-(VIII. TUBE RESEARCH AND DEVELOPMENT)kKak Jap A (l + Jaa)Z21 Val p2 a(3)kK a(1 +J PP)- A J az V2 P (4)whereJap Ha m X H dv with analogous meanings for J a' J a and Jpvol.Xm = magnetic susceptibility tensork2 - kZ(l +J ) kK a a c +I (+J ) + aa jkc o a a papkA = (jkc + I ) I + I (1 + JPP).K and A are obtained by interchanging a and p in the expression for Ka and Au'respectively. Finally, A = KaK - A A . The rest of the symbols have the samemeaning as those in reference 2.For a single input (vp2 = 0) the driving-point impedance of the cavity becomesz. = z (5)n 11Note that the expressions for driving-point and transfer impedances given in refer-ence 2 are special cases of the preceding expressions and may be obtained from thelatter by putting xm = 0.A case of particular interest is that of ferrites. The previous expressions simplifyconsiderably if this medium (the ferrite) is placed in a region of vanishing electric fieldand intense magnetic field. Then all four integrals of the type Iap (see ref. 2) becomevanishingly small and can be neglected.As an illustration consider a cylindrical cavity with a small piece of ferrite at thecenter of one of the faces, and a steady magnetic field along the axis of the cylinder.Here a and p represent the two degenerate TE111 modes (in space quadrature). Forthis particular case the input impedance (Eq. 5) becomes[k - k 2 (1 + TXc ) (1 + TXP) + (k-TK4)Z = jkclova l  2 2S[k2 a - k (1 + TX2 )] -(,Kk 2-33-(VIII. TUBE RESEARCH AND DEVELOPMENT)where T = volume of ferrite= a numerical factor depending on the dimensions of the cavityand X and K are defined by the expression for the magnetic susceptibility tensorXXm =jK0-j KX0and they are both functions of the frequency.A. D. Berk, B. Lax (Lincoln Laboratory)References1. J. C. Slater: Microwave Electronics, Van Nostrand, 1950, Ch. IV2. Quarterly Progress Report, Research Laboratory of Electronics, M. I. T. July 15,1952, pp. 60-63-34-

Tube Research and Development

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Tube Research and Development

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