A. MAGNETRON DEVELOPMENT. Prof. S. T. Martin J. G. Lawton A. G. Barrett R. R. Moats D. L. Eckhardt R. J. Renfrow. 1. High-Power 10.

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1 VI. TUBE RESEARCH AND DEVELOPMENT A. MAGNETRON DEVELOPMENT Prof. S. T. Martin J. G. Lawton A. G. Barrett R. R. Moats D. L. Ekhardt R. J. Renfrow 1. High-Power 10.7-Cm Magnetron a. Testing and design To failitate the onstrution of the high-power magnetron, the entire output setion, inluding the quarter wave transformer and window, has been redesigned. The new design is based on standard S band waveguide. The quarter wave transformer was hanged to inrease the external loading of the tube. Cold test measurements on a tube with the redesigned output setion give an external Q of 230. Work is in progress to build and test eight or more magnetrons. It is expeted that all of these tubes will have erami athode leads and that at least four of them will have thoria athodes. The hange to erami athode leads will neessitate some slight redesign of the anode, and the hange to the thoria athode will require lengthening of the end spaes and further old testing to detet and then eliminate possible spurious end spae resonanes. b. Thoria athodes Considerable diffiulty has been experiened in obtaining a large molybdenum-thoria athode for use in the magnetron. Trial athodes pressed in the split segment die were not of suffiient length or diameter after sintering, and an exessive amount of hourglassing was evident in the sintered athodes. The diffiulty was traed to the extremely fine partile size of the thoria powder used in the ompating proess, and after onsiderable experimentation a suitable partile size has been hosen and the problem solved. The mathematial investigation of the heat-onduting and dissipation properties of the end mounts for the thoria athode has been ompleted with use of the eletroni differential analyzer. The results show that if the thikness of the sheet tantalum or molybdenum used in the end mounts is in. or greater, there should be no diffiulty from failure of the end mounts at high temperatures. The experiment on platinum-brazing the tantalum end mounts to the thoria athode in a helium atmosphere has been suessfully ompleted. A small thoria athode and end mount together with the platinum solder were supported on a molybdenum spider inside a Vior tube. A tantalum ylinder around the athode provided the heating element. Pure helium was introdued through a fitting at the bottom of the Vior tube and permitted to flush for 20 minutes to insure an inert atmosphere. At the end of that time power was -31-

2 slowly applied to the tantalum ylinder by r-f indution. A moderate flow of helium was maintained while the temperature of the tantalum ylinder was brought to 1800 C; at this temperature the platinum solder flowed. The power was then shut off while the helium ontinued to flow for 40 minutes as the athode ooled. After another 20 minutes the athode had ooled to room temperature and was removed. No disoloration of the tantalum was observed, and a satisfatory braze had been effeted between the end mount and the athode.. 20-megawatt modulator A previous Progress Report mentioned the diffiulties enountered with the 20-Mw spark-gap modulator. More reently, r-f bridge measurements have been made on the 20-ohm pulse able, and pulse measurements have been made on the pulse-forming networks. A fundamental error in design alulations of the pulse-forming networks has been found. A new indutor for the l-pse pulse-forming network has been designed and will shortly be onstruted. 2. Magnetron Researh a. Mode interation in magnetrons A old test of the n = 3 resonane of a 718EY magnetron in the w-mode (n = 4) has been arried out using tehniques desribed in the last Quarterly Progress Report. These results show inreasing loading of the n = 3 mode as amplitude of ii-mode osillation inreases and anode urrent and voltage inrease orrespondingly. It is also observed that when there is preosillation anode urrent flowing, the eletron stream loads the n = 3 mode. A plot of standing wave ratio as a funtion of wavelength of the injeted signal for several onditions of operation is shown in Fig. VI-1. A series of tests is now under way to determine the effet of hanging the output oupling of the 718EY upon the phenomena desribed above. b. Noise properties of the preosillating magnetron Further work on this subjet has been postponed pending ompletion of the problem of manufaturing and mounting thoria athodes for the high-power magnetron.. Mode identifiation in magnetrons While the anode used in the high-power magnetron is designed to operate in the w-mode, n = 9, at 2800 Mse (n is the number of times the r-f pattern repeats itself around the periphery of the anode), osillation in other modes is possible. If the resonant frequenies orresponding to the various modes of resonane are determined, the starting voltages of all modes of operation as a funtion of magneti field an be -32-

3 determined by means of Hartree' s equation. These resonant frequenies have been alulated (1); the highest alulated frequeny, orresponding to n = 5, is 3455 Mse and the lowest alulated resonant frequeny, orresponding to n = 1, is 1442 Mse. o A < 20 0 Z Z ;5 W & 0 - NO PLATE POWER O - AVERAGE PLATE CURRENT MA: NO OSCILLA - TION - A - 7r- MODE OSCILLATION: AVERAGE PLATE CUR- RENT - 15 MA o~0~~ a~l o - r-mode OSCILLATION: AVERAGE PLATE CUR- RENT = 22MA 8=1220 GAUSS 500 PULSESSEC 5 SECPULSE B 00 P i -r a WAVELENGTH (CM) Fig. VI-1 Test of n = 3 resonane in 718EY magnetron (--mode: n = 4). In order to verify these alulations, a rotating probe is being onstruted whih will make possible the observation of the r-f field as a funtion of the angle of probe rotation. R-f energy is fed into the magnetron through its output setion from a klystron osillator. The signal piked up by the rotating probe is deteted by means of a rystal, and fed through slip rings to an amplifier and then to the vertial defletion plates of a athode ray osillograph. The sweep is synhronized with the rotation of the probe. The resulting pattern is therefore a plot of the magnitude of the r-f signal versus angle of rotation. The mode of resonane is identified from this pattern (2). In order to arry out these tests with available waveguides and klystron osillators, some of the measurements will be made on a wavelength-saled model of the original tube in whih all the dimensions were redued by a fator of Referenes (1) The resonant frequenies were alulated with the aid of the urves appearing in Fig. 3.10, Radiation Laboratory Series, 6, 101. (2) For some aspets of mode identifiation from the osillograph patterns, see Tehnial Report No. 99, Researh Laboratory of Eletronis, M. I. T. -33-

4 3. Cathode Researh The peuliar emission harateristis of the experimental diode reported in the last Quarterly Progress Report an be explained on the basis of the behavior of an insulator used for supporting and entering the athode. The tube onstrution is shown in Fig. VII-3, page 37, Quarterly Progress Report, Otober 15, When the athodes of the experimental tubes had been examined after proessing and emission measurements, a dark deposit was observed on both the athode oating and supporting insulator, while the nikel sleeve was still bright and shiny. A qualitative test for nikel in the darkened surfae of the insulator gave a strong positive indiation. The disoloration on both the insulator and athode oating was nikel evaporated from the anode during the prolonged proessing at high temperature. This nikel oating ats as a resistane in parallel with the diode; the resistane is variable with temperature. Suh a ondition aounts for the temperature sensitivity of the athode emission and the inrease in emission above the alulated spae-hargelimited emission. The nikel oating on the insulator will also disturb the axial symmetry of the eletri field, thereby making invalid the basi assumption under whih the Langmuir-Childs emission is alulated. B. MICROWAVE TUBES L. D. Smullin L. A. Harris A. Karp G. W. Zeoli 1. Mirowave Noise Studies Progress in studying the noise in an eletron beam at 2840 M may be desribed under the following headings: ompletion of noise-measuring equipment; ompletion of two suessful noise-study vauum tubes; ompilation of experimental data obtained with these tubes; development of a theory appliable to the ase under study; ompletion of design and ommenement of onstrution of a third vauum tube. a. Equipment for measuring noise at 2840 M The noise output from the resonant avity to whih the noise in the beam is oupled is in a band about 4 M wide entered at 2840 M, and is measured by omparing it with the noise from a standard soure. This standard soure, omprising a desk-lamp fluoresent tube, as oneived by Mumford (1), was desribed in the Progress Report, Otober 15, Sine then a signifiant hange in the onstrution has been adopted. The lamp (a 12-in. G.E. F8T5) is now loated obliquely aross the waveguide so that, like a tapered strip of resistive material, it is mathed over a very wide band, making a more useful laboratory instrument than the previous model. -34-

5 Fig. VI-Q CROSS -WIRE TUNER COUPLING HOLES Filter with variable loaded Q. A filter is needed so that the standard and unknown noise signals may have the same bandwidths, whih is neessary if a diret omparison is to be made. For this purpose a ylindrial 2 resonant avity was built as shown in Fig. VI-2. The ross-wire renders the avity asymmetrial with respet to the axis of the ylinder so that as the waveguides are rotated with respet to this wire (speial flanges were made to failitate this) various degrees of oupling an be ahieved at eah end. In this way the bandw Ldth and S.W.R. an be made equal to that of the avity in the noise-study vauum tube. The mirowave reeiver remains as desribed in the Otober report exept for a hange in the method of introduing L.O. power into the mixer. A diretional oupler is now used to minimize the amount of L.O. power inident on the noise soures. A noise measurement is essentially as follows. Notation: I = beam urrent - eletron harge = mean-sauare flutuation urrent in beam in band Af 2 = number by whih (ZelAf) may be multiplied to give i 2 (inludes spae-harge redution fator) R S = avity resonant impedane at the gap = V.S.W.R. of avity at resonane = beam oupling oeffiient A a k = attenuator setting expressed as a ratio output powerinput power = transmission of filter avity at resonane, expressed as a fration output power input power = Boltzmann's onstant T s = noise temperature of standard soure If A and Ax standard soure are the attenuator settings that give equal reeiver outputs for the and for the noise tube, respetively, then 2 A a k Ts(l + S )2 2A e I 2 R S (1) -35-

6 R is measured by determining the rate of hange of resonant wavelength with the insertion of a small metalli volume into the avity (2), derived in Referene 3. 2 evaluation of R and p. and j 2 is omputed from formulae The main unertainties in the noise measurements are in the COLLECTOR CAVITY BEAM ANODE X2 i r ---- b V 0 L L JQATHODE SPACE 2 SPACE I Fig. VI-3 Configuration of beamnoise tube. b. A basi theory The method of treatment whih seems to have the greatest potentialities for analyzing our problem may be alled the Llewellyn- Peterson Proedure (4, 5). It onsists of breaking up the various portions of the tube into parallel-plane diodes, using the end onditions of eah stage as the initial onditions for the next. An outline of our analysis follows. Notation: (see also Fig. VI-3 and the preeding notation) U = d- beam veloity at full voltage, V u q i v T = a- omponent of veloity = a- omponent of onvetion urrent density = a- total urrent (density) = a- voltage aross a diode setion = d- transit time T k = athode temperature 77 = ratio of eletron harge to mass For spae 1, the Llewellyn-Peterson linear equations are Vab = 0 = A1 il + B1 a + C 1 Ua (2) qb = D il +E* qa + F1 u (3) Ub = G 1 il + H 1 qa + I1 ua (4) and a similar set applies for spae 2. Beause spae 1 is a region of spae-hargelimited flow (see the original referenes (4, 5) where the oeffiients denoted by starred letters are evaluated), E = B = H* = 0, and we an solve our linear equations to get -36-

7 = (E D BA)( D A ) + (F -C D A )(I - G C A)] Ua (5) whih expresses the onvetion urrent modulation q, whih exites the avity, in terms of the veloity modulation ua at the virtual athode. Aording to Rak' s (7) analysis of noise in a spae-harge-limited diode 2 ua = (4- rr)(?i)kt Af. (6) Continuing to evaluate the oeffiients A, B,... finally obtain, aording to Referene 5, we an 2 (4 - r) ktk (U (7) r - (U T - U T2) (7) (plane ) 8 e In spae 2, beause the beam is not very dense, U T7 2 = X 2. If spae 1 were a planeparallel, spae-harge-limited diode we would have U T 1 = 3x 1, but we must note that we atually have a Piere-type eletron gun where the flow is between onentri spheres instead of parallel planes. The transit time of a spae-harge-limited, onentrisphere diode has not been evaluated in any known literature, so we have had to solve this problem, whose results are expressible as U T 1 = 3h x 1. The orretion fator h, whih turns out to be a universal funtion of the ratio of spherial radii, is shown in Fig. VI-4. (In our experimental gun h is about 1.5, and hene is important.) h Sa t- 4n " rr I Fig. VI-4 Fator for omputing transit time of a spae-harge-limited, onentrisphere diode, with initial veloities negleted. h = ratio of transit time of this diode to that of a spae-hargelimited parallel-plane diode of same A serious fault an be found in the foregoing analysis. The oeffiients A 1, B 1,... as evaluated in Referenes 4 and 5, for spae 1, involve in their omputation the spaeharge repulsion fores in the spae-harge-limited region. The evaluations of these fores in the Referenes are for flow between infinite, parallel planes, but in our Piere gun we have a onverging beam of small diameter, and it an be shown that the fores here are quite different from the ase of infinite, parallel planes. At present, partition effet an be treated theoretially only if we assume the ondition known in the literature as superposed flow. This would require that the probability of an eletron' s striking the edge of the hole be independent of the point in the athode -37-

8 whene the eletron ame. North (8) has worked out the theory' for this ase. Thus, if _ urrent interepted - urrent inident at plane = i ZZe I llf when p = 0 (r Z ), = i ZZe I(through)Af when p 'I 0 then (r Z ) 1= r Z + P (1- r Z ) (8) From this we see that if r Z is of the order of 0.05, p assume r Z =(r Z )I. must be less than 0.01 if we are to. Experimental tubes After three ases of athode ontamination and one envelope frature, we finally ompleted two tubes ontaining a Piere gun, a avity, and a olletor (Fig. VI-S). In one (No. 1-4) the distane X z was 118 in. and in the other (No. I-A-6), X z was lz in. CAVITY OUTPUT GETTER APPENDIX \ GUN LEADS Fig. VI-5 Tube for measuring shot-noise in an eletron beam at 2840 M. In designing I-A, some preliminary tests on the gun were made to find experimentally the distane from the anode at whih the beam has its minimum diameter, so that the avity ould be plaed at this point to keep p as low as possible. For this purpose the beam-exploring tehniques desribed elsewhere in this report were used. In tube I-A-6, p usually appeared to be less than 1 or Z perent, but we believe the real value of p to be quite negligible beause the stray urrent is interepted not at plane, but beyond the avity gap, where it has no effet on the measured noise. We are onvined of this beause appliation of a small bias voltage between the athode and athode-eletrode ould hange the apparent perentage intereption, but r Z would remain nearly onstant while doing this. Typial data obtained with these tubes are given in Figs. VI-6 and VI-7. In tube

9 onsiderable urrent was interepted when the athode and athode-eletrode were onneted together, but making the athode-eletrode a few volts positive with respet to the athode exerted a foussing effet apable of reduing the intereption. In Fig. VI-6 we have plotted r2 against perent urrent interepted, whih was varied in this way. Values of r 2 omputed from Eqs. (7) and (8) are also plotted, but no interpretations rc2 % CURRENT INTERCEPTED CATHODE TEMPERATURE (*K) 30 Fig. v--6 Data on noise fator, r 2, obtained with tube 1-4 at T k = 1105 K. Fig. VI-7 Data on noise fator, obtained with tube I-A-6 at zero "bias". 2, an be made beause we do not know just where the stray urrent is interepted nor to what extent the bias alters the urrent distribution in the beam or the transit time. In tube I-A-6, as previously mentioned, we believe p was negligible. In Fig. VI-7 we see that when the athode temperature is too low to provide adequate emission, r 2 is very high, but as soon as the spae-harge-limited ondition is established, f 2 is proportional to the absolute athode temperature, as predited by Eq. (7). We an also note that r2 dereases as V inreases, whih is in qualitative agreement with Eq. (9), but we annot find quantitative agreement regarding the inverse-square dependene on d. New onstrution To hek the dependene of F 2 on distane x 2, a new tube, having a metal bellows to permit hanging x 2, is now nearing ompletion. A. Karp -39-

10 Referenes (1) W. W. Mumford: B.S.T.J. 28:4, (1949). (2) S. C. Brown, D. J. Rose, et al: Tehnial Reports No. 140 (1949) and No. 66 (1948), Researh Laboratory of Eletronis, M. I. T. (3) A. H. W. Bek: Veloity-Modulated Thermioni Tubes (Cambridge, 1948). (4) F. B. Llewellyn: Eletron Inertia Effets (Cambridge, 1939). (5) F. B. Llewellyn, L. C. Peterson: Pro. I.R.E. 32, 144 (1944). (6) L. C. Peterson: Pro. I.R.E. 35, (1947). (7) A. J. Rak: B.S.T.J. 17:4, (1938). (8) D. O. North: R.C.A. Rev. 5: (1940). 2. Debunhing Fores in a Beam of Finite Diameter In the Llewellyn small signal analysis of the high-frequeny behavior of infinite, parallel-plane diodes, the debunhing fores in the eletron stream take on onsiderable importane. The appliation of these equations to beam devies (traveling-wave tube, et.) has raised some questions as to their diret appliability. In an effort to get a measure of the S. redution of the debunhing fores due to.'. the finite beam diameter, the following 27 problem was studied: a stationary, infinitely long beam of eletrons, radius a, Fig. VI--8 Density modulated beam. and density p= po +p os pz (Fig. VI-8). of harge of radius r and surfae density X os pz is The axial potential due to a thin ylinder and 1 = A I (pr) os pz (r < rl) 2 = BKo (pr) os pz (r > rl) where S1 1 (r) K (pr) + I (Pr) K(Pr) r B= o (r) 1 (pr) Ko (Pr)+ 1 (pr ) Irl(pr) Thus, to get the potential due to a solid beam we must integrate 1 and 2 from rl = 0 to r = a. This operation gives r p os p z o(pr) [a Kl(Pa) - pr KI(Pr) + K(Pr) Pr Il(Pr) = p os pz F r E 1 1) -40-

11 F l.o~ :::::::::-,,------' I I I 1',;, ",,;,;, ~.." ,.... ~... ~EDGE OF BEAM The axial field is E = _ ~ = (3 sin (3z (F) r az E Figure VI- 9 is a plot of F for the edge of the beam and the enter of the beam, against (3a. For f3a» 1, the value of F r=o approahes unity, and F r=a approahes 12. E = (3p E Sine the field in an infinite parallel plane flow is (sin f3z).03 Fig. VI-9 Redution in debunhing fore due to finite beam diameter. F gives the redution in debunhing fore due to finite beam diameter. The presene of a onduting drift tube will, of ourse, redue F still further. L. D. Smullin 3. Eletron Beam Measuring Tehnique In the development of eletron guns for traveling-wave tubes and other appliations it has beome neessary to develop a tehnique for observing the beams produed. The general sheme is shown in Fig. VI-lO. A suitable detetor sreen and olletor are mounted on a slug whih is free to slide on a tungsten rod within the evauated tube. This permits observing the beam shape as a funtion of distane from the anode of the gun. A large oarse sreen ylinder is plaed around the beam and detetor sreen to Fig. VI-IO Tube for exploring beam size of 2000 v-5!db. gun. -41-

12 ollet the seondary eletrons emitted by the detetor sreen and olletor. Eletron beams with power densities of the order of 30, 800, and 8000 wattsin 2 have been of partiular interest. The problem has been one of finding a suitable detetor sreen material for the different beam power densities. A willernite oated tantalum disk was used for the low power density beam, but due to the low urrent saturation of the willemite the relative urrent distribution over the spot ould not be determined. The low eletrial ondutivity of the willemite resulted in an aumulation of negative harge on the sreen. The exat effet of this negative harge upon the beam is unknown. A sreen made of in. thik platinum foil was used for the 800 wattsin 2 beam power density. A visible red spot was produed due to heating of the metal, but the high thermal ondutivity of the platinum made the exat beam diameter unertain. At the higher power density of 8000 wattsin 2 the platinum sreen melted. w 2 - DISTANCE TO ANODE 0-916" 1-0 xx.llno 4 ~ x- OBSERVER I U, o O- OBSERVER 2 Fig. VI-11 Data obtained using tungsten wire sreen with 2000 v-5ma gun. A n o I 2 3 I I DISTANCE FROM ANODE ( UNITS OF 332") To try to avoid spot spreading due to heat ondution a sreen onsisting of in. diam tungsten wire spaed in. apart was tried with moderate suess. Figure VI-11 shows data obtained by two different observers using this sreen, the disrepanies being due to the disontinuous nature of the sreen and the vagueness of the spot boundary. At present a fourth type of sreen is being tested. It onsists of a very thin oating of graphite on a thoria disk. The high melting points of arbon and thoria and the possibility of obtaining a oating of aquadag thin enough to have a high thermal resistane without too high an eletrial resistane makes this sreen seem very promising. G. W. Zeoli 4. Dense Eletron Beams in Axial Magneti Fields a. Theoretial Several aspets of this problem have been onsidered theoretially in the past quarter. The first of these is the effet of positive ions on the eletron beam. Complete spaeharge neutralization was assumed sine the foussing onditions for this ase are easily -42-

13 obtained, and beause it represents an extreme ase. These onditions are presribed by a ertain lous on the design harge desribed in the last Quarterly Progress Report. The question of whether suh neutralization will indeed take plae has also been onsidered. Although no definite answer has been found, several of the fators involved have been determined qualitatively. Loss of eletrons produed by ionization by motion along the beam will, of ourse, be greater than the similar loss of positive ions. It appears, however, that the positive-ion loss due to transverse motion will be greater than that of the seondary eletrons beause of the axial magneti field. A radial sorting effet, too, is present whih tends to produe a dipole layer that flattens the potential distribution aross the beam. Some detailed examination of the onditions at the end of the magneti field has been arried out. It is important to oordinate properly the eletron gun, the varyizig magneti field, and the potential distribution at the input end of the beam so that the restritions imposed in the analysis will be atually satisfied. A general treatment of this problem has not been attempted but ertain speifi methods are proposed. The magneti fields of the two foussing solenoids have been alulated. Sine the problem is set up in terms of flux linkages (see last Quarterly Progress Report) this alulation is essentially a mutual indutane problem. The alulated and measured values are in exellent agreement. b. Experimental Some of the experimental work on this problem has been started. Both of the foussing solenoids are omplete and, as mentioned above, their fields have been measured. A demountable vauum system designed to be used in onjuntion with a standard laboratory work table has been onstruted but not yet tested. Some parameters of the beam system have been hosen and the eletrodes are being fabriated. An eletron gun has been designed by means of the eletrolyti tank. This gun is now ready for assembly and test. Considerable time was taken up in developing a suitable heater for the large ringshaped athode. The heater power is of the order of 50 watts. The winding is bifilar and is shaped in a flat spiral by hooking it over a pin and winding it into the spiral grooves in the fae of a stainless-steel jig. A flat over failitates the winding and holds the wire in plae while it is being annealed. These heaters have been tested and found suitable for their purpose. It may be of interest that other types of heaters were formed in whih the jig or mandrel had to be dissolved hemially. In every ase the heater was warped beyond usefulness. It was finally proved that the hemial treatment alone was responsible for the warping and this method was abandoned. L. A. Harris -43-

14 C. THE GENERATION OF MILLIMETER AND INFRA-RED RADIATION BY ACCELERATED ELECTRONS The development of a 20-kv pulsed gun and bunhing avity is in the final stages of ompletion. The problem of spinning the tantalum eletrodes used in the Piere gun has finally been solved as indiated in the initial tests on the first of the two nearly built tubes. Peak urrents of the order of 70 perent of the alulated value have been obtained through a l-mm-diam iris, 14 in. deep, 2 in. from the anode; and of over 98 perent of the alulated value through a 1.5-mm iris. This urrent ( amp) will be more than suffiient for injetion into the mirowave aelerating avity whih is alulated to drive the eletrons up to 1.5 Mev. The first tube and aelerating avity are being assembled for measuring the eletron energy distribution. Following these measurements, the apparatus will be set to generate radiation at m for onveniene in detetion. P. D. Coleman -44-