Contains reports on three research projects

Fried, C.

Haus, Hermann A.

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VIII. MICROWAVE TUBE RESEARCHL. D. Smullin A. G. Barrett H. A. HausProf. L. J. Chu C. Fried C. R. RussellA. NOISE IN ELECTRON BEAMSThe shielding of the apparatus described in the previous Quarterly Progress Reporthas been completed, and very satisfactory semiautomatic operation has been obtained.The focusing magnet has been rewound and calibrated. With the improved focusing con-ditions the interception current was reduced to below 0. 1 percent for all but very weakmagnetic fields. Typical interception current curves are shown in Fig. VIII-1.The Brillouin field corresponding to the curves of Fig. VIII-1 is approximately 370gauss.A number of noise measurements have been made both for dc and for pulsedoperation. Figures VIII-2 to VIII-6 show some of these measurements. (If we assumethat the beam diameter d is 0.20 inch as calculated, the Brillouin fields corres-ponding to Figs. VIII-2 to VIII-5 are 262, 307, 362, and 370 gauss respectively.) Thecurves including the fine structure were well reproducible. Instability, however, wasaL U.bz 0.4wcrr 0.2Oz0- -02wo -0.4-0.6zMAGNETANODE SHIELDSI0.68Fig. VIII-1Interception current vs cavity position for convergent-flowpulsed electron beam V = 1500 volts; I = 16. 6 4a;pressure 7 0 X 10 - 7 mm.-35-a.50-5-10-15-, 9nINTERCEPTION CURRENT< 1.5% , H 258 GAUSSSHOT- NOISE LEVEL /10 15 20 25 30 32.5DISTANCE (CM)INTERCEPTION CURRENT < 1/10%, H- 430 GAUSSDISTANCE (CM)INTERCEPTION CURRENT < 1/O %, H - 598GAUSSo /SHOT- NOISE LEVEL0-5200 5 10 15 20 25 30 32.5DISTANCE (CM)Fig. VIII-2Noise power vs cavity position for convergent-flow dc electron beam. Icoll = 1. 7 ma;-7 colV = 700 volts; pressure 4. 5 x 10 mm; beam diameter = 0. 035 inch.MAGNETI I LC.) 0.68 0.62a"DISTANCE (CM)' ~_ tfiv 1l 4 1 ," \/"IINTERCEPTION CURRENT< 1/10% , H 344 GAUSS,,I \IJTERCEPTION CURRENT < V, I *-H 258 GAUSS5SHOT- NOISE LEVEL05-10-15-205 10 15 20 25 30 32.5DISTANCE (CM)INTERCEPTION CURRENT< 1/20% . H- 430 GAUSSSHOT-NOISE LEVaEL0-5-10-15-200 5 10 15 20 25 30 32.5DISTANCE (CM)DISTANCE (CM)DISTANCE (CM)Fig. VIII-3Noise power vs cavity position for convergent-flow dc electron beam. Icoll = 2. 8 ma;-7 chV = 1000 volts; pressure 5 x 10 mm; beam diameter w 0. 035 inch.0MAGNETM jOD SHEO GAVITYI5S0uJ -5P-inWLn2--1520.01zVii /550-5R -1010 15 20 25 30 32.5DISTANCE (CM)INTERCEPTION CURRENT < 1/20 % , H430 GAUSSSHOT -NOISE LEVELO 5 10 15 20 25 30 32.5DISTANCE (CM).0*02INTERCEPTION CURRENT< 2% , H * 258 GAUSSSHOT-NOISE LEVELFig. VIII-4Noise power vs cavity position for convergent-flow dc electron beam. Icoll = 4. 8 ma;-7V = 1500 volts; pressure = 8 x 10 mm; beam diameter~ 0.035 inch.oINTERCEPTION CURRENT< I% , H 344 GAUSS-10-5-20O 5 10 IS 20 25 30 325DISTANCE (CM)INTERCEPTION CURRENT < 1/20 % , H 602 GAUSSSSHOT- NOISE LEVEL-5-20O 5 10 15 20 25 30 32.5DISTANCE (CM)/ I / /vWczz25 25SHOT- NOISE LEVEL-510 -0-15 -15-20 -20S 5AwME S 7 ?L C7VITYII8 40.6210 15 20 25 30 32.5DISTANCE (CM)7!:..SHOT-NOISE LE/LSHOT- NOISE LEVEL0 5 10 15 20 25 30 32.5DISTANCE (CM)NOISE LEVEL-0-5-15-200 5 10 15 20 25 30 32.5DISTANCE (CM)H -602 GAUSSSHOT-NOISE LEVEL0 5 K) 15 20 25 30 32.5DISTANCE (CM)Fig. VIII-5Noise power vs cavity position for convergent-flow pulsed electron beam. Icoll = 16.6 ipa;-7V = 1500 volts; pressure~ 7 X 10 mm; beam diameter 0. 035 inch.H- 430 GAUSS////// /SHOT- I1050: -5-15-10(n -I5-20III/H 301 GAUSS H 344 GAUSSII I I / / /T /rH . 258 GAUSS/--_/I 11052Co~ ICcnWCU,SHOT-NOISE LEVEL56ANODE C6 2  YCL68 0.6210 15 20 25 30 325DISTANCE (CM)SHOT-NOISE LEVELO 5 10 15 20DISTANCE (CM)H 430GAUSS2015105SHOT-NOISE LEVEL-50 5 10 15 20 25 30 32.5DISTANCE(CM)H 581 GAUSSSHOT- NOISE LEVEL0 5 10 15 20 25 30 32.5DISTANCE (CM)Fig. VIII-6Noise power vs cavity position for temperature-limited pulsed electron beam.-coil = 1500 volts; pressure7IColl = 1. 1 La; V°= 1500 volts; pressure - 7 x 10 mm.25 30 32.5U- r,H -344 GAUSS--~-55-5(VIII. MICROWAVE TUBE RESEARCH)present in the growing waves from approximately 10 db above shot noise. The noisegrowing wave was considerably more prominent in the unneutralized pulsed beams thanin the completely neutralized dc operated ones. Therefore, it seems improbable thatthe growing noise was caused by the ions in the beam. Assuming that the noise curvesconsisted of a periodic standing wave and a superimposed growing wave, we couldobserve a delay of the growing part as the magnetic field was increased. For high mag-netic fields the growing part was absent in the drift space under observation. It mightwell be that in a longer drift space the growing part would still occur. The level of thesuccessive minima also changed with the magnetic fields, as shown in Figs. VIII-2,VIII-3, and VIII-4.Growing noise waves were present even in temperature-limited beams as Fig. VIII-6shows. A rise up to approximately 15 db above shot noise level could be observed. Againa delay of the growing portion was noticeable with the increase of the magnetic field.Increasing magnetic field also caused a decrease of the standing-wave ratio in the peri-odic noise waves. This decrease was due to the raising of the noise-wave minima andthe simultaneous lowering of the maxima as well.Further experiments will be made with confined-flow electron beams.C. FriedB. INTERNALLY COATED CATHODESTwo electron guns with internally coated cathodes have been designed for low-voltageas well as high-voltage operation. Both guns are under construction at present.C. FriedC. PROPAGATION OF SIGNALS ON ELECTRON BEAMSData have been taken, both on a direct current electron beam and a pulsed elec-tron beam, following the outline given in the last Quarterly Progress Report (1).A 3000-Mc/sec 1-mw signal, well above the noise level but still small enough for smallsignal theory to apply, was fed into one (fixed) cavity. The alternating beam current waspicked up by the other (movable) cavity. In continuous-wave operation, standing-waveratios of current as high as 41 db were observed at magnetic fields higher than 1. 5 timesthe Brillouin field. A typical curve is shown in Fig. VIII-7. The value of the Brillouinfield for this case was 300 gauss.A plot of the current standing wave under pulsed operation is given in the samefigure. The pulse repetition rate was 4000 cps, the length of the pulse 0. 75 4.sec. Itcan be seen that the plasma wavelength is considerably longer. This can be attributed-41-(VIII. MICROWAVE TUBE RESEARCH)323028262722920z 18S16o0 12(D4242- .00- / 0- P ULSED-\ BEAM VOLTAGE 1.5KVSBEAM CURRENT 3.2MA (CONTINUOUS WAVE)MAGNETIC FIELD 290 GAUSSI I! I I-O- CAVITY GAP 0.070 INCHESI I CAVITY HOLE DIAMETER 0.120 INCHES- MEASURED BEAM DIAMETER0.020 INCHES-I-2 4 6 10 12 14 16 20 22 24 26 28 30 32 34 361 2 4 6 8 IO 12 14 16 IB 20 22 24 26 28 30 32 34 36POSITION OF MOVABLE CAVITY (CM)Fig. VIII-7Standing-wave pattern of current.to the fact that ions are essentially absent under pulsed operation, whereas in con-tinuous-wave operation the focusing action of the ions confines the beam to a smallerdiameter. It may be that the smaller standing-wave ratio observed under pulsed opera-tion can be blamed on the shape of the pulse, which was not ideally square.At high beam current and low magnetic field, growing noise of the nature observedelsewhere (for example, see ref. 2) was known to exist. No anomalies were observedin the standing-wave pattern of the signal current.H. A. HausReferences1. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T. April 15,1953, p. 262. Quarterly Progress Report, Research Laboratory of Electronics, M.I. T. April 15,1953, p. 43-42-

Microwave Tube Research

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Microwave Tube Research

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