Contains reports on two research projects

Chu, L. J.

Harris, L. A.

Johnson, H. R.

Schreiter, E. F.

Smullin, Louis D.

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VII. TRAVELING-WAVE TUBESA. THREE-CM TRAVELING-WAVE AMPLIFIER TUBESStaff: Professor L. J. ChuL. A. HarrisH. R. JohnsonThe construction of auxiliary apparatus for use with 3-cm travel-ing-wave tubes has been completed. This apparatus includes the air-coreelectromagnet and the associated waveguide fixtures mentioned in the lastprogress report.Two completed tubes have been obtained. Although no extensivemeasurements have been made as yet, preliminary tests indicate a net gain ofa few db.In order to throw some light on the effects of various modes andthe presence of dielectric support rods, an experimental investigation of3-cm propagation along a cold helix has been started. For this purpose,an Esterline-Angus meter records the standing wave which is measured by ascrew-driven probe moving along the helix.B. SEVEN-MM TRAVELING-WAVE OSCILLATORStaff: L. D. SmullinWork has begun on the design of an oscillator for the regionabout 7 mm based on the traveling-wave principle. An iris-loaded, circularwaveguide will be used to provide the slow waves, and an electron beam willbe projected along the center of the tube. It is intended to use internalfeedback to produce oscillations. The output waveguide will be looselycoupled to the slow waveguide, so that most of the energy will be reflectedback to the cathode end of the slow guide, where it will be re-reflected,etc. The tube will be almost 40 wavelengths long, and oscillation will bepossible at any frequency for which the round-trip phase shift is a multipleof 2T radians. Oscillation will occur, however, only when the electronvelocity is roughly synchronous with the wave velocity. Thus, the oscilla-tor will be voltage tunable over a small range of frequencies (probably1 or 2 per cent).Present effort is being devoted entirely to the solution of thevarious mechanical problems involved. The most difficult problem is theconstruction of the loaded waveguide. This is a tube 0.407 in. I.D. withthin diaphragms or irises 0.003 in. thick with a 0.210-in. diameter hole,separated by rings, 0.407 in. O.D., 0.375 in. I.D., and 0.006 in. thick.The problem is to stack about 150 of these in the tube and braze them inplace so that all the irises are parallel and equally spaced. Since wechanged from copper to harder metals such as Monel or mild steel, progress-42-I--.l-r --L_. .I.-i----L-l--l-.-X--..^ ..-l -lli~yl~-- C llllll~l l~ ~~ ~~~- s8along this line has been encouraging. The proposed form of the tube isshown in Fig. VII-1..180 x .086 WAVE GUIDE1 SPLIT BLOCK30 RECESS FOR WINDOWFig. VII-1. Loaded-waveguide traveling-wave oscillator.An incidental development has been made of a coaxial copper-golddiffusion seal that allows a copper tube to be sealed to another within aslittle as in. from a glass feather-edge seal, Fig. VII-2. One of thecopper parts to be sealed is gold-plated, and the parts are assembled asshown with an Invar collar. Heating in air with a rough vacuum insidethe tube, to 40000 makes an excellent vacuum-tight bond without affectingthe glass seal. A number have been made with no particular effort to keepthe copper clean, and all have worked.^^rr ri *a.ASSGOLDFig. VII-2. Copper-gold diffusion seal.-43-_ 1~_ __^__1_1 1  ~_ __

Traveling-Wave Tubes

VII. TRAVELING-WAVE TUBESA. THREE-CM TRAVELING-WAVE AMPLIFIER TUBESStaff: L. D. SmullinProfessor L. J. ChuL. A. HarrisH. R. JohnsonE. F. SchreiterOf the two completed 3-cm tubes mentioned in the last progressreport, one had a poor vacuum and some broken leads, making it unsuitablefor use.Some measurements were made on the second tube, and its perform-ance appears quite promising. A beam current of about 8 ma to the collec-tor is easily obtained. The net gain of the tube has on occasion reached15 db though this figure cannot always be achieved. At gains of about13 db the tube has a tendency to break into oscillation. The output ofthe tube when oscillating is some 15 or 20 db higher than its amplifiedoutput, the frequency being in the 3-cm region, although erratic. The ampli-fication value at which the tube tends to become unstable was increased from9 db to 13 db by hanging a "curtain" of lossy cloth across the inside of thefocussing coil. More serious attempts to eliminate the instability have notyet been made although they are planned for the future. The frequency char-acteristic of the gain of the tube has not yet been measured, but cold testsindicate about 600 Mc/sec between half-power points. A more complete andsystematic series of measurements is now being made.Two other tubes were constructed, and although they are mechani-cally good, they show poor performance. These tubes will be slightly modi-fied to provide better magnetic shielding of the electron-gun region, andthereby to improve their performance.The probe measurements of the fields along a cold helix havebeen concluded. An approximate Fourier analysis of the standing wave patternindicated the presence of four waves along the helix: incident and reflectedwaves traveling with the velocity of light, and incident and reflected wavesof the slow helix mode. The results confirmed previous experience whichshowed that a gradual transition from the antenna (launching the helix wave)to the turns of the helix is more favorable to the excitation of the slowermode and tends to suppress the faster one. The radial attenuation character-istics of the faster mode and comparison with the field produced by anantenna alone both indicate that this faster wave is associated with thehelix and is not merely a radiation field.-53-II __I_VII. B. SEVEN-MM TRAVELING-WAVE OSCILLATORStaff: L. D. SmullinThe iris-loaded waveguide used to slow the phase velocity of thetraveling wave down to about one-tenth of the velocity of light has beenassembled by clamping alternate molybdenum discs and spacers in a molyb-denum tube. A K-Monel plug threaded into the end of the waveguide nearestthe cathode exerts the clamping force. A microscope has been arranged toexamine the interior of the structure after assembly to make sure thatnone of the discs is shorting. The structure described above was satis-factory in this respect after vacuum-fired at 8500C. The waveguide outputblock has been completed, and two alternative window designs are being madein the shop. One is a ceramic window using a disc of Alsimag No. 243 cera-mic 0.015 in. thick by 5/32 in. in diameter which will be brazed into acopied more or less directly from the waveguide outputs used on 1.25-cmmagnetrons. Since the tube is ready to be assembled now, the magnetrontype of window (glass in a kover cup) will be used on the first trial.(glass in a kovar cup) will be used on the first trial.A dummy tube was made up to test the electron-gun assembly.It consisted of the 2C39 cathode-grid assembly, an anode block with a 0.210-in. diameter hole, 1 in. long, and a collector. Satisfactory cathode emis-sion and grid control were obtained. Because of the liberal use of kovar,however, magnetic focusing was not too helpful. The actual tube will havemagnetic material only in the 2C39 gun (kovar) and at the output window.-54.~ -- ~~- -- - ----- --- -- ---- -------

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Traveling-Wave Tubes

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