Contains a report on a research project.Lincoln Laboratory (Purchase Order DDL B-00283)United States ArmyUnited States Air Force (Contract AF19(604)-5200)United States Navy, Office of Naval Research (Nonr-1841(49)

Morse, D. L.

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VII. MICROWAVE ELECTRONICSProf. L. D. Smullin Prof. A. Bers W. R. RummlerProf. H. A. Haus R. B. McCullough W. K. RushforthD. L. MorseA. TWO-GAP KLYSTRON CAVITIES1. IntroductionKlystron microwave amplifiers have, in the past, been considered as inherentlynarrow-bandwidth devices because resonant cavities have been used in these amplifiersfor coupling electromagnetic power into and out of the electron beam. The use of mul-tiple, stagger-tuned, intermediate cavities has resulted in such an increase in theoret-ical bandwidth that the output cavity, which was once considered to be broadband withrespect to the rest of the system, is now the limiting factor on bandwidth (1).The use of two-gap output cavities has been suggested (2) as a means of increasingbandwidth for the following reasons. The amount of rf power extracted from an elec-tron beam is inversely proportional to the equivalent shunt conductance of the outputcavity, and the Q of the cavity is inversely proportional to its shunt conductance, andthus the power extracted from the beam is directly proportional to the Q of the cavity.If two gaps with two coupled cavities are used, each gap need extract less power, andhence the Q of the cavities can be lower.Two-gap, double-tuned cavities, which have been built by Litton Industries, Inc.,San Carlos, California, and by Eitel-McCullough, Inc., San Bruno, California, showincreased bandwidth for equivalent output. Saharian (2) has presented a detailed theo-retical analysis of single-gap and two-gap coupled klystron output cavities.2. Description of Cavity and Equivalent CircuitA two-gap cavity has been constructed for the purpose of conducting tests in theabsence of an electron beam. This cavity is shown in Fig. VII-1. Each half of thecavity is tunable at frequencies around 3000 mec. The gap widths are for electrontransit angles of approximately 1 radian for a 100-kv beam. The transit angle betweenthe centers of the two gaps is 7r radians.The equivalent circuit for the cavity is shown in Fig. VII-2. The inductances Land Le represent the inductances of the septum and of the connector from the center*This work was supported in part by Purchase Order DDL B-00283 with LincolnLaboratory, a center for research operated by Massachusetts Institute of Technologywith the joint support of the U.S. Army, Navy, and Air Force under Air Force Con-tract AF 19(604)-5200; and in part by the U.S. Navy (Office of Naval Research) underContract Nonr-1841(49). Reproduction in whole or in part is permitted for any pur-pose of the United States Government.(VII. MICROWAVE ELECTRONICS)COAXIALOUTPUTLINESEPTUML LENDPIECE C L cFig. VII-1. Two-gap output cavity. Fig. VII-2. Equivalent circuit for thecavity of Fig. VII-1.conductor of the coaxial line to the center post of the cavity; the current generators iand i2 represent the currents induced in the gaps by the beam; and R is the load towhich power is delivered.3. Analysis of OperationThe type of circuit shown in Fig. VII-2 will have both a symmetric and an antisym-metric mode of operation. The current flow for each case is shown in Fig. VII-3.L L L LSC C cANTISYMMETRIC MODE SYMMETRIC MODEFig. VII-3. Modes of operation for the circuit shown in Fig. VII-2.This circuit can be operated in either or both of the two modes by adjusting the phasebetween i1 and i2. When only the antisymmetric mode exists, no output is seen becauseno net current traverses the center branch of the network. When only the symmetricmode exists, maximum output is seen across the load R.4. Experimental ResultsThe cavity was excited with a probe at each gap, and power from the coaxial outputwas measured as a function of frequency. With both halves of the cavity tuned to thesame frequency, maximum output power was observed for the symmetric excitation,and practically no output power was observed for the antisymmetric excitation. Withone side of the cavity slightly detuned, the antisymmetric mode became visible. This(VII. MICROWAVE ELECTRONICS)mode exhibited a higher Q than the symmetric mode, since only a small fraction of thetotal current flowed through the load R.Experiments were made with septum angles of 270', 2400, and 1800. The Q ofthe symmetric mode decreased as the septum angle was decreased because the result-ant increase in Ls caused more current to flow through the load. With a septum angleof 1800, as shown in Fig. VII-1, the approximate value of the loaded Q was 10.D. L. MorseReferences1. L. D. Smullin and D. L. Morse, Broadband-buncher klystron, Quarterly Prog-ress Report No. 56, Research Laboratory of Electronics, M.I.T., Jan. 15, 1960, p. 93.2. A. Saharian, Broadbanding of a Klystron Output Cavity, E.E. Thesis, Depart-ment of Electrical Engineering, M.I.T., Jan. 4, 1960.

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