Cabe, A.

Fetterman, H.R.

Kreischer, K.E.

Mulligan, W.J.

Temkin, R.J.

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PFC/JA-82-10A 100 kW, 140 GHz Pulsed GyrotronR.J. Temkin, K.E. Kreischer, W.J. Mulligan,A. MacCabe and H.R. FettermanPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139May 1982(To be published in InternationalMillimeter Waves, Vol. 3, 1982.)Journal of Infrared andBy acceptance of this article, the publisher and/or recipientacknowledges the U.S. Government's right to retain a nonex-clusive, royalty-free license in and to any copyright coveringthis paper.A 100 kW, 140 GHz Pulsed GyrotronR.J. Temkin, K.E. Kreischer, W.J. MulliganS. MacCabe and H.R. FettermanPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139The design and operation of a 100 kW, 140 GHz pulsedgyrotron are reported. To our knowledge, this is thehighest frequency at which high gyrotron output power(> 100 kW) has been achieved. Results are presented forgyrotron operation in the range of magnetic field from4 to 7 T, voltage from 23 to 80 kV and current up to 7.5 A.Near a value of magnetic field of 5.4 T, an output powerof 100 kW was obtained at 140.4 GHz in single mode opera-tion in the TE0 3 1 resonator mode.Key Words: Gyrotron, Electron Cyclotron Maser, MicrowaveTube, Microwave OscillatorIntroductionGyrotrons are an important source of high frequencymicrowave radiation for use in heating of magneticallyconfined plasmas [1]. In recent years, high power gyro-trons, exceeding 100 kW in power, have been built at fre-quencies of 28 [2], 35 [3,4], 45 [5], 60 [6,7], 86 18],and 100 [5] GHz. Gyrotron operation has been obtained atmuch higher frequencies, but not at high power levels(100 kW or higher). High power gyrotrons at frequenciesabove 100 GHz will be necessary for heating of plasmasconfined by magnetic fields exceeding 3.5 T. For example,application of electron cyclotron resonance heating (ECRH)to the TFTR tokamak will require gyrotrons in the 104 to140 GHz range. ECRH of the Alcator tokamak will requiregyrotrons in the 104 to 340 GHz range. Heating of fusionplasmas confined by fields below 3.5 T may also requireradiation at frequencies above 100 GHz. If the plasmafrequency exceeds the electron cyclotron frequency, whichis likely in high beta fusion machines, it will be neces-sary to employ electron cyclotron heating at a harmonic(of the confining magnetic field) in order to achieve goodwave penetration [9]. Heating at the second harmonic ofthe cyclotron frequency requires gyrotrons operating above100 GHz for plasmas confined by magnetic fields exceeding1.8 T.In this paper, we report on the design and operationof a 100 kW, 140 GHz pulsed gyrotron. To our knowledge,this is the first high power (> 100 kW) gyrotron to oper-ate at a frequency exceeding 100 GHz. The present gyro-tron incorporated many design features and approacheswhich are significantly different from those of lower fre-quency gyrotrons. The philosophy behind the present de-vice was to construct a gyrotron which would operate atshort pulse length and low duty cycle with interchangeableparts. This allows a variety of experiments to be easilycarried out without having to solve problems associatedwith heating of the tube. However, the selection of theresonator mode and the electron gun design are such thatthe present gyrotron design is within the accepted tech-nological limits for long pulse, high average power oreven cw operation. The results presented here representthe first phase of an ongoing experimental program. Fu-ture operation with modified tube components (resonator,window, etc.) will hopefully give a more complete under-standing of high frequency gyrotron operation.Gyrotron DesignThe basic configuration of the gyrotron is shown inFig. 1 and the operating parameters are listed in Table 1.The electron gun is a magnetron injection gun with acathode, control anode and ground anode. The detailedcomputer analysis and the construction of the electron gunwere carried out by Varian Associates, Inc. The theoreti-cal analysis of the electron gun resulted in a design witha beam having low velocity spread and a low cathode cur-rent desity, as specified in Table 1. A retractable gatevalve has been welded to the end of the electron gun. Byclosing the valve, the remainder of the tube may be2RESONATORGATE VALVE BITTER MAGNET PUMPING PORTCOLLECTOR OUTPUTWAVEGUIDE WINDOWFEEDTHROUGHGUN COILS 0 2 4 6 8 10INCHESCATHODEM. I.T. 140 GHz GYROTRONFIGURE 1brought up to air while maintaining vacuum conditionsin the gun. This allows easy interchange of components,such as resonators and windows, without adversely affec-ting the cathode lifetime or requiring reconditioningof the cathode.TABLE 1Gyrotron OperatingParametersDESIGNVALUEVOLTAGE (kV)CURRENT (A)FREQUENCY (GHz)MAGNETIC FIELD (T)OUTPUT POWER (kW)PULSE LENGTH (us)Avi /vBO/BkJk (A/cm2 )BEAM RADIUS (mm)CAVITY RADIUS (mm)6551405.41001 to 21.49<+3. 2%2521.823.48OPERATINGRANGE23-800-7.5110-1804-70-1001.0-1.5250-33.473The present experiments have been carried out with aresonator designed for TE0 31 mode operation. The resonatorconsists of a tapered cavity of the kind suggested byVlasov et al. [10,11]. The cavity has a straight sectionlength of five wavelengths, a downtaper (cutoff) angle of0.15' and an uptaper (output) angle of 4*. The theoreti-cal diffraction Q of the cavity is 1515 as calculatedusing the computer code of Fliflet and Read [12]. AnX-band model of the cavity was constructed and found tohave an experimental Q value of (1420 + 50) for the TE0 3 1mode. For an optimized device, we estimate that the over-all efficiency can be as high as 30%, using the theoriesof Nusinovich and Erm [13) and Chu et al. [14]. The start-ing current for the present resonator is calculated to be0.5 A for the TE031 mode [15,16]. The beam produced bythe electron gun is designed to interact with the secondradial maximum of the field of a TE0 3 1 mode. Placementof the beam at the second radial maximum was selected inorder to allow the electron gun to operate with reducedcathode current density and space charge relative to agun designed for beam placement at the first (innermost)radial maximum. The beam interaction with the RF fieldof the TE03 1 mode is about 1/3 less at the second radialmaximum than at the first. To compensate for this, ahigher Q cavity-is used. The use of a larger diameterelectron beam also requires a larger beam tunnel betweenthe gun and the resonator and larger diameter entranceand exit holes at the resonator. In principle, microwaveradiation generated in the cavity is completely cutofffrom leaking into the beam tunnel and gun regions. Inpractice, mode conversion in the tube can result in RFleakage. In the present short pulse experiments, thereis no evidence so far of problems arising from the useofa larger diameter electron beam.The electron beam is collected on the output wave-guide, which is 2.2 cm in diameter. The collector iselectrically isolated from the tube body so that collectorcurrent may be measured directly. The microwave outputis transmitted down the output waveguide and through aCorning 7940 fused quartz window 0.554 cm thick. Thedielectric constants of fused quartz have been recentlydetermined to very high accuracy at millimeter wave fre-quencies [17]. During operation, the pressure in thetube is ordinarily less than 4 x 10-8 Torr.4The magnet used for these experiments is a room tem-perature, cw, water-cooled copper solenoid available at theM.I.T. Francis Bitter National Magnet Laboratory. Themagnet has a bore diameter of 10.5 cm and can achievefields in excess of 10 T. Measurement of the magneticfield lines indicates that the field lines are, to highaccuracy, symmetric about the geometric center line ofthe magnet bore tube. This is qnite useful in aligningthe gyrotron tube. The electron gun was designed tooperate in the tail field of the main solenoid withoutany field shaping by auxiliary coils. For better controlof the magnetic field in the cathode region, two coilswere placed symmetrically about the cathode, each capableof generating up to 2.0 kG. In actual gyrotron operation,the optimum emission was obtained when the field generatedby these coils was of the order of 0.2 kG or less*.Experimental ResultsThe gyrotron has been successfully operated over awide range of parameter space. The operating range studiedso far is listed in Table 1. Figure 2 shows a survey ofgyrotron output power vs. magnetic field in the range 4.8to 7.0 T. For that study, the voltage was 74 kV for the6 A scan and 70 kV for the 1.5 A scan. The repetitionrate was 2.1 Hz. The output power scale was calculatedassuming the pulse length to be 1.lps at all values of themagnetic field. The actual pulse length varies in a rangeof about 1.0 to 1.5 us and depends oIu the current andvoltage. Hence, the power levels listed are only approx-imate. The data in Fig. 2 represent a continuous scan,and no attempt was made to optimize output power at :eachfield value. The scan was carried out by varying only thefield of the main solenoid. During this scan, the magneticmirror ratio between the cavity and gun regions remainedapproximately constant at about 25 to 1. The current,voltage, gun coil field and tube alignment were also heldconstant. The tube operated continuously and without arc-ing throughout the entire range of the scan. The outputfrom the gyrotron was sent down a 2.2 cm diameter copperwaveguide and onto a Scientech, Inc., disc calorimeter,Model 36-0001, with a 2.5 cm active surface. A calibrationof the response of the calorimeter at 140 GHz was madeusing a carcinotron. The calibration factor of 0.77 + 0.10is in good agreement with a previous calibration at thesame frequency by Foote, Hodges and Dyson [18].5II I I I 1 1 1 1W w ' a WO Uw W W W W60 -50-a.6A20- I.5A (x2)0 5o 58 62 66MAGNETIC FIELD (kG)Fig. 2 The gyrotron output power vs. magnetic fieldfor a continuous scan at 74 kV and 6 A and, withthe vertical height multiplied by two, at70 kV and 1.5A.In Fig. 2, the predicted locations of the transverseelectric (TE) modes of the resonator are indicated byarrows. Table 2 lists the TE modes, the theoreticalmode frequencies calculated using the Fliflet et al. theory[12) and the experimentally measured frequencies. Fre-quency measurements were carried out using a Hughes wave-meter and a diode detector. Good agreement is obtainedbetween theory and experiment.A stddy of the whispering gallery modes, TEmll,m > > 1, was undertaken in part as a diagnostic of theelectron beam spatial. quality. The whispering gallerymodes can only be excited by an electron beam locatednear the resonator wall. A search for these modes wascarried out by varying the beam voltage and current, themagnetic field of the main solenoid and gun coils and thetube alignment. The fact that these modes were eitherabsent, or resulted in lower output power is good confir-mation that the electron beam is reasonably well centered6TABLE 2GYROTRON 1ODESTHEORETICALFREQUENCY(GHz)128.0133.0137.5140.3145.1147.7156.5161.4161.8162.3EXPERIMENTALFREQUENCY(GHz)128.2137.5140.4145.2147.2156.4161.5162.3in the resonator, away from the walls." In general, ex-citation of the whispering-gallery modes may prove to bea useful diagnostic of gyrotron performance.Near a field of 5.4 T, the gyrotron operating con-ditions were adjusted so as to maximize the output in theTE0 3 1 mode. An output power of 100 kW was achieved atthe conditions listed in Table 3. The far field patternof the radiation emitted from the gyrotron was consistentwith that of a TE031 mode. The microwave emission pulseTABLE 3100 kW OPERATIONVOLTAGECURRENTEFFICIENCYPULSE LENGTHMODEMAGNETIC FIELDFREQUENCYBANDWIDTH80 kV7.5 A17%1.1 psTE0315.4 T140.4 GHz<+100 MHzMODE(TE)42181123103152191133114162110,117shape was monitored using a diode detector. Wavemetermeasurements indicated single mode emission in the TE031mode at 140.4 GHz. An upper limit on the emission band-width of +100 MHz is imposed by the wavemeter data, thelimit being set by the finite 0 of the wavemeter. Theexperimental value of overall efficiency, 17%, is somewhatless than the theoretical value of 30%. Higher efficiency,up to 21%, was achieved in operation at lower beam currents,between 2 and 4 A.Discussion and ConclusionsWe report the operation of a 100 kW, 140 GHz pulsedgyrotron.operating in single mode emission. To our know-ledge, this is the first operation of a high power gyrotronat a frequency exceeding 100 GHz. In fact, operation inthe electron cyclotron maser mode has previously resultedin emission at higher frequency and power levels [ref. 19and references therein], but only with relativistic beamenergies, high currents, rather broad emission bandwidthsand low efficiency. To our knowledge, the present experi-ments are also the first operation of a high power gyrotronat a magnetic field as high as 7.OT.The present results represent a first investigationof the operating characteristics of this gyrotron. Bymodifying the resonator and undertaking other changes, wehope to eventually improve the gyrotron performance andoutput power. The operation of the electron gun has provento be very satisfactory. The electron gun has produced abeam of up to 600 kW of beam power. That beam has beentransmitted from the gun through the resonator and to thecollector, through a magnetic mirror ratio of 25. Althoughthe beam quality has not been directly measured, the beamquality must be reasonably good in order to produce highoutput power radiation. Therefore, the electron gun repre-sents -an important advance in the design of magnetron in-jection guns for high power, high frequency gyrotrons.In the present experiments, we have chosen to locatethe electron beam at the second radial maximum of thecavity RF field. As previously noted, this beam locationsignificantly impacts both the electron gun design and theresonator design. Future gyrotrons operating in the highpower, high frequency regime are likely to also requireoperation with an electron beam of relatively large8diameter. Experience gained in operation of the presentgyrotron with a larger diameter electron beam should proveuseful in the design of electron guns and resonators forfuture high power, high frequency gyrotrons.Although the present experiments have been carriedout in short pulse operation, the gyrotron has been de-signed to be capable of an upgrade to long pulse or evencw operation. Such an upgrade would require significantchanges in the tube design to provide, for example, alarge collector area and cooling in many parts of thetube. However, the present design, in principle, shouldbe capable of high power operation.The present experiment demonstrates that high power,single mode operation can be achieved at very high fre-quencies with gyrotrons. Based on these results, we be-lieve that operation can be further extended to higherpower and higher frequency. Such gyrotrons are likely toprove useful in the heating of magnetically confinedplasmas.AcknowledgementsThis research was supported by the U.S. Department ofEnergy, Office of Fusion Energy, under contract numberDE-AC02-78ET-51013. The M.I.T. Plasma Fusion Center issupported by the Department of Energy. The National MagnetLaboratory is supported by the National Science Foundation.The authors wish to thank D. Stone, K. Felch, H. Jory andthe Varian Gyrotron Engineering Group for their effort onthe electron gun. We also wish to thank C. Chase for helpwith many aspects of this project and .R. Weggel, M. Leupold,L. Rubin and D.B. Montgomery for assistance in magnet de-sign and operation. We thank D. Cohn, R. Davidson, B. Lax,P. Woskoboinikow, K.J. Button, M. Afsar, C.M. Loring,M. Read, K.R. Chu, A. Fliflet and M. Caplan for very help-ful discussions and suggestions.References1. V.A. Flyagin, A.V. Gaponov, M.I. Petelin and V.K.Yulpatov, IEEE Trans. Microwave Theory and Tech. MTT-25(1977) 514.92. H. Jory, S. Evans, J. Moran, J. Shively, D. Stoneand G. Thomas, IEDM Technical Digest 12.1 (1980) 304.3. M.E. Read, R.M. Gilgenbach, R.F. Lucey, Jr., K.R. Chu,A.T. Drobot and V.L. Granatstein, IEEE Trans. Micro-wave Theory and Tech. MTT-28 (1980) 875.4. G. Mourier, Ph. Boulanger, P. Charbit, G. Faillon,A. Herscovici and E. Kaurmerer, Sixth Intl. Conf. onInfrared and Millimeter Waves, Conference Digest,IEEE Catalog No. 81CH1645-1 MTT (1981).5. A.A. Andronov, V.A. Flyagin, A.V. Gaponov, A.L.God'denberg, M.I. Petelin, V.G. Usov and V.K. Yulpatov,Infrared Physics 18 (1978) 385.6. H. Jory, Sixth Intl. Conf. on Infrared and MillimeterWaves, Miami (1981).7. J.J. Tancredi, Sixth Intl. Conf. on Infrared andMillimeter Waves, Miami (1981).8. A.V.-Gaponov, V.A. Flyagin, A.L. GoI'denberg, G.S.Nusinovich, Sh. E. Tsimring, V.G. Usov and S.N.Vlasov, Int. J. Electronics 51 (1981) 277.9. S. M. Wolfe, D.R. Cohn, R.J. Temkin, and K.E. Kreischer,Nuclear Fusion 19 (1979) 389.10. S.N. Vlasov, G.M. Zhislin, I.M. Orlova, M.I. Petelinand G.G. Rogacheva, Radiophys. and Quantum Electron12 No. 8 (1969) 972.11. R.J. Temkin, Intl. J. Infrared and Millimeter Waves2 (1981) 629.12. A.W. Fliflet and M.E. Read, Intl. J. Electronics 51,(1981) 475.13. G.S. Nusinovich and R.E. Erm, Elektronnaya Tekhnika,Ser. 1, Elektronika SV Ch, No. 2 (1972) 55.14. K.R. Chu, M.E. Read and A.K. Ganguly, IEEE Trans.Microwave Theory Tech. MTT-28 (1980) 318.1015. K.E. Kreischer and R.J. Temkin, Int. J. Infrared andMillimeter Waves, 1 (1980) 195.16. K.E. Kreischer and R.J. Temkin, Int. J. Infrared andMillimeter Waves 2'(1981) 175.17. M.N. Afsar and K.J. Button, Intl. J. Infrared andMillimeter Waves, 2 (1981) 1029.18. F.B. Foote, D.T. Hodges, and H.B. Dyson, Intl. J.Infrared and Millimeter Waves, 2 (1981) 773.19. R.E. Shefer and G. Bekefi, Intl. J. Electronics 51(1981) 569.11

100kW, 140 GHz pulsed gyrotron

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100kW, 140 GHz pulsed gyrotron

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