A C-band gyro-TWT

A C-BAND GYRO-TWT PATRICK FERGUSON AND ROBERT SYMONS VARIAN ASSOCIATES, INC. 611 HANSEN WAY PAL0 ALTO, CA. 94303 ( 4 15) 493-4000
ABSTRACT The o p e r a t i n g c h a r a c t e r i s t i c s of a gyrotron t r a v e l i n g wave tube (gyro-TWT) o p e r a t i n g a t C-band a r er e p o r t e d .D e t a i l e dt e s t i n g showed t h a ta th i g h e r beam voltage and c u r r e n t and with a magneticfieldprofileincreasinginmagnitude toward t h e rf output window, theoutput power kW a t 16.6% e f f i c i e n c y couldbeincreasedfrom50 t o 128 kW a t 24% e f f i c i e n c y . The maximum e f f i c i e n c y measured was 26% a t 120 kW. Experimentaldata on small and l a r g e s i g n a l g a i n , bandwidth,output power and e f f i c i e n c y will b e presented. The gyro-TWT employs a magnetron i n j e c t i o n gun t o provide an e l e c t r o n beam with a prescribed rat io. perpendicular to parallel velocity gun is a f i n i t e Inherentin t h i s typeofelectron spread in the two v e l o c i t y components. A n a l t e r n a t e method forproducinganelectron beam w i t h very low spreadin the v e l o c i t y components is presented. Design data will be presented.
The gyro-TWT was developedundercontractwith Rome Air Development Center, Advanced Research P r o j e c t s Agency and Ballistics Missile Defense Advanced TechnologyCenter.
2 1700 FIELD MAGNETIC PROFILE
ELECTRON GUN INPUT WAVEGUIDE
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Figure 1. Cross section of the C-band gyro-TWT with magnets F u r t h e r d e t a i l e d t e s t i n g has beenperformed. Output power versusfrequency is shown i nF i g u r e s beam power. Forthe 2 and 3 f o r twovaluesof high beam power mode, 65 kV beam voltage and 6 A beam c u r r e n t , t h e maximum output power is 120 kW a t 26% e f f i c i e n c y a t 5.2 GHz. A t t h i s frequency t h es m a l ls i g n a lg a i n is 21 dB and t h e l a r g e s i g n a lg a i n is 17 dB. The instantaneous 3 dB Correction the of impedance bandwidth is 7.251. mismatch a t t h e j u n c t i o n o f t h e i n t e r a c t i o n waveguide and o v e r s i z e d c o l l e c t o r would eliminate t h e power hole at 5.43 GHz, e f f e c t i v e l y increasing the bandwidth t o 8.8%. I n t h e low beam power mode, i . e . , a t 40 kV and 4 A, t h e maximum output power is 18.8 kW a t 9.8% e f f i c i e n c ya t 5.24 GHz. The small s i g n a lg a i n a t t h i sf r e q u e n c y is 14 dB and t h el a r g es i g n a lg a i n is 8.4 dB. The instantaneous 3 dB bandwidth is 9.3%. Table 1 shows e f f i c i e n c y and l a r g es i g n a l gain at a number ofoperatingpoints. These data suggest t h a t t h e g a i n and e f f i c i e n c y a r e dependent upon t h e beam c u r r e n t , t h e beam voltage, or both.Examinationof the d i s p e r s i o n r e l a t i o n s h i p publishedelsewhere ( 1 ) shows t h a tt h ec o u p l i n g f a c t o r between t h e waveguide mode and t h e beam modes is dependent upon t h e productofthe beam conductanceandthesquare of theperpendicular velocity of the electrons together with a third f a c t o r which is a f u n c t i o n o f t h e o l e c t r o n o r b i t s i z e and placement i n the waveguide. The square of the perpendioular velocity of the e l e c t r o n s is
The C-band gyro-TWT employs thefundamental cyclotron resonance interaction with the circul a r l y p o l a r i z e d T E l l dominantwaveguide mode. I n i t la1 experimental results includedmeasureas high as 24 dB small mentsofstablegain s i g n a la n d , 18 dB saturated ( 1 ) . A saturated porteroutput of 50 kW at an e l e c t r o n i c beam e f f i c i e n c y o f 16.65 was measuredwith a 3 dB s a t u r a t e d power outputbandwidthof 65 a t 60 kV beam voltage and 5 A beam c u r r e n t . The C-band gyro-TWT with associated magnetic is shown i n c o i l s and m a g n e t i c f i e l d p r o f i l e Figure 1. The s a l i e n tf e a t u r e so ft h e gyro-TWT and t h e s e n s i t i v i t y o f t h e o p e r a t i n g p a r a m e t e r s t o t h e six magnetic f i e l d coils havebeen described elsewhere ( 2 ) . The magnetic f i e l d a s a d j u s t e d f o r maximum e f f i c i e n c y h a s a 4.4% i n c r e a s e i n v a l u e from t h e r f i n p u t t o t h e end of t h e i n t e r a c t i o n waveguide.