11-2] 



THE KLYSTRON 



591 



Focus Electrode 



Cathode 

 Heater 



Input Output 

 Cavity Cavity 



Collector 



Electron Beam 



Fig. 11-16 A Two-Cavity Klystron Amplifier. 



gridded in Fig. 11-16. If RF power is supplied to this first cavity resonator 

 from an external source of power, an oscillating electromagnetic field will be 

 developed wholly within the cavity resonator. The electromagnetic field 

 developed by the driving signal does not reach the cathode; cathode current 

 is therefore unaffected by the level of drive power. 



With drive power supplied to the 

 first resonator, an electromagnetic 

 field is developed within the reso- 

 nator as shown in Fig. 11-7. The 

 electric field is largely concentrated 

 across the gap. When the electron 

 beam passes through this gap, the 

 electrons are acted upon by the 

 electric field. Electrons that pass 

 through the gap at the instant when 

 the electric field is in a direction to 

 speed them up are accelerated dur- 

 ing their passage through the gap. A 

 half-cycle later, the field will have 



reversed in direction and will decelerate the electrons that pass through at 

 that moment. When the electron beam enters the cavity gap, all electrons 

 are traveling at approximately the same velocity. When the beam leaves 

 the cavity gap, some electrons have been speeded up, and others have been 

 slowed down. This action is known as velocity modulation. 



After the electron beam leaves the first cavity and proceeds down the 

 drift tube (see Fig. 11-16), the electrons that have been speeded up will tend 

 to catch up with and bunch together with the electrons that passed through 

 the gap a half-cycle earlier but were slowed down by the field in the cavity. 

 Clumps or bunches of electrons will form along the beam. A lumpy or 

 bunched electron beam is called density-modulated . A Fourier analysis of a 



Fig. 11-17 Electromagnetic Field In- 

 side a Re-entrant Cavity Resonator. 



