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BELL SYSTEM TECHNICAL JOURNAL 



medium, and small velocities, respectively.^ As the ball first thrown up 

 takes a longer time to return than the second, and the third takes a shorter 

 time to return than the second, when the balls return the time intervals 

 between arrivals will be less than between their dei)artures. Thus time- 

 position "bunching" occurs when the projection velocity with which a 

 uniform stream of particles enters a retarding iield is progressively decreased. 

 Figure 3 demonstrates such bunching as it actually takes place in the 

 retarding field of a reflex oscillator. :\n electron crossing the gaj) at phase A 



R-F VOLTAGE 

 ACCELERATING 



FOR ELECTRONS 

 FROM CATHODE 



R-F VOLTAGE RETARDING 



FOR ELECTRONS 

 FROM CATHODE 



FOR ELECTRONS 



RETURNING TOWARD 



/ CATHODE \ 



T T 



Fig. 3. — The drift time for transfer of energy from the bunched electron stream to the 

 resonator can be deduced from a plot of gap voltage vs time. 



is equivalent to the first ball since its velocity suffers a maximum increase, 

 an electron crossing at phase B corresponds to the ball of velocity ^o where 

 for the electron Vq corresponds to the d.c. injection velocity, and finally an 

 electron crossing at j^hase C corresponds to the third ball since it has suffered 

 a maximum decrease in its velocity. The electrons tend to bunch about the 

 electron crossing at phase B. To a tirst order in this process no energy is 

 taken from the cavity since as many electrons give up energy as absorb it. 

 The next step in the process is to bring back the grou])ed electrons in 

 such a phase that they give the maximum energy to the r.f. field. Now, 

 f of a cycle after the gap voltage in a reflex oscillator such as that shown in 

 Fig. 1 is changing most rai)i(lly from accelerating to retarding for electrons 



^ The reader is not advised to try this experimentally unless he has juggling experience. 



