Sec. 21.2] 



THE CYCLOTRON 



479 



source and the dees, and partly by recombination. Furthermore, ions can 

 be started on stable orbits during only a part of the electric-field cycle. 

 Those which start long before the dee voltage has reached a maximum value 

 are ultimately defocused and strike the dees; those starting long after the 

 dee voltage has passed a maximum value may be accelerated and find stable 

 orbits, but they still have little probability of reaching the target. These 

 ions receive only a fraction (approximately one-half or less) of the accelerating 



A 



ROTARY 

 CONDENSER 

 SHIFTS 

 FREQUENCY 



OSCILLATOR 



-200 KV DC 



VACUUM TANK 

 (GROUND POTENTIAL) 



BEAM 



INCREASE IN MASS DUE 



TO RELATIVISTIC EFFECT 



10% 



VELOCITY 43% 



VELOCITY 

 OF LIGHT 



Fig. 128. Diagram of the principal components of the 184-in. cyclotron of the Radiation 

 Laboratory, University of California. [Reprinted by permission of the Radiation Laboratory. ] 



voltage at each transit past a gap and must perform correspondingly more 

 cycles; hence, they traverse a greater path length to reach the maximum 

 radius and energy. The probability of collision with the residual gas in the 

 tank becomes greater and, in general, such ions are either deflected by 

 collisions to the dee walls or are knocked out of resonance with the electric 

 field. The ions that successfully reach the target, therefore, are those which 

 start during an interval in the electric-field cycle shortly after it has passed 

 maximum voltage, as illustrated in Fig. 129. 



For those ions which have successfully entered circular orbits three features 

 of the cyclotron are essential to their subsequent acceleration to the maximum 

 radius and energy: (1) stability to small oscillations superimposed on the 

 normal spiral path; (2) the tendency of ions to remain in or near the median 

 plane between the pole pieces; and (3) the stability of resonance between the 

 ion rotational frequency and the oscillating electric field. The necessity for 



