CHAPTER 21 

 THE CYCLOTRON 



21.1. Description. The cyclotron, or magnetic resonance accelerator, 

 first proposed and developed by Lawrence [1], enables high energies to be 

 imparted to heavy charged particles by repeated accelerations across a com- 

 paratively small electric field. Ions formed at the center of a uniform mag- 

 netic field of 10,000 gauss or more are accelerated in the gap between two 

 hollow D-shaped electrodes, known as dees, to which a high-frequency poten- 

 tial of 10 to 100 kv is applied in opposite polarity, as shown in Fig. 126. An 



Fig. 126. Schematic diagram of the cyclotron. A, deflector; B, vacuum lock used without 

 foil window for internal target; C, target; D, dees; E, dee stems; F, filament leads to ion 

 source at center; G, aluminum foil used as a vacuum seal for external targets. 



ion starting from the center of the magnetic field is constrained to move in 

 nearly circular orbits of slowly increasing radius while accelerated by succes- 

 sive impulses in traversing the gap between the dees. Synchronization of the 

 high-frequency electric field with the frequency of rotation of the particle 

 ensures that the maximum dee voltage occurs at, or just before, the instant 

 the particle passes the gap. During the remainder of its path in the field-free 

 region within the dees, the particle is unaffected by the changing electric 

 field which has reversed its polarity when the particle again enters the next 

 gap. Two impulses, therefore, are experienced by a particle in each cycle. 



The magnetic field does not contribute to the energy of an accelerated 

 particle and serves only to constrain the particle to a circular path whose 

 radius at any instant depends on the velocity. 



r = 



mv 



7h 



H\ 



476 



2mn V sin <p 



