To Termlnol 



CHAPTER 20 

 THE ELECTROSTATIC GENERATOR 



20.1. Description. The most successful form of electrostatic generator 

 for the acceleration of heavy charged particles is that first proposed by Van 

 de Graaf [1] and subsequently developed by Van de Graaf and others [2-6]. 

 High potentials are produced by the accumulation of charge on large closed- 

 surface conductors, often spheres, called terminals. The terminals are 

 charged to the desired electrostatic potential by means of an endless belt 

 of nonconducting material serving as a charge conveyer. Charge is sprayed 

 on at the grounded end of the belt and subsequently removed by a collecting 

 comb in the terminal. The charge collected appears only at the surface of 

 the terminal, leaving the interior a field-free region as required by elementary 

 principles of electrostatics. Ions formed by a source within the terminal are 

 accelerated through a vacuum tube running from the terminal to ground or 

 to a second terminal charged to the opposite polarity. The maximum energy 



imparted to a particle is eV, or in terms of 

 electron volts nV, the charge number of the 

 particle times the potential difference between 

 the ends of the accelerating tube. This places 

 a severe limit on the energies derivable from 

 such machines as compared to resonance and 

 induction accelerators since the maximum 

 energy to which a particle can be accelerated 

 is only equal to the voltage to which the ter- 

 minals can be charged. The maximum volt- 

 age is established when the rate of charge 

 leakage by corona discharge from the terminal 

 equals the transport of charge by the belt. 

 In the largest units thus far built this potential 

 has exceeded 5 million volts [2]. 

 20.2. Construction and Operation. A typi- 

 cal belt charging system is shown in Fig. 123. To achieve the highest equi- 

 librium voltage requires the greatest practicable charge transport; belts are 

 operated at saturation charge, made as wide as the mechanical design permits, 

 and operated at the highest practicable speed — usually several thousand feet 

 per minute. Cotton and paper belts have proved to be more practicable in 

 large installations than silk, rayon, or linen, partly because of their consider- 



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Fig. 123. Charging system for 

 electrostatic (Van de Graaff) 

 generator. A, spray combs; B, 

 charging belt; C, collector and 

 shield; D, metallic pulley. 



