Sec. 20.2] 



THE ELECTROSTATIC GENERATOR 



473 



ably lower cost but also because of better electrical properties [1,3]. Charge 

 is sprayed on the belt by a spray wire stretched across the belt at a dis- 

 tance of approximately Y% in. and charged to a potential ranging from 5 to 

 20 kv as determined by the desired equilibrium voltage on the terminal. The 

 charge conveyed to the terminal is then removed with an efficiency of 35 to 60 

 per cent by a collecting comb consisting of a metal strip containing numerous 



Fig. 124. Schematic diagram of the Van de Graaff electrostatic generator. A, terminal: 

 aluminum shell; B, ion source: filament, grid, and capillary cathode through which ions 

 pass into accelerating tube; C, corona ring; D, drift tube; E, accelerating tube insulator 

 section; F, electromagnet for magnetic separation of ion beams; G, target; H, charge belt; 

 /, power generator for source and other equipment; /, auxiliary belt for driving power 

 generator; K, cylindrical type support insulator; L, tripod type support insulator. 



sharp points directed toward the belt near the terminal pulley. Charge of 

 opposite sign again is sprayed on the belt in the terminal and removed at the 

 ground end, thus doubling the terminal charging rate. The current delivered 

 to the belt by a charging system has been shown to be approximately [1] 

 I — k{V — V ) 2 , where k is an empirical constant, V is the spray-wire volt- 

 age, and V is the spray-wire threshold. The threshold voltage for sharp- 

 pointed combs is zero; for wires of small diameter, it is usually several 

 kilovolts. 



The vacuum accelerating tube, illustrated in Fig. 124, is constructed of a 

 series of insulating sections made of glass, pyrex, or porcelain which are 

 separated by metallic rings and vacuum-sealed. The metal rings support 

 external corona rings, which ensure uniform potential distribution along the 



