Sec. 20.2] THE ELECTROSTATIC GENERATOR 475 



down gap for a particular voltage. This permits a very considerable reduc- 

 tion in the physical dimensions which become an important consideration in 

 high-voltage installations. It allows, in addition, rigid control of the atmos- 

 phere and, in particular, the humidity. The essential features of a high- 

 pressure electrostatic generator are the same as in units operated at atmos- 

 pheric pressure. They are however built compactly and enclosed in a steel 

 tank designed to withstand a pressure of 10 atm or more. Because of the 

 small spacing between the terminal, accelerating tube, and the grounded tank 

 wall, it is necessary to shield the entire unit with closely spaced hoops of 

 aluminum tubing to ensure a uniform potential distribution and to reduce 

 sparking to the tank wall. 



The acceleration of protons and alpha particles in an accelerator of this 

 type does not provide strictly homogeneous beams of particles. Hydrogen, 

 for example, introduced into the ionization chamber is injected into the 

 accelerator tube both as H + and H^~. Both particles reach the target with 

 the same kinetic energy, but the molecular ion splits on impact into two H + 

 particles, each with only half the original kinetic energy. Similarly, when 

 accelerating alpha particles, both He + and He"^ are produced by the source. 

 Energetically homogeneous beams can be obtained readily, however, by 

 passing the accelerated beam through a magnetic field, or analyzer, to sepa- 

 rate the particles H + and H£ or He + and He 4- *" before they reach the target. 



REFERENCES FOR CHAP. 20 



1. Van de Graaf, R. J., K. T. Compton, and L. C. Van Atta: Phys. Rev., 43, 149 (1933). 



2. Van Atta, L. C, D. L. Northrup, C. M. Van Atta, and R. J. Van de Graaf :Phys. 

 Rev., 49, 761 (1936). 



3. Tuve, M. A., L. R. Hafstad, and O. DAmv.P/m. Rev., 48, 315 (1935). 



4. Trump, J. G., and R. J. Van de Graaf: Phys. Rev., 55, 1160 (1939). 



5. Wells, W. H., R. O. Haxby, W. E. Stephens, and W. E. Shoupp: Phys. Rev., 58, 162 

 (1940). 



6. Williams, J. H., L. H. Rumbaugh, and J. T. Tate: Rev. Sci. Instruments, 13, 202 (1942). 



