1296 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



ION 

 COLLECTOR 

 -20 VOLTS 



BEAM-FORMING 

 ^ ELECTRODE 

 VOLTS 



COLLECTOR 



1600,1800 OR 



2000 VOLTS 



^^S9W<R)W^''- timmmifMF- ■ 



T 



CATHODE 

 VOLTS 



ACCELERATOR 

 1800 VOLTS 



HELIX 



-300 



-200 -100 100 



POTENTIAL DIFFERENCE BETWEEN HELIX 

 AND ACCELERATOR IN VOLTS 



200 



Fig. 6 — Effect of electrode voltages on ion bombardment of the cathode in a 

 prototype of the M1789. In this e.xperiment the helix voltage was varied while the 

 positive ion current to a monitor electrode behind a hole in the cathode was meas- 

 ured. Curves are shown for the collector voltage greater than, equal to, and less 

 than the accelerator voltage. During this experiment the accelerator voltage was 

 held constant at 1800 volts with a resulting beam current of 40 ma. The experi- 

 ment was performed on a continuously pumped sj'stem with the pressure main- 

 tained at 2 X 10-' mm Hg. The helix ID was 80 mils, the cathode diameter 300 

 mils, and the cathode hole diameter 20 mils. These curves show that the ion 

 bombardment of the cathode can be reduced by as much as a factor of 20 by prop- 

 erly arranging the voltage profile. 



