THE BELL SYSTEM 



TECHNICAL JOURNAL 



VOLUME XXXVI JULY 1957 number 4 



Copyright 1957, American Telephone and Telegraph Company 



Noise Spectrum of Electron Beam in 

 Longitudinal Magnetic Field 



By W. W. Rigrod 



(Manuscript received January 21, 1957) 



Measurements of induced noise currents along drifting cylindrical elec- 

 tron beams have shown that noise fluctuations propagate as space-charge 

 waves in the same fashion as RF signals of the same frequency. On many 

 such beams, however, the regular standing-wave noise pattern is interrupted, 

 after some drift distance, by a smooth steep increase in noise current, fol- 

 lowed by slow, shallow undulations. This "growing noise" phenomenon, 

 discovered by Smullin and his co-workers at M.I.T. several years ago, is 

 the subject of study in this paper. Its importance is considerable, in a nega- 

 tive way, because it has hampered the development of medium-power travel- 

 ing-wave-tube devices with acceptably low noise figures. 



The experimental measurements show the growing noise pattern to be the 

 result of a two-stage process. Its primary cause is rippled-beam amplifica- 

 tion of noise fluctuations over a wide band of microwave frequencies, much 

 higher than the usual observation frequency. This explains its elusiveness. 

 In the second stage, noise energy is transferred to lower frequencies, due to 

 intermodulation and other non-linear processes within the gain band. As 

 the beat- frequency noise increments are excited by continuous arrays of fre- 

 quency pairs, their standing-wave patterns overlap one another, resulting in 

 a smooth growing-noise pattern. 



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