Noise Spectrum of Electron Beam in 

 Longitudal Magnetic Field 



Part II — The UHF Noise Spectrum 



By W. W. Rigrod 



(Manuscript received January 21, 1957) 



Sharp peaks are found in the UHF spectrum (10 to 500 mc) of an elec- 

 tron beam, emanating from a shielded diode. In the presence of a longitudi- 

 nal magnetic field, the strongly rippled beam displays an additional set of 

 peaks whose frequencies are proportional to the field strength. The largest 

 of these, just above the cyclotron frequency, is connected with the overlap of 

 a dense cluster of particle orbits, passing close to the beam axis. It can attain 

 amplitudes of 65 db above background noise. 



The transverse distribution of UHF noise power is found to agree with 

 that for ideal Brillouin flow , even in rippled beams. With long ripple wave- 

 lengths, two noise maxima are found to flank each beam waist. A small- 

 signal wave analysis explains this pattern, and affords some insight into the 

 energy-exchange processes in rippled-beam amplification. The reduction in 

 "growing noise" due to positive ions is attributed to increased cancellation 

 of net radial beam motion, due to overlap in particle orbits near the axis. 



I INTRODUCTION 



The reader is referred to Part V for a description of the experimental 

 apparatus and its operation. In this paper, measurements of noise power 

 in the same electron beam are described, with freciuencies chiefly in the 

 10- to 500-mc range, and relatively weak magnetic fields. For the UHF 

 measurements, a calibrated coaxial step attenuator and a super-regenera- 

 tive receiver (the Hewlett-Packard 417-A VHF Detector) are used. Rela- 

 tive noise-power amplitudes at fixed freciuencies are measured as before, 

 in terms of changes in attenuation between probe and receiver required 

 to restore constant receiver output. To obtain qualitative information, 

 however, such as the location of noise maxima along the beam, the series 

 attenuation is fixed. The receiver output is amplified, rectified, and per- 



855 



