PART II — THE UHF XOISE SPECTRUM 877 



at other frequencies. Here the energy suppHed by the ac field is coupled 

 most effectively to the free electrons at the resonant frequency, increas- 

 ing their dc kinetic energy until the gas breaks down. The circular ac 

 charge motions due to the dc magnetic and the ac electric fields are 

 superimposed on high-velocity random motions, similar to the radial 

 motions in the drifting beam. 



The UHF peaks observed at harmonics of the proper frequency may 

 simply be due to the non-linear character of the beam, when excited by 

 the high-level fundamental oscillations. The other faint satellite peaks, 

 near 0.5 coc and 0.707 Wc , seem to be associated with the unneutralized 

 space-charge density at the beam waist. 



The conspicuous role played by crossover electrons in the waist region 

 of rippled beams, due to the tendency of their orbits to overlap there, 

 leads one to re-examine their influence on rippled-beam amplification. 

 As seen in the previous section, this gain process depends on the average 

 value of (vr/r) at each cross-section plane of the beam. The fraction of 

 all electrons which penetrate to the beam axis depends on competition 

 between the unneutralized space-charge forces and the particle's trans- 

 verse kinetic energy. An increase in positive ion density tends to make 

 the potential depressions at beam waists broader and shallower, and 

 thereby increase the number of crossover electrons as well as the axial 

 distance over which they reach the axis. The net effect is to reduce the 

 average value of | fr | over a greater portion of the ripple wavelength, and 

 thus reduce the net gain of the space-charge wave. This may explain 

 why the "growing noise" phenomenon tends to be inhibited by an in- 

 crease in positive ion density. 



VII CONCLUSIONS 



Evidence is found of oscillations with frequencies in the 10- to 500-mc 

 region inside of an electron-gun diode. There is some basis for associating 

 them with electron-field interaction in the retarding region of the diode. 

 Another type of narrow-band noise peak is found near the waists of a 

 strongly rippled beam in a longitudinal magnetic field, with frequencies 

 proportional to the field strength. The strongest of these, at about 1.05 

 times the angular cyclotron frequency, co^ , as well as its harmonics, can 

 be explained by the resonant behavior of a short section of the beam, in 

 which the average transverse velocity is nullified by overlap in particle 

 orbits. Fainter satellite peaks, near 0.5 Wr , 0.707 Uc , and o)^ , respectively, 

 accompany the dominant frequenc3\ 



In a drifting beam launched from a shielded electron gun and focused 

 by an axial field, the transverse distribution of noise (or signal) intensity 

 is found to agree with that predicted for ideal Brillouin flow. Despite 



