1332 



THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



focusing system similar to that employed in the M1789, we estimate the 

 actual beam diameter (for 99 per cent of the current) to be about 65 

 mils (Jb/a = 0.7). However, the current density distribution is peaked 

 at the center of the beam because of the effect of thermal velocities of 

 the electrons. Thus an effective h/a of 0.5 is not unreasonable. 



Maximum Power Output ' 



Fig. 35 shows the maximum power output as a function of beam cur- 

 rent both immediately after rf drive is applied and after the tube has 

 had time to stabilize. We see that at high rf power outputs the fading 





3.0 



2.5 



iJ 2.0 



> 



u 



z 



ly 1.5 

 o 

 u. 

 H] '-0 



0.5 





 4.0 



3.5 



3.0 



^2.5 

 U 



z 



y 2.0 



u 



IL 



li 1.5 



1.0 



0.5 



4.0 



4.5 5.0 5.5 6.0 6.5 



FREQUENCY IN KILOMEGACYCLES PER SECOND 



7.0 



7.5 



Fig. 38 — Ratio of electronic efficiency to gain parameter C as a function of 

 frequency. The efficiencies used for this comparison are all before fading. The dot- 

 ted line.s are estimated from the Tien theory corrected for the intrinsic loss of the 

 helix. The curves in (a) are for the case of the heli.x voltage adjusted for the maxi- 

 mum low-level gain and those in (b) for the case of the helix voltage adjusted for 

 maximum power output. 



