Why a circuit sever affects traveling-wave tube efficiency

The effect of a circuit sever on traveling-wave amplifier efficiency has been calculated from the large-signal traveling-wave tube equations. Measurements made on high-power traveling-wave amplifiers, which utilize a severed coupled-cavity interaction circuit, have confirmed the calculated results. The results show that a traveling-wave amplifier with C = 0.1 and QC = 0.25 must have 26 db or more growing-wave small-signal gain beyond the sever to obtain the maximum theoretical efficiency. If the growing-wave gain is less than this value, efficiency is reduced; for example, if the gain is 4 db less, the efficiency is reduced by one half. The mechanism which causes this reduction of saturated output power can be understood from the computed electron distribution in phase and velocity. From these phase-velocity diagrams, the degradation in efficiency is seen to be caused by the debunching of the beam by space-charge forces in the region between the sever and the point where the microwave signal builds up to the value it previously reached before the sever. Further study of the computer results shows that the loss in efficiency caused by insufficient gain beyond the sever cannot be made up by depressed collector operations, and that the use of over-voltage to obtain increased efficiency in a severed tube may not be effective because of the resulting reduction in gain that over-voltaging causes.