Effects of high-velocity secondary electrons in traveling-wave tubes

The presence of a beam of high-speed secondary electrons in a traveling-wave tube affects the amplification and cross-modulation characteristics of the tube. When primary electrons strike the collector electrode, secondary electrons are produced. The high-speed secondary electrons have a velocity approximately equal to the primary electrons and can form a reverse beam which serves as a feedback mechanism within the traveling-wave tube. Coupling of the signal to the reverse beam and the influence of the reverse beam on the forward amplification system are investigated. The principal effects of this secondary beam are that a second-order fluctuation is superimposed on the usual variation of gain as the helix voltage is varied, and that the cross-modulation characteristics of the tube are materially altered, with the possible production of "negative" cross modulation. Experimental results demonstrate the existence of these effects and indicate that they are absent when the secondary beam is eliminated.