Measurement of small-signal parameters in traveling-wave tube theory

An experimental procedure is described which allows the accurate determination of parameters needed to calculate the small-signal gain of traveling-wave tubes using the equivalent transmission-line model of Pierce. A movable electromagnetic probe is used to measure the gain per unit wavelength along the helix and the attenuator and also to measure the initial loss. From these data the gain parameter C' and the space-charge parameter Q'C' are determined simultaneously. The value of C' enables the growth parameter x' to be obtained from the gain per unit wavelength. Excellent agreement with theory has been obtained for the variation of C' and Q'C' with beam-to-helix radius ratio b/a and with perveance, and for the variation of the growth parameter x' with C' and Q'C'. The change of initial loss with helix voltage for various C' and Q'C' has been verified. Finally the growth parameter in the attenuator has been found to be positive, as expected from field theory as well as the equivalent transmission-line model.