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An analysis of the propagation characteristics of coupledcavity (CC) delay lines in traveling-wave tubes (TWTs) under hot conditions is proposed. Using this approach a necessary impedance level at the end of the periodic line is derived that leads to minimum input reflections during operation. From this matching condition a required cold input reflection can be predicted which is in general different from the one commonly used. INTRODUCTIONThe design of delay lines for coupled-cavity traveling-wave tubes (CC-TWTs) is commonly performed using cold parameters, i.e. without considering the electron beam. During the manufacturing process the design of the delay line is verified by means of cold measurements, e.g., of the input reflection over frequency. Once approved, the tube is assembled and evacuated. However, a common observation is that the previously measured reflection changes in the presence of the electron beam. In this paper we study the necessary hot load condition for the input section of a CC-TWT. In the next section the underlying models and the derivation of the hot dispersion characterictics are introduced. After that the hot load condition is applied to a generic input section of a CC-TWT.

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