A comparison of L-band helix TWT experiments with CHRISTINE, a 1-D multifrequency helix TWT code

Abstract

Extensive experimental measurements were carried out to test the accuracy of the parametric helix traveling-wave tube (TWT) code, CHRISTINE. The model is one-dimensional, with beam electrons represented as rigid disks. Multifrequency interactions are supported and the RF circuit can be optionally represented with cold-test data, a sheath helix model, or a recently implemented tape helix model. Simulations using the tape helix model are shown to be in good agreement with experimental measurements of an L-band TWT over a broad (250-MHz) frequency range. In the intermediate and saturated power regimes, the modeled and measured TWT gain versus frequency agree to better than 0.4 dB, with deviations explained by strong reflections at the output window that are not accounted for in the code. Single-tone experimental and simulated drive curves agree to better than 1 dB in the small- and large-signal regimes; relative phase shift simulations agree to within experimental measurement accuracy in the small-signal regime and to within 75% in the large-signal regime. Two-tone experimental and modeled data exhibit similarly good agreement, with CHRISTINE accurately predicting the effect of frequency-dependent gain variations on the TWT output response and third- and fifth-order intermodulation products.