Interaction Impedance for Space Harmonics of Circular Helix Using Simulations

Design of traveling-wave tubes requires accurate estimate of interaction impedance for the fundamental and nonfundamental space harmonics of the slow wave structures such as circular helix. Previous work based on Fourier decomposition has shown that the interaction impedance for the fundamental space harmonic can be estimated accurately using simulations. Here, we consider the interaction impedance for the nonfundamental space harmonics; the accuracy of the simulation results is assessed by comparison with those obtained from the tape-helix model. We present illustrative results for the on-axis and off-axis interaction impedance values for the fundamental and the -1th space harmonic of the circular helix based on simulations. Together with Fourier decomposition, the simulation-based approach can provide accurate estimate of the interaction impedance. This can be particularly useful in many practical cases where an accurate analysis or method of measurement of interaction impedance is not available.