Study on a High-Power W -Band Extended Interaction Klystron With Efficiency Toward 44%

In order to overcome the low efficiency (usually less than 25%) of millimeter-wave and terahertz (MMW/THz) klystrons, an exploratory study is conducted to improve the efficiency in the design of W-band extended interaction klystrons (EIKs). In this article, fast large-signal simulations using KlyC code are comprehensively used for the first time to accurately conduct full-scale parameterization of EIK, it is found that the upper limit of terahertz klystron efficiency is no longer solely dominated via the beam perveance, but rather restricted by various factors, including effective impedance, ohmic loss, cavity arrangement, and so on. The physical mechanism involved is so complex that the preliminary parametric sortation is mandatory to achieve very high efficiency (HE). Particle-in-cell (PIC) simulation results verified our theoretical evaluations, demonstrating that the W-band EIK proposed and optimized in this article could deliver up to 44.5% electronic efficiency and 39.2% RF efficiency around 100 GHz, with operating beam voltage of 50 kV and beam current of 2 A. The output power and saturation gain could achieve 39.2 kW and 65.5 dB, respectively, in which no instability is observed. In addition, a novel beam optics system (BOS) based on hybrid permanent magnets is proposed. Such compact BOS is exclusively developed to deal with the challenge induced by severe radial beam expansion in this EIK, which is the side-effect of the high intensity of beam-wave interactions. This study on high-power (HP) HE compact W-band EIK reveals the true potential of such types of devices in the MMW/THz regime.