Multipactor is a nonlinear ac discharge in which a high frequency rf field creates an electron avalanche sustained through secondary electron emission from metallic or dielectric surfaces. Multipactor discharge can adversely affect various rf systems, such as telecommunications systems, high power electromagnetic sources, and accelerator structures. The restricted frequency spectrum and the cluttered satellite orbits require a single spacecraft to perform the same or enhanced functions which previously required several satellites. This necessitates complex multi-frequency operation for a much-enlarged orbital capacity and mission, where the requirement of high power rf payload significantly increases the threat of multipactor. This work provides a comprehensive understanding of multipactor discharge driven by two-frequency rf fields. The study provides important results on single and two-surface multipactor, including multipactor mitigation, migration of electron trajectory, and frequency domain analysis.We use Monte Carlo simulations and analytical calculations to obtain single surface multipactor susceptibility diagrams with two-frequency rf fields. We present a novel multiparticle Monte Carlo simulation model with adaptive time steps to investigate the time dependent physics of the single surface multipactor. The effects of the relative strength and phase of the second carrier mode as well as the frequency separation between the two carrier modes are studied. It is found that two-frequency operation can reduce the multipactor strength compared to single-frequency operation with the same total rf power. Migration of the multipactor trajectory is demonstrated for different configurations of the two-frequency rf fields. Formation of beat waves is observed in the temporal profiles of the surface charging electric field with small frequency separation between the two carrier modes. We study the amplitude spectrum of the surface charging field due to multipactor in the frequency domain. It is found that for the single-frequency rf operation, the normal electric field consists of pronounced even harmonics of the driving rf frequency. For two-frequency rf operation, spectral peaks are observed at various frequencies of intermodulation product of the rf carrier frequencies. Pronounced peaks are observed at the sum and difference frequencies of the carrier frequencies, at multiples of those frequencies, and at multiples of the carrier frequencies. We also study two surface multipactor with single- and two-frequency rf fields using Monte Carlo simulations and CST. The effects of the relative strength and phase of the second carrier mode, and the frequency separation between the two carrier modes on multipactor susceptibility are studied. Regions of single and mixed multipactor modes are observed in the susceptibility chart. The effect of space charge on multipactor susceptibility and the time dependent physics is also studied.
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