To suppress the onset of multipactor breakdown on a dielectric surface, a graded permittivity dielectric material is introduced to replace a classical single uniform dielectric material. The electromagnetic field simulation results show that the peak value of the microwave electric field will increase when the center of the microwave window is constructed from a material with a high relative permittivity. However, when the edge of the microwave window is made of such a material, the peak of the microwave electric field will decrease. In addition, the process of multipactor breakdown is investigated using the 2D particle-in-cell method. The results show that the maximum density of electrons that occurs when the edge of the microwave window is constructed from a high-relative-permittivity material is only 66.05% of that which occurs with a uniform dielectric material. As a result, the threshold of multipactor breakdown can be improved. The results reported in this paper can be used to guide the design of microwave windows.
Negative and positive ions crossing the anode-cathode gap of a magnetically insulated transmission line (MITL) can cause non-negligible current loss and energy deposition on the electrodes, which may lead to the formation of anode plasma and the growth of cathode plasma. Furthermore, gap closure could occur due to the expansion of cathode plasma and anode plasma. In this paper, a model for magnetic insulation of both negative ion flow and positive ion flow is developed. The operating voltage V of the MITL is expressed as a function of the total current I0 and the boundary current Ib. The total current and the boundary current of the MITL are derived at saturated and self-limited flows, respectively. In addition, particle-in-cell simulations are implemented for the validation of the theoretical model. The thickness and density of the magnetically insulated ion layers are analyzed, and an empirical expression for space charge factor g is obtained through simulation results. This work extends the understanding of magnetically insulated ion flow in MITLs.
The coaxial wiggler with periodic permanent magnet is one of the ways to realize portable and practical high power microwave, which has a relatively small volume and low weight. However, the electromagnetic mode competition will significantly affect the output power at a high electron current. Based on the dispersion curve considering the effect of the electron beam, we propose a method of judging whether the mode can oscillate as the main mode in the coaxial magnetic wiggler. We analyze the wave–beam interaction by the first-order perturbation and predict some modes that seem impossible in traditional methods. It is verified by 3D particle-in-cell simulation that these electromagnetic modes will be the operating modes.
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