This article proposes a new type of metamaterial slow wave structure with larger space-charge-limited current for high-power microwave sources. In order to avoid the anisotropy effect and improve the uniformity of electromagnetic field distribution, the slow wave structure consisted of two all-metal units arranged by a modified method of broadside-coupled complementary split ring resonant (CSRR) geometry. Moreover, the conductor on the periphery of the CSRR has been removed, and a short shift tube was added to the units for the purpose of obtaining larger space-charge-limited current. High-frequency analysis shows that the new slow wave structure has the basic characteristics of metamaterials, including double negative electromagnetic parameters, peculiar dispersion, and high coupling impedance. More importantly, transmission simulation shows that the space-charge-limited current of the structure is at least 12 kA, which is about twice the maximum beam current that the hollow circular waveguide of the same size can transmit, and it proves that the design method is effective and feasible. Applying the structure to a microwave source will achieve the goal of high-efficiency and high-power output.

In recent vacuum electronic devices, metamaterials have shown an obvious advantage of miniaturization. Due to increasing miniaturization demand, research of metamaterials in high-power microwave (HPM) field is now also a hotspot. For better applications of metamaterials, there are still two issues to be solved, the low space limit current of the structure and the low uniformity of the working electric field. To solve these problems, we construct a novel all-metal metamaterial structure by applying a 45° rotational arrangement in space and a four-support rod structure. This new metamaterial structure has a great uniformity of the axis electric field, and its electric field fluctuation is reduced from 8.6% to 2.8%. Based on this metamaterial structure, we proposed a novel relativistic slow wave structure, and its working mode has a negative dispersion characteristic. Using the method of expanding the electron beam channel, the space limit current of the relativistic slow wave structure is greatly improved to 12.3 kA, and the particle simulation method is used to prove the increase in the space limit current. In addition, it is found that this slow wave structure has higher coupling impedance, which provides a better working environment for an annular relativistic electron beam. Therefore, this metamaterial structure has great potential in the HPM field.

A novel L-band metamaterial slow wave structure with a weak space charge effect and high field uniformity is proposed. The slow wave structure is composed of two identical all-metal resonant elements. In order to weaken the space charge effect, a long drift tube is adopted as a metal joint, connecting the front periodic unit with the rear one. Moreover, the novel metamaterial structure exhibits higher field uniformity by rotating one of the resonant elements 45° relative to the other. Electromagnetic calculation indicates that the average fluctuation in the electric field dropped from 3% to 0.5%. Based on the novel metamaterial slow wave structure, an L-band Cerenkov oscillator model is built, and a preliminary particle-in-cell simulation is carried out. Under the condition of an acceleration voltage of 500 kV, beam current of 8 kA, and magnetic field of 1 T, a 1.64 GW average output microwave power at 1.30 GHz is obtained. The corresponding conversion efficiency is 41%. The simulation results prove the advantages of the metamaterial Cerenkov oscillator, such as low magnetic field, low impedance, miniaturization, and high conversion efficiency.

In this paper, a novel high-efficiency L-band Metamaterial Relativistic Oscillator (MRO) with high input and output power under relatively low guiding magnetic field has been proposed. In order to increase the power level while decreasing the guiding magnetic field, through our theoretical and simulation investigations on previously designed metamaterial (MTM) structure, we found that the thickness of the ring and the length of the drift tube are the two key factors that can improve the power handling capacity of the MTM structure and field uniformity. Through transmission analysis and electron conductance calculation of the novel MRO based on the improved MTM structure, preliminary MRO design is done. Within CST particle simulation of the proposed MRO and after optimizations, we computationally got a pure 718 MW TE10 output signal at 1.49 GHz using a pulsed electron beam with energy of 700 keV and current of 2 kA under 1 T guiding magnetic field, and the corresponding conversion efficiency is 51.3%. Relevant results of particle simulation verified the previous high frequency analysis. A comparison between our proposed MRO and the previous one reveals that while keeping the high efficiency and miniaturization merits of MRO, the power handling capacity has been increased and the novel MRO works more stable under low guiding magnetic field.

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