Microwave Sources Based On The Metamaterials

Metamaterials are artificial structures which have many unique electromagenetic properties, and the studies of different types of metamaterials have attracted many researchers in recent ten years. The first realization of the metamaterial enabels a variety of unprecedented applications such as cloaking and antennas, and so on. The vacuum electron devices are important microwave sources which have been widely used in military and civil fields. As present, the main serious challenges of the vacuum electron devices are from the increasing demands from the applications, such as the miniaturization, high electronic efficiency and high power of the devices. The study of the metamaterial vacuum electron devices is just begging at home and abroad, but which is very important to the development of the new types of the vacuum electron devices.In this paper, the applications of the metamaterials to the vacuum electron devices have been comprehensively researched, including the design of the metamaterial and research of the corresponding slow-wave structure (SWS),and the beam-wave interaction, and so on. The main contents and innovation points of this thesis are as follows:1. The design principles of the metamaterials for the vacuum electron devices are given by analysing their common physical characteristics, based on which a new all-metal metamaterial has been proposed. The new metamaterial is suitable for the vacuum electron devices, and the effective parameters also have been extracted from the S parameter retrieval algorithm. The results exhibit that the metamaterial has the negative effective permittivity and positive effective permeability.2. An S-band new slow-wave structure based on the metamaterial has been proposed, and the results of the extracted effective parameters show that this SWS can be equivalent to a double-negative material in the band of the fundamental mode. This metamaterial SWS is appropriate to be used in the pencil beam backward wave oscillators (BWO). The dispersion and interaction impedance characteristics of the metamaterial SWS with the changes of the parameters also have been calculated, the results predict that this metamaterial SWS has the advantages of miniaturization and high interaction impedance compared to the conventional SWSs. Experiments have been done to measure the transmission characteristics of the metamaterial SWS, and the measured results of the S-parameters are in good consistent with the simulated results.3. An S band BWO with a coaxial line loop used as the signal output system is designed on basis of the metamaterial SWS. The corresponding beam-wave interaction results are presented here: a 2.454 GHz signal with the peak output power of 4.0 MW is generated, while the electronic efficiency of the device is 31.5%. The analyses of the electronic fields of the particle-in-cell (PIC) predict that there is great potential for the BWOs based on this metamaterial SWS to have high electronic efficiency.4. In order to efficiently take advantage of the high interaction impedance for the metamaterial SWS, a rectangular output coupler which has higher power capacity and energy coupling efficiency is proposed. The parameters of the metamaterial SWS have been optimized and the focusing longitudinal magnetic field is calculated in theory.Then an S band BWO is designed, the period number of the SWS is only 7, which means the miniaturization for the longitudinal length of the device. The PIC results show that when the beam voltage and the current are 240 kV and 20 A, respectively,the peak output power the BWO is 4.5 MW, while the electronic efficiency can reach above 90%.5. An improved SWS of which the bandwidth has been obviously increased compared to the metamaterial SWS is presented, while the interaction impedance is still as high as that of the metamaterial SWS. Based on the improved SWS, an S band BWO also has been designed, the PIC results show that the performances of the bandwidth, the saturated peak output power and the electronic efficiency have been improved significantly. The the maximum value of the saturated peak output power is about 41 MW, while the electronic efficiency is still up to 66%. Another S band BWO which can operate at low voltage is designed by changing the parameters of the improved SWS and redesigning the signal output coupler: when the beam current is 3 A, the maximum average output power of the BWO is 45 kW, while the electronic efficiency is about 40%, far higher than that of the conventional BWOs.