| Klystrons are a kind of vacuum electron devices which can exchange energy between electron beam and electromagnetic wave through velocity modulation and density modulation based on the mechanism of transition radiation.They are widely used in large-scale scientific facilities,radars,communications,and medical fields because of their advantages of high power,high efficiency,high gain,low phase noise,and long life.However,the conventional high power klystrons suffer from large sizes and heavy weights due to the limitation of frequency scaling laws,which significantly affects the development costs and application promotion of klystrons.Therefore,developing a new type of miniaturized microwave klystrons while maintaining their high power and high efficiency is an urgent problem to be solved in the field of vacuum electron devices.This dissertation introduces the metamaterial into the high-frequency structure of the klystron and combines them with the second harmonic bunching technique and the multi-electron beam technique,which provides a new idea for realizing miniaturized,high power and high efficiency microwave klystrons.The main work and innovations of this dissertation include:1.The theoretical and experimental studies of a 714 MHz miniaturized single-electron beam klystron based on single-bridge complementary electric split ring resonator（Ce SRR）metamaterial unit cells are carried out.On the basis of the design principle of Ce SRR-loaded waveguide,a metamaterial-inspired resonant cavity with a diameter only half of the conventional resonant cavity is proposed by loading two single-bridge Ce SRRs with significant subwavelength characteristics in the resonant cavity.The effect of the Ce SRRs on the electric field and the resonant frequency of the fundamental mode of the resonant cavity,and the effective characteristic impedance of the resonant cavity are studied.The frequency-tuning method of the metamaterial-inspired input cavity is analyzed.The test results of the reflection characteristics confirm the miniaturization characteristics of the metamaterial-inspired resonant cavity.A miniaturized metamaterial-inspired high-frequency structure is constructed by connecting the metamaterial-inspired resonance cavities through drift tubes.CST simulations predict that the metamaterial-inspired klystron can achieve 2.28 MW of the output power and 57%of the electronic efficiency at 714 MHz with an electron beam of 100 k V/40 A and a longitudinal magnetic flux density of 720 G.Compared with the conventional high-frequency structure,the transverse and longitudinal dimensions of the metamaterial-inspired high-frequency structure have been reduced by 1/2 and 1/3,respectively.In addition,thermal analyses show that the temperature rise of the metamaterial-inspired output cavity of about 3.8°C is able to satisfy the operational requirements.2.The theoretical studies of a miniaturized high-efficiency metamaterial-inspired single-electron beam klystron at 324 MHz in the China Scattered Neutron Source（CSNS）are investigated.A resonant cavity loaded with one double-bridge Ce SRR with a low gap transit angle is proposed to achieve shortening of the resonant cavity in the longitudinal and transverse lengths.The analysis shows that the resonant cavity is able to avoid mode competition.The electronic efficiency of the metamaterial-inspired klystron is improved by introducing the second harmonic bunching technique.The improvement of electron bunching in the klystron by the second harmonic cavity is theoretically investigated,as well as the effect of the resonant frequency of the second harmonic cavity on the electron bunching.A jointing simulation of the beam-wave interaction of the electron-optical system and the metamaterial-inspired high-frequency structure is performed,with a predicted output power of 2.97 MW at 324 MHz and an electronic efficiency of 57.2%.The proposed klystron is 6.5%higher in efficiency relative to a conventional klystron loaded with a second harmonic cavity,and the volume of the metamaterial-inspired high-frequency structure is scaled down by 37%compared to that of the conventional counterpart.By suppressing the second harmonic component in the output signal and electrons returning,as well as the breakdown risk and thermal analyses,the klystron is predicted to operate stably.3.The experimental studies of the miniaturized high efficiency metamaterial-inspired single-electron beam klystron at 324 MHz in the CSNS were carried out.The metamaterial-inspired high-frequency structure is machined and assembled.The resonant frequency of each resonant cavity and the external quality factor of the input and output cavities were tested.Error analyses were performed on the cold-test results,and the methods of tuning the diameters of the cavity segments and the lengths of the transit gaps were used to optimize the resonant frequencies of the resonant cavities.Simulations of the corresponding electron gun,magnetic focusing configuration,collector,and output window,as well as the designing,assembled,and testing of the mechanical structures were carried out.Hot-tests are forthcoming and will be reported later.4.The simulation studies of a 714 MHz metamaterial-inspired multi-electron beam klystron are carried out.The problems of high beam voltage,large longitudinal length,electronic efficiency limitation,and narrow bandwidth in metamaterial-inspired single-electron beam klystron can be solved by introducing the multi-electron beam technique in the microwave high power metamaterial-inspired klystron.A double-bridge multi-hole Ce SRR is proposed for constructing the metamaterial-inspired resonant cavity based on six electron beam channels.The stability of the metamaterial-inspired input cavity based on six electron beam channels is analyzed,and the effects of the energy coupling and the length of the transit gap on the uniformity of the electric field intensity distribution of the quasi-TM010 mode in each electron beam channel of the metamaterial-inspired output cavity are investigated.Compared with the single-electron beam klystron loaded with single-bridge Ce SRRs at the same frequency,the electron beam voltage of the multi-electron beam metamaterial-inspired klystron is reduced from 100 k V to 60 k V,the single electron beam current is reduced from 40 A to 10 A,the electronic efficiency is improved from 57%to 62.5%,the longitudinal length of the high-frequency structure is shortened by 12%,and the 1 d B bandwidth is enlarged from 4.7 MHz to 13.2 MHz.At the same time,the transverse area of the metamaterial-inspired multi-electron beam high-frequency structure is reduced by 30%compared with conventional counterpart at the same operating frequency.The comparative analyses show that the metamaterial-inspired multi-electron klystron has some difficulties in the transmission and focusing of electron beams,and the reliability of the breakdown risk can be further improved. |
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