An L-band Coaxial Relativistic Backward Wave Oscillator With Tunable Frequency

The relativistic backward-wave oscillator (RBWO) is one of the most promising high-power microwave (HPM) sources and has wide potential for industrial and military applications due to its essential characters such as high power, high efficiency and high repetitive rate. Recently, there are extensive reports on RBWOs operating in the high frequency region (S-, X-band, and millimeter wave), but discussions on low operation band (L-, P-band) are rarely scanty. The main reason is that the dimension and the guiding-magnetic system of the low operation band RBWOs are especially large and complicated. Meanwhile, the RBWO with tunable frequency provides an effective approach to achieve extensive applications of HPM. Based on the above discussion, the L-band coaxial RBWO with tunable frequency is proposed in this dissertation. Experiments are carried out to verify the initiative idea, theoretical analysis and simulation results. It is demonstrated that an L-band, gigawatt level HPM can be generated effectively with a low guiding-magnetic field. The main content and innovative work are listed as follows.Firstly, the coaxial slow-wave structures (SWSs) are investigated systematically. Compared with the circular waveguide, the cutoff frequency of the transverse electromagnetic (TEM) mode is 0 and the space-charge limiting current is much higher in the coaxial waveguide, thus the transversal dimension of the coaxial waveguide can be reduced and the coaxial device has the potential of high power. Naturally, the coaxial SWSs are chosen in this paper. The dispersion relations of the SWSs with arbitrary geometrical structures are studied in detail under the condition of finite thickness of the beam. The dispersion curves and coupling impedance of the SWSs with the cosinoidal, trapezoidal and rectangular corrugations are obtained by numerical calculation. It is clearly found that the cosinoidal dispersion curve possesses the widest frequency range, the largest phase speed and the smallest coupling impedance. The corresponding characteristics of the trapezoidal and rectangular corrugations become inferior in order. And the longitudinal electrical field distribution of theπmode of the quasi-TEM wave conforms to the"surface wave"property. As the period and phase shift of the SWS decreases, the coupling impedance becomes higher. If the depth of the SWS decreases, the coupling impedance becomes smaller. In addition, the coupling impedance increases first and then decreases when the radius of the inner SWS increases. Considering the space charge effects of the beam, the growth rate of the quasi-TEM mode is calculated. Analytical results show that the growth rate increases along with the enlargement of the depth of the SWS, becomes smaller when the period of the SWS and the thickness of the beam increases, and possesses a maximum if the diode voltage varies.Secondly, the coaxial SWS with the outer trapezoidal corrugation and the coaxial extractor structure are designed to reduce the size and increase the efficiency of the L-band device. The transversal mode selection is achieved using the property of"surface wave"of the coaxial SWS to excite the quasi-TEM mode without the higher TM modes and it is proved that the coaxial SWS may decrease the transversal dimension of the SWS sections. Meanwhile, the relationship between the radius of inner-conductor and the resonance frequency is analyzed in detail, and it shows that the resonance frequency decreases obviously with the increasement of the inner-conductor radius. Moreover, the S-parameter method is employed to investigate the longitudinal resonant characteristic of the finite-length SWS. It is proved that the introduction of a well designed coaxial extractor to the slow-wave devices can make contributions to the more compact device and avoid the destructive competition between various longitudinal modes. Namely, the coaxial RBWO with a well designed coaxial extractor can realize the longitudinal mode selection and enhance the beam-wave efficiency.Thirdly, the mechanism of HPM generation from the device is investigated thoroughly by the 2.5-dimensional full electromagnetic particle-in-cell (PIC) code, and related physical images are presented. The dependences of the output microwave power, efficiency and frequency on structural parameters, beam parameters and guiding-magnetic field are analyzed intensively. It is shown that the frequency is lowered from 1.63GHz to 1.51GHz when the inner-conductor radius increases from 0.5cm to 2.5cm. And the efficiency varies in the range of 35.4-27.7%. After optimization, a microwave with frequency of 1.60GHz, power of 2.78GW and efficiency of 35.4% is obtained under the condition of diode voltage at 700kV, beam current at 11.2kA and guiding-magnetic field at 1.1T. Further, the basic design principles and processes for the compact and low frequency RBWO are presented. According to that, the P-, S-band compact coaxial RBWOs are provided, respectively. Additionally, effects of the plasma produced by the explosive emission in the inner-surface of SWSs and the collisional ionization of the pre-filled gas on the performance of the device are simulated sufficiently. Typical results showed that the transversal electrical field at the surface of the SWS is useful to prevent the emission of the secondary electrons. The lighter ions produced by the explosive emission have greater impacts on the output microwave. The density of the plasma produced by the collisional ionization is relative to the nucleus number of the pre-filled gas.Finally, experiments are carried out to demonstrate the operating mechanism and characteristics of the L-band coaxial RBWO with tunable frequency. On the Torch-01 accelerator, when the diode voltage is 713kV, the current is 11.4kA and the guiding-magnetic field is 0.8T, the radiated microwave with frequency of 1.58GHz, power of 1.07GW and pulse duration of 38.8ns is generated. Its efficiency is 13.2% and its main mode is TM01 mode. This result is the first experimental report on the L-band RBWO at home and abroad. Particularly, the tunable characteristics and mode-competition mechanism are studied in detail. It is found that the frequency decreases from 1.64GHz to 1.58GHz when the inner-conductor radius increases from 0.5cm to 1.5cm. An S-band microwave with power of 390MW could be obtained after removing the inner-conductor. As the depth of the collector changes, the tunable frequency varies in the range of 40MHz. If the diode voltage and guiding-magnetic field are lower and the device is non-homocentric, the asymmetric quasi-TE11 mode could be generated. On the other hand, the mode-competition mechanism could be restrained efficiently by increasing the diode voltage and guiding-magnetic field and adjusting the non-homocentric structure. Furthermore, experiments are carried out on the long pulse accelerator. When the diode voltage is 700kV and guiding-magnetic field is 0.9T, the microwave is obtained with power of 1.2GW at the frequency of 1.58GHz. The pulse duration of the radiated microwave is 86.4ns, which exhibits the RBWO's promising application in terms of the long pulse.