A Compact P-band Coaxial Relativistic Backward Wave Oscillator

Currently, the relativistic backward wave oscillators (RBWOs) operating in thefrequency regime of L-band, S-band, C-band and X-band have been fully developed, butresearches on the P-band RBWOs are rare. The main reason is that the P-band RBWO is solarge and cumbersome that it is difficult to fabricate and manipulate in experiments.Coaxial RBWO has the potential to realize compactness for the P-band RBWO, but thereare still many important issues to be solved. In addition, the P-band high power microwave(HPM) has the potential applications both in military and industrial areas, and theinvestigation on the P-band RBWO is of importance. Therefore, a compact P-band coaxialRBWO is proposed based on the results of theoretical studies, physical analysis andparticle simulation in this dissertation. Experiments of the compact P-band RBWO arecarried out and a P-band, gigawatt level HPM can be generated effectively. The contents ofthe dissertation are listed as follows.1. Space charge limiting current of a relativistic electron beam propagating in acoaxial slow wave structure (SWS) is derived and calculated. An equivalent analysismethod for investigating the space charge limiting current is given based on the numericalresults. With this method, the effects of the configuration parameters on space chargelimiting current can be determined obviously. The arbitrary mode dispersion equation ofthe coaxial SWS with arbitrary periodic profile is derived and calculated. With thenumerical results, the validity of our dispersion equation for calculating dispersion curve isverified, and the variations of dispersion curve related to the configuration parameters inthe coaxial SWS with only outer conductor ripple and the coaxial SWS with both inner andouter conductor ripple is presented. In addition, the accuracy of the mix mode dispersioncurve is verified and the excitation of the asymmetric-mode in the experiment for thecoaxial RBWO is studied theoretically.2. The large longitudinal dimensions and the long saturation time of the microwavesignal are two main shortcomings for the conventional P-band coaxial RBWO. In order toovercome these shortcomings, the physical analysis on the coaxial SWS is performed withnumerical calculation and particle simulation. Numerical results show that compared withthe conventional coaxial SWS with only outer conductor ripple, the coaxial SWS with bothinner and outer conductor ripples can remarkably enlarge the coupling impedance for the-1st space harmonics of the quasi-TEM and can largely enhance the temporal growth.Therefore, the coaxial SWS with both inner and outer conductor ripples is chosen as theSWS of our novel P-band RBWO. The parameters of the SWS are chosen using the resultsof the linear theoretical analysis, and then the operating frequency and mode of the noveldevice are given theoretically. The SWS period number is chosen by particle simulation. The simulation results show that three periods SWS not only has a compact structure, butalso has a wide region of single-frequency operation and relatively high efficiency andoutput power in a wide range of the diode voltage. Based on the above mentioned analysis,a compact P-band coaxial RBWO is proposed.3. The compact P-band coaxial RBWO is optimized with a2.5-dimension fullelectromagnetic PIC code. With the diode voltage of585kV and the beam current of7.85kA guided by a magnetic field of0.8T, a microwave with frequency of900MHz, powerof1.5GW and efficiency of about33%is obtained. The simulation results also show thatthe operation mode of the device is quasi-TEM mode, and the operating mechanism of thedevice is the mechanism of the backward wave oscillator. In addition, the effects of theadjustable parameters in the experiment and the additional parameters caused by theprocess of the machining and assembling are presented and discussed in detail.4. The extension and improvement studies on the compact P-band coaxial RBWO areperformed. It is demonstrated that the idea of designing a compact P-band coaxial RBWOis also feasibility for the L-band RBWO. A principle of design for the low-band coaxialRBWO is presented with the results of P-band coaxial RBWO and L-band coaxial RBWO.A P-band coaxial RBWO, which can operate at low magnetic field, is proposed and apermanent magnet system is designed for it. The simulation results of the RBWO confirmthe feasibility of permanent magnet, which can be used for the guiding magnetic field. Thecollector of the compact device is improved for the long pulse and repetitive rate operation.Then a collector design method is proposed. The main merit of this method is that it canavoid the use of particle simulation optimization repeatedly, and thus can save time greatly.The SWS and collector of the compact device is improved for increasing the beam-waveconversion efficiency. With the diode voltage of585kV and the beam current of7.85kAguided by a magnetic field of0.8T, the improved device can generate a microwave withfrequency of900MHz, power of2.2GW and efficiency of48%.5. The experiments of the compact P-band coaxial RBWO are performed. Theexperimental results with the dielectric-copper cathode shows that with the diode voltageof570kV and the beam current of8.0kA guided by magnetic field of0.86T, the P-bandmicrowave with frequency of897MHz is obtained. The microwave power is measured tobe1.47GW and the efficiency is approximately32%. The experimental results are in goodagreement with the results of particle simulations. In addition, increasing the guidingmagnetic field to1.2T, a microwave with power of3.14GW, efficiency of20%and pulsewidth of about47ns is obtained at the diode voltage of995kV and the beam current of15.5kA.