| Relativistic Klystron Amplifier (RKA) is a promising candidate in the field ofpower combination of High Power Microwave (HPM) due to its essential characteristicssuch as high output power, high gain, controllable frequency and phase-locking. Aimingat the correlative problem existed presently in the investigation of RKA, here proposedan improved S-band RKA which has an input cavity with dual coupling holes, an outputcavity with an axial symmetric coupling hole, and a drift tube coated with absorbingmaterial. Four significant issues, i.e., injection matching, the design of the improvedinput and output cavities, the method for depression of parasitic modes andphase-locking characteristics are studied systematically. The main content and resultsinvolved in this dissertation is depicted as following.The properties of the annular Intense Relativistic Electron Beam (IREB) fordriving RKA are studied numerically, and then the effects on the bunching distance ofthe IREB are analyzed based on the small signal space-charged theory. The beamloaded resonant frequency and quality factor which are satisfying the matchingconditions of the input cavity under IREB loading are obtained by using the equivalentcircuit method combining a2.5-dimensional (2.5D) Particle-in-Cell (PIC) code. Basedon the above results, the matching injections of the input cavity of the RKA are realizedin both2.5D and3D PIC simulations. This can support the design of the RKA inputcavity.The improved input cavity with dual coupling holes is proposed. The couplingholes are placed symmetrically, and the area of each hole is small, which is beneficial tothe uniform of the field distribution. The input cavity is fabricated and tested. The coldtesting results of the improved input cavity agree well with the simulation ones. Thefollowing experimental results indicate that the injected microwave can be wellabsorbed by the input cavity according to the designed parameters.The improved output cavity with an axially symmetric coupling hole is proposed.Possessing a big annular coupling hole and an isolated coupling structure, the outputcavity is featured by a low external quality factor and less possibility of excitingparasitic modes. The output cavity is analyzed and designed primarily with highfrequency analysis software. The parameters of the output cavity are further determinedwith the3D PIC code, and the simulated power efficiency of25%is obtained. In thehigh power experiments, an output microwave power of700MW and a powereffieciency of more than20%are obtained with this improved output cavity.The suppression through adopting the drift tube coated with absorbing material isproposed and the related3D PIC simulations are performed. PIC simulations show thatthe oscillation of the main parasitic mode, i.e. TE11mode in the drift tube, is strongly order modes in the middle cavity. The simulation results show that an absorbing rate of30%for the coated material is enough to suppress the oscillation of the parasitic mode.The following experimental results of the RKA with the drift tube of50%absorbingrate show that the parasitic mode oscillation is effectively suppressed in the diodevoltage regime below500kV.The S-band RKA with three cavities is systematically studied with PIC codes.With the2.5D PIC code, the influence of the three cavities on the RKA operation isstudied by order, and the systematic design method is given for the simplified twodimensional structure of the RKA. A novel beam collection method capable ofsignificantly improving the power efficiency of RKA is proposed and investigated.After a serial optimization, the power of the output microwave is obtained as1.15GWwhen setting the diode voltage of510kV, current of6.7kA, guiding magnetic field of1.5T and input microwave power of500kW, which indicates a high power efficiency of33%and a gain of33.6dB. With the3D PIC code, the effects of the differentparameters of the improved output cavity on the RKA output microwave properties areexplored. Through the simulation, the HPM with parameters of an output power of930MW, an efficiency of25%and a gain of32dB is obtained under the conditions of asegment guiding magnetic field of1.3T, a diode voltage of500kV, a current of7.4kAand an input power of600kW. For obtaining better understanding of the RKAphase-locking properties, the other PIC simulations are carried out. In the simulations,the effects of the factors such as the voltage waveform provided by the accelerator, thestructural parameters of the RKA and the guiding magnetic field on the characteristicsof phase-locking are studied in detail. Based on the simulation results, the evaluating ofthe influence factors on the RKA phase-locking properties is given, which can provideguidness for the phase control in power combining based on the RKA.Experimental study of the improved S-band RKA in a high-power range is carriedout based on Torch-01accelerator by using a large-power S-band magnetron as themicrowave seeder. Two types of the experimental results are obtained. Under theconditions of diode voltage488kV, current5.6kA, pulse width50ns, input power720kW and input frequency2.84GHz, the microwave radiation is generated with theparameters: the power of about300MW, the frequency of2.84GHz, the efficiency ofgreater than10%, the gain of about26dB, the pulse width of50ns, and the radiationpattern of TM01mode. There is almost no parasitic mode oscillation within the outputmicrowave pulse. The output microwave can achieve its phase-lock in less than10nsand the phase-lock lasts for more than30ns, with a phase jitter less than±7degrees.Under the conditions of diode voltage505kV, current6.5kA, pulse width50ns, themain frequency of the microwave is2.84GHz, and the radiated power reaches700MW,which indicates a power effiency of22%and a gain of30dB. The radiation pattern of the main mode is still TM01mode，but the oscillation of the parasitic modes occurs. Thepossible reasons and suppression measures for the occuring of the parasitic modes aregiven. |
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