Investigation Of A Novel High-power Microwave Source Based On Transition Radiation Effect

To generate high power microwave (HPM) with higher power, higher frequencyand longer pulse width is always one of the hottest topics in the HPM domain. In theHPM generators based on transit radiation, the transit-time oscillator (TTO) is attractivebecause of its simplicity and compactness, yet it is very difficult to realize high-poweredand long-pulsed operation, which has been greatly limited by the conducting foilstructures. Without foils structure, the relativistic klystron is expected to operate at highpower and long-pulse. However, due to a long drift-tube, the intense relativistic electronbeam (IREB) would probably degrade in its transportation process, and the externalmagnetic field is also required more strictly. Besides, in pursuit of higher frequency,owing to the reduced dimensions, both the devices are confronted with the limititationof power capacity.Because of these reasons, a novel foilless HPM generator based on transit radiationis put forward. With a low external guiding magnetic field, such a device is morecompact than the klystron and has the advantages of high output power, high operationfrequency and long-pulsed operation. In the paper, the proposed device is systematicallyinvestigated by theoretical analysis, particle-in-cell (PIC) simulation and experiments,and its investigation foundation is eventually established.Firstly, the modulating field distributions are analytically solved by using modematching method. The numerical calculation shows that the modulating fields in thestructure have the trait of quasi body wave. From the field distribution of the singlecavity, the normalized beam-loading conductance ratio of the multi-cavity structure,indicating the beam-wave energy interchange, is derived. Consequently, the operatingvoltage range can be easily obtained.Secondly, the simulation and experimental results from the L-band device arecompared. With a620keV,25kA electron beam guided by an external magnetic fieldof0.5T, a3.5GW microwave at1.64GHz is obtained and the corresponding efficiencyreaches22.6%. The experimental results are almost consistent with those of the PICsimulation. The initial investigation testifies that the novel device is capable ofgenerating high output power. Simultaneously, it also indicates that the designed devicehas many shortcomings in the long-pulsed operation, which can provide a good guidefor the next structure improvement.Further, an improved device based on transition radiation is designed at X-band. Inorder to realize long-pulsed operation, special attentions are focused on thecathode-anode gap, the distance between the extractor and the electron collector, and theaxial electric field strength on the structure surface. With a710keV,14.5kA electronbeam guided by an external magnetic field of0.8T, a2.5GW microwave at9.38GHz is obtained in the simulation, and the corresponding efficiency is about24.3%. Thehighest axial electric field strength on the surface of electrodynamic structure is only590kV/cm, which is lower than the RF breakdown threshold in vacuum.Based on the PIC simulation, the elementary experiment of the improved X-banddevice is carried out in our laboratory. With a780kV,13.5kA electron beam guided byan external magnetic field of~0.8T, a9.34GHz,1.1GW microwave has beenexperimentally obtained and the corresponding efficiency is about10.5%. The resultsare basically consistent with those obtained in the simulation with the similar beamparameters (~1.5GW). The elementary investigations indicate that the improved deviceis hopeful of long-pulsed operation, which has laid a good foundation for the cominglong-pulsed experiments.Finally, considering the realistic demands, studies of the novel device are furtherwidened to the multi-frequency and phase-locking domains. In the generation ofmulti-frequency HPM, the novel structure shows many merits. With a500kV,15kAelectron beam guided by an external magnetic field of0.8T, a dual-frequency HPM hasbeen obtained in the simulation. The output power is about2.37GW and thecorresponding efficiency reaches31.6%. In the phase-locking, preliminary studies haveshown that, phase-locking among several devices with the same operation frequency ispossible when the input RF power is relatively low (~MW), and the operation frequencyand phase can be effectively controlled by the external signal when the input RF powergets to100MW.