Study On Pulse Combination For High Power Microwaves

The strong electric field accompanying with the high-power microwaves(HPM)would ionize the material in the transmission line to form a plasma.With the growth of the plasma density,the plasma frequency will increase.When the plasma frequency is higher than the transmitted microwave frequency,the microwave transmission will cut off due to absorption and reflection of the plasma,which is called microwave breakdown.Thus,it is difficult to obtain a microwave pulse with power greater than the GW level by a single HPM system.It is an effective solution to this problem to synthesize multiple HPM pulse to form a HPM of higher power level by means of multiple waveguide channels,which has attracted considerable interest in recent years.Multi-channel coupled output components such as waveguide filters,duplexers,and combiners,which are widely used in conventional microwave systems,generally operate at fundamental modes with low power capacity.Enlarging the dimensions of these components seems to be an effective way to increase the power capacity,but it suffer from lower transmission efficiency due to the high-order modes excitation and coupling.Meanwhile,the summation of the E-field casued by the multi-mode superposition would lead to breakdown,which severely limit the power capacity of the waveguide components.Therefore,it is very important to investigate the techniques to combine multiple HPM pulses to achieve high output power.In this paper,with the consideration of high power capacity characteristics of over-moded circular waveguide,a new type of over-moded circular waveguide combiner is designed at first based on the over-moded waveguide filtering and mode control theory.Secondly,after studying the breakdown mechanism of the waveguide under the ns-level pulse strong field,the power capacity problem of the over-moded circular waveguide combiner operating with the ns??s interval pulses is analyzed.Based on above work,a HPM over-moded circular waveguide combiner with high power capacity and high efficiency is developed,which can realize the HPM pulse combination with high peak power and high pulse energy on GW level.Finally,high peak power level output,which is several folds of a single HPM source,is realized.The main works of dissertation are as follows:1.The mode excitation,coupling and power capacity problems in overmoded wav guides are analyzed.Firstly,the mode fundamental functions of three typical modes in the over-moded waveguides are analyzed and the cascade formulas are established.Secondly,a genetic algorithm is used to analyze the high-order modes excited by the strong discontinuous structure in the over-moded waveguide.Theoretical calculation results are compared with the HFSS software simulation,which lays a foundation for the optimizing design of the over-moded filter.Then,the method to solve the mode control and filtering problems in the over-moded circular waveguide is proposed.Through the method and the above analysis,an over-moded circular waveguide filter with non-uniform periodic expansion of radial size is realized.Finally,three X-band over-moded circular waveguide filters for different operating modes are designed.2.The design of the high power capacity over-moded waveguide combiner.An HPM combiner is proposed to combine two-way TM01 mode microwaves directly by means of the over-moded circular waveguide.Based on the experimental research,the design and fabrication of the over-moded HPM combiner are further improved.Besides,low power measurement and high power test are also carried out.The measured results show that the combiner has a transmission efficiency greater than 95%,and the power capacity is greater than 4 GW/20 ns and 2 GW/100 ns respectively,which can satisfy the requirements under HPM conditions at GW level.3.Theoretical research on GW level pulses combination.The theoretical analysis and numerical simulation of the single-particle motion law in the waveguide under strong field conditions are carried out.A Matlab simulation program is compiled to calculate single-particle motion to analyze the particle trajectory,transit time and magnetic field influence.After analyzing the influences of HPM thermal effects,gas desorption and diffusion,a PIC simulation model is established.The PIC simulation indicates that at the falling edge of any pulse,the plasma will rapidly develop in the combiner,causing the output power of the latter pulse to decrease.The theory analysis shows that the strong electric field formed by the superposition of two synchronous pulses will pull the electrons away from the plasma quickly,thus inhibiting the rapid increase of plasma density.Therefore,when two pulses with the time interval limited in ns??s range are input,the plasma density changes little within the pulse width.Therefore,combined pulse outputs with higher power and energy efficiency can be achieved by producing high power beat wave.4.Experimental research on GW level pulses combination.Using one 4GW/20ns and one 2GW/100ns HPM sources with ns-level time accuracy,the high-power pulse combination experiments is carried out.The combined pulse output of ns??s time interval is realized,and the beat wave of several folds the power level of the short pulse source is achieved.It is found from the experiment that the plasma discharge induced by the HPM pulse will strongly affect the output power of the latter pulse,and the recombination time of plasma is on the order of ?s level.The reliability test in 30 Hz repetitive mode is carried out to verify the power capacity and stability of the HPM combiner.The combination regularity and typical waveforms of different trigger intervals are obtained,which can verify the PIC simulation results.