磁绝缘线振荡器中模式竞争的物理分析和数值模拟

Competitions among modes in magnetically insulated transmission line oscillator

作为一种紧凑型高功率微波器件,磁绝缘线振荡器在起振过程中容易出现模式竞争现象,如果不能对其进行有效抑制,可能导致器件的最终输出性能下降.由于磁绝缘线振荡器中波束互作用区通常采用同轴盘荷波导作为其慢波结构,因此本文从同轴盘荷波导中几个可能被相对论电子束激发的低阶本征模与电子束之间的色散关系入手,分析了三种类型的模式竞争的特点、产生的可能原因以及削弱方法.基于以上分析,给出了一种高功率紧凑型L波段磁绝缘线振荡器的物理模型,并利用全电磁三维粒子程序对其进行了冷腔和热腔的数值模拟.结果表明,由于结构不完全对称和电子发射可能存在一定的非均匀性,器件运行初期互作用区有竞争模式HEM11模出现,与理论分析一致;起振一小段时间后(10 ns左右),互作用区基模增长加快,高阶模被抑制.进一步优化后器件在基模获得了高效率和高功率微波输出,饱和时输出功率约为8.1 GW,输出效率达到了18%,模式纯度约为97%.本文研究结果可为磁绝缘线振荡器运行过程中出现的竞争模式的识别和输出性能优化提供理论参考和依据.

As a compact and high power microwave source,the competitions among various modes are prone to appear in the initial stage of the development of the radiated electromagnetic field in a magnetically insulated transmission line oscillator(MILO).If the mode competitions are not controlled effectively,the output characteristics of the MILO may decline in the end.As is well known,the operating mode of MILO is generally designed on theπmode of the TM00 mode and the coaxial disk-loaded waveguide is usually adopted as a slow-wave structure for beam-wave interaction in MILO.Therefore from the dispersion relations between the electron beam and the lower order electromagnetic modes(including TM00,TM01 and HEM11 modes)in the slow-wave structure,the characteristics and possible suppression methods of the three kinds of mode competitions are analyzed simply.The first kind mode competition is between the different axial modes of the fundamental TM00 mode.In this case,the electromagnetic field of the competition mode is also axially symmetric and its frequency is slightly lower than that of theπmode.The second is the competition between the TM00 and higher order TM01 mode.In this case,the competition frequency is rather higher than that of theπmode(TM00).The third is the competitions between the TM00 and low order asymmetric HEM11 modes.In this case,the competition frequency is slightly higher than that of the main mode.Appropriately choosing the radii of the anode vanes,the number of the anode cavity and the load length of the cathode,the corresponding mode competition intensity can be weakened.Based on the obtained results above and the existing model of the MILO,a compact high output power Lband MILO is proposed.Numerical studies of the mode competitions and output characteristics are carried by using the three dimensional particle-in-cell code.Cold-cavity test shows that in the low frequency range,the easily stimulated electromagnetic modes are theπmode of TM00 and HEM11 modes with frequencies of 1.61 GHz and 1.77 GHz,respectively.The numerical results of hot-cavity verify that the competition in the initial stage comes mainly from the asymmetric HEM11 mode due to the fact that there exists the strut in the output region,the Cartesian coordinates are adopted during the simulation,and totally symmetry cannot be guaranteed.In addition,electron beam emission from the cathode is not ideally even.But stable and high output microwave power is obtained in the end in the L-band MILO by being optimized.The output power and efficiency are 8.1 GW and 18%respectively,and the mode purity reaches about 97%.