Competitions among modes in magnetically insulated transmission line oscillator

Abstract

<sec>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. </sec><sec>As is well known, the operating mode of MILO is generally designed on the π mode of the TM<sub>00</sub> 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 TM<sub>00</sub>, TM<sub>01</sub> and HEM<sub>11</sub> 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 TM<sub>00</sub> mode. In this case, the electromagnetic field of the competition mode is also axially symmetric and its frequency is slightly lower than that of the π mode. The second is the competition between the TM<sub>00</sub> and higher order TM<sub>01</sub> mode. In this case, the competition frequency is rather higher than that of the π mode (TM<sub>00</sub>). The third is the competitions between the TM<sub>00</sub> and low order asymmetric HEM<sub>11</sub> 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. </sec><sec>Based on the obtained results above and the existing model of the MILO, a compact high output power L-band 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 the π mode of TM<sub>00</sub> and HEM<sub>11</sub> 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 HEM<sub>11</sub> 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%. </sec>