Microwave Generation due to Degenerate Interaction of Cherenkov and Cyclotron Beam Modes

对于磁化的电子束，存在三个不同的束波，它们可以贡献微波的产生。它们是慢回旋波和快回旋波以及束流空间电荷波。在零磁场下，传统的切伦科夫相互作用和回旋共振相互作用退化。该研究项目提出了简并相互作用产生微波的理论和实验研究。在零磁场下，应该考虑垂直和纵向扰动的束流运动。在传统切伦科夫和回旋共振相互作用的简并束流相互作用中，束流-真空边界的表面电荷起着至关重要的作用。对于轴对称的TM模和非轴对称的HE和EH混合模，由于简并相互作用而产生的不稳定性。在100A以上的大电流区和弱相对论能区，简并相互作用优于传统的切伦科夫相互作用。实验证明了一种无引导磁场的大直径后向波振荡器。这款BWO简单紧凑。电子束可以在没有引导磁场的情况下传播，这是由于背景气体(氦或氮)的自电离在离子聚焦区产生等离子体。对于氮气和氦，观察到BWO微波产生的气体压力分别为5-10mTorr和100-160mTorr。振荡模为轴对称TM模和非轴对称TE模，可用简并相互作用解释。TM模式的输出功率小于1kW，相当于电子效率小于0.004%。电子效率低可能是由于大直径光束的轮廓控制不佳所致。

For a magnetized electron beam, there exist three distinct beam waves, which can contribute microwave generations. They are the slow and the fast cyclotron waves and the beam space charge wave. At zero-magnetic field, the conventional Cherenkov and cyclotron resonance interactions degenerate. This research project has proposed microwave generations due to the degenerate interaction and includes theoretical and experimental studies.At zero-magnetic field, vertical as well as longitudinal perturbed beam motions should be taken into account. In the degenerate beam interaction of the conventional Cherenkov and cyclotron resonance interactions, surface charge at beam-vacuum boundary plays an essential role. Instabilities due to the degenerate interaction occurs for axisymmetric TM mode and non-axisymmetric HE and EH hybrid modes. In the high current region above 100 A and in the weakly relativistic energy region, the degenerate interaction is superior to the conventional Cherenkov interactionA large diameter backward wave oscillator (BWO) without guiding magnetic field is demonstrated experimentally. This BWO is simple and compact. Electron beam can propagate without guiding magnetic field due to generation of plasma in ion-focused regime by self-ionization of background gas (helium or nitrogen). The gas pressures, at which microwave generations of the BWO are observed, are 5-10 mTorr and 100-160 mTorr for nitrogen and helium, respectively. The oscillation modes are axisymmetric TM and non-axisymmetric TE modes, which can be explained by the degenerate interaction. The output power of the TM mode is less than 1kW, which corresponds to electronic efficiency less than 0.004 %. The low electronic efficiencies might be ascribed to poor profile control of the large diameter beam.