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

对于磁化的电子束，存在三种不同的束波，它们可以产生微波。它们是慢回旋波、快回旋波和束流空间电荷波。在零磁场下，常规切伦科夫和回旋共振相互作用发生简并。本研究计划提出利用简并相互作用产生微波，包括理论和实验研究。在零磁场下，应考虑垂直和纵向扰动束运动。在常规切伦科夫相互作用和回旋共振相互作用的简并束相互作用中，束-真空边界的表面电荷起着至关重要的作用。对于轴对称TM模和非轴对称HE、EH混合模，均存在由简并相互作用引起的不稳定性。在100 A以上的强流区和弱相对论能区，简并相互作用比常规切伦科夫相互作用优越上级。该返波管结构简单、紧凑。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，电子束可以在没有引导磁场的情况下传播。观察到BWO产生微波的气压对于氮气和氦气分别为5-10 mTorr和100-160 mTorr。振荡模式是轴对称TM和非轴对称TE模式，这可以用简并相互作用来解释。TM模式的输出功率小于1 kW，对应的电子效率小于0.004%。低的电子效率可能归因于大直径光束的差的轮廓控制。

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.