Spectral analysis of a THz radiation source based on high-harmonic interaction in a hybrid cavity

We consider a novel concept of a miniature vacuum electronic device (VED), targeting coherent electromagnetic wave generation in the THz range, but not limited to the latter. While in a traditional VED a preformed mono-energetic electron beam is interacting with a slow-wave structure or a resonant cavity, the hybrid cavity under consideration allows for interaction of electron bunches accelerating or oscillating in a Penning trap-like static field configuration integrated with a resonant cavity, formed partially by the trap's electrodes and partially by a dielectric Bragg structure. As a first step, we analyze the energy exchange spectrum between the oscillating bunches and the cavity eigenmodes. While the dielectric breakdown limits the maximal frequency of bunches' oscillations to an order of 0.1 THz, a significant high-harmonic interaction with cavity modes above the fundamental is possible for a large, comparable with the cavity dimensions, oscillating dipole. Proper design of the cavity may allow for interaction of a selected high harmonic of bunch oscillations with a single mode of the cavity, providing conditions for an energy-efficient generation of a virtually monochromatic THz wave.