Research on a V-band relativistic transittime oscillator with a focusing cathode under a low guiding magnetic field

Reducing guiding magnetic fields for high-power microwave (HPM) sources is important for HPM systems to replace widely used coils with permanent magnets [1–2]. For high-frequency HPM sources, high magnetic fields are often required to confine the lateral motion of intense electron beams in the narrow transportation channel [3–5]. To meet the demand for permanent magnet packaging for a V-band relativistic transit-time oscillator (TO), a novel focusing cathode is introduced in the TTO to reduce the required magnetic field. Theoretical analyses and Particle-in-cell simulations are employed to investigate the influence of focusing cathode on electron beam transportation, and output microwaves. Especially, the comparison between conventional annular cathode and novel focusing cathode is made to show their effects on electric field near cathode surface, electron beam current, and electron beam envelope to deepen the insights of TTO with the focusing cathode. When loading an electron beam with a voltage of 393 kV, and a current of 5.25 kA, the TTO with a focusing cathode is capable of outputting 566 MW microwaves at 58.6 GHz under a low magnetic field of 0.6 T, corresponding to a conversion efficiency of 27.3 %. Parametric sensitivity analysis also indicates the V-band TTO can steadily operate across a wide range of voltage ($370-490 \mathrm{kV}$) and magnetic field (0.6-1.2 T).