Theoretical calculation and particle-in-cell simulation of a multi-mode relativistic backward wave oscillator operating at low magnetic field

Abstract

Increasing the dimensions of high power microwave devices is an efficient method to improve the power capacity. However, an overmoded structure usually results in mode competition and a low beam-wave conversion efficiency. In this paper, a multi-mode operation mechanism is used to avoid mode competition and increase the efficiency. The calculation results of nonlinear theory of beam-multimode interaction show that the optimized conversion efficiency is up to 48% when TM01 mode, TM02 mode, and TM03 mode are all considered. As only the TM01 mode, TM02 mode, or TM03 mode is taken into account independently, the corresponding efficiency is 38%, 22%, or 20%. Based on this, a multi-mode relativistic backward wave oscillator is proposed with the ratio of the mean diameter of the slow wave structure (SWS) to the wavelength of the output microwave to be 3.5. The non-uniform SWS is used to increase the beam-wave conversion efficiency, and a combined reflector is adopted to reflect partial of the mixed microwave modes and make the device compact. The particle-in-cell simulations show that as the diode voltage is 1.1 MV, the beam current is 22.8 kA, and the external magnetic field is 0.76 T, the conversion efficiency is 45%, and the output microwave of 11.3 GW is the mixed modes of TM01 mode, TM02 mode, and TM03 modes with the corresponding power ratio of 74%, 7%, and 19%, respectively.

A high-efficiency C-band coaxial transit time oscillator with a dual-cavity extractor under low-magnetic fields

“…Firstly, the available magnetic flux density region with high efficiency in our simulation is much lower than that in reference [5] and reference [6]. The reason is that Cerenkov devices have multiple SWS periods while TTOs have less cavity periods.…”

“…As a well-known, efficient, and widely used device, the typical RBWOs with low magnetic field are compared with the CTTO here. Reference [5], reference [6] and this paper present different methods to suppress the radial divergence of the electron beam and increase the power efficiency under low magnetic field in PIC simulation. In reference [5], the efficiency is 30% ∼ 45% at 9.9 GHz when the external magnetic field is between 0.6 T and 0.76 T. Its power saturation time is about 22 ns.…”

“…Reference [5], reference [6] and this paper present different methods to suppress the radial divergence of the electron beam and increase the power efficiency under low magnetic field in PIC simulation. In reference [5], the efficiency is 30% ∼ 45% at 9.9 GHz when the external magnetic field is between 0.6 T and 0.76 T. Its power saturation time is about 22 ns. In reference [6], the efficiency is 30% at 16.15 GHz when the magnetic flux density is between 0.85 T and 1.1 T in simulation.…”

“…However, the large electron beam energy spread due to the radial divergency of the electron beam under low magnetic fields will reduce the efficiency in these HPM sources. It is well known that the relativistic backward wave oscillator (RBWO) is HPM source with high efficiency, which can work at a low magnetic field [5,6]. One feasible method is to enlarge the overmoded ratio with multi-mode operation that is used to avoid mode competition and increase the efficiency of the overmoded HPM devices in reference [5].…”

“…It is well known that the relativistic backward wave oscillator (RBWO) is HPM source with high efficiency, which can work at a low magnetic field [5,6]. One feasible method is to enlarge the overmoded ratio with multi-mode operation that is used to avoid mode competition and increase the efficiency of the overmoded HPM devices in reference [5]. Another method in reference [6] is to reduce the diode voltage and the cyclotron resonance magnetic flux density which is over the working guiding magnetic flux density.…”