A high-efficiency long-pulse relativistic backward wave oscillator (RBWO) operating in the C band is described. The electrodynamical structure of the RBWO consists of a resonance reflector, a slow-wave structure (SWS), an extractor, and a coaxial collector. To achieve high electron beam-electromagnetic wave interaction efficiency, a SWS for the RBWO is designed comprising of two structural segments with different corrugations that enables the gradually decreased electron beam axial velocity to better match the spatially constant electromagnetic wave phase velocity of the operating mode of the RBWO. To suppress electron emission and plasma formation on the inner surface of the electrodynamical structure, which is caused by a strong RF electric field, the RF electric field magnitude is optimized in computer simulations to be less than 700 kV/cm. A coaxial collector is designed to slow down the expansion of the collector plasma. The design of the RBWO is optimized in numerical simulations and its operation is validated in practical microwave-generation experiments. An output power of 3.3 GW is obtained in a practical experiment with a 796-kV, 10.3-kA electron beam guided by an external magnetic field of 2.2 T. The RBWO efficiency and microwave pulse width are measured in the experiment and found to be about 40% and 100 ns, respectively.

Studies of a high-efficiency, long-pulse relativistic backward wave oscillator

Relativistic backward wave oscillator (RBWO) is a promising high-power microwave (HPM) generator. In past decades, many efforts have been devoted to improving its practicality, in which the repetitively operating RBWO has drawn much attention. However, two bottlenecks once limited its development. One is the generation of massive anode plasmas or even damage of the collector, and the other is the RF breakdown of electrodynamic structures. This article describes an RBWO which can operate repetitively with 3-GW microwave power by mitigating these two problems. First, a titanium collector, which shows better performance in resisting electron beam bombardment than the conventional stainless steel collector, is adopted. Meanwhile, water-cooling technology is applied to expedite heat dissipation of the collector. Second, since our investigations demonstrate that an anodic mechanism, not the conventional cathodic mechanism, is the main cause for RF breakdown of electrodynamic structures under strong guiding magnetic field, the titanium material is also used to manufacture the electrodynamic structures of the RBWO in order to increase its power capacity. The advantage of the titanium RBWO is verified in experiments. The application of these techniques, as well as some important surface treatment techniques such as polishing, ultrasonic washing, and vacuum baking, leads to the stable repetitive operation of the RBWO with 3-GW output power and 25-ns microwave duration under a repetition rate of 50 Hz. The successive operation time of the device is up to 20 s.

Research on 3-GW Repetitively Operating Relativistic Backward Wave Oscillator

We efficiently enhance the power capacity of klystron-like relativistic backward wave oscillators (RBWOs) with a TM021 mode extraction cavity. Theoretical analysis and simulation results are shown in detail. Theoretically, an electron cluster will be steadily decelerated in the whole region of the TM021 extraction cavity if the transiting time in the cavity approximates the period of microwaves. As a result, the energy of the electron beam will be efficiently extracted by the TM021 mode field. Due to the larger size, the electric field strength in a TM021 cavity can be much smaller than that in a TM020 cavity. In addition, once the extraction cavity breaks down, the emission electron will absorb less energy from the TM021 mode field than the TM020 mode field. In PIC simulations, for an X-band klystron-like RBWO with a frequency of 9.6 GHz and an output power of 2.3 GW, the maximum longitudinal electric field (Ez) in the extraction cavity decreases from 610 kV/cm to 320 kV/cm, when the TM020 extraction cavity is replaced with a TM021 extraction cavity. PIC simulation results agree well with the theoretical analysis.We efficiently enhance the power capacity of klystron-like relativistic backward wave oscillators (RBWOs) with a TM021 mode extraction cavity. Theoretical analysis and simulation results are shown in detail. Theoretically, an electron cluster will be steadily decelerated in the whole region of the TM021 extraction cavity if the transiting time in the cavity approximates the period of microwaves. As a result, the energy of the electron beam will be efficiently extracted by the TM021 mode field. Due to the larger size, the electric field strength in a TM021 cavity can be much smaller than that in a TM020 cavity. In addition, once the extraction cavity breaks down, the emission electron will absorb less energy from the TM021 mode field than the TM020 mode field. In PIC simulations, for an X-band klystron-like RBWO with a frequency of 9.6 GHz and an output power of 2.3 GW, the maximum longitudinal electric field (Ez) in the extraction cavity decreases from 610 kV/cm to 320 kV/cm, when the TM020 extraction cav...

Power capacity enhancement for klystron-like RBWOs with a TM021 extraction cavity

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of the relativistic backward wave oscillator (RBWO). In this study, segmented slow-wave structure (SWS) rings, which can be observed directly using a scanning electron microscope without any post-processing, are used to study the breakdown in a RBWO. After undergoing high-power microwave experiments, the working surface of the SWS rings was found to exhibit corrugated morphologies at the sub-mm to mm scale, and a large number of craters in the vicinity of the corrugated morphologies and a mass of scattered droplets were visible on the surface. It was deduced that the unusual morphologies resulted from electron bombardment of the working surface. The observed structures and droplets provide new evidence that helps to explain the mechanism for RF breakdown in the RBWO.

Mechanism of radio frequency breakdown on metal surfaces in relativistic backward wave oscillator

The effect of the local electric field on the radial oscillation of the electron beam in the low-magnetic-field foilless diode is studied theoretically and experimentally. The results obtained with different methods show good coherence. The radial oscillation amplitude of the electron beam affected by the local electric field depends on the electric field intensity and the phases of electrons entering and leaving the local electric field region. It is found that the enhanced radial electric field near the drift tube head results in the change of the radial oscillation amplitude with the anode–cathode gap. Meanwhile, a method to effectively suppress the radial oscillation is proposed. Optimizing the anode–cathode gap and the tilt angle of the drift tube head can effectively adjust the matching state of the electron phase and the local electric field, and thus the transverse momentum of the electron beam can be greatly reduced due to the negative effect of the radial electric field.

Effect of Local Electric Field on Radial Oscillation of Electron Beam in Low-Magnetic-Field Foilless Diode

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of high-power microwave (HPM) generators. In this paper, the initial stage of RF breakdown in HPM generators with a strong external guiding magnetic field has been carefully analyzed. It is thought that the initial breakdown process involves electron emission induced by an intense local field, electron acceleration in the electric field, and electron bombardment on the structure’s surfaces. Strong external guiding magnetic fields play a key role in the RF breakdown process of HPM generators. They concentrate the impact energy and make it more destructive. To suppress RF breakdown, feasible solutions include lowering the local field strength, decreasing the energy of the field-emitted electrons obtained from the RF field, reducing the deposited energy density, and improving the material’s resistance to electron bombardment.

RF Breakdown of the Resonant Reflector in a Relativistic Backward Wave Oscillator

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RF Breakdown of the Resonant Reflector in a Relativistic Backward Wave Oscillator