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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Due to the ironless structure of the toroidal winding in the tubular linear voice coil motor (TLVCM), the winding thickness is one of the critical parameters that have an impact on air-gap flux density, and hence affects the copper loss at rated thrust. This paper investigates and determines the thickness of the ironless toroidal winding of the TLVCM for the purpose of a minimum copper loss. Magnetic equivalent circuit (MEC) method is used to analyze the air-gap flux density with respect to winding thickness, and the corresponding copper loss at the rated thrust is assessed by considering the slot fill factor. The optimal solution is obtained according to the U-shaped curve of the copper loss. Moreover, the finite-element analysis is combined to validate the results of MEC method, and a good agreement is reached. Two prototypes of TLVCMs having similar slot fill factor are made, one of which is with the optimal winding thickness. The expected current at the rated thrust is obtained by a test platform of static thrust, so as to obtain the corresponding copper loss for each prototype. The experimental results show that the TLVCM with optimal winding thickness features a lower copper loss. Hence, the attempt to minimize the copper loss by designing the thickness of the ironless winding is highly effective. [ABSTRACT FROM AUTHOR]

The two-way power combiner consisting of two TM01–TE11 serpentine mode converters with a common output has demonstrated high-power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators (RBWOs). To realize channel power combination for four phase-locked X-band RBWOs, we propose an all circular waveguide four-way power combiner based on the previous two-way power combiner. Four TM01 modes are first combined into two TE11 modes with two TM01–TE11 mode two-way power combiners; then, through two separate 90° bending waveguides and tapered waveguides, the two TE11 modes are combined into one TE11 mode with a TE11–TE11 mode two-way power combiner. Separate calculations show that at 9.22 GHz, the conversion efficiency of the TM01–TE11 mode two-way power combiner is 95.4%, the transmission efficiencies of 90° bending waveguide and tapered waveguides, and the combination efficiency of the TE11–TE11 mode two-way power combiner are close to 100%. Integrated simulations indicate that as four incident ports are fed with TM01 modes with power of 2.5 GW at each port and phase of 0°, 180°, 0°, and 180°, respectively, the total combination efficiency is 94.5%, and the maximum electric field in the four-way power combiner is 774 kV/cm. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Metallic cathodes can be used to generate intense relativistic electron beams for high-power microwave devices. Many previous papers have demonstrated that metallic cathodes usually have a limited lifetime. In this paper, three metallic cathodes, namely, stainless steel cathode, copper cathode, and aluminum cathode, are researched with a repetitive-operation accelerator and an X-band relativistic backward-wave oscillator. The cathode surface morphologies are observed with a scanning electron microscope before and after lifetime test. The experimental results demonstrate that the stainless steel cathode has a very limited lifetime ( <10^3 pulses), the copper cathode has a moderate lifetime ( \sim 10^4 pulses), and the aluminum cathode has a surprisingly long lifetime (at least 10^5 pulses). The different lifetimes of the three cathodes may originate from the different situations of regeneration of microprotrusions on the cathode surface. The large number of regenerated microprotrusions which possess large microscopic field enhancement factors supports the long lifetime of the aluminum cathode. [ABSTRACT FROM PUBLISHER]

Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator. [ABSTRACT FROM PUBLISHER]

Numerical and experimental studies on a coaxial vircator with radial dual-cavity premodulation are presented. The physics of mechanisms on microwave frequency determination and effects on microwave generating efficiency are discussed. It is proved that the microwave frequency is mainly determined by the modulation frequency, while the optimum operating frequency is determined by the intrinsic frequency of the beam itself. When the two frequencies match, the maximum efficiency is yielded. A structure capable of generating 6.0 GW microwave, 17% efficiency, and 50% frequency tuning bandwidth has been obtained by numerical simulations. The preliminary experiments were carried out on the MC55 accelerator. A 2.0 GW output microwave was experimentally obtained and the corresponding device efficiency is about 7%. [ABSTRACT FROM PUBLISHER]

As the impedance of the foilless diode for a relativistic backward wave oscillator and the transmission line of the high-current accelerator are mismatching, a method is brought forward to study the voltage and current transient process of the foilless diode by analyzing the voltage and current wave reflection and superposition. In addition, by theoretical analysis, particle-in-cell numerical simulations and experiment based on the SINUS-881 accelerator, the effects of periodic oscillations of growth rate in the high-voltage pulse's rising edge applying to the diode from the Tesla-type accelerator and the time when the cathode begins to emit on transient process are discussed. In addition, the characteristics of rising edges of voltage and current are meticulously discerned. [ABSTRACT FROM AUTHOR]

This paper gives a numerical model to deal with the thermal effect of field-induced thermal-assisted current passing through the microprotrusion on the cathode surface of a planar high-voltage vacuum diode. The model of protrusion is based on a one-dimensional truncated cone, whose physical and thermal properties have been assumed temperature dependent. The influences of space charge effect and Nottingham effect are included. According to the simulations, the delay times of explosive electron emission of cathodes made of different materials are given and compared with the experimental results, which seems to be in agreement. [ABSTRACT FROM AUTHOR]