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Relativistic backward wave oscillators (RBWOs) have the characteristics of high power and high repetition rate. Reducing the magnetic field strength and the weight of the external permanent magnet (PM) is a significant development direction of RBWOs. In previous research, an X-band RBWO enclosed with a PM has achieved a power efficiency of 50%. However, the PM used in these papers requires a magnetic field magnitude of over 0.68 T, which leads to a weight exceeding 400 kg. The RBWO designed in this paper operates in coaxial TM01 mode, and the radial distance between its cathode and the surface of the slow wave structures reaches 7 mm. Under the condition of a low magnetic field, this design can provide a wide electron beam channel to avoid the rubbing of the electron beam envelope on the inner and outer conductors. Particle-in-cell simulation results have demonstrated that this RBWO achieves an output microwave power of 3 GW with a power efficiency of 50% enclosed in a PM with a magnetic field strength of 0.43 T and a weight of 74 kg. [ABSTRACT FROM AUTHOR]

In this paper, a comparative investigation of a C-band long-pulse high-efficiency klystron-like relativistic cavity oscillator is carried out by using numerical simulation and practical experiments. It is indicated that owing to the improper configuration of the solenoid, some returning electrons originating from the cathode supporting rod would hit the entrance of the diode and shunt the input electric power. As a result, the startup process of the device is delayed and the saturated output power is decreased as well. In the initial experiment, the output power and the pulse duration are only about 3.2 GW and 78 ns, respectively. To eliminate the influence of the returning electrons, by increasing the turn number of the winding at both ends of the solenoid, the magnetic field at the diode region is enhanced so that the returning electrons could be guided to the shielding bowl in front of the insulator. As the shielding bowl is under the same potential as the cathode, the returning electrons would not shunt the input power. In contrast, after solenoid compensation, the output power of the device is increased to 4.4 GW at a diode voltage of 700 kV and a current of 14 kA, whereas the pulse duration is extended to 96 ns. Also, the corresponding conversion efficiency achieves about 45%. [ABSTRACT FROM AUTHOR]

Explosive emission cathodes are extensively used in high power microwave sources. Growing requirements are highlighted in the performance of the explosive emission cathode with the development of the high power microwave technology. In order to improve the emission properties of the graphite cathode, modifications using SiC particles or whiskers are carried out by the in situ chemical vapor reaction method. The experiments demonstrate that SiC whiskers accelerate the explosive emission turn-on speed of the graphite cathode for large field enhancement factors and improve the emission uniformity due to the surface flashover plasma generation mechanism. SiC particles increase the emission capability of the graphite cathode, possibly corresponding to the dielectric properties of SiC particles. These results suggest that the SiC whiskers modified graphite cathode is suitable for applications under low magnetic field and large emission area conditions, while the SiC particles modified graphite cathode has a brighter application prospect in the long-time stable operation. [ABSTRACT FROM AUTHOR]

In this paper, a transmission line with five spiral-wires in parallel was designed. The transit time and the characteristic impedance of the transmission line are 200 ns and 14 Ω, respectively. Its slow wave coefficient is close to 13. The insulating medium of the transmission line is glycerin. In order to investigate the breakdown strength of glycerin under a hundred-nanosecond pulse, simulations and experiments are conducted. The results indicate that the breakdown strength of glycerin under hundred nanoseconds reached 653 kV/cm, which is a 400 kV/cm increase compared with that under a microsecond pulse. However, under hundred nanoseconds, the breakdown strength of glycerin in transmission is unaffected by pressure within 1 MPa, and pressure is conducive to the insulation recovery of glycerin. [ABSTRACT FROM AUTHOR]

An improved X-band coaxial transit time oscillator is proposed in this paper. First, the device uses a non-uniform three-gap modulation cavity to improve the clustering of electron beams under a low magnetic field and to increase the depth of the fundamental current modulation. The operating mode of the modulation cavity is the 2π/3 mode of TM01. Second, the dual-cavity extraction structure works in π mode, which can continuously and intensively extract the electron beam energy. The physical characteristics of the device are studied through simulation and experiment. The experimental results indicate that the high power microwave with a frequency of 8.38 GHz and a power of 1.78 GW is generated when the diode voltage is 520 kV and the guiding magnetic field is 0.65 T, yielding a pulse width of 27 ns and an efficiency of 32%. [ABSTRACT FROM AUTHOR]

Simulations for the thermal conductivity of carbon fibers (CFs) are carried out based on a molecular dynamics (MD) approach. Our calculations are geared toward providing a general trend for thermal conductivity and its dependence on sample length and to probe the suitability of this material for high power applications from the thermal management standpoint. Since abundant data are not available for carbon fibers, calculations are first carried out in silicon to validate the predictions. The MD results yield a bulk thermal conductivity of 148 W m−1 K−1 for silicon in good agreement with published reports. However, for CF, a much lower bulk value of ∼14 W m−1 K−1 is predicted. This suggests that thermal management would likely be an issue for this material and that CF emitters of lengths below 2 µm should be avoided. The predicted increases in thermal conductivity with temperature may help alleviate the issue to a minor degree. Carbon nanotubes would likely be a better alternative in this context of field emitter arrays. [ABSTRACT FROM AUTHOR]

The energy consumption of high-power microwave sources has attracted much attention in the research field of high-power microwaves. One way of decreasing the energy consumption is to reduce the guiding magnetic field. In this work, a high-efficiency transit-time oscillator is proposed with a low guiding magnetic field. In order to enhance the modulation depth of electrons and increase the conversion efficiency under a low guiding magnetic field, two bunchers are adopted to modulate the electron beam. 2.5D simulation results show that the proposed device generates microwaves with a power of 3.6 GW and a frequency of 4.31 GHz. The power conversion efficiency is 40% and the modulation depth is 125% when the guiding magnetic field is only 0.5 T. In addition, a 3D simulation verifies the results. [ABSTRACT FROM AUTHOR]

In this paper, the lattice Boltzmann method is used to simulate the diffusion and ionization process of a two-dimensional energetic electron beam that is injected into the background gases under the sub-atmospheric pressure, and the characteristics of such plasmas are revealed. It is found that the penetration distance of the electron beam from the exit of the electron beam reduced to 10% of the maximum value at the axis is about 35 cm, whereas the envelope of the plasma distribution is like a spindle with the penetration distance of about 130 cm, under the pressure of 5 kPa. The gas pressure has a significant impact on the penetration distance of the electron beam and the plasma that is produced. The increase in gas pressure will reduce the penetration distance of the electron beam and the plasma because higher pressure will increase the collisional probability between the electrons and the gas molecules, and the energy transfer becomes stronger. [ABSTRACT FROM AUTHOR]

In the study of breakdown in high power microwave (HPM) devices, researchers pay attention to the plasma effects caused by material ablation under electron bombardment. In this paper, the destruction of material under the bombardment of electrons with a range of energies around 100 keV is investigated. The simulation results reveal that low-energy electrons are more likely to cause ablation damage to the material, with the location of maximal energy deposition density close to the material's surface. The experiment has a good agreement with the simulation results. Compared with high-energy electrons, the damage traces bombarded by low-energy electrons are more serious, possessing the characteristics of large size and shallow depth. This abnormal physics phenomenon provides a reference for the breakdown of HPM devices and the surface treatment of materials by pulsed electron beams. [ABSTRACT FROM AUTHOR]

High efficiency and short saturation time have great significance on the practical work of high power microwave devices. A dual-band Cerenkov microwave generator with high-efficiency based on electrical tuning is presented in this paper. Two-segment slow wave structures for the C-band and X-band separately are adopted, which are isolated by a drift cavity. The collimating hole is replaced by a resonant reflector, which can effectively shorten the saturation time. Moreover, in order to realize dual-band microwave output, the electrical tuning method is used in this paper. In particle-in-cell simulation, when the guiding magnetic field is 0.7 T, the output microwaves at the X-band can be obtained with an efficiency of 38% and saturation time for nearly 20 ns; when the guiding magnetic field is 1.5 T, the output microwaves at the C-band can be obtained with an efficiency of 32% and saturation time for nearly 20 ns. Thus, microwaves at the X-band and C-band can be generated separately by modulating the guiding magnetic field, which can be realized just by altering the current in the magnetic coil. It should be mentioned that such a modulating method is much more convenient than mechanical modulation. [ABSTRACT FROM AUTHOR]

The generation of high-power nanosecond radio frequency pulses is obtained by means of sequential commutation of silicon high voltage switches periodically placed in a nonlinear transmission line. The coaxial transmission line had the inner conductor made in the form of a corrugated periodic structure. The dispersive characteristics of the transmission line were optimized for RF generation by an electromagnetic shock front using numerical simulation. In the experiment, nanosecond RF pulses at frequency near 1 GHz were obtained for 2 kV incident pulses. The peak-to-peak amplitude of the excited oscillations reached about 30% of the amplitude of the incident pulse. [ABSTRACT FROM AUTHOR]

The design of compact, lightweight devices has become an important development direction of high-power microwave source devices, with the reduction of the external magnetic field being the leading innovation. By analyzing the transmission equation of the intense relativistic electron beam (IREB), it can be known that magnetic field strength in the diode region has great influence on the transmission quality of the IREB. In this paper, a new structure that introduces a soft magnetic material as the cathode base is proposed. Considering that soft magnetic materials have the capability to concentrate and enhance the magnetic field, the proposed structure leverages this merit to improve the magnetic field strength in the diode area under certain low external magnetic field conditions. Thus, the proposed structure is capable of effectively improving the emission and transmission quality of the IREB without any increase of the external guiding magnetic field. As for the device design, the new structure was introduced into a previously proposed C-band transit time oscillator, and the particle-in-cell simulation was carried out. The results showed that high power microwaves with a power of 2.7 GW are generated with a corresponding frequency of 40% when the electron beam voltage is 550 kV, the beam current is 12 kA, and the guiding magnetic field is 0.3 T. [ABSTRACT FROM AUTHOR]

Coaxial slow wave structures have always been used to enhance the power handling capacity in the relativistic O-type Cherenkov oscillators generally. However, little attention has been paid to the regularity and the problem it may bring by using large average radius. The aim of this study was to investigate the high-frequency characteristics of coaxial slow wave structures in order to evaluate the influence of the radial dimension comprehensively. The results indicate that a large overmoded ratio can make the power handling capacity increase significantly, but as a result, it will bring some problems such as low efficiency, long starting time, and the difficulty in mode control. In addition, this paper also gives some suggestions and precautions for using large radial dimensions. The conclusion of this paper has strong applicability for the design of coaxial slow wave structures. [ABSTRACT FROM AUTHOR]

The breakdown of polymer insulators at a nanosecond scale is a serious problem for high-voltage devices and pulsed power systems. This paper reviews recent developments in research on the mechanisms and characteristics of the breakdown of polymers at a nanosecond scale at the Northwest Institute of Nuclear Technology. An improved avalanche breakdown model is proposed, and a formula for the strength of the electric breakdown (EBD) is formulated based on it. The formative time lag of the avalanche breakdown model is then developed to verify it. Two research groups were formed to study the characteristics of the breakdown. One focused on the characteristics of single-pulse breakdown, such as the effects of size, polymer type, electrode, pulse width, and pulse polarity on EBD, with the aim of deriving expressions for EBD. The other research group examined cumulative (multi-pulse) characteristics of the breakdown with the aim of deriving a formula for the lifetime of the polymer. A method to design solid insulation structures is finally proposed. [ABSTRACT FROM AUTHOR]

We found that the electron drift length, L1, between a diode area and a resonant reflector in a relativistic backward wave oscillator operating at a low guiding magnetic field has a periodical influence on device power, which is a special characteristic due to periodical transverse motions of electron beams. In this paper, electron motion characteristics under both TM02 and TM01 fields are investigated using single-particle simulation, and the result shows that the condition of beam expansion is dependent on the beam state that spatially meets the TM02 standing-wave field. A larger beam expansion will lead to a larger axial energy spread in the modulation area, which is unfavorable for efficient beam-wave interaction. Furthermore, rapid transverse expansion of electron beams can be observed under a TM01 travelling-wave field, which is the main cause for the obvious decline in the current collected by the collector as the output power reaches saturation. [ABSTRACT FROM AUTHOR]

Various configurations of corrugated coaxial Slow-Wave Structures (SWSs) are utilized in O-type Cherenkov oscillators to achieve different goals. This is the reason why a strategy targeting the proper selection of corrugated coaxial SWSs configurations deserves to be further studied. In this paper, high frequency characteristics of different configurations of corrugated coaxial SWSs are calculated and analyzed. The analysis suggests that the coaxial SWSs with corrugations on inner conductor only are characterized by higher coupling impedance and higher growth rate of microwave oscillations, while the coaxial SWSs with corrugations on the outer conductor only are characterized by higher power capacity. Due to the low power capacity, coupling impedance and growth rate, the coaxial SWSs with corrugations on both inner and outer conductors are not suitable for O-type Cherenkov oscillators desired to operate in the quasi-TEM mode. However, if these devices are desired to operate in the TM01 quasi-π mode, the SWSs with both inner and outer conductor corrugations may be a better choice to make it happen. [ABSTRACT FROM AUTHOR]

A novel coaxial relativistic Cerenkov generator (CRCG) operating in quasi-transverse electromagnetic mode is proposed for generating Ku-band high-power microwave radiation. It is found that using a cathode with a shielding electrode can decrease the diffusing electric field in the diode, thereby allowing a CRCG with the proposed diode structure to operate in a low guiding magnetic field. Taking advantage of the two-sectional slow wave structure, the output power and conversion efficiency are improved. Furthermore, a simple optimization rule is proposed for the length of the drift cavity and is verified by numerical simulation. In a particle-in-cell simulation, when the diode voltage and beam current are 550 kV and 4.2 kA, respectively, the microwave with an output power of 1 GW at 15.3 GHz is achieved in a guiding magnetic field of 0.35 T, yielding a conversion efficiency of 43%. [ABSTRACT FROM AUTHOR]

The mechanism and realization of a cross-band relativistic Cherenkov oscillator based on the cathode adjustment are presented. The device adopts coaxial dual-annular cathode to emit the inner and the outer electron beams, independently. When the inner annular cathode is extended and the outer annular cathode is retracted, the electron beams are emitted from the inner cathode and interact with the internal electromagnetic structure to generate Ka-band microwaves. On the contrary, the outer electron beams interact with the external electromagnetic structure to generate X-band microwaves. Thus, the cross-band microwave generation can be achieved only by adjusting the protrusion and retraction of dual-annular cathode. By the particle-in-cell (PIC) method, the physical process of beam-wave interaction in the device is studied, and the high efficiency internal-external double electromagnetic structures are designed. Simulation results show that the X-band microwave with a power of 1.50 GW and a frequency of 8.55 GHz is generated, corresponding to an efficiency of 32%. Meanwhile, the Ka-band microwave with power of 650 MW and frequency of 31.80 GHz is generated, corresponding to efficiency of 28%. [ABSTRACT FROM AUTHOR]

In this paper, we study an X-band klystron-like relativistic backward wave oscillator (RBWO) with a pre-modulation cavity, modulation ridge, drift tube in the slow wave structure (SWS) and extraction cavity in both theoretical and experiment. PIC simulations show that the conversion efficiency can be up to 62% with output power of 3.0 GW. In experiment, when the diode voltage is 660 kV, and the diode current is 6.4 kA, the output power is 1.9 GW, and the conversion efficiency is 45%(±5%). And we find that the conversion efficiency mainly suffers from the breakdown in the extraction cavity and the plasma from the cylindrical collector. To suppress the breakdown and plasma effect, the breakdown electric field threshold in RF structures(especially in the extraction cavity) had better be larger than 700 kV/cm. Moreover, the distance between the position where the electrons are collected and the end of extraction cavity should be larger than 20 mm in case of the cylindrical collector. [ABSTRACT FROM AUTHOR]

A coaxial line with periodically tapered inner rode corrugated ripples is introduced to act as a distributed Bragg reflector to increase output microwave power of relativistic diffraction generator (RDG). The RDG device consists of an overmoded cross-section, two-sectional disk-loaded cylindrical slow-wave structure (SWS), and coaxial reflector is placed upstream at the first SWS section. The RDG device with tapered Bragg reflector is investigated by full three-dimensional Particle-in-Cell (PIC) simulations and experiments in this paper. The structural parameters of the reflector are optimized for favorable characteristic of mode selection and frequency response. It is found that oscillation modes varied at different magnetic field magnitude. The measured radiation power at 29~31 GHz is over 300 MW for TM01 mode and over 700 MW for TM11 mode. Results show that the reflector has a great impact on performance of the device. Operating at proper conditions, the radiation power is remarkably enhanced. [ABSTRACT FROM AUTHOR]