Search

Radio frequency (RF) breakdown is one of the crucial factors limiting the power capacity of high-power microwave (HPM) generators. In this paper, a waveguide cavity has been designed to study quantitatively the influence of temperature on high-gradient RF breakdown. The two planes of the waveguide cavity are divided into a strong-field side and a weak-field side with the feeding of microwave signals. The temperature of the strong-field side of the waveguide cavity can be adjusted from ambient temperature (25 °C) to 435 °C by loading a resistive wire heater. The breakdown threshold of the waveguide cavity decreases apparently with the increase in the temperature. The breakdown threshold at 408 °C is 751 kV/cm, which is about 150 kV/cm lower than that at ambient temperature. In addition, the higher the temperature, the more pronounced the pulse shortening. Under the same microwave power injection, the averaged pulse width of HPM after passing through the waveguide cavity at 408 °C is equal to 17.6 ns, shortened by about 1/3 compared to that at ambient temperature. The quantitative study of the influence of temperature on the RF breakdown provides a new guideline for exploring breakdown suppression methods in HPM generators. [ABSTRACT FROM AUTHOR]

An oversized Ku-band Cerenkov oscillator with pure TM01 mode output is proposed. By utilizing a separated slow-wave structure (SSWS), the resonant characteristic of the operating mode is preserved, whereas the resonant characteristics of the high order electromagnetic modes are destroyed. As a result, only the expected mode can be stimulated, and undesired beam–wave interactions are suppressed effectively. In terms of an oversized Cerenkov oscillator, the usage of SSWS shows great potential for obtaining pure operating mode output. By utilizing particle-in-cell simulation, microwaves with an output power of 4.5 GW and frequency of 14.1 GHz are obtained, when the beam voltage is 0.9 MeV, and the beam current is 12.9 kA. The percentage of the operating mode is up to 99.5% and exceeds 99% in a wide range of the beam voltage. [ABSTRACT FROM AUTHOR]

This paper presents a design method of the relativistic backward wave oscillator at low magnetic field, which can improve the efficiency by 29% in the particle in cell simulation. The core of this method is to introduce a beam filtering ring. The beam filtering ring takes the characteristic of the radial position change as the electron oscillates. The structure manipulates the axial current, so that a large proportion of the electrons expected to be in the accelerated phase in the slow-wave structure is absorbed by the structure. It greatly enhances the bunching of the beam in the RF field and improves the beam-wave conversion efficiency significantly. The particle in cell simulation results reveal that at a permanent magnet with a magnetic induction intensity of 0.68 T, the output microwave power of the relativistic backward wave oscillator with a beam filtering ring is 5.9 GW, and the conversion efficiency can be up to 54% when the diode voltage is 890 kV and the beam current is 12.2 kA. [ABSTRACT FROM AUTHOR]

For the high power microwave (HPM) generator, damage on an RF structure surface is harmful for a long time operation. In this paper, we investigate damage traces on slow-wave structure (SWS) of the relativistic backward-wave oscillator (RBWO) with a low guiding magnetic field. By theoretical and experimental analysis, we find that damage traces on SWS surface are mainly caused by the bombardment of beam electrons pulled back. Moreover, it is suggested that damage traces are not caused by RF breakdown, the bombardment of secondary electrons, and the bombardment of particles from collector plasma. Importantly, to decrease damages and prolong the working life of the RBWO with a low guiding magnetic field, the electric field which pulls electrons to the RF structure surface (saying Ez and −Er) should be decreased by special design for the RF structure, such as blending and parameters adjustment. Those results may offer benefit guides for study of the RBWO with a low guiding magnetic field. [ABSTRACT FROM AUTHOR]

An annular cathode is generally fixed on a cylindrical cathode holder in a magnetically insulated coaxial foilless diode. Electrons emitted from cathode plasmas or the cathode holder may easily move backward under the action of the quasi-static electric field and axial magnetic field, leading to backward current loss, which will seriously reduce the efficiency of high power microwave sources, especially under the condition of a low guiding magnetic field. A novel storage electrode immersed in the uniform magnetic field is proposed to suppress the backward current. Reverse electrons will be decelerated and blocked by the storage electrode. Under the guiding magnetic field of 0.66 T, over 66% of the backward current is suppressed (from 3.6 kA to 1.2 kA) when the storage electrode is used, and furthermore, the waveforms of the diode voltage and current are improved significantly. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The breakdown problem in the relativistic backward wave oscillator (RBWO) limits the microwave pulse energy and should be solved. The slow wave structure (SWS) is the most key component of the RBWO. The breakdown of SWS may be triggered by field-induced electron emission and electron bombardment on the wall. This paper demonstrates that the space charge field of the relativistic electron beam (REB) can enhance the electric field forbidding field-induced electron emission and decrease the electric field igniting the RF breakdown, and therefore, the RF breakdown can be significantly suppressed. However, the enhanced electric field will enlarge the radial displacements of electrons, especially the high-energy electrons. When the RBWO operates with a low guiding magnetic field or a long-pulse REB, this situation will worsen and some electrons may possess large enough radial displacements and bombard on the SWS surface with the help of radial oscillation of REB and radial expansion of cathode plasmas. This may aggravate the breakdown of SWS. [ABSTRACT FROM AUTHOR]

An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM01 mode and TM02 mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field. [ABSTRACT FROM AUTHOR]

The relativistic backward wave oscillator (RBWO) is one of the most important high-power microwave generators. Its stability and reliability must be further improved to meet the demands of key applications. The factors that limit its performance are varied but include radio frequency breakdown and mode competition. Based on a C-band RBWO, the mode competition was carefully analyzed and the effects on the performance of the RBWO were revealed. As shown in the studies, for an axisymmetric system, asymmetric mode competition probably still exists. Therefore, a three-dimensional particle-in-cell model was configured to investigate the mode competition. The theoretical and experimental results confirmed that the competing mode for a C-band RBWO was EH21. Changing the gap between the cathode and anode was found to be effective for suppressing the asymmetric mode. [ABSTRACT FROM AUTHOR]

In the repetitive experiments of relativistic backward wave oscillators operating at either low or high magnetic field, we found that the spiral traces appeared on the surfaces of the tube heads (entrance to the reflector and slow wave structure). Three possible causes: electrons emitted from the cathode base, cathode plasma, and cathode negative ion, are analyzed. Among them, the cathode negative ion is the most likely factor leading to the traces. The negative ions are generated by electron impact-ionization and ion impact-ionization of molecules that are desorbed from the cathode by the Ohmic heating of explosive electron emission. Due to the larger mass of negative ion, it cannot be restricted effectively by the magnetic field and will basically move along the electric field line. Many negative ions bombard the tube head with a Larmor radius of several centimeters, and the traces are connected to form spiral lines. The viewpoint of cathode negative ion current is further confirmed by the comparative experiments that show that a graphite cathode after being immersed in water for 12 h results in heavier traces, while a larger anode and cathode gap leads to slighter traces. Some suggestions are provided to reduce the production and the effect of cathode negative ion current. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The directional coupler is an important online power measurement device based on coupling principles. It is widely used for real-time monitoring of the performance of high-power microwave devices. However, insufficient power handling capacity limits further applications of the coupler in higher-pulsed energy measurements. From a theoretical and numerical analysis, the plasma effects of breakdown on the coupler are investigated. The plasma is found to break the fixed phase relationship between the coupling holes and eventually the directivity of the coupler. As the plasma density increases, the isolation of the coupler decreases rapidly, and thus, a negative power flux in the secondary rectangular waveguide increases gradually. Simultaneously, the positive power flux fluctuates and even terminates ahead of the microwave pulse in the main circular waveguide. The conclusions provide a good interpretation of the experimental phenomena. [ABSTRACT FROM AUTHOR]

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO. [ABSTRACT FROM AUTHOR]

An overmoded rectangularly corrugated relativistic surface wave oscillator is designed and theoretically analyzed to produce the high power terahertz radiation with a TM01 mode. The design is fully verified and optimized by 2.5D particle-in-cell simulations. A high power terahertz wave is obtained with the output power of 285 MW at the frequency of 0.148 THz with conversion efficiency 20% when beam voltage is about 400 kV, current 3.8 kA, and the magnetic field 6 T. [ABSTRACT FROM AUTHOR]

The klystron-like relativistic backward wave oscillator (RBWO) combines the transition radiation with Cerenkov radiation and has demonstrated microwave output of high power and high efficiency. The coaxial slow wave structure device can produce microwave with a lower frequency in a smaller cross section. For the purpose of high efficiency, low frequency, and miniaturization, a coaxial klystron-like RBWO with a premodulation cavity is presented. Particle-in-cell simulations show that a microwave with power of 1.15 GW and frequency of 2.1 GHz is generated with conversion efficiency of 48%, whereas for the device with a reflector, the efficiency is 38%. [ABSTRACT FROM AUTHOR]

Klystronlike relativistic backward wave oscillator (RBWO) can produce microwave power exceeding 5 GW with a high efficiency larger than 40%. In the experiment of klystronlike RBWO, for about 1 MV peak diode voltages, increasing magnetic field from 1.43 to 1.89 T slowed the impedance collapse until it was suppressed completely. The introduction of a stainless steel obstructing ring aggravated the impedance collapse, whereas replacing the stainless steel obstructing ring with a flat stainless steel provided a more stable impedance variation during the pulse duration. These impedance collapses did not affect microwave generation seriously and may be attributed to the radial expansion of cathode plasma initialing from the cathode shank so that part of reverse currents were collected at the anode wall, contributing to the measured diode current. On the other hand, it was found that microwave generation shot-to-shot reproducibility was closely related to the diode impedance variation. When there was no or very low microwave measured, diode impedance collapse appeared at the latter of the pulse. The microwave generation shot-to-shot reproducibility was improved greatly after the electron collector was enlarged on radius with 1 mm. A possible explanation is that the anode plasma produced from electron collector expands axially and enters the diode region at a very high velocity of several mm/ns. The movement of the anode plasma in the beam-wave interaction region affects the microwave generation, which reduces the microwave power during the whole pulse duration significantly. [ABSTRACT FROM AUTHOR]

We demonstrate both theoretically and experimentally the possibility of the generation of powerful microwave pulses by channel power summation of two X-band phase-locked relativistic backward wave oscillators (RBWOs). A modulated electron beam induced by an external signal can lead the microwave field with an arbitrary initial phase to the same equilibrium phase, which is determined by the initial phase of the external signal. A high-current dual-beam accelerator was built to drive the two RBWOs. An external signal was divided into two channels with an adjusted relative phase and injected into the two RBWOs through two TE10-TEM mode converters. The generated microwaves were combined with a power combiner consisting of two TM01-TE11 serpentine mode converters with a common output. In the experiments, as the input power for each channel was 150 kW, the two RBWOs output 3.1 GW and 3.7 GW, respectively, the jitter of the relative phase of two output microwaves was about 20°, and the summation power from the power combiner is 6.2 GW, corresponding to a combination efficiency of 91%. [ABSTRACT FROM AUTHOR]

This paper reports the space-charge limited current (SLC) and virtual cathode behaviors in one-dimensional grounded drift space. A simple general analytical solution and an approximate solution for the planar diode are given. Through a semi-analytical method, a general solution for SLC in one-dimensional drift space is obtained. The behaviors of virtual cathode in the drift space, including dominant frequency, electron transit time, position, and transmitted current, are yielded analytically. The relationship between the frequency of the virtual cathode oscillation and the injected current presented may explain previously reported numerical works. Results are significant in facilitating estimations and further analytical studies. [ABSTRACT FROM AUTHOR]

A klystron-like relativistic backward wave oscillator with a ratio of transverse dimension to free-space wavelength being about four is presented. In the beam-wave interaction region, the electron beam interacts with surface wave and volume wave simultaneously. The cathode holder plays an important role in the reflection of backward waves. A guard electrode, an electron collector ring, and a reflection ring are used to optimize the beam-wave interaction. The particle in cell simulation results reveal that microwaves with a power of 2 GW and a frequency of 12.3 GHz are generated with an efficiency of 42% when the diode voltage is 400 kV, the beam current 12 kA, and the magnetic field 0.48 T. [ABSTRACT FROM AUTHOR]