High Power Microwave Oscillator Research Articles

Design of a cross-band frequency hopping high power microwave oscillator with permanent magnet package

A cross-band frequency hopping high power microwave (HPM) oscillator with permanent magnet package was proposed. The oscillator comprises of an inner Ku-band hollow oscillator and an outer C-band coaxial oscillator. By comparing with other cross-band HPM oscillators, this device is quite compact and high efficiency, for the guiding magnetic field is as low as 0.43 T, which is provided by a 74 kg permanent magnet. Since the cathode can be adjusted telescopically to produce annular electron beams at different radius, different bands HPMs can be generated in the inner and the outer slow wave structure. Verified by particle-in-cell simulation, a HPM with a frequency of 7.8 GHz and a power of 3.1 GW is achieved for C-band, corresponding to a power efficiency of 45%. Respectively, a Ku-band HPM output with a frequency of 14.4 GHz and a power of 1.3 GW were obtained, corresponding to a power efficiency of 45%.

A three-band frequency hopping high power microwave oscillator based on magnetic field tuning

The frequency hopping technology is one of the most significant research directions for high-power microwave (HPM) devices. This paper presented a novel HPM oscillator with frequency hopping across C, X, and Ku bands based on magnetic field tuning. A coaxial transit time oscillator (TTO) is nested onto the outer conductor of the hollow relativistic Cherenkov microwave oscillator, which forms a dual electromagnetic structure with a single-annular cathode. When the electron beam is guided by gradient magnetic fields, it interacts with TTO to produce Ku-band HPMs. If the gradient magnetic field changes into the uniform magnetic field, the electron beam would enter the relativistic Cherenkov microwave oscillator, and the frequency of generated microwaves decreases, which are decided by the strength of the magnetic fields according to the cyclotron resonance absorption theory. In the particle-in-cell simulation, when the diode voltage and gradient magnetic field are 580 kV and 0.5 T, respectively, a Ku-band HPM output with a frequency of 13.9 GHz and a power of 2.09 GW is obtained, corresponding to power efficiency of 42%. When the magnetic field transforms into uniform, the device produces an X-band HPM output with a frequency of 9 GHz and a power of 2.4 GW at a diode voltage of 683 kV and a magnetic field of 0.7 T. When the voltage and magnetic field strength are increased to 699 kV and 1.5 T, respectively, the device generates a C-band HPM output with a frequency of 4.5 GHz and a power of 2.1 GW. The corresponding conversion efficiency of the X-band and C-band Cherenkov microwave oscillators is 35% and 30.7%, respectively.

Analysis and design of overmoded coaxial Ku-band Cerenkov high power microwave oscillator with high efficiency

An overmoded coaxial Ku-band Cerenkov high power microwave oscillator with high efficiency is proposed. The distance between the inner and outer conductors of the slow wave structure (SWS) in the Cerenkov oscillator is smaller than the space wavelength of output microwave. The overmoded structure can increase the power capacity of the oscillator by increasing the transversal size of the device. The higher-order modes are introduced into the beam-wave interaction region intentionally, so that the intense relativistic electron beams (IREB) interacts with the fundamental and higher-order modes. Dual-mode operation leads impure mode component to output microwave, so the simple trapezoid inner conductor is designed to achieve mode purification and output pure TEM mode. The extractor is added at the end of the SWS, which enhances the axial electric field near the location and increases the modulation depth of the electron beam current passing through the region. The particle-in-cell (PIC) simulation results show that under the input power of 3.5 GW, the output microwave power is 1.8 GW, and the corresponding working efficiency is 51.4%, which is much higher than that of similar periodic devices. The mode of output microwave is pure TEM mode, and the maximum surface electric field intensity is 0.96 MV cm−1, which means that the probability of radio-frequency breakdown is low.

A Novel Cross-Band Frequency Hopping Gigawatts Class High-Power Microwave Oscillator

A novel cross-band frequency hopping giga- watts (GWs) class high-power microwave (HPM) oscillator is proposed in this brief, which consists of an inner Ku-band coaxial oscillator, an outer <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula> -band coaxial oscillator, a beam transfer channel, and an extraordinary external magnetic field system. Compared with ordinary design, the external magnetic field of this device is generated by three closely spaced solenoids. Through the adjustment of the current passing through each solenoid, two kinds of magnetic field distribution can be generated, each guides the beams into a separate frequency beam-wave interaction region. The preliminary particle-in-cell (PIC) simulation results were as follows: when the currents passing through the three solenoids were the same of 700 A, a HPM output with a frequency of 14.4 GHz, and a power of 1. 5 GW was obtained, achieving a power efficiency of 37%; when the three solenoid currents were 1800 A, 225, and 460 A, respectively, a HPM output with a frequency of 8.2 GHz and a power of 1.6 GW was obtained, achieving a power efficiency of 40%.

Theory of Peer-to-Peer Locking of High-Power Gyrotron Oscillators Coupled with Delay

Peer-to-peer locking is a promising way to combine the power of high-power microwave oscillators. The peer-to-peer locking of gyrotrons is especially important because arrays of coupled gyrotrons are of special interest for fusion and certain other applications. However, in case of coupled microwave oscillators, the effect of delay in coupling is very significant and should be taken into account. In this article, we present the model of two delay-coupled gyrotrons. We develop an approximate theory of phase locking based on the generalized Adler’s equation, which allows for the treatment of in-phase and anti-phase locking modes. We also present a more rigorous bifurcation analysis of phase locking by using XPPAUT software under the limitation of small delay time. The structure of the phase-locking domains on the frequency-mismatch–coupling-strength plane of parameters is examined. Finally, we verify the results by numerical simulations in the case of finite delay time. The simulations reveal various regimes, including peer-to-peer locking, the suppression of one gyrotron by another, as well as the excitation of one gyrotron by another.

High-Voltage Drivers Based on Forming Lines with Extended Quasi-Rectangular Pulses for High-Power Microwave Oscillators

The paper considers such modifications of an ordinary pulse-forming line (PFL) as double-width and triple-width forming lines (DWFL, TWFL) built around the PFL by nesting one and two additional uncharged lines, respectively, into its free volume inside the inner conductor of the PFL. The theoretical analysis is supported by simulation and experimental data, showing that the TWFL provides a 3-fold increase in the voltage pulse width and that it can be further increased by an arbitrary integer factor k. The results of the numerical simulations also show the electric field behavior and other features, including the edge effect in the TWFL. The proposed method opens up new opportunities for designing compact high-power microwace (HPM) sources.

PIC Simulation of the Coherent Cerenkov– Cyclotron Radiation Excited by a High-Power Electron Beam in a Crossed-Elliptical Metamaterial Oscillator at S-Band

We present an S-band high-power microwave (HPM) oscillator based on crossed-elliptical metamaterial (MTM) rings in a circular waveguide and powered by a high-power electron beam. Particle-in-cell (PIC) simulations demonstrate that when an output horn antenna of radius of 200 mm is used, an output power of 443 MW of a backward wave TE41 mode below its cutoff frequency at 2.80 GHz can be generated using a pulsed electron beam with a voltage of 990 kV and a current of 1.736 kA in a 1.8-T solenoidal magnetic field, corresponding to an electronic efficiency up to 26%. Meanwhile, another degenerate frequency of 2.80 GHz of a forward wave in the TM01 mode with an output power of 9 MW is also observed. When an output horn antenna of radius 150 mm is used, this crossed-elliptical MTM oscillator operates mainly in a forward TM01 mode at 2.80 GHz with an output power of 325 MW at an electronic efficiency of 19%, and at the same time, a backward wave TE41 mode below its cutoff frequency of 2.80 GHz with an output power of 8 MW can be observed. Simulations indicate that this slow-wave structure (SWS) consisting of a periodically crossed-elliptical MTM loaded circular waveguide can support degenerate mode operation. This result suggests that this configuration is favorable for mode switching and this work seeks to design a compact HPM source.

Investigation of an X-band Cerenkov-type high power microwave oscillator driven by sheet electron beam

An X-band Cerenkov-type high power microwave (HPM) oscillator driven by a sheet electron beam is proposed and designed in this paper. In order to improve the efficiency of simulation optimization, a research method is proposed in which the large caliber coaxial Cerenkov-type HPM oscillator is taken as an initial model, which is equivalent to that of the planar slow-wave structure (SWS) by using the similarity between the planar SWS and the large caliber coaxial SWS. In addition, the effectiveness of the equivalent method is verified from the geometric structure, the field distribution of the operating mode, dispersion characteristics, and the preliminary particle simulation results of two devices. Based on the results of an X-band large caliber coaxial Cerenkov-type HPM oscillator, an X-band Cerenkov-type HPM oscillator driven by a sheet electron beam is investigated by the 3D particle-in-cell code. The simulation results show that the power conversion efficiency can reach 26.1% at the frequency of 11.9 GHz with the output power of 0.3 GW, while the sheet electron beam has the energy of 486 kV and it carries the current of 2.4 kA conducted by the magnetic field of 2.5 T.

Design and simulation study of double side-cavity gridless reltron with dual extraction sections

Reltron is a high power microwave (HPM) oscillator with many attractive features. Here, we report a computer-aided design and simulation-based study of a double side-cavity (DSC) gridless reltron, which is an advancement of the conventional reltron. Commercially available software package CST Microwave Studio (MWS) has been used for the device simulation. The modulation section has been designed and studied by obtaining the S-parameters. From the electromagnetic (EM) field patterns and the frequencies of normal modes, we identify an extra mode in a DSC reltron, in addition to the three normal modes of a conventional reltron – (0, , and π). Furthermore, the whole device has been modeled and simulated using particle in cell (PIC) solver to study the device performance under different post acceleration (PA) potentials. The use of the dual extraction section results in a peak efficiency of 38.6%.

Development of the Concept of High-Power Microwave Oscillators with Phase Locking by an External Signal

By simulations and experiments we develop a method for generation phase locking of a Ka-band subgigawatt nonstationary relativistic backward-wave oscillator (BWO) with an external radio pulse, the duration of which is considerably greater than the pulse duration of a driven BWO. The level of input power (about 20 kW), which provided phase locking with a total spread of about 1.2 rad, is determined both for the case of competition with an electromagnetic disturbance from a steep front of the beam current and the case of a slow current rise without phase locking in the self-excitation mode. The phase locking effect is achieved with a significant difference in the frequencies of the external radio pulse and driven BWO (about 1 GHz) and hence differs from the amplification mode.

MILO Performance Improvement Study—An Equivalent Circuit Approach

In this article, a high-power microwave (HPM) oscillator magnetically insulated line oscillator (MILO) design study has been carried out to improve its performance. Impedances of different subsections of the device have been matched. An equivalent circuit approach, which is simpler and easy to handle, has been adopted for this purpose. First, the input impedance of the tapered inner conductor of the coaxial structure has been matched with the impedance of the choke disks. Then, the extractor inner radius and extractor gap impedance have been optimized at the output section. This impedance matching scheme provides the maximum efficiency of the device with the maximum RF transmission. The optimum position of the stub to match the impedance with the output side has also been analyzed. To validate our concept, initially, an experimentally reported L-band MILO has been simulated using commercial particle-in-cell (PIC) code, which provides an RF output power of 1.38 GW with the efficiency of 7.8%. The PIC simulation results have been found in agreement of ~5% with the experimental values. Furthermore, the PIC simulation with the optimized design parameters has improved the RF output power of 2.5 GW with an efficiency of 14.4%.

Theoretical research on TEM mode feedback for compact design of an X-band triaxial klystron amplifier

A relativistic triaxial klystron amplifier (TKA) is considered to be one of the most promising high power microwave (HPM) sources for coherent power combination in high frequency bands. In general investigations, the TEM mode leakage and asymmetric modes' competition are considered to play unfavorable roles in the normal operation of TKA. Therefore, mode reflectors with high reflection efficiency in both the TEM mode and the asymmetric modes are employed and expected to significantly suppress the transmissions of the microwaves between the cavities. So the cavities can be separated from each other, while the long drift length is necessary to achieve the deep modulation. Therefore, the size of TKA is noticeably larger compared to other HPM oscillators, which requires large magnetic coil and huge energy consumption. In our present research, it is found that the TEM mode leakage can be utilized to form positive feedback between the cavities, which is significant for electron beam modulation and furthermore to make the devices more compact. In this paper, the mechanism of TEM mode feedback between the cavities is discussed and the conditions of positive and negative feedback are achieved. Besides, an especially designed reflector with high reflection coefficients in asymmetric modes and low reflection coefficients in the TEM mode is designed to utilize the TEM mode and suppress the asymmetric modes. Based on the above research, a TKA adopting positive TEM mode feedback, whose total length is decreased by 23.6% compared to the previous design, is presented. A high power microwave with a power of 2.25 GW and a frequency of 8.4 GHz is generated with a 690 kV, 9.3 kA electron beam excitation and a 40 kW radio-frequency signal injection. Meanwhile, there is insignificant self-excitation of the parasitic mode in the proposed structure in three-dimensional particle-in-cell simulation, and the relative phase shifts between the injected signals and the output microwaves are kept locked after the amplifier becomes saturated.

Experimental investigation of an S-band triple cavity oscillator

The triple cavity oscillator (Tritron) is an improved high power microwave (HPM) oscillator from the dual cavity oscillator (Bitron), in which an intermediate cavity is introduced to enhance the beam modulation depth and improve the conversion efficiency by optimizing the phase between the modulated beam and the field in the intermediate cavity. There are only three cavities in the device, which makes it fit for developing compact HPM source. In this paper, an S-band Tritron is proposed and investigated experimentally. By means of adjusting the axial position of the support posts, the self-oscillation phenomenon observed in the experiment is efficiently suppressed without affecting the characteristics of the working mode. An output power of 1.8GW with the pulse width of 46ns and the frequency of 2.46GHz is obtained in the experiment under the condition of the 0.7T guiding magnetic field, and the axial length of the interaction region is only 11cm.

Coupled-mode analysis for two-dimensional coaxial Bragg structures with helical ripples

Making use of field expansion and the equivalent boundary, an analytical model is set up to describe the multi-mode intercoupling in two-dimensional coaxial Bragg structures with either one or both of the conductors corrugated with helical ripples. The coupled-mode equations and the explicit formulae of the coupling coefficients for all possible mode combinations are derived, and the general mode coupling rules are discussed. Based on the analytical model, the dependence of the coupling coefficients on structural parameters of the two-dimensional coaxial Bragg structures with overmoded operation is investigated by numerical calculation. The results show that the additional conductor provides more flexibility for adjustment of coupling coefficients compared with other types of two-dimensional Bragg structures with a single conductor. Taking advantage of this feature, a two-dimensional coaxial Bragg structure can provide strong reflection with high mode purity under overmoded operation by suppressing the parasitic coupling using an optimized combination of corrugated outer and inner conductors. In this way, the structure has potential applications in the design of high-Q improved Bragg resonators for high-power microwave oscillators operating in the millimeter wave or terahertz range.

The modified scheme of optimized in simulations Cherenkov type high-power microwave oscillator without guiding magnetic field

The compactness and miniaturization of high-power-microwave (HPM) systems are drawing more and more attention. Based on this demand, HPM generators without a guiding magnetic field are being developed. This paper presents an X-band Cherenkov type HPM oscillator without the guiding magnetic field. By particle-in-cell codes, this oscillator achieves an efficiency of 40% in simulation. When the diode voltage and current are 620 kV and 9.0 kA, respectively, a TEM mode microwave is generated with a power of 2.2 GW and a frequency of 9.1 GHz. In this oscillator, electrons are modulated in both longitudinal and radial directions, and the radial modulation has a significant effect on the energy conversion efficiency. As analyzed in this paper, the different radial modulation effects depend on the phase matching differences of the microwave and electrons. The modified scheme of simulations achieves a structure with an efficient longitudinal beam-wave interaction and optimized radial modulation.

High-Power Microwave Oscillator of Twistron Type with Efficiency of 55% for Electron Energy of 180 keV

Highly effective mode of generation of a relativistic microwave oscillator of twistron type is calculated by numerical simulation using an axisymmetric version of completely electromagnetic PiC-code KARAT. With accelerating voltage of 180 kV, electron beam current of 1.2 kA, and 1.9 T guiding magnetic field strength, the simulated stationary generation power is 120 MW at an operating frequency of 10.7 GHz. The efficiency of electron beam power conversion to electromagnetic radiation is 55%, and the electronic efficiency of the oscillator calculated from the change of the initial electron energy is 64%.

Study of virtual cathodes formation during beam-wave interaction in the reltron oscillator

In the present work, a high power microwave oscillator—reltron has been analyzed to investigate the virtual cathode formation mechanism during the beam-wave interaction. In reltron, a side coupled modulation cavity is used as its RF interaction structure containing three metal grids along the longitudinal direction. The space charge current responsible for the virtual cathode and its steady state electric field distribution has been analyzed. Space charge and beam impedance conditions for efficient device operation have been demonstrated. It has been shown that during the beam-wave interaction in the device, first a virtual cathode forms in the post-acceleration gap, and then the second virtual cathode develops between the first and second grids of the modulation cavity. These two virtual cathodes co-exist and cause the formation of a third virtual cathode between the second and third grids. At this instant, only the third virtual cathode remains, and for sustained device oscillation, this process repeats periodically in the device. The present study would be useful in understanding the beam-wave interaction mechanism as well as the design and development of efficient reltron devices.

Investigation of Tritron as a high power microwave oscillator

In order to improve the conversion efficiency of the dual cavity oscillator named Bitron, a new type of triple cavity oscillator named Tritron is proposed and investigated in this paper. An intermediate cavity is introduced into the device to enhance the beam modulation depth and the conversion efficiency. The theoretical study shows that the fundamental harmonic current modulation coefficient could exceed 1.16 when the modulated electron beam enters the intermediate cavity at an optimized phase. Accordingly, the maximum theoretical efficiency could exceed the efficiency of the Bitron of 58%. Moreover, an S-band Tritron is designed and verified by the particle-in-cell simulation. An output power of 2.6 GW with a frequency of 2.42 GHz is obtained in the simulation. Compared with the efficiency of 22.2% obtained in the Bitron, the efficiency of the Tritron increases to 41.3%.

Activation of the explosive-emission-cathode in conditions of artificial initiation of field emission

Low inertial transition of the field emission into explosive electrons emission at the cathode and related fast-rise-time electron beam formation are of interest in a phase-stable excitation of relativistic high-power microwave oscillators. We present experiments on the cathode activation using emission initiation during specially prepared advance voltage pulse (prepulse) possessing variable amplitude of tens-hundreds kilovolts and the width adjustable from tenths to units of nanosecond. Such a prepulse can be separated in time with a main accelerating pulse as well as adjoined to the fast-rise-time voltage front.

Preliminary experimental investigation of an X-band Cerenkov-type high power microwave oscillator without guiding magnetic field

Among high power microwave (HPM) generators without guiding magnetic field, Cerenkov-type oscillator is expected to achieve a relatively high efficiency, which has already been realized in X-band in our previous simulation work. This paper presents the preliminary experimental investigations into an X-band Cerenkov-type HPM oscillator without guiding magnetic field. Based on the previous simulation structure, some modifications regarding diode structure were made. Different cathode structures and materials were tested in the experiments. By using a ring-shaped graphite cathode, microwave of about one hundred megawatt level was generated with a pure center frequency of 9.14 GHz, and an efficiency of about 1.3%. As analyzed in the paper, some practical issues reduce the efficiency in experiments, such as real features of the electron beam, probable breakdown regions on the cathode surface which can damage the diode, and so forth.