High Power Microwave Sources Research Articles

Research on a broad-band X-band high power relativistic klystron amplifier

This paper introduces a high-power broadband X-band klystron amplifier that achieves an output power of 157 MW and a 3 dB operating bandwidth of 6.7%. The amplifier employs an explosive emission diode with high impedance, which balances the requirements of high power and broadband. The multi-gap input and output cavities are designed to operate at two different longitudinal modes within the operating frequency band, resulting in a flat absorption rate and output efficiency. Moreover, an eight-stage stagger-tuned bunching section is implemented to achieve a uniform fundamental harmonic current modulation depth across the frequency band. Simulation results indicate that when the diode voltage is 550 kV and the beam current is 550 A, the amplifier can achieve a maximum power of 157 MW with an efficiency of 51.9% at the central frequency of 9.8 GHz. Furthermore, within the 3 dB operating bandwidth of 6.7% (670 MHz), the power output remains higher than 80 MW. This novel klystron amplifier structure exhibits exceptional performance in terms of bandwidth, power, and efficiency, thus validating the effectiveness of the design in enhancing bandwidth and providing a solid foundation for the broadband design of high-power microwave sources.

A Compact V-Band Transit Time Oscillator with Reflective Modulation Cavity

Improving compactness is essential for high-power microwave (HPM) sources. In this paper, a novel reflective modulation cavity is proposed and investigated in a V-band relativistic coaxial transit-time oscillator (RCTTO). The cold cavity analyses and particle-in-cell simulations show that the reflective modulation cavity has larger reflection coefficients of TEM mode and stronger electron beam modulation capability when compared with a uniform modulation cavity. When the input diode voltage is 391 kV, the beam current is 4.91 kA, and when the guiding magnetic field is 0.6 T, the compact V-band RCTTO produces an output microwave power of 518 MW (conversion efficiency of 27.0%). Compared with the original RCTTO, the compact V-band RCTTO featuring a reflective modulation cavity exhibits a 24.8% increase in output power and a 5.4% improvement in efficiency, and the axial length of the magnetic field uniform region is reduced by 24.2%. The compact V-band RCTTO also demonstrates a broad operation voltage range, indicating potential for stable operation with voltage fluctuations in experiments. Furthermore, the reflective modulation cavity can be integrated into other high-frequency O-type HPM devices to enhance compactness, thereby diminishing the demands on the magnetic field region, which is advantageous for the future permanent packaging of HPM sources.

Calculations for preemptive surface adsorbate drive-off to minimize plasma formation during operation of high-power microwave sources

Thermal driven desorption of surface impurities is probed based on coupled Monte Carlo–heat flow–molecular dynamics simulations. Such adsorbates can lead to plasma formation during the operation of high-power microwave systems with various negative outcomes and so need to be curtailed. Our study attempts to obtain temperature thresholds for desorbing different surface contaminants such as C2, O2, CO, and CO2. The results show that carbon-based adsorbates on copper (chosen as an example anode material) could be ejected at a relatively modest surface temperature of 650 K. On the other hand, reactive species such as oxygen are very stable due to their large cohesive energies. Our calculations further suggest the benefit of using a platinum coating layer, as the noble metal is robust with strong resistance to oxidation.

Enhancing power capacity and performance of wide-drift-channel transit-time oscillators for long-pulse high-power microwave generation

This paper addresses the challenge of extending the pulse duration of high-power microwave (HPM) sources, a critical focus in HPM technology. Transit-time oscillators (TTOs) typically modulate electron beams by forming standing waves through the cutoff of the TM01 mode in the modulation cavity. This study proposes enhancing the power capacity of TTOs by widening the drift channel. To enhance modulation capability without compromising power capacity, front and rear reflectors are strategically introduced. In particle-in-cell simulations, the device exhibits an output power of 1.65 GW at a frequency of 12.6 GHz, with an efficiency of 33.6%. The saturation time for the output microwave is 30 ns, and the maximum internal electric field strength is 860 kV/cm. The presented TTO, with design improvements, offers a promising solution for applications requiring prolonged pulse durations in high-power microwave sources.

Annular Beam Driven Metamaterial Backward Wave Oscillator

Metamaterials (MTMs) are synthetic materials designed to have characteristics that "may not be readily available in nature," such as negative permittivity, reversed Doppler Effect, reversed Cherenkov Effect, and negative Refractive Index. These characteristics have motivated researchers to analyze and investigate the use of MTMs for modelling high-power microwave (HPM) radiation sources. One of the most potential HPM sources is an annular beam-driven Backward-Wave Oscillator (BWO) based on the Cerenkov mechanism. The devices that use the Cerenkov mechanism are preferred due to their larger bandwidth. Its high output power and repetition operations make it a promising source. In this paper, an annular beam-loaded metamaterial BWO is simulated in order to investigate and comment on the possibility of metamaterial or metamaterial-inspired structures replacing the slow-wave structures (SWSs) in vacuum tube devices. Performance parameters of several BWO configurations loaded with different metamaterial-inspired conductor rings are compared to rippled SWS-based BWOs. The results suggest that fewer metamaterial rings with solid cross-section (CS) inside the BWO lead to a closer match in generated output power and frequency with the output power generated using a rippled SWS.

Design and Development of Electromagnet Power Supply for Burst Mode Repetitive High Power Microwave Sources

Single frequency High-power microwave sources like Backward Wave Oscillator, Relativistic Magnetron etc., require a high magnetic field of about 1T to 4T for efficient operation. Single-shot HPM systems usually consist of pulsed magnetic systems. However, for repetitive HPM generation, a continuous magnetic field is required. To obtain a continuous high magnetic field in the required volume usually three solutions are worldwide preferred i.e., permanent magnet, DC electromagnet with cooling arrangements and superconductor-based magnetic coil. All these three systems are bulky, expensive and technologically demanding. In this paper, a novel power supply has been suggested that discharges a charged capacitor through a magnetic coil keeping the discharging current constant for a small period like hundreds of milliseconds to a second. An IGBT switch is used to discharge the capacitor1. The duty cycle of the IGBT switch is controlled using a current feedback signal from the hall sensor, thus keeping the current steady at a preset reference value. An experimental setup has been developed using a 300mF capacitor. A constant current up to 500A is achieved for 200mS. This system is scalable. For a longer duration of operation, more capacitor modules need to be added. Details of design, development and experimental results are presented in this paper.

Design, development and characterization of Blumlein Pulse Forming Line based pulsed power system for High Power Microwave source research

This paper describes a complete cycle of designing, developing, and characterising a Pulsed Power System (PPS) and generating microwaves using the relativistic source. For low impedance relativistic High Power Microwave (HPM) sources, Blumlein Pulse Forming Line (BPFL) based high voltage pulsed power systems are the most suited. An HPM source such as a virtual cathode oscillator (VIRCATOR) operating in nanosecond durations consists of a primary voltage source that charges a Marx pulse generator’s capacitor bank over a long time. The requirements of a diode load are met by compressing the output pulse width of a Marx generator by a BPFL. This article describes the novel design, construction and characterization of a circulating De-Ionized (DI) water insulated BPFL pulsed power system. A novel approach of insulating gas filled Marx generator of this class/rating is attempted. A pulse power system consisting of high pressure gas insulated Marx generator and circulating DI water-based BPFL was developed and experimented with the resistive load as well as HPM source. The PSPICE simulation of the Blumlein circuit for various load conditions is described. Criticalities in the design of end bushings are elaborated along with the development and testing of a compact BPFL-based pulsed power system.

Multi-objective design method for high-power microwave source devices based on preferred PIC/gNSGA-II

To solve the challenging problem of multi-objective optimization for high-power microwave sources, a specialized preference-based multi-objective optimization method, namely PIC/gNSGA-II, is proposed. Additionally, to achieve the quantitative optimization of the frequency spectrum and power of high-power microwave source devices, objective functions for both the spectrum and power are designed. PIC/gNSGA-II is employed to carry out structural optimization tests on relativistic backward wave oscillators (RBWO). The optimized results produced novel device structures that meet the specified frequency bands: in the X-band, the average output power is 845.85 MW, and the operating frequency is 9.53 GHz; in the Ku-band, the average output power is 167.25 MW, and the operating frequency is 14.73 GHz. This demonstrates that by controlling the parameters of such objective functions, the code can simultaneously optimize the spectrum and power of RBWOs. Comparing the optimization results of PIC/gNSGA-II with those of NSGA-II, PIC/gNSGA-II demonstrates better optimization efficiency and the ability to design device structures that meet various performance criteria.

Internal-locking relativistic magnetron in transversely opposed driven scheme

A cascaded relativistic magnetron array with symmetric feeder structure was first proposed as a multi-port phase-coherent high-power microwave source, which is intrinsically equipped with high structural symmetry. Two symmetrically positioned slow-wave structures surround the feeder structure, which reduces axial electron drifting in each resonant system and ensures the phase-locking process. In this paper, a theory of structure-provided coupling coefficient and oscillator-required coupling coefficient is proposed as the phase-locking prerequisites. The method is evaluated by adopting two A6-type resonant systems. The symmetrically driven cascaded relativistic magnetron array employs a typical π-mode with an anode voltage of 450 kV and an axial magnetic field of 0.47 T. The phase-locked state was achieved in 17 ns with a jitter less than 5 deg. The total output power exceeds 2.3 GW at a frequency of 2.15 GHz, and the power flow in each output port exceeds 350 MW. The transversely opposed driven scheme can be combined with other phase-locking patterns for additional uses, and further optimization of resonant system could be applied for enhancing device performance. The theory of coupling prerequisites is also sufficient for analyzing other cascaded relativistic magnetrons.

Investigation on a high-power V-band transit-time oscillator under a low guiding magnetic field with a focusing cathode

Low guiding magnetic fields are favored for high-power microwave sources in practical applications for the sake of compactness and miniaturization, especially for permanent magnet packaging to replace the bulky and heavy solenoid system. A novel focusing cathode is proposed in a V-band transit-time oscillator to reduce the requirement for guiding magnetic fields. Particle-in-cell simulation results indicate that in the diode region, the radial electric field and electron beam current are reduced with the focusing cathode, further leading to improved transmission of the electron beam under a low guiding magnetic field. With an input of diode voltage of 393 kV, a beam current of 5.25 kA, and a low guiding magnetic field of 0.6 T, the transit-time oscillator outputs a microwave with an average power of 566 MW and a frequency of 58.6 GHz, resulting in a conversion efficiency of 27.3%. The proposed V-band transit-time oscillator with a focusing cathode can also stably work under a large range of input voltage and magnetic field, desired for further research with permanent magnet packaging.

An open-end high-power microwave-induced fracturing system for hard rock

Microwave pre-treatment is considered as a promising technique for alleviating cutter wear. This paper introduces a high-power microwave-induced fracturing system for hard rock. The test system consists of a high-power microwave subsystem (100 kW), a true triaxial testing machine, a dynamic monitoring subsystem, and an electromagnetic shielding subsystem. It can realize rapid microwave-induced fracturing, intelligent tuning of impedance, dynamic feedback under strong microwave fields, and active control of microwave parameters by addressing the following issues: the instability and insecurity of the system, the discharge breakdown between coaxial lines during high-power microwave output, and a lack of feedback of rock-microwave response. In this study, microwave-induced surface and borehole fracturing tests under true triaxial stress were carried out. Experimental comparisons imply that high-power microwave irradiation can reduce the fracturing time of hard rock and that the fracture range (160 mm) of a 915-MHz microwave source is about three times that of 2.45 GHz. After microwave-induced borehole fracturing, many tensile cracks occur on the rock surface and in the borehole: the maximum reduction of the P-wave velocity is 12.8%. The test results show that a high-power microwave source of 915 MHz is more conducive to assisting mechanical rock breaking and destressing. The system can promote the development of microwave-assisted rock breaking equipment.

Influence of gas pressure and gas type on pseudospark electron beam

Pseudospark discharge is a discharge that occurs in the left band of the Paschen curve. According to previous studies, an electron beam will be generated in the initial stage of pseudospark discharge. This electron beam has the advantages of high energy, high current and self-confinement. It has a promising application in high-power microwave sources, surface treatment of metal materials, etc. To investigate the characteristics of this electron beam, a pseudospark discharge experimental platform is established in this paper. A pseudospark chamber is designed for pseudospark discharge. To trigger the discharge, a high voltage pulse trigger signal is generated by an avalanche transistor Marx circuit and a trigger unit is designed to generate trigger electrons. Faraday Cup is utilized to collect the electron beam and measure the electron waveform. In order to quantitatively characterize the electron beam, we define six electron beam characteristics, including trigger delay, peak current, electron beam charge, electron beam width, electron beam loop current ratio and electron beam loop charge ratio. To investigate the influence of gas pressure and gas type on electron beam characteristics, we experimentally obtained results of electron beam characteristics for different gas pressures and gas type. We mainly investigated the change rule of the electron beam characteristics when the gas pressure changes between 6 Pa and 20 Pa and the similarities and differences of the electron beams under the two gas environments, argon and nitrogen. We find that gas pressure can be used for the modulation of electron beam time delay. The width of the electron beam in the case of nitrogen has a very stable relationship with the gas pressure. Gas pressure and gas type are two important means of controlling the characteristics of pseudospark electron beams.

Multi-Objective Optimization of High-Power Microwave Sources Based on Multi-Criteria Decision-Making and Multi-Objective Micro-Genetic Algorithm

Multi-Objective Optimization of High-Power Microwave Sources Based on Multi-Criteria Decision-Making and Multi-Objective Micro-Genetic Algorithm

GW-Level Ku-Band Compact Coaxial TTO With Permanent Magnet Packaging Potential

When the O-type high-power microwave (HPM) sources are extended to the high band, the current density of the intense relativistic electron beam will increase, which requires a higher intensity of the guiding magnetic field. The coil of the excitation system will be bulkier, which is not conducive to the development of the device toward the direction of light and small. The transit time oscillator (TTO) has attracted much attention due to its ability to output GW-level HPMs in the small number of cavities. In order to replace solenoid with permanent magnet, the compact coaxial TTO with a low magnetic field is designed in this article. The coaxial structure reduces the need for magnetic field strength. In addition, the familiar reflector is removed and the axial size of the beam-wave interaction region is reduced, which not only reduces the risk of electron beam diffusion under low magnetic field, but helps to reduce the weight of permanent magnets in the future. In particle-in-cell (PIC) simulation, the proposed coaxial TTO can output more than HPMs of 1.0 GW under the magnetic field less than 0.4 T with the efficiency is over 30%. Therefore, it is feasible to use permanent magnet packaging for the device.

A Compact Repetitive Marx Generator for Generating Intense Electron Beams for HPM Sources

A 500 kV, 80 Hz repetition rate compact Marx generator has been designed and developed for producing high intense relativistic electron beams for high-power microwave (HPM) sources. This Marx generator comprises eight bipolar stages (16 stages of unipolar) using energy storage capacitors for achieving improved performance in repetition mode operation. The maximum charging voltage per stage is 50 kV, producing an erected voltage of 800 kV. However, the load voltage will vary according to the impedance of the load connected across the generator. Custom-designed energy storage capacitors were developed to allow voltage reversal of up to 30%–50%, enabling the system to deliver a higher current. A small pulse transformer-based triggering unit with an output voltage of 30 kV was used for triggering the generator. The system’s performance was tested in pulse repetition frequency (PRF) mode with resistive and diode loads and achieved a repetition rate of 80 Hz. An energy density of ~460 J/m3 was achieved in the developed system. A vircator device was connected and tested to benchmark the Marx generator for its repetitive and burst mode operations. The test results show the measured RF output power of 100 MW.

A C-Band Relativistic Magnetron With a Novel Extraction Structure

Relativistic magnetron (RM) is a promising high-power microwave (HPM) source and major hotspot in the field of HPM research, whose advantages include high efficiency, low magnetic field, and compact configuration. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> -band RM with a novel extraction structure is presented and investigated numerically in this article. Compared with the traditional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> -band RM with rectangular extraction slots (CRMRS), the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> -band RM with novel extraction slots (CRMNS) has three advantages. On the one hand, mode competition is suppressed effectually, and so its operation is more stable. On the other hand, the power efficiency is enhanced. Moreover, the power capacity is enhanced, which is advantageous to increase the output power. The typical simulation results are as follows: HPM is generated with a peak power of 2.53 GW at an efficiency of 58% and a frequency of 4.3 GHz, when the voltage is 650 kV, the current is 6.7 kA, and the magnetic field is 0.347 T.

Initial testing of ohmic heating through double flux swing during electron cyclotron start-up in the QUEST spherical tokamak

A power-supply system was developed for Ohmic heating (OH) to double × 1018 the amount of change magnetic flux in the primary central solenoid (CS) on the QUEST spherical tokamak. Two power supplies are connected with stacks of insulated-gate bipolar transistors, and sequentially operated to generate positive and negative CS currents. This bipolar power-supply system is controlled via a field-programmable gate array, which guarantees the safety of the entire system operation. The new OH system, assisted by electron cyclotron heating, enables the stable generation of plasma currents exceeding 100 kA. Moreover, the achieved electron density over the wide range in the major radial direction exceeds the cut-off density for one of the high-power microwave sources in QUEST. This strategy yields target plasmas for future experiments with the electron Bernstein wave.

Study on explosive emission characteristics of the silicon carbide modified graphite cathodes with varied morphologies

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.

Simulation and experimental study of a K-band extended interaction oscillator for microwave processing systems.

High-power microwave sources have been widely applied for material processing in scientific research and manufacturing. The development of stable, high-frequency, high-power microwave sources is essential for achieving efficient microwave processing. This study proposes using a square doubly reentrant coupled-cavity as the slow-wave resonant structure in a K-band extended interaction oscillator (EIO). This design allows for ease of fabrication and high-power capability. The EIO is designed to operate in single 0-mode. The simulation results show that the competing π/5-mode can be effectively suppressed by properly choosing the width and location of the output coupler. The simulation and experiments successfully demonstrate stable, single-mode, tunable, high-performance operation of the EIO. The experimental measurements show a maximum output power of 1.776 kW (18.56% electronic efficiency), and a wave frequency of 24.324GHz at a beam voltage of 17.4kV and beam current of 550 mA. The EIO microwave source is suitable for interdisciplinary applications that require higher heating rates and greater uniformity.

Analytical Design and Simulation of a 9.3-GW Tesla Transformer for HPM Sources

This article presents a step-by-step design and modeling of a high-voltage pulse generator (PG) based on a Tesla transformer (TT) with an open-magnetic core to drive a low-impedance high-power microwave (HPM) source. Our design uses the OFF-resonance mode of operation for higher voltage transformation ratios. The pulse-forming line of the transformer is filled with deionized water for a higher energy density of the line and an increased output pulse duration. Maxwell equations are used to establish the transformer parameters such as the magnetizing and leakage inductances, and subsequently, the geometry of the TT for optimum power transfer between the resonant circuits. The required circuit parameters and geometry of the generator producing up to 250 kV and peak power of 9.3 GW when matched to a 6.7- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> load were determined through analytical design and validated through simulations.