High Power Microwave Transmission Research Articles

Heterogeneous CPU-GPU Accelerated Subgridding in the FDTD Modelling of Microwave Breakdown

Microwave breakdown is crucial to the transmission of high-power microwave (HPM) devices, where a growing number of studies have analyzed the complex interactions between electromagnetic waves and the evolving plasma from theoretical and analytical perspectives. In this paper, we propose a finite-difference time-domain (FDTD) scheme to numerically solve Maxwell’s equation, coupled with a fluid plasma equation for simulating the plasma formation during HPM air breakdown. A subgridding method is adopted to obtain accurate results with lower computational resources. Moreover, the three-dimensional subgridding Maxwell–plasma algorithm is efficiently accelerated by utilizing heterogeneous computing technique based on graphics processing units (GPUs) and multiple central processing units (CPUs), which can be applied as an efficient method for the investigation of the HPM air breakdown phenomena.

Eigenmode Analysis for High Power Microwave Transmission and Its Application

Eigenmode Analysis for High Power Microwave Transmission and Its Application

A compact high-power microwave TM01-TE01 mode converter

In circular waveguides, the TE01 mode has the lowest transmission loss, which is very suitable for long-distance transmission of high-power microwaves (HPMs). The output mode of HPM sources is mainly the TM01 mode; however, there are few research studies on mode converters of TM01-TE01. In this paper, a high efficiency HPM TM01-TE01 mode converter is designed; compared with the traditional TM01-TE01 mode converters, the structure of the mode converter is compact and easier to process. It is mainly composed of an input circular waveguide, a tapered rectangular waveguide, a 90° bent rectangular waveguide, and an output circular waveguide. A prototype with a center frequency of 2.4GHz is fabricated and HPM experiments are carried out. The transmission efficiency of this device reaches 99.8% in the simulation, and the measured transmission efficiency is more than 98%. Additionally, the measured power handling capacity is more than 1 GW, which is consistent with simulation. This designhas important reference significance for the designof long-distance power transmission devices and HPM mode converters.

Design Procedure for a Broadband TE11/HE11 Mode Converter for High-Power Radar Applications

The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical gyro-TWT. However, since the HE11 hybrid mode is not suitable for electron-beam-wave interaction in this kind of vacuum electron device, an additional mode converter is required. The present paper addresses the design procedure of a broadband high-power mode converter, designed for a helical gyro-TWT intended for future broadband high-power radar applications in the W-band. The interaction mode of the helical gyro-TWT under consideration can be easily transferred to the circular waveguide TE11 mode. Therefore, a TE11 ↦ HE11 mode converter is addressed here. The design procedure is based on a scattering matrix formalism and leads to a high HE11 mode content of ≥98.6% within the considered frequency range from 92 GHz to 100 GHz. Inside this frequency band, the mode content is even better and reaches ≈ 99.7% at ≈95 GHz. This allows broadband frequency operation of a helical gyro-TWT and is suitable for broadband high-power radar applications.

A Broadband TE01–TE11 Mode Converter With Elliptical Section for Gyro-TWTs

The mode converter is a key component in a gyrotron traveling-wave tube (gyro-TWT) application system, which can transform the output circular TE $_{0n}$ modes (in a gyro-TWT loaded with lossy ceramic dielectric or photonic bandgap interaction structure, the output mode is normally a TE $_{0n}$ mode) into a mode conducive to transmission and emission (linearly polarized Gaussian-like HE11 mode). In particular, the bandwidth of the TE01–TE11 mode converter is the most important challenge in the conversion sequence. In this paper, based on the deformation of the transverse cross section and the modified multi-frequency iteration synthesis technique along the longitudinal propagation axis, a novel method for synthesis of a broad bandwidth mode converter is proposed. Then, an application of the method to a wideband serpentine TE01–TE11 mode converter with an elliptical section is given. This novel mode converter including transitional waveguides achieves a 19.18% relative bandwidth (conversion efficiency over 95%) at center frequency 30.5 GHz, which increases nearly 195% compared with the conventional circular design, and its structure length is 1151.65 mm. Overall, this method offers a novel solution to the requirement of broad bandwidth in the high-power microwave transmission system.

Compact circular waveguide TM 02 ‐TE 11 mode converter

Using the concept of a metallic cylinder and a linearly tapered metallic baffle loaded in a circular waveguide to convert TM02 circular waveguide mode into TE11 circular waveguide mode, a novel and compact circular waveguide TM02-TE11 mode converter is designed, fabricated, and measured. The detailed design methods, experimental setups and results are presented in this study. Simulation results show that the purity of output fundamental TE11 mode of the converter exceeds 99.6% for an operating frequency of 9.4 GHz and the power capability is as high as 1.66 GW, the mode conversion efficiency is more than 90% from 9.23 to 9.83 GHz band and the relative bandwidth is about 6.38%. A good agreement between measurements and simulations is observed. The circular waveguide TM02-TE11 mode converter has many advantages, such as the simple structure, coaxial input and output, high conversion efficiency, proper bandwidth and high power-handling capability. When used in the field of high power microwave transmission and radiation, the proposed converter could significantly enhance the compactness of all the radiation systems.

Polarizer miter bends for high-power microwave transmission: Ohmic loss and cooling

Waveguide miter bends with grooved mirrors are used to alter the polarization in oversized waveguides. The grooves increase the ohmic loss on the mirrors, and this increase is a function of the incident and output polarizations. In this paper, theoretical calculations of ohmic loss are compared with other available theory and experimental data. The optimum orientation and location of the polarizer miter bends for the lowest maximum ohmic loss is determined from the theory. Then the capability of polarizer miter bends to handle the heat loading at ITER is evaluated, using theoretical calculations of the ohmic loss and ANSYS modeling of the mirror cooling.

Study of periodic surface profile on improving the window capacity at single and repetitive pulses

The surface breakdown of dielectric windows seriously limits the transmission of high power microwaves (HPM), and has blocked the development of microwave technology in recent decades. In this paper, X-band HPM experiments of window breakdown at the vacuum/dielectric interface and the atmosphere/dielectric interface at single and repetitive pulses were conducted. The cross-linked polystyrene (CLPS) dielectric window with a periodic surface profile can significantly improve the breakdown threshold at single and repetitive pulses. Furthermore, the flat surface layer of CLPS was discovered to be carbonized to a depth of several millimeters and filled with electrical trees at repetitive pulses. Theoretical models were built to understand the underlying physics behind the phenomena in experiments. With the analysis of the electron resonance process breaking the molecular bond and the temperature rise caused by the traversing current in the dielectric material, a microscopic explanation for the carbonization of the dielectric window was introduced.

Power monitor miter bends for high-power microwave transmission

Two miter bends are described for monitoring the power transmitted in an oversized corrugated wave-guide. One has an array of holes in its mirror that couples a small fraction of the incident power to a rectangular waveguide directly machined into the mirror. Millimeter-wave detectors on the outputs of this miter bend can respond very rapidly to the transmitted power, but the coupling is sensitive to the mode purity in the oversized waveguide. The other miter bend monitors the power by measuring the rise in temperature of the cooling water passing through the mirror. The mirror is well isolated from the miter bend housing to prevent heat from neighboring waveguides from reaching the mirror. The measurement requires about 200s to reach steady state, but it is relatively insensitive to mode purity. The measurement does require knowledge of the input polarization.Thermo-mechanical analyses of the miter bends indicate that they are capable of reliable operation with 1.5MW transmitted through them. High-power long-pulse 170GHz tests of these miter bends at the Japan Atomic Energy Agency (JAEA) are described.

Rectangular grooves suppressing multipactor across high power microwave dielectric window

Dielectric window is an important component of high power microwave (HPM) generation device. However, breakdown easily occurs on vacuum/dielectric interface with the development of HPM device. Surface breakdown limits the generation and transmission of HPM, and becomes the bottle neck of HPM technology development. For further understanding the breakdown mechanism and improving the threshold, the theoretical and experimental investigation of multipactor suppressed in rectangular grooves simulation model is built in this paper. From the dynamic analysis and PIC simulation, the movement of multipactor electron in grooves is obtained and the physical process of rectangular grooves suppression breakdown is simulated. Breakdown experiments of polytetrafluoroethylene (PTFE) under an S-band (2.86 GHz) HPM are conducted, and the obtained results correspond to the suppression theory.

Enhancement of the electric field threshold for microwave breakdown by crossed magnetic field

The enhancement of breakdown threshold is of benefit to the high power microwave transmission. We propose a magnetic field in the transverse direction to the electric field to enhance the breakdown threshold. A theory of electric field threshold with crossed magnetic field for short pulse is developed, and verified by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. The result shows that the crossed magnetic field can enhance the breakdown threshold significantly.

High power microwave propagation properties in radio frequency breakdown plasma

The effect of plasma on the microwave propagation properties is investigated experimentally by using a high power microwave combiner. The plasma is generated by radio-frequency breakdown on the coupling slit of the combiner through changing the power or pulse width of the prime microwave source. The plasma diffuses from the slit to the high power microwave transmission channel, and induces the absorption of microwave energy and the rotation of microwave polarization. The results show that the spread velocity of plasma is about 1 μs/cm, and the duration is about 5 μs. The polarization rotating angle is determined by electron density and microwave frequency. The maximum rotating angle is 4.1° while the number of electron is about 3.7×1015.

Measurements of Secondary Electron Emission From Dielectric Window Materials

Dielectric window is an important component of high-power microwave (HPM) devices. However, surface breakdown easily occurs at the vacuum/dielectric interface when HPM pass through the dielectric window. This greatly limits transmission of HPM and makes the window a bottle neck of HPM technology development. Secondary electron emission (SEE) from dielectric window plays an important role in its surface breakdown. This paper studies the total SEE (including true secondary electrons and backscattered electrons) coefficients of several inorganic and organic dielectric materials including polytetrafluoroethylene, polyethylene, alumina ceramic, and machinable ceramic. The measurements are implemented by using pulsed electron beam impacting the materials with energies from 200 eV to 5 keV. In addition, surface desorbed gas property is studied with the quadrupole mass spectrometer. The performances of different materials are evaluated. The obtained results are useful for the selection of HPM window materials.

Gas breakdown driven by L band short-pulse high-power microwave

High power microwave (HPM) driven gas breakdown is a major factor in limiting the radiation and transmission of HPM. A method that HPM driven gas breakdown could be obtained by changing the aperture of horn antenna is studied in this paper. Changing the effective aperture of horn antenna can adjust the electric field in near field zone, leading to gas breakdown. With this method, measurements of air and SF6 breakdowns are carried out on a magnetically insulated transmission-line oscillators, which is capable of generating HPM with pulse duration of 30 ns, and frequency of 1.74 GHz. The typical breakdown waveforms of air and SF6 are presented. Besides, the breakdown field strengths of the two gases are derived at different pressures. It is found that the effects of air and SF6 breakdown on the transmission of HPM are different: air breakdown mainly shortens the pulse width of HPM while SF6 breakdown mainly reduces the peak output power of HPM. The electric field threshold of SF6 is about 2.4 times larger than that of air. These differences suggest that gas properties have a great effect on the transmission characteristic of HPM in gases.

空气和SF6气体击穿对微波传输的影响

空气和SF₆气体击穿对微波传输的影响

Tree-like breakdown phenomena of dielectric window under X-band high power microwave in vacuum

Surface breakdown of dielectric window seriously limits the transmission of high power microwave (HPM), which blocks the development of microwave technology. Under X-band HPM with the power level of 1 GW at 9.4 GHz, dielectric breakdown experiments were conducted with several dielectric window materials including polytetrafluoroethylene (PTFE), low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polymethyl methacrylate (PMMA) in vacuum. The influence of different surface treatments for PTFE with notching and polishing on its breakdown characteristics is also studied. After breakdown, the photographs of damaged dielectric samples are analyzed with macroscopic and microcosmic observations. There are obvious tree-like breakdown phenomena occurred on the upstream face of the dielectric window along the direction of microwave electric field. The damage is significantly affected by the different surface treatments. It is considered that the treelike breakdown channels are closely related to the applied microwave field. Based on in-depth observation on the tree failure inside the transparent PMMA sample, it was found that, the tree developed along the surface and also inside of the material. A physical model is proposed to describe the dielectric window breakdown under HPM. It is suggested that the electric field results in the tree developing along the material surface. Possible reason for the tree invading into the material is analyzed. Moreover, several kinds of possible manners are presented to describe the tree initiation and development inside the dielectric window.

Investigation on Dielectric Window Treelike Breakdown and Suppression Under HPM in Vacuum

Surface discharges of dielectric window seriously limit the generation and transmission of high-power microwaves (HPMs), which block the development of microwave technology. An S-band (2.856-GHz) HPM experimental system is established. Several kinds of dielectric window materials, i.e., polyethylene, polytetrafluoroethylene, and polymethyl methacrylate, are investigated under S-band HPMs in vacuum, and the characteristics of destroyed dielectric samples are analyzed with macroscopic and microcosmic observation. Treelike breakdown channels are found on the upstream face of dielectric window along the direction of microwave electric field, and a breakdown development model with an increase of cumulative breakdown times is proposed. Dielectric surface breakdown suppression technologies are also studied, i.e., surface-polishing and surface-notching treatments. The experimental results show that the polishing treatment on the dielectric surface can decrease breakdown degree and increase surface breakdown threshold. The surface grooves that are parallel to the direction of electric field will reduce the surface breakdown threshold, while the grooves that are perpendicular to the direction of electric field can suppress the surface breakdown obviously. The effect is influenced by the depth and width of grooves on the dielectric surface, and correlative theoretical analysis is discussed.

200 MW S-band traveling wave resonant ring development at IHEP

The resonant-ring is a traveling wave circuit, which is used to produce high peak power with comparatively smaller stored energy. The application to be considered is its use as a high power simulator mainly for testing the klystron ceramic output window, as well as for high power microwave transmission devices. This paper describes the principle of a resonant ring and introduces the structure and property of the newly constructed traveling wave resonant ring at IHEP. Our goal is to produce a 200 MW class resonant ring at 2.856 GHz with a pulse length of 2 μs and repetition rate of 25 Hz. The installation, commissioning and testing of the ring have been completed and a peak power of 200 MW at 3 μs has been achieved. The conditioning results show that all the parameters of the resonant ring reach the design goals.

Dielectric window breakdown in oxygen gas: Global model and particle-in-cell approach

Dielectric window breakdown is a major issue in the transmission of high power microwave (HPM) radiation. Breakdown at the dielectric window decreases the power transmitted to a target, and if the electron density increases significantly, it can reflect a large fraction of HPM radiation to the source. Dielectric window breakdown from vacuum multipactor to collisional microwave discharge was previously investigated using the particle-in-cell Monte Carlo collision (PIC-MCC) model for noble gases. A global model (GM) with pressure-independent enhanced electron energy distribution function (EEDF) was also developed to study the breakdown in noble gases. In this work, the previous PIC-MCC model and GM are extended to include oxygen cross sections, and used to study microwave breakdown in oxygen. The GM with pressure-independent enhanced EEDF enables more efficient study of parameter space due to its simplicity and speed. The PIC-MCC model and GM indicate that the breakdown time for oxygen is less than that for argon below 500Torr. Above 500Torr, the high collision frequency reduces the effective electron temperature, and the depleted tail leads to faster reduction of ionization than dissociative attachment, and hence longer breakdown times.

Contributing Factors to Window Flashover under Pulsed High Power Microwave Excitation at High Altitude

One of the major limiting factors for the transmission of high power microwave (HPM) radiation is the interface between dielectric-vacuum or even more severely between dielectric-air if HPM is to be radiated into the atmosphere. Surface flashover phenomena which occur at these transitions severely limit the power levels which can be transmitted. It is of major technological importance to predict surface flashover events for a given window geometry, material and power level. When considering an aircraft based high power microwave platform, the effects on flashover formation due to variances in the operational environment corresponding to altitudes from sea level to 50,000 feet (760 to 90 Torr; 1 Torr=133.3 Pa) are of primary interest. The test setup is carefully designed to study the influence of each atmospheric variable without the influence of high field enhancement or electron injecting metallic electrodes. Experimental data of flashover delay times across different materials, such as polycarbonate, Teflonreg, and high density polyethylene as a function of background pressure and gas type, air, N2, argon are discussed. An empirical relationship between flashover field amplitude and delay time is given.