Microwave Breakdown Research Articles

Microwave breakdown of low pressure N2 gas in microgaps

The pressure dependence of the breakdown of nitrogen gas by a microwave field in microgaps down to 13 μm has been measured. Fits to a phenomenological function suggest that microwave breakdown in small gaps requires a higher collision frequency, νc. If the effective electric field on an electron in a microgap is interpreted as Eeff=Eoνc2/(νc2+ω2) and then as the gap size is reduced, we find that the collision frequency needed to produce a minimum breakdown electric field shifts from νc = ω for large gaps to νc = 2ω for small gaps.

Modeling statistical variations in high power microwave breakdown

Flashover of high power microwave (HPM) vacuum isolation windows presents a serious design limitation of megawatt class HPM systems. The delay time from HPM radiation incident on the window to flashover development on the atmospheric side is critical. Previously developed modeling efforts have yielded reasonable correlation with experimentally observed average delay times while failing to capture any statistical variations. Simply preseeding the volume with an initial electron density is identified as inadequate to describe the source of initiatory electrons. The process of field assisted electron detachment is examined and shown to be a probable candidate for the initiatory electron generation.

Ionization–diffusion plasma front propagation in a microwave field

A 1D model of the expansion of a collisional plasma under the combined effect of diffusion and ionization is presented. It is shown that a simple quasi-neutral model of the plasma using an effective diffusion coefficient can accurately describe the plasma front propagation. The effective diffusion coefficient describes the transition from free electron diffusion in the plasma front to ambipolar diffusion in the bulk. Comparisons with ‘exact’ solutions from a drift-diffusion–Poisson model show excellent agreement not only in the simple case of a constant ionization frequency, but also when the plasma front propagation is due to microwave breakdown. In the latter case the plasma model is solved together with Maxwell's equations and the ionization frequency in the front is modulated in time due to the formation of standing waves in the plasma front region, leading to the formation of plasma patterns. The effect of electron–ion recombination on the observed plasma pattern is discussed.

线极化高功率微波双工器空间谐波的选择性

A novel diplexer for the coupling output of GW level microwaves has been investigated, which consists of an array of periodically spaced cylindrical rods. The factors influencing the selection of spatial harmonics for the diplexer were researched, and some important rules for the design of the diplexer were obtained. A cylindricalrod diplexer for the coupling output of S/X band GW level microwaves was designed accordingly. The low power experiments revealed that, its S-band reflection and X-band transmission efficiencies were both higher than 97%. In further high power experiments, the diplexer was operated with microwaves of 1.8 GW and 80 ns, and no microwave breakdown was observed.

Investigation of the delay time distribution of high power microwave surface flashover

Characterizing and modeling the statistics associated with the initiation of gas breakdown has proven to be difficult due to a variety of rather unexplored phenomena involved. Experimental conditions for high power microwave window breakdown for pressures on the order of 100 to several 100 torr are complex: there are little to no naturally occurring free electrons in the breakdown region. The initial electron generation rate, from an external source, for example, is time dependent and so is the charge carrier amplification in the increasing radio frequency (RF) field amplitude with a rise time of 50 ns, which can be on the same order as the breakdown delay time. The probability of reaching a critical electron density within a given time period is composed of the statistical waiting time for the appearance of initiating electrons in the high-field region and the build-up of an avalanche with an inherent statistical distribution of the electron number. High power microwave breakdown and its delay time is of critical importance, since it limits the transmission through necessary windows, especially for high power, high altitude, low pressure applications. The delay time distribution of pulsed high power microwave surface flashover has been examined for nitrogen and argon as test gases for pressures ranging from 60 to 400 torr, with and without external UV illumination. A model has been developed for predicting the discharge delay time for these conditions. The results provide indications that field induced electron generation, other than standard field emission, plays a dominant role, which might be valid for other gas discharge types as well.

Pattern formation and propagation during microwave breakdown

During microwave breakdown at atmospheric pressure, a sharp plasma front forms and propagates toward the microwave source at high velocities. Experiments show that the plasma front may exhibit a complex dynamical structure or pattern composed of plasma filaments aligned with the wave electric field and apparently moving toward the source. In this paper, we present a model of the pattern formation and propagation under conditions close to recent experiments. Maxwell’s equations are solved together with plasma fluid equations in two dimensions to describe the space and time evolution of the wave field and plasma density. The simulation results are in excellent agreement with the experimental observations. The model provides a physical interpretation of the pattern formation and dynamics in terms of ionization-diffusion and absorption-reflection mechanisms. The simulations allow a good qualitative and quantitative understanding of different features such as plasma front velocity, spacing between filaments, maximum plasma density in the filaments, and influence of the discharge parameters on the development of well-defined filamentary plasma arrays or more diffuse plasma fronts.

Computational Studies of Filamentary Pattern Formation in a High Power Microwave Breakdown Generated Air Plasma

Simulations of the dynamics of high power microwave breakdown of air at atmospheric pressure and 110 GHz are presented. The model reproduces well the formation and motion of filamentary plasma arrays observed experimentally with fast camera imaging. The numerical model is based on finite-difference time domain solutions of Maxwell equations coupled with a simple fluid description of the plasma growth and diffusion. The computational procedure is discussed in details along with numerical experiments, to show the sensitivity of the results to different numerical parameters.

Microwave Multipactor Breakdown Between Two Cylinders

An analysis has been made of the microwave breakdown threshold for multipactor in an open structure comprising two parallel cylinders, approximating, e.g., parts of a helix antenna. The electron motion in the corresponding electromagnetic field is analyzed by separating the motion into a slowly varying drift velocity (driven by the ponderomotive force due to the electric field inhomogeneity) and a rapidly oscillating part (driven by the oscillating electric field). Furthermore, the curvature of the cylindrical surfaces of emission is shown to give rise to a new effect that implies a loss of electrons. This leads to a more stringent multipactor breakdown condition for the two-wire structure than for the classical situation corresponding to the case of two plane parallel infinite plates. The importance of this effect is determined by the ratio of the cylinder radii and the distance between the cylinders, and it is shown that when this ratio is small, multipactor can only occur for surfaces having very large secondary emission coefficients. A detailed analysis is also made to determine the lowest voltage between the cylinders at which multipactor becomes possible.

On the microwave breakdown stability of a spherical hot spot in air

An analysis is made of the basic physical conditions under which a small local microwave-induced breakdown region in a gas may develop to extended ‘global’ breakdown. The analysis describes the different nonlinear stages of the microwave breakdown process. In the first stage, the increasing breakdown plasma density suppresses the electric field in the breakdown region to reach a quasi-stationary state with constant electron density. The subsequent heating of the gas due to absorption of microwave power in the breakdown plasma is then analysed and the corresponding steady state for the thermal evolution is found including the temperature dependence of the breakdown electric field. The stability properties of the stationary state are examined and it is found that there exists a critical (unstable) radius of the initial breakdown plasma region such that initial regions smaller than this critical dimension will shrink to ultimately vanish whereas plasma regions larger than the critical dimension will grow indefinitely and transform the local breakdown region into full scale ‘global’ breakdown. The practical implication of this model is to give an order of magnitude estimate for the critical size of hot spots, regions of enhanced field and intensified heating in rf systems.

Microwave Breakdown of Air by Nanosecond and Subnanosecond Ka-Band Pulses

The initial stage of atmospheric air breakdowns in Ka-band pulsed microwave fields was observed for the microwave power increasing to a maximum within a characteristic time of ~300 ps. Pulsed microwaves were produced by relativistic BWOs with output powers of ~170 and ~500 MW (4-ns and 300-ps FWHMs, respectively). The effects of air breakdown in the fields of microwave pulses of this type are pronounced when the radiation is extracted through the vacuum window of a horn antenna and when it is channeled in passing through waveguides and quasi-optical lines. Of particular interest are modes in which repetitive pulsed microwaves are generated.

Single and repetitive short-pulse high-power microwave window breakdown

The mechanisms of high-power microwave breakdown for single and repetitive short pulses are analyzed. By calculation, multipactor saturation with electron density much higher than the critical plasma density is found not to result in microwave cutoff. It is local high pressure about Torr class that rapid plasma avalanche and final breakdown are realized in a 10–20 ns short pulse. It is found by calculation that the power deposited by saturated multipactor and the rf loss of protrusions are sufficient to induce vaporizing surface material and enhancing the ambient pressure in a single short pulse. For repetitive pulses, the accumulation of heat and plasma may respectively carbonize the surface material and lower the repetitive breakdown threshold.

Experimental study of a multipactor discharge on a dielectrics surface in a high-Q microwave cavity

Results from experimental studies of multipactor discharges on the surfaces of various dielectrics placed in a high-Q cylindrical microwave cavity excited at the TE013 mode in the X-band are presented. The thresholds for the onset and maintenance of a multipactor discharge on quartz, polycrystalline diamond, lithium fluoride, and Teflon surfaces possessing different roughness are determined. It is shown that, in such a resonance system, a steady multipactor discharge can operate without transition into the stage of microwave breakdown of the desorbed gas. It is found that, due to long-term action of the discharge, a thin carbon-containing film is deposited on the dielectric surface, which leads to an increase in the breakdown threshold.

Combining microwave beams with high peak power and long pulse duration

The beam combining results with a metal dichroic plate illuminated by the S/X band gigawatt level high power microwaves are presented. According to the previous experiments, the microwave breakdown problem becomes obvious when the peak power and the pulse duration increase, thus, several methods for enhancing the power handling capacity have been considered, and the metal dichroic plates are redesigned to handle the S/X band high power microwaves. Then the design, fabrication, and testing procedure are discussed in detail. The further experimental results reveal that, operated on the self-built accelerator Spark-04, the radiated powers from the S and X band sources have reached 1.8 GW with pulse durations of about 80 ns, and both beams have been successfully operated on the selected dichroic plate without microwave breakdown.

Microwave breakdown in waveguide filters, numerical simulation, and comparison to experiments

Microwave breakdowns in output multiplexer filters, boarded on telecommunications satellites, can affect the device and damage it. During tests at atmospheric pressure, these breakdowns are generally initiated in the vicinity of a noble metal coated frequency adjustment screw. A modeling of the microwave breakdown ignition is developed taking into account the thermoelectronic emission from the screw and the noble metal atom vaporization. The influence of field strength is particularly investigated to predict the breakdown delay after the application of the microwave power. Then, a specific structure has been designed and built and a dedicated measurement procedure has been proposed. As it will be shown, the test structure experimental behavior agrees well with the theoretical one. Finally, the results given by the numerical modeling of microwave breakdown are compared to the measurements performed in a five-pole filter of an output multiplexer.

Theory and Modeling of Self-Organization and Propagation of Filamentary Plasma Arrays in Microwave Breakdown at Atmospheric Pressure

High power microwave breakdown at atmospheric pressure leads to the formation of filamentary plasma arrays that propagate toward the source. A two-dimensional model coupling Maxwell equations with plasma fluid equations is used to describe the formation of patterns under conditions similar to recent experiments and for a wave electric field perpendicular to the simulation domain or in the simulation domain. The calculated patterns are in excellent qualitative agreement with the experiments, with good quantitative agreement of the propagation speed of the filaments. The propagation of the plasma filaments is due to the combination of diffusion and ionization. Emphasis is put on the fact that free electron diffusion (and not ambipolar diffusion) associated with ionization is responsible for the propagation of the front.

Preliminary investigation on the design and experiment of a spatial filter for dual band high power microwave

A spatial filter/diplexer for gigawatt level high power microwave is theoretically and experimentally investigated in this paper. The spatial filter consists of several cylindrical rods which are located in the near field of the applied radiating antenna. The S/X dual band microwaves are reflected and transmitted by the spatial filter in the same propagation directions. In the low power experiments, the transmitted and the reflected patterns are very close to that of the applied antenna. The measured reflection and transmission efficiencies, calculated from the power densities in the far-field region, are about 96% and 97% respectively. In addition, the spatial filter has the isolation level higher than 25 dB and the cross polar level much lower than -25 dB. Under the illumination by S/X band high power microwave with a pulse magnitude of about 1.5 GW and duration of about 100 ns, no microwave breakdown is observed.

不同气压下介质表面高功率微波击穿的数值模拟

不同气压下介质表面高功率微波击穿的数值模拟

Electromagnetic design of a novel microwave internal combustion engine ignition source, the quarter wave coaxial cavity igniter

New ignition sources are needed to operate the next generation of lean high-efficiency internal combustion engines. A significant environmental and economic benefit could be obtained from these lean engines. Towards this goal, the quarter wave coaxial cavity resonator (QWCCR), igniter was examined. A brief overview of the QWCCR's history is presented, followed by an electromagnetic analysis of the resonator. Microwave and radio-frequency gas breakdown at atmospheric and higher conditions are reviewed briefly. The presented analysis focuses on geometric and material parameter relations compared with performance characteristics, such as resonator quality factor and developed tip electric field which initiates a single-electrode high-pressure microwave gas breakdown. Conductor surface losses, dielectric losses, and radiation losses of the device are considered in the analysis, for which parameters of a prototype QWCCR were chosen for construction and evaluation of a resonator. Improved designs allowed successful integration of a QWCCR into a Briggs and Stratton engine. Given the capability of precise control, high power, and high energy delivery, and the opportunity for further optimization, the QWCCR has the potential to deliver more energy per ignition than a conventional spark plug and thus should be considered for application as a lean ignition source.

Theory of Filamentary Plasma Array Formation in Microwave Breakdown at Near-Atmospheric Pressure

Recently reported observations of filamentation during high power microwaves breakdown of near-atmospheric pressure gas are explained using a one-dimensional fluid model coupled to a theoretical wave-plasma model. This self-consistent treatment allows for time-dependent effects, plasma growth and diffusion, and partial absorption and reflection of waves. Simulation results, consistent with experiments, show the evolution of the plasma filaments spaced less than one-quarter wavelength, the sequential discrete light emission propagating back toward the source, and the diffusion and decay of the plasma. The model allows examination of many features not easily obtained experimentally, including dependence on field strength and frequency, pressure, and gas composition, which influence the breakdown and emission properties, including the spacing and speed of propagation of the filaments.

MO‐D‐BRD‐01: Pushing the Limits of Microwave Linear Accelerators

We will review recent advances for ultra‐high gradient‐linac. We will show how the research on the basic phenomena of microwave breakdown in ultra‐high vacuum structure, led to novel geometrical designs that allow electron linac to achieve gradients in excess of 140 MV/m. Also, with the usage of copper alloys, one would hope to push this gradient to 180 MV/m. these advances can be applied to proton linacs. We will present suggested designs for future proton linacs, which would push the gradient to more than 70 MV/m. These will operate at high frequency in the X/Ku band. With this technology one hopes to design compact proton therapy machines at energies of ∼ 250 MeV, with fine control of energy. The compactness of the linac might allow the whole system to be installed on a single gantry.