Multipactor Breakdown Research Articles

Multipactor analysis of 431 MHz L-shaped inductive output tube cavity

Gridless inductive output tubes (IOTs) offer compact size and high-power amplification at sub-GHz frequencies. Minimizing cavity dimensions in the interest of compactness leads to smaller gaps, which may cause multipactor discharge under high-power operating conditions. The uncontrolled electron growth resulting from multipactor breakdown can lead to undesired effects including surface damage and system failure. This paper performs a parallel-plate multipactor analysis for a high-Q, L-shaped, aluminum, 431 MHz cavity designed for a gridless IOT to be operated in the MW-power regime. The cavity gap is 27 mm, and diameter is 339 mm. Multipactor susceptibility regions are calculated for non-zero emission energy, half-cycle, and non-half-cycle multipactor using a semi-analytic approach and a standard aluminum secondary electron yield (SEY) curve. The analytical results are validated with particle-in-cell simulation in CST Studio. Simulation results show a voltage range of 6.4–19 kV, compared to the analytically calculated values of 8.2 and 18.3 kV for the lower and upper bounds, respectively. Fluorocarbon coating as a means to reduce secondary electron emission is simulated, which shows 46% reduction in peak particle population with an 11.2 nm PTFE coating, with further reduction as coating thickness increases. The results show that the L-shaped cavity is a suitable choice for this IOT design as it does not exhibit single-surface multipactor and will not develop two-surface multipactor at full-power operation.

Investigation of sheath structure in surface flashover induced by high-power microwave

Flashover is a major limiting factor for the transmission and miniaturization of high-power microwave (HPM) devices. We conducted a study to investigate the developmental process of surface flashover on HPM dielectric windows through particle-in-cell-Monte Carlo collision simulations. A one-dimensional spatial distribution and three-dimensional velocity distribution model is established, encompassing the entire process of surface flashover, which includes electrode field emission, single-surface multipactor, outgassing, and gas breakdown. The nonuniform mesh generation method is employed to enhance the simulation accuracy. The growth rates of electron and ion densities increase as gas pressure rises. Additionally, the discharge transitions gradually from multipactor to gas ionization dominance. Notably, a space-charge-limited (SCL)-like sheath occasionally forms during an rf cycle near the surface under intermediate background pressure (∼0.05 Torr). The SCL-like sheath cannot exist stably. Instead, it periodically disappears and appears as the rf electric field changes. The underlying physics are explained by the variations of the rf electric field, which lead to the variations in the surface charge density, thereby affecting the normal electric field. The normal electric field interacts with the spatial distribution of charged particles, ultimately leading to the formation of the SCL-like sheath. This work may facilitate a comprehensive understanding of the developmental processes of surface flashover.

Effects of external magnetic and electric field on multipactor and plasma breakdown of high-power microwave window

In this work, we investigated the effects of an external magnetic field, a DC electrostatic field, and a normal rf electric field on the multipactor and plasma ionization breakdown process near a microwave window by performing kinetic particle-in-cell/Monte Carlo collision simulations, and the underlying mechanism is also given. The magnetic field, parallel to the surface and perpendicular to the tangential rf field, can effectively suppress the electron multipactor process by delaying the electron incidence on the dielectric window and push the plasma breakdown bulk away from the dielectric window. However, when the magnetic field is too strong, the mitigation effect is not significant, and may even enhance the multipactor process at the beginning of the plasma breakdown. The external DC electrostatic field, perpendicular to the surface, can inhibit electron multipactor when it points toward the surface. On the other hand, when the DC electric field direction is reversed, then the electron multipactor process is found to be promoted, and the gas ionization bulk is closer to the dielectric window. The external normal rf electric fields perpendicular to the surface with small amplitudes are found to be capable of promoting the multipactor process. With increasing the amplitude of normal rf electric field, the multipactor process can be suppressed to some degree at the initial stage of the plasma breakdown and the gas ionization bulk region is kept away from the dielectric window surface.

Investigation on Multipactor in Double-Sided Dielectric-Loaded Microwave Components

This article presents a study of the multipactor effect in a double-sided dielectric-loaded parallel-plate waveguide. To this end, a 1-D electrostatic particle-in-cell model is established, which considers the radio frequency electric fields, space charge fields, and surface charge fields generated by the accumulated charge on the dielectric surface. The dynamic evolution of multipactor breakdown for different operating voltages and dielectric materials with different secondary electron yield (SEY) properties is investigated by numerical calculation. The results obtained show that multipactor does occur self-extinguishing phenomenon due to the presence of the surface charge fields under certain circumstances in the double-sided dielectric-loaded parallel-plate waveguide. Moreover, a comparative study of multipactor between metal, single-sided, and double-sided dielectric-loaded parallel-plate waveguides is carried out. The simulation results show that the self-extinguishing time of the multipactor in the double-sided dielectric-loaded model is less than that in the single-sided dielectric-loaded model. Finally, a multipactor susceptibility diagram for the SEY curve of different bottom dielectric materials in the parallel-plate waveguide is constructed. The results show that the single-sided dielectric-loaded microwave components are more beneficial to suppress the multipactor effect. The results reported in this study can be used to guide the design of space high-power microwave components.

Study of the Multipactor Effect in Groove Gap Waveguide Technology

This article presents a theoretical and experimental comparative study of the different multipactor threshold values obtained in the rectangular waveguide (RW) and the groove gap waveguide (GGW). To this end, the multipactor effect has been first analyzed in an RW with a recently developed theoretical model. Then, the multipactor breakdown power levels in the equivalent GGW have been predicted with an accurate electron tracking code, showing a significant increment compared with the RW case. In order to validate these results, two <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -plane WR-90 RW transformers have been designed with a full-wave electromagnetic simulation tool. The central sections of these transformers have been implemented in RW and GGW, respectively, and their multipactor breakdown power levels have also been predicted. The two designed transformers have been fabricated in aluminum and then measured in terms of electrical response (scattering parameters) and RF multipactor effect (power threshold values). All the experimental results agree well with the set of simulated data, thus fully validating the performed study.

Multipactor experiments on an S-band coaxial test cell

Radio frequency vacuum electronics are prone to multipactor discharges. These electron discharges, driven by secondary electron emission, can disrupt and damage devices and are particularly important in satellite communication systems. We present results from a new S-band coaxial multipactor test cell which demonstrates scaling to much higher frequencies (3.05GHz) than previous coaxial experiments (10-150MHz). The multipactor breakdown threshold has been found to agree very well with our earlier simulated predictions. The significant effect from multipactor self-conditioning has also been demonstrated and characterized. Future experiments will use this test cell to investigate various multipactor mitigation strategies.

A novel low-pass filter based on dielectric impedance inverters to enhance the multipactor breakdown threshold

This work presents a novel low-pass filter based on using dielectric materials, in the critical areas of the structure, to prevent high-power space condition adverse effects, like multipactor breakdown. It is shown that the use of dielectric pieces in the inverters of the filter leads to increased gaps between metallic parts in the irises. In addition, by completely filling up the air space in critical gaps, the multipactor breakdown thresholds can be increased. Moreover, they also lead to a volume reduction, increasing compactness of the structure. To demonstrate the proposed concept, a prototype has been manufactured with an aluminum housing and a Teflon dielectric material, and tested for multipactor effect at ESA-VSC Laboratory. Tests confirmed that the manufactured filter has good electrical performance with cut-off frequency 13.6 GHz, insertion losses 0.32 dB, return losses better than 15 dB, and additionally does not show multipactor breakdown up to the maximum power tested in the experimental facility (5.5 kW).

A Novel Multipactor Suppression Circulator Using Transformation Dielectrics

Due to the multipactor discharge breakdown, ferrite circulators have become the most susceptible component losing efficacy in high-power applications such as satellite payload systems. To date, effective suppression method of multipactor without affecting the electrical performance while keeping a high level of stability during in-orbit operation is still a technological challenge. In this article, a novel method based on the composite dielectric transformation is proposed for the considerable improvement of the multipactor breakdown threshold. The transformation medium is composed of dielectric rings of different permittivity, which could realize the uniform field transformation from the susceptibility region of multipactor to the dielectric/vacuum interface without acute change and discontinuity. In addition, the transformation medium is designed to guarantee little perturbance of the electrical performance and great convenience of fabrication and assembly. The experimental results of the S-band circulator using this method demonstrated that while keeping very low insertion loss and isolation level, this method has improved more than 9 dB (≥850%) of multipactor threshold, showing very promising application potentials in antimultipactor designs for the spacecraft payload system.

Improving the threshold of multipactor using a graded permittivity dielectric window

To suppress the onset of multipactor breakdown on a dielectric surface, a graded permittivity dielectric material is introduced to replace a classical single uniform dielectric material. The electromagnetic field simulation results show that the peak value of the microwave electric field will increase when the center of the microwave window is constructed from a material with a high relative permittivity. However, when the edge of the microwave window is made of such a material, the peak of the microwave electric field will decrease. In addition, the process of multipactor breakdown is investigated using the 2D particle-in-cell method. The results show that the maximum density of electrons that occurs when the edge of the microwave window is constructed from a high-relative-permittivity material is only 66.05% of that which occurs with a uniform dielectric material. As a result, the threshold of multipactor breakdown can be improved. The results reported in this paper can be used to guide the design of microwave windows.

Multipactor Analysis in Circular Waveguides Excited by TM01 Mode

A series of detailed numerical simulations are used to investigate the properties of multipactor breakdown in circular waveguides propagating the TM01 mode. A Monte Carlo model is constructed to track the motion of the electrons, study the multipactor scenarios, and predict the multipactor thresholds. The theoretical and numerical analyses indicate that the product of the frequency and the gap ( ${f}\cdot {D}$ ) affects both the intensity of the ponderomotive force and its spatial distribution, which results from the nonuniformity of the radio frequency (RF) field and significantly influences the electrons’ trajectories and multipactor trends. The decrease in ${f}\cdot {D}$ results in a remarkable enhancement in the magnitude of the ponderomotive force, while the maximal intensity gradually moves toward the half radius R/2 area. Low values of ${f}\cdot {D}$ correspond to high ponderomotive potential, which sustains the short-range electrons and triggers the single-sided multipactor. In contrast, high values of ${f}\cdot {D}$ correspond to low ponderomotive potential, contributing to long-range electrons and exciting the double-sided multipactor. Fitting to the susceptibility diagram produces the border line and a modified ${f}\cdot {D}$ threshold of ( ${f}\cdot {D}$ )th $\approx ~338.4$ GHz mm, which separates the susceptibility diagram into single-sided, double-sided, and mixed-sided zones. The initial electron energy influences their trajectories at high ${f}\cdot {D}$ and low RF power. This effect tends to dominate the multipactor behavior in the mixed-sided region.

Experimental Validation of Multipactor Effect for Ferrite Materials Used in L- and S-Band Nonreciprocal Microwave Components

This paper reports on the experimental measurement of power threshold levels for the multipactor effect between samples of ferrite material typically used in the practical implementation of L- and S-band circulators and isolators. For this purposes, a new family of wideband, nonreciprocal rectangular waveguide structures loaded with ferrites has been designed with a full-wave electromagnetic simulation tool. The design also includes the required magnetostatic field biasing circuits. The multipactor breakdown power levels have also been predicted with an accurate electron tracking code using measured values for the secondary electron yield (SEY) coefficient. The measured results agree well with simulations, thereby fully validating the experimental campaign.

Evolution of multipactor breakdown in multicarrier systems

Many high-power radio frequency communication and navigation systems utilize multicarrier systems. Testing and analyzing such systems for multipactor breakdown have no universal approach, and current practices range in their degree of conservatism. A common approach involves considering the number of signal crossings above the single carrier breakdown threshold. Previous experimental work [Hubble et al., Proceedings of the 9th International Workshop on Multipactor, Corona, and Passive Intermodulation, Noordwijk, The Netherlands (April 2017)] evaluated multicarrier breakdown in two-tone systems and determined a relationship between carrier spacing and required signal crossings to achieve breakdown. This work builds upon the previous effort to include broader carrier spacing and additional tones, as well as the use of innovative center-conductor diagnostics to measure real-time electron population growth and decay.

Multipactor Effect Characterization of Dielectric Materials for Space Applications

The objective of this paper is to advance the state of the art in the characterization of the multipactor effect in dielectric materials. The materials studied are the most commonly used dielectrics in space applications, namely, Alumina, Rexolite, Rogers RT5870, Rohacell, Teflon, and Ultem 1000. In this paper, a new family of coaxial waveguide components, covering the $L$ - and $S$ -bands, with a wideband, low-pass response has been designed, and six different prototypes have been specifically optimized and manufactured. The six prototypes have then been used to simulate and measure the multipactor breakdown susceptibility charts for the six dielectric materials investigated. Finally, the simulation results are compared with the results of the measurement campaign indicating good agreement.

Enhancement of the Multipactor Threshold Inside Nonrectangular Iris

Multipactor breakdown is studied inside the capacitive iris of a rectangular waveguide with a skewed slot along its longitudinal cross section. Both the iris length and height are assumed to be small compared to the electromagnetic wavelength. Therefore, the quasi-static approximation is applied so as to describe the RF field distribution inside the iris gap, whereas a 2-D model is used to analyze the electron motion. The peculiarities of RF field structure are studied using the conformal mapping approach, which shows that the electric field lines can be approximated by circular arcs when the iris length is much larger than its height. The electron motion inside the iris gap is analyzed using different analytical approaches as well as numerical simulations. The simplest analytical consideration is based on the general theory of multipactor between curved surfaces. Within the more sophisticated model, the fringing field effect on electron motion inside the iris is calculated assuming circular structure of electric field lines. It is demonstrated within all approaches that the electron losses within the iris gap increase considerably with the skew angle, deviating from the rectangular iris shape. As a result, the multipactor becomes impossible at a relatively small value of this angle.

Multipactor susceptibility chart of coaxial transmission lines with stationary statistical modeling

Multipactor breakdown is a detrimental electromagnetic phenomenon caused by resonant secondary electron emissions synchronizing with field oscillation, which frequently takes place in powerful microwave devices and accelerating structures. Regarded as the principal failure mode of space microwave systems, multipactor may cause the performance to degenerate or even hardware operation to deteriorate catastrophically, thus multipactor becomes a major limitation in promoting the further development of space communication technology. Meanwhile, higher power capacity and volume integration accordingly lead to continuously growing multipactor hazard. In order to prevent multipactor from occurring, the accurate predictive technique to determine multipactor susceptibility has become a key issue for the mechanical design and performance optimization of microwave devices in the ground stage. Compared with the existing approaches to investigating the multipactor, statistical theories are able to conduct multipactor threshold calculation and mechanism analysis, with the stochastic nature of secondary emission fully considered from the probabilistic perspective. Currently, stationary statistical theory of multipactor has been developed for efficient multipactor threshold analysis of the parallel-plate geometry. However, it has not been further extended to the coaxial geometry which is commonly involved in radio frequency (RF) systems. For this reason, the stationary statistical modeling of the coaxial multipactor with all influencing factors considered is detailed in this paper. Due to the field nonuniformity and the secondary emission randomness, analytic equation of electron trajectories in the coaxial geometry is approximately derived by using the perturbation approach. Based on the implicit correlation between electron emission velocity and transit time, the joint probability density function is constructed for the calculation of the probability density distribution of electron transit time. Afterwards, a system of integral equations for depicting electron multiplication process in the coaxial geometry is formulated and solved with a novel and general iteration method. Finally, this stationary statistical theory is applied to the full multipactor susceptibility chart of coaxial transmission lines with typical coating materials in space engineering, such as silver, copper, alumina and alodine. A comparison shows that the calculation results are in reasonable agreement with the experimental measurements provided by the Europe Space Agent. What is more, there exists significant difference between multipactor susceptibility curves of the parallel-plate geometry and the coaxial geometry. This research is of great significance for optimizing the mechanism design and material selection of multipactor-free microwave devices.

Stationary statistical theory of two-surface multipactor regarding all impacts for efficient threshold analysis

Statistical multipactor theories are critical prediction approaches for multipactor breakdown determination. However, these approaches still require a negotiation between the calculation efficiency and accuracy. This paper presents an improved stationary statistical theory for efficient threshold analysis of two-surface multipactor. A general integral equation over the distribution function of the electron emission phase with both the single-sided and double-sided impacts considered is formulated. The modeling results indicate that the improved stationary statistical theory can not only obtain equally good accuracy of multipactor threshold calculation as the nonstationary statistical theory, but also achieve high calculation efficiency concurrently. By using this improved stationary statistical theory, the total time consumption in calculating full multipactor susceptibility zones of parallel plates can be decreased by as much as a factor of four relative to the nonstationary statistical theory. It also shows that the effect of single-sided impacts is indispensable for accurate multipactor prediction of coaxial lines and also more significant for the high order multipactor. Finally, the influence of secondary emission yield (SEY) properties on the multipactor threshold is further investigated. It is observed that the first cross energy and the energy range between the first cross and the SEY maximum both play a significant role in determining the multipactor threshold, which agrees with the numerical simulation results in the literature.

Experimental demonstration of improving resonant-multipactor threshold by three-dimensional wavy surface

A proof-of-principle experiment is presented demonstrating the suppression of multipactor breakdown in a coaxial multipactor device with three-dimensional periodic wavy surfaces. By changing the power and pulse width of the microwave source, threshold behavior near breakdown was obtained for this wavy-surface structure and a smooth-surface structure used for comparison. With a wide pulse width at a suitable power, the coefficient of reflection for the smooth-surface structure was found to increase, whereas the coefficient of transmission decreased. For the wavy-surface structure, a similar behavior appeared, only when the microwave pulse had a width of order of a few seconds. Accompanied by changes in transmission power characteristics, distinct increases in the second and third harmonic components were evident for the smooth-surface structure. These experimental results demonstrate that the wavy-surface structure effectively suppresses multipactor breakdown with the suppression increasing with the pulse width.

Multipactor Breakdown Threshold Reduction Due to Magnetic Confinement in Parallel Fields

In certain high-power RF devices, such as isolators and circulators, a static magnetic field is present in parallel with the oscillating RF electric field. This parallel magnetic field imposes electron Larmor motion, which restricts the trajectories of electrons accelerated by the RF field. In open geometries, magnetic confinement reduces electron losses in the multipactor region and can reduce the breakdown threshold. In this paper, the effect of magnetic confinement on multipactor breakdown threshold was measured experimentally using a stripline fixture with an externally applied uniform magnetic field. Multipactor threshold power was reduced from the unmagnetized case by as much as 3 dB at magnetic fields of about 0.5 kG. Multipactor gap aspect ratio has a significant impact on the degree of threshold reduction.

Center conductor diagnostic for multipactor detection in inaccessible geometries.

Electron collecting current probes are the most reliable diagnostic of multipactor and radiofrequency (RF) ionization breakdown; however, stand-alone probes can only be used in test setups where the breakdown region is physically accessible. This paper describes techniques for measuring multipactor current directly on the center conductor of a coaxial RF device (or more generally, on the signal line in any two-conductor RF system) enabling global multipactor detection with improved sensitivity compared to other common diagnostics such as phase null, third harmonic, and reflected power. The center conductor diagnostic may be AC coupled for use in systems with a low DC impedance between the center conductor and ground. The effect of DC bias on the breakdown threshold was studied: in coaxial geometry, the change in threshold was <1 dB for positive biases satisfying VDC/VRF0<0.8, where VRF0 is the RF voltage amplitude at the unperturbed breakdown threshold. In parallel plate geometry, setting VDC/VRF0<0.2 was necessary to avoid altering the threshold by more than 1 dB. In most cases, the center conductor diagnostic functions effectively with no bias at all-this is the preferred implementation, but biases in the range VDC=0-10V may be applied if necessary. The polarity of the detected current signal may be positive or negative depending on whether there is net electron collection or emission globally.

CNES – Chalmers – IAP – ONERA - XLIM activities in the domain of high RF power breakdown phenomena

Multipactor breakdown is an important potential failure mechanism in many different microwave devices working under close to vacuum conditions. Applications range from space borne RF equipment to high-power microwave generators. The basic physics involved in the multipactor phenomenon is well known for the case of two infinite pallel plates made of metal. However, most realistic RF device geometries involve inhomogeneous RF electric fields and curved field lines and sometimes also dielectric material. The purpose of this paper is to set up methodologies to determine the Multipactor threshold in such situations.