Tiancun Hu

Secondary electron emission from rough metal surfaces: a multi-generation model

We develop a multi-generation model to examine secondary electron emission (SEE) from a rough metal surface. In this model, the traces of both primary electrons (PEs) and secondary electrons (SEs) are tracked by combining the electron scattering in the material and the multi-interaction with the rough surface. The effective secondary electron emission yield (SEY) is then obtained from the final states of the multi-generation SEs. Using this model, the SEE properties of the surfaces with rectangular and triangular grooves have been examined. We find that a rectangular groove can be used for effective SEE suppression. For a triangular groove, the criterion of SEY enhancement/suppression has been achieved, indicating that a small groove angle is required for effective SEE suppression, especially for a high PE energy. Furthermore, the SEE properties for some random rough surfaces are examined and some preliminary results are presented. Accordingly, our model and results could provide a powerful tool to give a comprehensive insight into the SEE of rough metal surfaces.

Effect of rough surface morphology on secondary electron emission from metal surface

Surface morphology is one of the main factor affecting the secondary electron (SE) emission from a rough metal surface. To overcome the limitation of only using the roughness to reveal the SE emission properties of a rough surface morphology, the fluctuation correlation length, which represents the spatial frequency of surface fluctuation, is thus introduced. In addition, the effects of the rough surface morphology on SE emission properties from the metal surface, which considers both the surface roughness and the fluctuation correlation length, are examined in this work. On the basis of the mechanism of electron interaction with morphology including the shading, multigeneration, and oblique effects, the SE emission properties versus the rough surface morphology, primary electron energy, and incident angle can be reasonably explained. The results further reveal the effect of surface morphology on SE emission, which gives a comprehensive insight into the control of SE emission properties using surface morphology.

Monte Carlo Analysis of Occurrence Thresholds of Multicarrier Multipactors

The newly discovered “long-term” multicarrier multipactors that can be induced by low-energy electric fields potentially threatens the reliability of spaceborne microwave systems. It also causes a new technical challenge to the prediction of the global occurrence threshold of multicarrier multipactors. In this paper, we propose efficient approaches to rapidly compute the fluctuation of space electrons in the evolving process of a multicarrier multipactor, based on which Monte Carlo approaches are developed to efficiently find the global “worst case” waveforms and the occurrence thresholds for both “single-event” and “long-term” multipactors. The proposed approaches can be used for wideband devices, providing an efficient integrated solution for the multicarrier multipactor analysis for practical high-power microwave devices with inhomogeneous field distribution.

An efficient multipaction suppression method in microwave components for space application

Multipaction, caused by the secondary electron emission phenomenon, has been a challenge in space applications due to the resulting degradation of system performance as well as the reduction in the service life of high power components. In this paper we report a novel approach to realize an effective increase in the multipaction threshold by employing micro-porous surfaces. Two micro-porous structures, i.e., a regular micro-porous array fabricated by photolithography pattern processing and an irregular micro-porous array fabricated by a direct chemical etching technique, are proposed for suppressing the secondary electron yield (SEY) and multipaction in components, and the benefits are validated both theoretically and experimentally. These surface processing technologies are compatible with the metal plating process, and offer substantial flexibility and accuracy in topology design. The suppression effect is quantified for the first time through the proper fitting of the surface morphology and the corresponding secondary emission properties. Insertion losses when using these structures decrease dramatically compared with regular millimeter-scale structures on high power dielectric windows. SEY tests on samples show that the maximum yield of Ag-plated samples is reduced from 2.17 to 1.58 for directly chemical etched samples. Multipaction testing of actual C-band impedance transformers shows that the discharge thresholds of the processed components increase from 2100 W to 5500 W for photolithography pattern processing and 7200 W for direct chemical etching, respectively. Insertion losses increase from 0.13 dB to only 0.15 dB for both surface treatments in the transmission band. The experimental results agree well with the simulation results, which offers great potential in the quantitative anti-multipaction design of high power microwave components for space applications.

Fabrication of Porous Ag/TiO 2 /Au Coatings with Excellent Multipactor Suppression

Porous Ag/TiO2/Au coatings with excellent multipactor suppression were prepared by fabrication of porous Ag surface through two-step wet chemical etching, synthesis of TiO2 coatings by electroless-plating-like solution deposition and deposition of Au coatings via electroless plating. Porous structure of Ag surface, TiO2 coatings on porous Ag surface and Au coatings on porous Ag/TiO2 surface were verified by field-emission scanning electron microscopy, the composition and crystal type of TiO2 coatings was characterized by X-ray photoelectron spectroscopy and X-ray diffraction. Secondary electron yield (SEY) measurement was used to monitor the SEY coefficient of the porous Ag coatings and Ag/TiO2/Au coatings. The as-obtained porous Ag coatings were proved exhibiting low SEY below 1.2, and the process was highly reproducible. In addition, the porous Ag/TiO2/Au coatings showed excellent multipactor suppression with the SEY 1.23 and good environmental stability. It is worth mentioning that the whole preparation process is simple and feasible, which would provide a promising application in RF devices.

Prediction of passive intermodulation between rough waveguide flanges based on fractal theory

Passive intermodulation (PIM) products are spurious frequency signals generated by nonlinear components and devices. It has an important impact on the performance of multi-carrier communication system in satellites. System performance can be severe degraded once the passive intermodulation products are also transmitted in the reception band.

Passive intermodulation analysis for mesh antennas

Passive Intermodulation (PIM) becomes a serious electromagnetic interference due to the development of high-sensitivity microwave communication systems, especially large deployable mesh antennas. In this paper, a 3D geometrical model which can describe the precise metal-metal contact is established. The metal-metal contact is described by a nonlinear equivalent circuit which is regarded as PIM source. Then the contact current is scattered to far-field, and PIM value of one meter quadrate mesh sample is evaluated, which agrees with the experiment result.

A complex wire used for mesh reflector antenna

Large reflector antenna contains a large number of metal-insulator-metal (MIM) junctions. When illuminated by high-power microwave radiation, passive intermodulation (PIM) products may disturb satellite communication system. The general materials used for space-borne antenna are stainless steel coated with nickel, gold-plated tungsten wire, gold-plated molybdenum wire etc. This paper presents a new complex wire, produced by gold-plated molybdenum wire and polyimide fiber. Passive intermodulation risk can be reduced, due to the improvement of the thermal stability of the wire gauze. The test results show that the mechanical properties of the complex wire meet the requirement of mesh antenna.

Improvement multipactor discharge of microwave components by micro-porous surface

A method for the improvement multipactor discharge of microwave components is described in this paper. It is a kind of surface treatments with regular micor-porous structure by the photolithography technique or irregular micro-porous structure by chemical etching process on silver plated coatings. Their influences of the micro-porous structure for secondary emission are compared. A C-band impedance transformer is used to valid multipactor discharge level by three-dimensional simulations and experimental results. Results show that this method can improve greatly the multipactor discharge of microwave components in theory and experiments.

Analysis of ridge waveguide with substrates of metamaterials with zero index of refraction

In this paper, based on peculiar electromagnetic characteristics of metamaterials, peculiar properties of ridge waveguide with substrates of metamaterials with zero index of refraction are analyzed. These results provide some interested insights for potential applications.