Wanzhao Cui

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.

Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory

This paper presents a statistical theory for the initial onset of multipactor breakdown in coaxial transmission lines, taking both the nonuniform electric field and random electron emission velocity into account. A general numerical method is first developed to construct the joint probability density function based on the approximate equation of the electron trajectory. The nonstationary dynamics of the multipactor process on both surfaces of coaxial lines are modelled based on the probability of various impacts and their corresponding secondary emission. The resonant assumption of the classical theory on the independent double-sided and single-sided impacts is replaced by the consideration of their interaction. As a result, the time evolutions of the electron population for exponential growth and absorption on both inner and outer conductor, in response to the applied voltage above and below the multipactor breakdown level, are obtained to investigate the exact mechanism of multipactor discharge in coaxial lines. Furthermore, the multipactor threshold predictions of the presented model are compared with experimental results using measured secondary emission yield of the tested samples which shows reasonable agreement. Finally, the detailed impact scenario reveals that single-surface multipactor is more likely to occur with a higher outer to inner conductor radius ratio.

Exploration of the relationship between emitted energy spectrum and multipactor threshold

The relationship between emitted energy spectrum and multipactor threshold of impedance transformer is investigated numerically in this paper. Secondary electron yield remains the same, while the emitted energy spectrum changes. Numerical simulations are provided for different cases of emitted energy spectrum. We analyze the practicability of this method in theory and simulation. Simulation results demonstrate that multipactor threshold increase when the proportion of electrons in the low energy of emitted energy spectrum increases simultaneously. The conclusion of this simulation provides the direction of changing emitted energy spectrum for improving the multipactor threshold.

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.

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.

Generation of Coherent Multicarrier Signals for the Measurement of Multicarrier Multipactor

Many radio frequency measurement systems require multicarrier signals with coherent phases between carriers with different frequencies. A typical example is the signal generator which is used in multicarrier multipactor (MM) testing systems. In this paper, we propose a new approach for the generation of coherent multicarrier signals based on a state-of-the-art phase-locked loops technique. Different from existing systems that rely on a heavy use of commercial microwave instruments, this generator can be implemented with on-shelf chips and devices. As an example, an ultrahigh frequency band generator is implemented and how to implement the in-system calibration of this generator in an experimental multipactor testing system is demonstrated. Based on the proposed approach, performing a quantitative experimental research on MM becomes affordable for an ordinary microwave laboratory, which may promote the in-depth experimental study in this area. It can also be used in a wide range of applications such as MIMO and phased array systems.

A general route towards secondary electron yield suppression based on graphene

Secondary electron yield suppression is of great importance for high power microwave components in satellite. Great efforts have been made to find an easy and effective way to suppress the secondary electron emission process. However, its still a challenge. In this paper we report a new approach for secondary electron emission suppression which can realize reducing the secondary electron yield from 2.12 to 1.09. Compared with the traditional techniques, this approach makes use of graphene which is deposited by remote plasma enhanced chemical vapor deposition on the surface of the components to realize the suppression of the secondary emission. Raman spectroscopy was used to character the quality of graphene. The nano-membrane of grapheme further enhances the suppression effect and contributes little to the insertion loss of the surface. Our technique shows great potential in space applications.

The secondary electron facility at China Academy of Space Technology (Xi'an)

The secondary electron emission characteristics of the space metal material are the important parameters to determine the surface charge rate and charge balance potential of the spacecraft. And it is also an important basis for material selection, analysis and design of high power microwave components of space vehicle. In order to accurately measure the secondary electron emission characteristics of space metal materials, the Academy of Space Technology has been developing a secondary electron facility with good performance and mature technology. The multi-functional test-stand is designed and constructed to evaluate characteristics of secondary electron emission of metals in ultrahigh vacuum, such as its emission yield, its energy spectrum and XPS. At present, this facility allows working in a large energy range (between 20eV up to 30KeV), a large temperature range (down to 0°C and up to 400°C), a variety of radiation sources (electrons, photons and ions), and at high vacuum level (10−8Pa). This paper mainly introduces the measurement principles and structures of the secondary electron emission characteristics of the platform, and presents a series of typical test results on the basis of the principles and functions of the test.

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.