Yongning He

Suppression of secondary electron yield by micro-porous array structure

We study secondary electron yield (SEY) suppression for metal materials using a roughened surface with a micro-porous array. First, we perform a Monte Carlo simulation of the electron trajectory in a single cylindrical well using a phenomenological model of secondary electron emission and the SEY suppression efficiency of a micro-porous array. The simulation results show that the SEY of a roughened surface is affected significantly by the aspect ratio of the micro-pores and the surface porosity of the metal plate. Then, to verify the simulation results, we produce a micro-porous array on metal plates using photolithography and measure their SEYs. We show that the micro-porous array structure can efficiently suppress the SEY of metal materials, and the measurements agree quantitatively with the corresponding simulation results. Finally, we derive an analytical formula to evaluate easily the SEY suppression efficiency of the Ag micro-porous array. In total, the micro-porous array proposed in this paper offers an alternative to SEY suppression in related areas such as multipactor effects in satellite payloads or electron cloud effects in accelerators.

Enhanced Performance of a Self‐Powered Organic/Inorganic Photodetector by Pyro‐Phototronic and Piezo‐Phototronic Effects

Self-powered photodetectors (PDs) have long been realized by utilizing photovoltaic effect and their performances can be effectively enhanced by introducing the piezo-phototronic effect. Recently, a novel pyro-phototronic effect is invented as an alternative approach for performance enhancement of self-powered PDs. Here, a self-powered organic/inorganic PD is demonstrated and the influences of externally applied strain on the pyro-phototronic and the photovoltaic effects are thoroughly investigated. Under 325 nm 2.30 mW cm-2 UV illumination and at a -0.45% compressive strain, the PDs photocurrent is dramatically enhanced from ≈14.5 to ≈103 nA by combining the pyro-phototronic and piezo-phototronic effects together, showing a significant improvement of over 600%. Theoretical simulations have been carried out via the finite element method to propose the underlying working mechanism. Moreover, the pyro-phototronic effect can be introduced by applying a -0.45% compressive strain to greatly enhance the PDs response to 442 nm illumination, including photocurrent, rise time, and fall time. This work provides in-depth understandings about the pyro-phototronic and the piezo-phototronic effects on the performances of self-powered PD to light sources with different wavelengths and indicates huge potential of these two effects in optoelectronic devices.

Investigation into anomalous total secondary electron yield for micro-porous Ag surface under oblique incidence conditions

We study the dependence of total secondary electron yield (SEY) of micro-porous silver surface on incident angle of primary electron. First, we produce regular and random micro-porous surfaces on silver plated aluminium samples by photolithography pattern process and direct chemical etching method, respectively. Second, we study the dependence of the SEY characteristics of these surfaces on the primary electron incident angle from 0° to 60°. The experimental results show that micro-porous surface with large porosity can suppress SEY effectively either for normal incidence or oblique incidence. The maximum SEY of the directly chemical etched sample can reach a 45% reduction relative to the original flat sample. We also show the anomalous experimental phenomenon that the SEY of directly chemical etched sample with large porosity does not increase with incident angle. Third, we explain the complicated SEY dependence on incident angle qualitatively by the secondary electron trapping effect in micro-pores as well as the Monte Carlo simulation based on phenomenological model of secondary electron emission. Finally, we suggest a third order polynomial formula to describe the SEY dependence on incident angle which will be useful in particle simulation. In total, we demonstrate that micro-porous surface with large porosity is effective in SEY suppression under both normal and oblique incidence and thus the proposed direct chemical etching method is prospective in related areas such as multipactor improvement.

Thermal evaporated hyperbranched Ag nanostructure as an effective secondary-electron trapping surface coating

We study secondary electron yield (SEY) suppression of silver using a hyperbranched nanostructure obtained by thermal evaporation. First, we perform thermal evaporation at different residual gas pressures for studying the influence of pressure on surface morphologies. A self-assembled hyperbranched Agnanostructure has been achieved at 100 Pa. Then, we further investigate the detailed formation process of the self-assembled hyperbranched Agnanostructure qualitatively and find it to be dominated by “screening effect”. Finally, we study the obvious SEY suppression effect of this special structure. We show that 100 Pa is the best process condition within our experimental scope from the SEY suppression point of view. It exhibits maximum SEY (δmax) of ∼0.9. We also show that the combining of this nanostructure with the micro-porous surface we developed before can further improve its SEY suppression effect which leading to a δmax of ∼0.8. We propose a novel 2D rectangular-hemisphere hybrid trap model to perform numerical simulation of secondary electron dynamics for interpretation of the experimental results. In total, this work provides guidance to controllable preparation of low SEY metallic surfaces for potential applications in particle accelerators, RF microwave components and satellite systems.

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.

Positive bias and vacuum chamber wall effect on total electron yield measurement: A re-consideration of the sample current method

The measurement of the total secondary electron yield (TEY, δ) is of fundamental importance in areas such as accelerator, spacecraft, detector, and plasma system. Most of the running TEY facilities in the world are based on the kind of bias strategy. The applied bias can assist in the collection of the secondary/primary electrons. In the prevailing sample current method, the TEY is obtained by the measurement of the current from the sample to ground with a negative/positive bias applied to the sample. One of the basic assumptions in this method is that the positive bias can retain most of the electrons emitted by the sample. This assumption is generally recognized based on the seeming fact that the low energy secondary electrons dominate the emitted electrons. In this work, by considering the full electron energy spectrum including both the true secondary and backscattered electrons, we give a new insight in this TEY measurement method. Through the analytical derivation as well as the Particle-in-Cell num...

In Situ Test of Thickness and Sheet Resistance of Conductive Nanomaterial Using Microwave Cavity

Both thickness and sheet resistance of conductive nanomaterial deposited on glass substrate have been simultaneously measured using the TE011 mode cavity method. Measurements have been performed in two cavities with resonant frequency of ~10.8 and ~17.7 GHz, respectively. Measurement errors of thickness and sheet resistance can reach below 10%. Compared to existing methods, the presented method does not require prior knowledge on either film thickness or conductivity; what is more, by using the film as one of the end plates of the cavity, this method is noncontact and has little requirements on sample shape. These advantages suggest that the presented method has potential to be used in situ of monitoring the metal film characteristics such as that in microelectronic fabrication industry.

Empirical equation for carrier power dependence of passive intermodulation product

This paper proposes an empirical equation for input carrier power level dependence of 3rd order reflected passive intermodulation (PIM) product. Three kinds of microwave passive devices, namely, directional coupler, N type coaxial adapter and microstrip line, are measured by the PIM test setup in the 900MHz band. To obtain systematical experimental results, the power levels of the two carriers are tuned in the following three ways: 1. Keep the two carriers with equal amplitude and simultaneously increase both of them; 2. Keep one of the carriers constant and increase the other carrier; 3. Keep the sum of the two carrier power constant and change their ratio. Enlightened by the classic polynomial theory of intermodulation, an empirical equation based on polynomial is proposed to describe the relationship between input carrier power and PIM product. It is found that the empirical equation describes the measurement data very well with the fitting parameters in the equation vary for different devices. The empirical equation can be used as a simple tool of PIM evaluation when engineers tackle problems on site.

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.

Passive-intermodulation analysis between rough circular waveguide flanges using Weibull distribution

In this paper, the Weibull distribution is employed to characterise the surface topography in the analysis of the passive intermodulation(PIM) between rough circular waveguide flanges, where the PIM level as a function of different system parameters such as the applied pressure, roughness, layer thickness and the power ratio has been developed assuming that the PIM level for a particular set of parameters is known. The proposed method is more significant when the contact surface needs to be described by parameters like the skewness or the kurtosis.