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Secondary electron emission (SEE) is a fundamental phenomenon of particle/surface interaction, and the multipactor effect induced by SEE can result in disastrous impacts on the performance of microwave devices. To suppress the SEE-induced multipactor, an Ni (111) surface covered with a monolayer of graphene was proposed and studied theoretically via the density functional theory (DFT) method. The calculation results indicated that redistribution of the electron density at the graphene/Ni (111) interface led to variations in the work function and the probability of SEE. To validate the theoretical results, experiments were performed to analyze secondary electron yield (SEY). The measurements showed a significant decrease in the SEY on an Ni (111) surface covered with a monolayer of graphene, accompanied by a decrease in the work function, which is consistent with the statistical evidence of a strong correlation between the work function and SEY of metals. A discussion was given on explaining the experimental phenomenon using theoretical calculation results, where the empty orbitals lead to an electron trapping effect, thereby reducing SEY. [ABSTRACT FROM AUTHOR]

Plasmonic devices work at visible and near-infrared frequencies, where a large number of bound electrons (polarization) in metals are excited. However, the existing particle-in-cell (PIC) software cannot take into account these bound electrons. In this paper, a new PIC simulation method is developed to study plasmonic devices, which has the ability to model both free electrons and bound electrons in metals. In this study, surface plasmon polaritons (SPPs) are excited by depositing excess electrons at one end of the metal film of thickness 140 nm (surrounded by air), thus initiating nonequilibrium in the electron density to start the oscillations. The spectrum, mode size, and propagation length of SPPs are calculated to confirm the electrical excitation of SPPs and characterize their properties. The excited SPPs are of a broad range of frequencies. Unlike SPPs excited by inelastic electron tunneling, the spectrum of SPPs excited in this way is not determined by e V b i a s but only by the properties of metals and dielectrics, and therefore the excited SPPs are intrinsic. The mode sizes of the intrinsic SPPs in the Ag–air geometry are 30 nm in Ag and 120 nm in air. The propagation length is ∼730 nm. The excited SPPs are converted into radiation modes by a sub-wavelength slit etched on the back side of the film to enable the detection of the radiation modes. [ABSTRACT FROM AUTHOR]

The high power microwave window breakdown characteristics of N2–SF6 mixtures are investigated with 3D particle-in-cell and Monte Carlo collision simulation. The space and density distributions of electrons and ions are obtained. The results show that the threshold of breakdown increases with the ratio of SF6 when E/P is large. However, when E/P is small, the threshold of breakdown in 70% of SF6 and 30% of N2 is greater than that of pure SF6. This phenomenon is also observed in experiments. The theory analyses show that the energy loss of electrons is mainly caused by excitation collisions with N2 when the average energy of electrons Te is less than 6 eV and is dominated by excitation and ionization collisions with SF6 when Te is greater than 6 eV. When E/P is small, the proportion of low energy electrons is large and Te increases with the ratio of SF6. Therefore, the effective ionization rate first decreases and then increases as the ratio of SF6 increases. Thus, the optimal ratio for improving the insulation properties is 60%–80% SF6 when E/P is small. When E/P is large, the proportion of high energy electrons increases. Therefore, the effective ionization rate and density of electrons decrease as the ratio of SF6 increases. The maximum threshold of breakdown occurs when the ratio of SF6 is 100%. [ABSTRACT FROM AUTHOR]

With the introduction of the gas diffusion model, the surface flashover at the insulator–vacuum interface perpendicular to electrodes is simulated with the particle-in-cell method. The distributions of density of electrons, ions, and neutral gas molecules are obtained, which is consistent with images diagnosed in the experiment. The gas pressure at the dielectric surface near electrodes is much higher than it is at other locations. Furthermore, the processes of vacuum surface flashover for angled dielectric insulators are analyzed. The results show that negative charges will accumulate on the dielectric surface when the negative angle is large. The electric field produced by negative charges weakens the normal electric field and emission current on the cathode triple junction. Moreover, the process of secondary electron emission is completely suppressed when the negative angle is large. With the development of gas desorption, the breakdown will evolve from field emission into gas ionization. Therefore, when the angle is negative, the threshold of surface flashover first decreases then increases with the angle, which is in good agreement with laws obtained in experiments. This study can provide a deep understanding to the vacuum surface flashover. [ABSTRACT FROM AUTHOR]

To suppress microwave window breakdown, an external parallel dc bias electric field is applied to sweep out the electrons from the discharge zone. The development of microwave window breakdown under an external dc electric field is investigated using particle-in-cell and Monte Carlo collision simulation. The results show that the number of multipactor electrons decreases with the strength of the external unidirectional dc electric field when the size of the microwave window is finite. In addition, the threshold of breakdown is strongly influenced by the direction of the external bidirectional dc electric field. When the electric field is directed from the center to the edge of the microwave window, breakdown occurs more readily than in the absence of the electric field. However, when the electric field is directed toward the center of the microwave window, the discharge process starts at the edge of the window and develops to the center of the window. This is because a smaller density of electrons at the window edge results in a smaller gas pressure and the low gas pressure above the dielectric surface thus makes formation of the plasma more difficult. As a result, the early stage of breakdown is suppressed and the maximum transmitting power is enhanced. [ABSTRACT FROM AUTHOR]