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To accelerate the design of a high-power microwave device, a deep learning assisted multi-objective optimization method is used to optimize a Ka-band relativistic backward-wave oscillator (RBWO) operating with a low magnetic field. Particle-in-cell simulation results show that the optimized RBWO with a tooth-shaped slow wave structure (SWS) can generate microwave pulses with an output power of 1.24 GW and an operating frequency of 26.8 GHz under a diode voltage of 623.3 kV, and the diode current is 6.56 kA at a guiding magnetic field of 0.8 T. Compared with the original RBWO, the output power of the optimized RBWO has been increased by 201.2%, and the beam-to-microwave conversion efficiency has increased from 10.0% to 30.3%. The detailed analysis reveals that in an overmoded RBWO with low guiding magnetic fields, the introduction of a tooth-shaped SWS is beneficial to mode competition, improves output power, and decreases microwave starting time. [ABSTRACT FROM AUTHOR]

An extension of a first-principle combined Monte Carlo method is proposed in this work to obtain the secondary electron emission characteristics of anisotropic crystal Al2O3. Unlike isotropic crystal Cu, density functional theory calculations reveal that the q-dependent energy loss function of Al2O3 in all directions is different. Therefore, an interpolation algorithm is introduced in the Monte Carlo method to determine the loss of energy and inelastic mean free path of electrons. The simulation results are in good agreement with experimental data. This method can be further used to simulate the secondary emission yield of other anisotropic crystal materials. [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]

The formation and evolution of the cathode plasma in small-gap magnetically insulated transmission lines (MITLs) may lead to enormous current loss and even gap closure. This issue has been investigated through particle-in-cell/Monte Carlo collision simulations. Based on gas desorption from the cathode, the plasma is formed at the cathode when the emitted electrons collide with the desorbed gas contaminants. The expansion velocity and the electron and ion density distributions of the cathode plasma were analyzed. Rapid expansion of the plasma occurs when plasma instability increases due to a transverse magnetic mode in the MITL. Factors affecting the expansion velocity such as voltage amplitude and rate of gas desorption were examined. The simulation results indicated that a relatively high desorption rate of neutral gas resulted in a higher expansion velocity for the cathode plasma. This work extends our knowledge and understanding of cathode plasma dynamics in high-current MITLs. [ABSTRACT FROM AUTHOR]

Negative and positive ions crossing the anode-cathode gap of a magnetically insulated transmission line (MITL) can cause non-negligible current loss and energy deposition on the electrodes, which may lead to the formation of anode plasma and the growth of cathode plasma. Furthermore, gap closure could occur due to the expansion of cathode plasma and anode plasma. In this paper, a model for magnetic insulation of both negative ion flow and positive ion flow is developed. The operating voltage V of the MITL is expressed as a function of the total current I0 and the boundary current Ib. The total current and the boundary current of the MITL are derived at saturated and self-limited flows, respectively. In addition, particle-in-cell simulations are implemented for the validation of the theoretical model. The thickness and density of the magnetically insulated ion layers are analyzed, and an empirical expression for space charge factor g is obtained through simulation results. This work extends the understanding of magnetically insulated ion flow in MITLs. [ABSTRACT FROM AUTHOR]

The curved structure of a coaxial magnetically insulated transmission line (MITL) causes it to generate a nonuniform azimuthal magnetic field, which can disturb the electron flow and lead to a higher current loss when compared with a straight MITL. In this paper, the current loss mechanism and nonuniformity of the azimuthal magnetic field inherent to curved MITLs are investigated using theoretical analysis and particle-in-cell simulations. Results show that the critical current for magnetic insulation of the curved MITL increases with nonuniformity of the azimuthal magnetic field. The magnitude of the azimuthal magnetic field of the curved MITL can be described by a cosine distribution, lower in the outer region and higher in the inner region of the curved MITL. Nonuniformity of the azimuthal magnetic field decreases as electron current increases. Finally, an empirical expression relating to the structural parameters of curved MITLs is obtained for nonuniformity of the azimuthal magnetic field, which can be used to estimate the critical current for magnetic insulation in curved MITLs. This work extends the understanding of magnetic insulation in curved coaxial transmission lines. [ABSTRACT FROM AUTHOR]