In comparison with conventional lasers, topological lasers are more robust and can be immune to disorder or defects if lasing occurs in topologically protected states. Previously reported topological lasers were almost exclusively based on the first-order photonic topological insulators. Here, we show that lasing can be achieved in the zero-dimensional corner state in a second-order photonic topological insulator, which is based on the Kagome waveguide array with a rhombic configuration. If gain is present in the corner of the structure, where the topological corner state resides, stable lasing in this state is achieved, with the lowest possible threshold, in the presence of uniform losses and two-photon absorption. When gain acts in other corners of the structure, lasing may occur in edge or bulk states, but it requires substantially larger thresholds, and transition to stable lasing occurs over much larger propagation distances, sometimes due to instabilities, which are absent for lasing in corner states. We find that increasing two-photon absorption generally plays strong stabilizing action for nonlinear lasing states. The transition to stable lasing stimulated by noisy inputs is illustrated. Our work demonstrates the realistic setting for corner state lasers based on higher-order topological insulators realized with waveguide arrays.

The thickness effect of solid dielectrics means the relation between the electric breakdown strength (E_BD) and the dielectric thickness (d). By reviewing different types of expressions of E_BD on d, it is found that the minus power relation (E_BD = E₁d^−a) is supported by plenty of experimental results. The physical mechanism responsible for the minus power relation of the thickness effect is reviewed and improved. In addition, it is found that the physical meaning of the power exponent a is approximately the relative standard error of the E_BD distributions in perspective of the Weibull distribution. In the end, the factors influencing the power exponent a are discussed.

With the introduction of an external circuit model and a gas desorption model, the surface flashover on the plane insulator-vacuum interface perpendicular to parallel electrodes is simulated by a Particle-In-Cell method. It can be seen from simulations that when the secondary electron emission avalanche (SEEA) occurs, the current sharply increases because of the influence of the insulator surface charge on the cathode field emission. With the introduction of the gas desorption model, the current keeps on increasing after SEEA, and then the feedback of the external circuit causes the voltage between the two electrodes to decrease. The cathode emission current decreases, while the anode current keeps growing. With the definition that flashover occurs when the diode voltage drops by more than 20%, we obtained the simulated flashover voltage which agrees with the experimental value with the use of the field enhancement factor $\beta$ = 145 and the gas molecule desorption coefficient $\gamma =0.25$ . From the simulation results, we can also see that the time delay of flashover decreases exponentially with voltage. In addition, from the gas desorption model, the gas density on the insulator surface is found to be proportional to the square of the gas desorption rate and linear with time.

The possibility of using single-stepped cavities to replace the common tapered cavities was studied using particle-in-cell simulations in an A6 magnetron with diffraction output (MDO). The replacing of the tapered cavities by the single-stepped cavities in a 12-cavity MDO increases the interaction space where the charged particles interact with the induced RF waves. The electronic efficiency of the 12-cavity MDO with single-stepped cavities driven by the transparent cathode [2] of GW output power level can be as high as 73% for α = 18.2°, 74% for α = 17.5°, and 72% for α = 12.5° at β = 32°, where α is the angle between the outer wall and z-axis, and β is the angle between the inner wall and z-axis. The depth of single-stepped cavities is changed when α is changed, which results in different frequency range of magnetron operating modes. When a 400-kV voltage pulse of 10-ns duration is applied to a transparent cathode or a solid cathode, the output power can be as high as 1 GW. Without loss of generality, for α = 12.5° at β = 32°, the peak efficiency around 70% of 12-cavity MDO with single-stepped cavities design occurs at the voltage (V ~ 400 ± 50 kV). The results presented in this paper provide references for relativistic magnetron mode selection or mode switching experiments when choosing the input parameters (magnetic field and accelerating voltage) allowing the magnetron to operate in the desired operation mode.

The breakdown of polymer insulators at a nanosecond scale is a serious problem for high-voltage devices and pulsed power systems. This paper reviews recent developments in research on the mechanisms and characteristics of the breakdown of polymers at a nanosecond scale at the Northwest Institute of Nuclear Technology. An improved avalanche breakdown model is proposed, and a formula for the strength of the electric breakdown (E_BD) is formulated based on it. The formative time lag of the avalanche breakdown model is then developed to verify it. Two research groups were formed to study the characteristics of the breakdown. One focused on the characteristics of single-pulse breakdown, such as the effects of size, polymer type, electrode, pulse width, and pulse polarity on E_BD, with the aim of deriving expressions for E_BD. The other research group examined cumulative (multi-pulse) characteristics of the breakdown with the aim of deriving a formula for the lifetime of the polymer. A method to design solid insulation structures is finally proposed.