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Potassium (K) channels show the highest variability and most frequent alterations in expression in many tumor types, and modulation of K+ channels may represent a new window for cancer therapy. In previous work, we found that a terahertz (THz) field incident along the z-axis with a frequency of 51.87 THz increased the ion flux through K+ channels. In practice, it is difficult to ensure that the incident electromagnetic (EM) wave is strictly parallel to the direction of channel ion flow. In this paper, we found by changing the direction of the applied electric field that the EM wave of a specific frequency has the largest ion flux when the incident direction is along the ion flow, and the smallest ion flux when the incident direction is perpendicular to the ion flow, and that overall the EM wave of this frequency enhances the ion flow of the K+ channel. Changes in the direction of the applied field at a specific frequency affect the stability of the φ dihedral angle of the GLY77 residue and alter the ion permeation mechanism in the selectivity filter (SF) region, thus affecting the ion flux. Therefore, this frequency can be used to modulate K+ fluxes by THz waves to cause rapid apoptosis in potassium-overloaded tumor cells. This approach consequently represents an important tool for the treatment of cancer and is expected to be applied in practical therapy. [ABSTRACT FROM AUTHOR]

Vacuum surface flashover is the main factor limiting the development of pulsed power system. In this work, a quantitative dynamical model is built to investigate the vacuum surface flashover from field emission to secondary electron emission avalanche (SEEA). The results show that positive charges accumulating on the dielectric surface caused by field emission electrons play a dominant role when the normal electric field on the dielectric surface Edc is small. The growth of the number of positive surface charges exponentially decreases with the distance from the cathode triple junction (CTJ). With the increase in the density of positive surface charges, the growth of positive surface charges mainly depends on the secondary electrons cascade. As a result, the saturation state of Edc will first occur on the dielectric surface at a distance hundreds of micrometers from the CTJ. In addition, the effect of dielectric permittivity on positive surface charges is considered. The density of positive surface charges equals (1+εr) ε0Edc rather than 2ε0Edc. Particle-in-cell simulations are employed to verify the correctness of the theory model, and the agreement of comparisons is favorable. Furthermore, the space charge effect is discussed. When SEEA reaches saturation, electrons can significantly change the spatial distribution of the electric field. [ABSTRACT FROM AUTHOR]

Pre-ionization caused by runaway electrons is an important mechanism for negative streamer development. The aim of this paper is to investigate the runaway criteria and overvoltage threshold of electrons at the initial stage of streamer development in air, with the self-developed 3D particle-in-cell with Monte Carlo Collision code. First, numerical simulations are performed with fixed number of electrons to study the runaway criteria in nonrelativistic cases. This method takes the stochastic fluctuations of collisions into account and solves the major shortcomings of theoretical approach. The simulated critical electric field is less than that of the theoretical approach, and the amplitude of the difference increases with electron energy, due to the "tunneling effect" caused by the stochastic fluctuations of collisions. Then, simulations of negative streamers at various applied voltages are performed to investigate the overvoltage threshold. A more intuitive method, searching energetic electrons in front of the negative streamer head, which corresponds to the nature of runaway electrons, is applied to determine the generation of runaway electrons. Electrons that escaped a certain distance ahead of the streamer can be observed at 30 kV. Thus, the overvoltage threshold for runaway electrons can be roughly estimated as 3.3 in our simulations, which is about three times less than the previously published one. At last, with the redefined overvoltage threshold, the figure of regions of breakdown development for various mechanisms depending upon the overvoltage in air is updated. [ABSTRACT FROM AUTHOR]

The breakdown characteristics of C4F7N/CO2 mixtures under dc condition are investigated with a particle-in-cell method. The processes of elastic, excitation, ionization, and attachment collisions are all considered. The elastic cross section of electron scattering from C4F7N is modeled through quantum mechanical spherical complex optical potential formalism. Based on the cross-sectional data, the dielectric strength of pure C4F7N is studied. The threshold of breakdown is in good agreement with the experiment. Besides, the dielectric strength of C4F7N/CO2 mixtures is also obtained. The threshold of breakdown in 80% C4F7N and 20% CO2 is larger than that of pure C4F7N when the gas pressure is larger than 1300 torr. The results show that the energy loss of electrons is mainly caused by excitation collision with CO2 when the energy of electrons ${T}_{e}$ is smaller than 5.5 eV. When ${E}/{P}$ is small, the proportion of low-energy electrons is large and ${T}_{e}$ increases with the ratio of C4F7N. Therefore, the effective ionization rate first decreases and then increases as the ratio of C4F7N increases. [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]

The microwave window breakdown due to the plasma formation greatly limits the power handling capability of high-power microwave systems. However, the experimentally-observed fast plasma propagation cannot be explained using previous theory or simulation results. In this paper, the photoionization is considered to investigate the mechanism of microwave window breakdown at the air/dielectric interface by particle-in-cell simulation. The results show that photoelectrons produced by high-speed photons can profoundly promote discharge above the air/dielectric interface. Then a fast plasma formation and propagation occurs. The speed of plasma propagation can reach 1 × 106 m s−1, which agrees well with experiments. As a result, the transmitting power is attenuated more seriously than the case without the photoionization. Furthermore, the effects of size of microwave window, gas pressure, strength of microwave electric field and distribution of microwave electric field on the plasma propagation are investigated. The results show that the total number of electrons is nonlinearly increasing with the size of microwave window when a uniform microwave electric field is applied. The speed of the plasma propagation exponentially increases with the strength of microwave electric field. Therefore, the photoionization is an indispensable process in the microwave window breakdown with high-strength microwave electric field. [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]

Plasma pattern transition in a symmetric hybrid structure cavity device at micrometer scale is researched through microplasma interaction in intervening microchannel between adjacent cavities while manipulating electric field strength. Plasma distribution reconfiguration in central (objective) cavity is observed when sidearm (donor) cavities are ignited. As long as coupling effect occurred by modulating the electric field strength in the sidearm cavities, stable plasma pattern transition in objective cavity is obtained, exhibiting plasma pattern split from one circular spot (initial pattern) to two small circular spots (transited pattern), along with plasma peak emission intensity displacement over 100 μm to its equilibrium position. The shape of transited plasma patterns are depending on the coupling effect from sidearm cavities. The two circular spots unsymmetrically distributed if either donor cavity is ignited, and the ratio of average emission intensity between the two plasma spots is over 30%, however, which is less than 4% if coupling symmetrically occurred. The electrical and optical properties of central microplasma are also modulated, that the breakthrough voltage is decreased by 22% and emission intensity is improved by ∼30%, by means of plasma coupling. The microplasma pattern formation at micrometre scale and manipulation of the electrical properties in microscale cavity implies significant value in the application of plasma transistor and signal processing. [ABSTRACT FROM AUTHOR]

The excitation dynamics and self-oriented plasma coupling of a micro-structure plasma device with a rectangular cross-section are investigated. The device consists of 7 × 7 microcavity arrays, which are blended into a unity by a 50 μm-thick bulk area above them. The device is operated in argon with a pressure of 200 Torr, driven by a bipolar pulse waveform of 20 kHz. The discharge evolution is characterized by means of electrical measurements and optical emission profiles. It has been found that different emission patterns are observed within microcavities. The formation of these patterns induced by the combined action between the applied electric field and surface deactivation is discussed. The microplasma distribution in some specific regions along the diagonal direction of cavities in the bulk area is observed, and self-oriented microplasma coupling is explored, while the plasma interaction occurred between cross adjacent cavities, contributed by the ionization wave propagation. The velocity of ionization wave propagation is measured to be 1.2 km/s to 3.5 km/s. The exploration of this plasma interaction in the bulk area is of value to applications in electromagnetics and signal processing. [ABSTRACT FROM AUTHOR]

Microchannel plates (MCPs) are widely used as electron, ion, and X-ray detectors. The gain factor and time resolution of the MCP are strongly dependent on its operating and geometry parameters (applied voltage, length-to-diameter ratio, bias angle, and electrode penetration depth). Measurements about this dependence are sparse and do not cover the full range of the parameters. In this paper, 3-D single channel models are developed in computer simulation technology studio suit to systematically and comprehensively calculate the MCP gain and time resolution for various operating and geometry parameters. Furman secondaries electron emission model is employed in our simulation. Simulated result of the gain versus bias voltage is validated by the available experimental data. Finally, geometry parameters of L = 373.6~\mu \textm , D =10~\mu \textm , $h_{{{\text {in}}}}=h_{{{\text {out}}}}= 5~\mu \text{m}$ , and \theta =12^{\circ } are proposed to optimize the MCP performances. Simulation results show that the gain, mean transit time, and transit time spread of the optimized MCP are expected to reach 128 012, 128 ps and 19 ps at the applied voltage of 1000 V. [ABSTRACT FROM AUTHOR]

This paper deals with the theory of plasma development in the nanosecond high-power microwave breakdown at the dielectric/air interface. Utilizing the Poisson and Boltzmann equations, the basic physics are explored and investigated. Corresponding to the incident electric field, electrons moving back and forth bring about the negative charge accumulation at the interface. This process generates an extra electric field, strengthening the total amplitude. Simultaneously, deviations of electrons and ions lead to a sheath region providing a space charge field. Secondary electrons are strongly accelerated in the sheath region, eventually leading to an ionization avalanche in a very short time. Both the densities and energies of particles reach the maximums near the sheath region, and that is why the illumination is most significant near the interface. The theory is in good accordance with an experiment conducted under the same conditions. [ABSTRACT FROM PUBLISHER]

A positive streamer in a strong electric field in air at 1-atm pressure is investigated with particle-in-cell/Monte Carlo collision method including a photoionization model. The simulation results of streamer discharge in parallel plate air gap with and without the photoionization model are compared. It is shown that the seed electrons produced by photoionization generate secondary avalanches which lead to the formation of the streamer. No streamer is observed without photoionization model. [ABSTRACT FROM AUTHOR]