


卷 44, 编号 5 (2018)
- 年: 2018
- 文章: 7
- URL: https://journals.rcsi.science/1063-780X/issue/view/11696
Plasma Dynamics
Simulation of the Plasma Afterglow in the Discharge Gap of a Subnanosecond Switch Based on an Open Discharge in Helium
摘要
The phenomenon of subnanosecond electrical breakdown in a strong electric field observed in an open discharge in helium at pressures of 6–20 Torr can be used to create ultrafast plasma switches triggering into a conducting state for a time shorter than 1 ns. To evaluate the possible repetition rate of such a subnanosecond switch, it is interesting to study the decay dynamics of the plasma remaining in the discharge gap after ultrafast breakdown. In this paper, a kinetic model based on the particle-in-cell Monte Carlo collision method is used to study the dynamics of the plasma afterglow in the discharge gap of a subnanosecond switch operating with helium at a pressure of 6 Torr. The simulation results show that the radiative, collisional-radiative, and three-body collision recombination mechanisms significantly contribute to the afterglow decay only while the plasma density remains higher than 1012 cm−3; the main mechanism of the further plasma decay is diffusion of plasma particles onto the wall. Therefore, the effect of recombination in the plasma bulk is observed only during the first 10–20 μs of the afterglow. Over nearly the same time, plasma electrons become thermalized. The afterglow time can be substantially reduced by applying a positive voltage Uc to the cathode. Since diffusive losses are limited by the ion mobility, the additional ion drift toward the wall significantly accelerates plasma decay. As Uc increases from 0 to +500 V, the characteristic time of plasma decay is reduced from 35 to 10 μs.



Oscillations and Waves in Plasma
Linear Transformation of Electromagnetic Wave Beams of the Electron-Cyclotron Range in Toroidal Magnetic Configurations
摘要
The field structure of quasi-optical wave beams tunneled through the evanescence region in the vicinity of the plasma cutoff in a nonuniform magnetoactive plasma is analyzed. This problem is traditionally associated with the process of linear transformation of ordinary and extraordinary waves. An approximate analytical solution is constructed for a rather general magnetic configuration applicable to spherical tokamaks, optimized stellarators, and other magnetic confinement systems with a constant plasma density on magnetic surfaces. A general technique for calculating the transformation coefficient of a finite-aperture wave beam is proposed, and the physical conditions required for the most efficient transformation are analyzed.



Theory of Electromagnetic Surface Waves in Plasma with Smooth Boundaries
摘要
A theory of nonpotential surface waves in plasma with smooth boundaries is developed. The complex frequencies of surface waves for plasma systems of different geometries and different profiles of the plasma density are calculated. Expressions for the rates of collisionless damping of surface waves due to their resonance interaction with local plasma waves of continuous spectrum are obtained. The influence of collisions in plasma is also considered.



Nonlinear Phenomena
Nonlinear Right-Hand Polarized Wave in Plasma in the Electron Cyclotron Resonance Region
摘要
The propagation of a nonlinear right-hand polarized wave along an external magnetic field in subcritical plasma in the electron cyclotron resonance region is studied using numerical simulations. It is shown that a small-amplitude plasma wave excited in low-density plasma is unstable against modulation instability with a modulation period equal to the wavelength of the excited wave. The modulation amplitude in this case increases with decreasing detuning from the resonance frequency. The simulations have shown that, for large-amplitude waves of the laser frequency range propagating in plasma in a superstrong magnetic field, the maximum amplitude of the excited longitudinal electric field increases with the increasing external magnetic field and can reach 30% of the initial amplitude of the electric field in the laser wave. In this case, the energy of plasma electrons begins to substantially increase already at magnetic fields significantly lower than the resonance value. The laser energy transferred to plasma electrons in a strong external magnetic field is found to increase severalfold compared to that in isotropic plasma. It is shown that this mechanism of laser radiation absorption depends only slightly on the electron temperature.



Dusty Plasma
Ion-Flow-Induced Excitation of Electrostatic Cyclotron Mode in Magnetized Dusty Plasma
摘要
The stability of electrostatic cyclotron mode is investigated in a flowing magnetized dusty plasma in the presence of strong ion–neutral collisions. In the high magnetic field limit, when the dust magnetization becomes important, it is expected that the collective behavior of magnetized dust grains suspended in the near-sheath region substantially influences the dispersion properties of electrostatic modes. The growth/damping of the collective excitation is significantly controlled by such parameters as the ion–neutral collision frequency, Mach number, and magnetic field strength. In our case, the explicit dependence of the Mach number on the magnetic field and collision frequency has been taken into account and possible implications on the stability of the mode is analyzed. Streaming instability of cyclotron modes may be important to understand issues related to the interaction mechanism between dust grains and other associated phenomena like Coulomb crystallization, phase behavior, transport properties, etc., in the relatively strong magnetic field limit, which is currently accessible in the DPD (Kiel University) and MDPX (PSL, Auburn University) experiments.



Low-Temperature Plasma
Characteristics of a Pulse-Periodic Corona Discharge in Atmospheric Air
摘要
Pulse-periodic corona discharge in atmospheric air excited by applying a voltage pulse with a subnanosecond or microsecond rise time to a point electrode is studied experimentally. It is shown that, at a voltage rise rate of dU/dt ~1014 V/s, positive and negative ball-shaped streamers with a front velocity of ≥2 mm/ns form near the point electrode. As dU/dt is reduced to 1010−1011 V/s, the streamer shape changes and becomes close to cylindrical. The propagation velocity of cylindrical streamers is found to be ~0.1 mm/ns at dU/dt ~ 2 × 1010 V/s. It is shown that the propagation direction of a cylindrical streamer can be changed by tilting the point electrode, on the axis of which the electric field strength reaches its maximum value. It is established that, for the negative polarity of the point electrode and a microsecond rise time of the voltage pulse, a higher voltage is required to form a cylindrical streamer than for the positive polarity of the point electrode.



Initiation of Positive Streamers near Uncharged Ice Hydrometeors in the Thundercloud Field
摘要
Since the threshold electric field required for breakdown of air is much higher than the maximum field strength measured in thunderstorm clouds, the problem of lightning initiation still remains unsolved. According to the popular hypothesis, lightning can be initiated by a streamer discharge in the field enhanced near a hydrometeor. To verify the adequacy of this hypothesis, the development of a positive streamer propagating along the thunderstorm electric field in the vicinity of an ice needle at an air pressure corresponding to an altitude of 5 km (which is typical of the lightning initiation conditions) was simulated numerically. The hydrometeor dimensions are determined at which streamers can be initiated at different strengths of the thunderstorm electric field.


