Laqun Liu

Rescaling of microwave breakdown theory for monatomic gases by particle-in-cell/Monte Carlo simulations

A particle-in-cell/Monte Carlo code is developed to rescale the microwave breakdown theory which is put forward by Vyskrebentsev and Raizer. The results of simulations show that there is a distinct error in this theory when the high energy tail of electron energy distribution function increases. A rescaling factor is proposed to modify this theory, and the change rule of the rescaling factor is presented.

Magnetically insulated theory with both electron and ion flows

Both the ion emission from anodesurface and the electron emission from cathodesurface may occur in the magnetically insulated transmission line (MITL) with a very high pulsed power and a very large current density. A model for the MITL with both electron and ion flow is developed. In this model, physical quantities (such as space-charge sheath thicknesses and flow currents) in the MITL are theoretically analyzed, and the specific expression for the voltage on the line by the terms of currents is derived. Furthermore, particle-in-cell simulations are carried out to verify the theoretical results.

Theory of plasma propagation from microlayer discharges in vacuum window breakdown

The theory of plasma propagation from microlayer discharges in vacuum window breakdown is proposed and compared with particle-in-cell simulations and experimental results. In the field of vacuum window breakdown, one contradiction in the previous experiments is that the range of plasmas from gas discharges is much wider than the range of desorption gases. In this paper, we proposed a theoretical model to solve this contradiction. In this model, the critical plasma, provided by the discharges in the microgas layer, is propelled by the space charge field that is caused by different speeds between ions and electrons. The propelling electric field is determined by the electron density and the electron temperature Te in the propagation dimension. With the propelling electric field, the plasma consisting of electrons and carbon ions spreads about 1 cm within just 40 ns, if kTe is 100 eV and the plasma density is 2.5 × 1024 m−3 in the gas microlayer.

PIC simulations of conical magnetically insulated transmission line with LTD generator: Transition from self-limited to load-limited flow

Based on the 3-dimensional Particle-In-Cell (PIC) code CHIPIC3D, with a new circuit boundary algorithm we developed, a conical magnetically insulated transmission line (MITL) with a 1.0-MV linear transformer driver (LTD) is explored numerically. The values of switch jitter time of LTD are critical parameters for the system, which are difficult to be measured experimentally. In this paper, these values are obtained by comparing the PIC results with experimental data of large diode-gap MITL. By decreasing the diode gap, we find that all PIC results agree well with experimental data only if MITL works on self-limited flow no matter how large the diode gap is. However, when the diode gap decreases to a threshold, the self-limited flow would transfer to a load-limited flow. In this situation, PIC results no longer agree with experimental data anymore due to the anode plasma expansion in the diode load. This disagreement is used to estimate the plasma expansion speed.

The ionization rate under a general magnetic field for microwave breakdown

The ionization rate under an extra magnetic field is studied by theory and particle-in-cell/Monte Carlo Collision simulations. The result shows that a magnetic field always decreases the ionization rate if 3ω   υm. The effect of the magnetic field on the ionization rate fades away when the angle between the magnetic field and the electric filed approaches to zero. Furthermore, the peak ionization rate among different magnetic fields is almost independent of ω. This peak ionization rate is in direct proportion to the gas pressure in the low pressure region, while it is about in inverse proportion to the gas pressure in the high pressure region.

The effect of the H2 density on the electron energy distribution in H− ion sources

The electron energy distribution in H(-) ion sources is studied. By three-dimensional Monte Carlo simulations (3D-MCC) and cross sections of collisions, the crucial collisions in the JAEA 10A are chosen to develop theoretical models for the low electron temperature and the high electron temperature, respectively. The effects of the H2 density on the low electron temperature and the high electron temperature in H(-) ion sources are predicted with analytic expressions and verified by 3D-MCC. The results show that both the low and high electron temperatures become lower while n(H2) increases.

PIC simulation of the anode plasma in a high-power hollow cathode diode

In this paper, the evolution and dynamics of anode plasmas in high-power hollow cathode diodes were studied by particle-in-cell (PIC) simulation. The simulation results show that the ion flow emitted by the anode plasma layer and the increase of the electron current caused by the ion flow will cause a significant decline in the diode impedance in a short time. In addition, the expansion of the anode plasma layer will cause the diode impedance to decrease. The PIC simulation technique is also applied to a high-power hollow cathode diode of a 1.0 MV-LTD generator for anode plasmas, and the PIC simulation results were compared with the experimental data.

Upstream and downstream multipactor of dielectric window by electromagnetic PIC simulations

By using the electromagnetic Particle-In-Cell method, the multipactor of the dielectric window on the upstream side is observed directly even without presetting extra normal electric fields and compared to that on the downstream side. This paper shows: First, with the initial emission of electrons to provide extra normal electric fields, the secondary electron avalanche is much faster than that on the downstream side. Second, even without the initial emission of electron to provide extra normal electric fields, the secondary electron avalanche occurs on the upstream side, while it is nowhere to be found on the downstream side.

PIC simulation of the vacuum power flow for a 5 terawatt, 5 MV, 1 MA pulsed power system

In this paper, a 5 Terawatt, 5 MV, 1 MA pulsed power system based on vacuum magnetic insulation is simulated by the particle-in-cell (PIC) simulation method. The system consists of 50 100-kV linear transformer drive (LTD) cavities in series, using magnetically insulated induction voltage adder (MIVA) technology for pulsed power addition and transmission. The pulsed power formation and the vacuum power flow are simulated when the system works in self-limited flow and load-limited flow. When the pulsed power system isn’t connected to the load, the downstream magnetically insulated transmission line (MITL) works in the self-limited flow, the maximum of output current is 1.14 MA and the amplitude of voltage is 4.63 MV. The ratio of the electron current to the total current is 67.5%, when the output current reached the peak value. When the impedance of the load is 3.0 Ω, the downstream MITL works in the self-limited flow, the maximums of output current and the amplitude of voltage are 1.28 MA and 3.96 MV, and the ratio of the electron current to the total current is 11.7% when the output current reached the peak value. In addition, when the switches are triggered in synchronism with the passage of the pulse power flow, it effectively reduces the rise time of the pulse current.In this paper, a 5 Terawatt, 5 MV, 1 MA pulsed power system based on vacuum magnetic insulation is simulated by the particle-in-cell (PIC) simulation method. The system consists of 50 100-kV linear transformer drive (LTD) cavities in series, using magnetically insulated induction voltage adder (MIVA) technology for pulsed power addition and transmission. The pulsed power formation and the vacuum power flow are simulated when the system works in self-limited flow and load-limited flow. When the pulsed power system isn’t connected to the load, the downstream magnetically insulated transmission line (MITL) works in the self-limited flow, the maximum of output current is 1.14 MA and the amplitude of voltage is 4.63 MV. The ratio of the electron current to the total current is 67.5%, when the output current reached the peak value. When the impedance of the load is 3.0 Ω, the downstream MITL works in the self-limited flow, the maximums of output current and the amplitude of voltage are 1.28 MA and 3.96 MV, and ...

Global optimization methods to design vacuum electronic devices

In this paper, we try to adopt the global optimization method to design vacuum electronic devices. Based on the platform of three dimensional particle-in-cell (PIC) CHIPIC, the modules of Particle Swarm Optimization (PSO) and Genetic Algorithm (GA) are designed to optimize vacuum electronic devices, respectively. The comparisons of PSO and GA are implemented to optimize the slow wave period structure (SWS) of a relativistic backward wave oscillator (RBWO). The results show that the performances (optimization result and convergence speed) of PSO are better than that of GA in the cases of a small population size.