cell-Monte Carlo Collision Research Articles

Modelling of plasma discharge in a filament negative ion source

Most of the currently operating Negative ion Neutral Beam Injectors (N-NBIs) exploit filament powered sources for the generation of beam ions. Being widely used in fusion experiments, the filament arc technology has been thoroughly investigated and optimized over the years, allowing to achieve excellent performances in terms of extracted beam optics. The source geometry, the magnetic field topology and the arc power strongly influence both the plasma discharge and the background gas properties and, consequently, the beam features. In this framework, this contribution describes a numerical investigation of the plasma properties in a filament-powered negative ion source, performed by means of a 2D3V Particle In Cell-Monte Carlo Collisions (PIC-MCC) code. Specifically, we discuss plasma formation by the thermionic electrons emitted by the filaments, investigating their interaction with the background gas. We also study the plasma diffusion through the magnetic filter field and how the latter modifies plasma density, electron temperature and plasma potential along the axial direction when approaching the plasma facing electrode.

An Analysis of Sheath Development of Cathode During the Instability Stage of Vacuum Arc

The purpose of this article was to investigate the dynamic evolution laws of the plasma between the gap during the instability phenomena in low-current vacuum arc using the simulation date comparison with the experimental data. The instability phenomena are characterized by high-frequency peaks of the arc voltage which are prior to the actual current chopping. Those peaks, which reach a level of more than ten times the normal arc voltage, have typical correspondence with the fluctuations of arc current and will result in a sudden chopping of the current through interaction with the elements of circuit. The fluctuations of arc voltage were classified into the types of long pulses and short pulses because of different characteristics on the duration time and oscillation morphology, which can summarize the peaks on the instability phenomena. The concepts of prearc cathode sheath and the cathode-emission intensity are proposed to reasonably explain the difference and connection between these two spikes. The distribution and motion of particles in the gap at different cathode-emission intensity, which demonstrating the development direction of the prearc cathode sheath and the relationship with the cathode-emission intensity, are simulated by the particle in cell-Monte Carlo collision (PIC-MCC). As a result, the relationship in the development of the prearc cathode sheath, the voltage spikes, and the intensity of cathode emission is summarized.

Wide-range characteristics of beam perveance and saddle point potential of LIPS-200 ion thruster

Both the long-life and multi-mode versions of LIPS-200 ion thruster are under investigation in LIP (Lanzhou Institute of Physics). To confirm the feasible ranges of the beam current and accel (abbreviation for accelaration) grid potential to apply to the thruster, the wide-range beam perveance (the state of beam focus) and saddle point potential (the lowest potential along beamlet centerline) characteristics of LIPS-200 are studied with a test-verified PIC-MCC (Particle in Cell-Monte Carlo Collisions) model. These characteristics are investigated with both the initial and the eroded states of the accel grid aperture diameter. The results show that the feasible ranges of these parameters with respect to perveance/crossover (overfocused) limit extend as the operating time accumulates, while the feasible range of accel grid potential narrows due to a reduced EBSF (electron backstreaming failure) margin. The feasible ranges determined by the initial condition are: (i) the beam current up to 0.981 A, and (ii) the accel grid potential up to −85 V. A 23% enlargement of the aperture diameter would bring up to 48 V of EBSF margin loss.

Numerical Simulation Of A Bi-directional Plasma Thruster For Space Debris Removal

A spacecraft during mission typically switches from chemical propulsion to electric propulsion once it lifts out of the Earth's gravity as the thrust requirement to drive it reduces substantially. Consequently, electric propulsion technology is commonly used for deep space mission, satellite orbit keeping and orbit correction. In the last few decades, the amount of man-made space junk (space debris) has increased enormously and has become a potential danger for space stations, space shuttles and other live satellites. A bi-directional plasma thruster, mounted on a satellite, can be used to remove space debris during satellite operation (Takahashi et al., Sci. Rep., vol. 8, 2018, p. 14417). A directed ion beam ejected from a plasma thruster imparts a net force on space debris to decelerate and facilitates manoeuvring and re-entry of space debris into the Earth's atmosphere where it can burn out. We present a detailed 1D3V PIC-MCC (particle in cell-Monte Carlo collision) simulation of a bi- directional plasma thruster. To this end, a PIC-MCC solver which resolves thruster axial direction and all three velocity dimensions is used to study a magnetic nozzle plasma thruster with both ends open (bi-directional plasma thruster). We show that such a bi-directional plasma thruster can be used to accelerate–decelerate a live satellite and also to remove space debris by altering the magnetic field spatial profile in the plasma expansion region. A detailed study is presented.

Investigation of variable aperture on the performance and lifetime of ion thruster

Beam flatness is an important parameter that determines the performance and the lifetime of a gridded ion thruster. To improve the beam flatness of the 30 cm (LIPS-300) ion thruster, variable aperture ion optics that adapts to the decreasing ion density as the radius increases is proposed. It is the ion optics that the screen grid surface is divided into several zones, where the aperture diameter in each zone is determined by the ion density and the electron temperature upstream of the screen grid. The beam current density in the central area is artificially reduced. A particle in cell-Monte Carlo collision model is applied in this work to investigating the effect of variable aperture on the perveance and the maximum beam current per aperture by simulating the extraction, focusing and acceleration processes of ions. Taking into account the engineering implementability, the screen grid surface is divided into four zones. The hole diameter in each zone is decreased from 1.95 mm to 1.8 mm, 1.9 mm, 1.8 mm and 1.7 mm, respectively. The simulation results show that the maximum ion density in the center area of grid is decreased by 10.6% and 6.99%, while it is increased by 6.49% and 22.3% in the edge region, respectively. The beam flatness of the variable aperture ion optics is improved from 0.69 to 0.88. The erosion rate is decreased by 31.9%, but the total beam current is also decreased by 7.15%. The simulation results can provide a valuable reference of the development of the ion thruster.

PIC-MCC Investigation on the Influences of Gas Medium and Flashover on the Multipacting Cathode Operation

In this article, the influences of outgassing and flashover on the performance of a novel multipacting cathode are numerically investigated by using particle in cell-Monte Carlo collision (PIC-MCC) method, including the conditions of weak, medium, and strong outgassing. The numerical results demonstrate that under the condition of weak outgassing (0.1 Torr), multipacting is hardly influenced by gas ionization. Multipacting dominates the entire physical process. Due to the further improved output current by additional electrons generated from ionization, the weak outgassing can help cathode performance enhancement. Under the condition of medium outgassing (1 Torr), gas ionization first boosts multipacting developing and then tends to suppress multipacting process. During the initial stage, multipacting dominates the entire physical process. When multipacting tends to saturate, gas ionization will dominate the whole physical process. The rapidly increased output current will finally decrease gap voltage and induce the diode breakdown. Under the condition of strong outgassing (1 atm), multipacting is suppressed by gas ionization from beginning to end. Gas ionization dominates the whole physical process all the time. The strong effect of ionization collision will finally induce streamer discharge and diode breakdown.

Study of the influences of different factors on the charged particles absorbed by the post-arc anode during the post-arc sheath expansion process in vacuum circuit breakers

The dielectric recovery process has decisive effects on the current interruption process in vacuum circuit breakers, which has attracted the special attention of researchers. In commercial vacuum interrupters, ions and electrons of the residual plasma between the contact gap are separated and then fly to the electrodes under the effect of the transient recovery voltage after current zero. During this period, the post-arc current forms. During the formation of the post-arc current, ions also enter the post-arc anode due to their thermal motion. Therefore, the number of net electrons, which form the post-arc current, is only part of the total electrons between the contact gap at current zero. During the post-arc sheath expansion process, almost all electrons in the contact gap will enter the post-arc anode under the effects of the transient recovery voltage. If the proportion of ions and electrons, which enter the post-arc anode, can be obtained, the total number of electrons and consequently the residual plasma density between the contact gap could be estimated from the integration of the post-arc current. In this paper, the influences on the absorption of charged particles by the post-arc anode of some factors, e.g., rising rate of transient recovery voltage and metal vapor, have been simulated and discussed with a one-dimensional particle in cell-Monte Carlo collision model.

Influence of the Decelerator Grid on the Optical Performance of the Ion Thruster

In order to reduce the erosion of the ion thruster accelerator grid, which is caused by charge-exchange (CEX) ions, the 2-grid optical system is added to a decelerator grid to block the reflux CEX ions. The previous experiment and simulation results have proven that the decelerator grid can effectively reduce the Pit and Groove erosion. However, the influence of the decelerator grid on the optical performance has not yet been studied well. In this paper, a three-dimensional Immersed Finite Element Method-Particle in Cell-Monte Carlo Collision (IFE-PIC-MCC) algorithm was adopted to investigate the effect of the decelerator grid on the optical performance under crossover and normal circumstances. Results show that the decelerator grid has no effect on the focusing state and the distribution of beam ions. It also has little effect on the CEX ions from the upstream and extraction (center) regions. However, it has great influence on the downstream CEX ions. When the upstream plasma number density is small, the decelerator grid will cause most of the downstream reflux CEX ions to impinge on the accelerator grid aperture barrel, resulting in the significant increase of the Barrel erosion of the accelerator grid. With the increase of the upstream plasma number density, the downstream reflux CEX ions tend to impact the downstream surface of the decelerator grid, which means the decelerator grid begins to block the downstream backflow of CEX ions.

Current pulse characteristic analysis of typical void and point partial discharge based on PIC‐MCC method

The partial discharge (PD) in a void of epoxy solid insulation is a typical kind of PD behaviour, its occurrences may lead to complete breakdown of the insulation system. The microscopic process of particle movement is simulated here by using particle in cell-Monte–Carlo collision (PIC-MCC) method, which possesses great significance for researching typical discharge. The simulation results of void discharge and point discharge show detailed motion process and the Townsend discharge phenomenon. The PD current pulse waveform possesses similar trends of steep rising edge and gentle falling edge compared with Gaussian function. The behaviour of discharge events is influenced by many factors, such as electric field strength, void thickness, and polarity effect. Thus, the characteristics of PD are discussed and compared by adjusting these simulation parameters to find out internal connection and performance differences.

A 3D particle model for the plume CEX simulation

ABSTRACTCharge Exchange (CEX) ion is the main factor causing the plume pollution. The distribution of CEX ions is determined by the distribution of beam ions and neutral atoms. Hence, the primary problem in the study of the plume is how to accurately simulate the distribution of beam ions and neutral atoms. At present, the most commonly used model utilised for the plume simulation is the analytical model proposed by Roy for the plume simulation of the NASA Solar Technology Application Readiness (NSTAR) ion thruster. However, this analytical model can only be applied to the ion beam with small divergence angles. In addition, the analytical model is no longer applicable to the simulation for the plume of a new type of ion thruster that appeared recently, which is called the annular ion thruster. In this paper, a 3D particle model is proposed for the plume simulation of ion thrusters consisting of the particle model for beam ions, the Direct Simulation Monte Carlo (DSMC) model for neutral atoms and the Immersed Finite Element-Particle In Cell-Monte Carlo Collision (IFE-PIC-MCC) model for CEX ions. Then, the plume of the NSTAR ion thruster is simulated by both Roy's model and the 3D particle model. The simulation results of both models are then compared with the experimental results. It is shown that the numerical results of the 3D particle model agree well with those of the analytical model and the experimental data. And this 3D particle model can also be used for other electric thrusters.

Particle in Cell-Monte Carlo Collisions of a Plasma Column Driven by Surface Wave Plasma Discharges

Particle in Cell-Monte Carlo Collisions of a Plasma Column Driven by Surface Wave Plasma Discharges

Study of the operating parameters of a helicon plasma discharge source using PIC-MCC simulation technique

In this work, a Particle in Cell-Monte Carlo Collision simulation technique is used to study the operating parameters of a typical helicon plasma source. These parameters mainly include the gas pressure, externally applied static magnetic field, the length and radius of the helicon antenna, and the frequency and voltage amplitude of the applied RF power on the helicon antenna. It is shown that, while the strong radial gradient of the formed plasma density in the proximity of the plasma surface is substantially proportional to the energy absorption from the existing Trivelpiece-Gould (TG) modes, the observed high electron temperature in the helicon source at lower static magnetic fields is significant evidence for the energy absorption from the helicon modes. Furthermore, it is found that, at higher gas pressures, both the plasma electron density and temperature are reduced. Besides, it is shown that, at higher static magnetic fields, owing to the enhancement of the energy absorption by the plasma charged species, the plasma electron density is linearly increased. Moreover, it is seen that, at the higher spatial dimensions of the antenna, both the plasma electron density and temperature are reduced. Additionally, while, for the applied frequencies of 13.56 MHz and 27.12 MHz on the helicon antenna, the TG modes appear, for the applied frequency of 18.12 MHz on the helicon antenna, the existence of helicon modes is proved. Moreover, by increasing the applied voltage amplitude on the antenna, the generation of mono-energetic electrons is more probable.

Influence of the axial magnetic field on sheath development after current zero in a vacuum circuit breaker

After current zero, which is the moment when the vacuum circuit breaker interrupts a vacuum arc, sheath development is the first process in the dielectric recovery process. An axial magnetic field (AMF) is widely used in the vacuum circuit breaker when the high-current vacuum arc is interrupted. Therefore, it is very important to study the influence of different AMF amplitudes on the sheath development. The objective of this paper is to study the influence of different AMF amplitudes on the sheath development from a micro perspective. Thus, the particle in cell–Monte Carlo collisions (PIC–MCC) method was adopted to develop the sheath development model. We compared the simulation results with the experimental results and then validated the simulation. We also obtained the speed of the sheath development and the energy density of the ions under different AMF amplitudes. The results showed that the larger the AMF amplitudes are, the faster the sheath develops and the lower the ion energy density is, meaning the breakdown is correspondingly more difficult.

Study of terahertz generation via the interaction of two-color ultra-short laser pulses with water vapor plasmas

In this work, using a two dimensional particle in cell-Monte Carlo collision simulation scheme, the Terahertz (THz) generation process via the interaction of a two-color ultra-short laser pulses with the water vapor gas (H2O) is examined. The background gas pressure and various laser parameters, e.g., its pulse shape, duration, and waist, are varied, and their effects on the temporal variation of the generated current density, THz electric field, and THz spectral intensity are studied. It is shown that the best pulse shape generating the THz signal radiation with the highest intensity is a trapezoidal pulse. Moreover, the intensity of generated THz radiation is increased at the higher pulse durations and waists. In addition, at the higher water vapor gas pressures, the time to peak of the generated current density is shifted to the earlier moments. Finally, it is observed that, for the laser pulses with the intensities of about 8 × 1013 W/cm2, the water vapor triatomic molecules are a proper source for the THz radiation generation under the illumination of high power ultra-short two-color laser pulses.

Kinetic study of terahertz generation based on the interaction of two-color ultra-short laser pulses with molecular hydrogen gas

In this work, using a two dimensional particle in cell-Monte Carlo collision simulation scheme, interaction of two-color ultra-short laser pulses with the molecular hydrogen gas (H2) is examined. The operational laser parameters, i.e., its pulse shape, duration, and waist, are changed and, their effects on the density and kinetic energy of generated electrons, THz electric field, intensity, and spectrum are studied. It is seen that the best pulse shape generating the THz signal radiation with the highest intensity is a trapezoidal pulse, and the intensity of generated THz radiation is increased at the higher pulse durations and waists. For all the operational laser parameters, the maximum value of emitted THz signal frequency always remains lower than 5 THz. The intensity of applied laser pulses is taken about 1014 w/cm2, and it is observed that while a small portion of the gaseous media gets ionized, the radiated THz signal is significant.

Influences of different gases on the terahertz radiation based on the application of two-color laser pulses

In this work, using a two-dimensional Particle In Cell-Monte Carlo Collision simulation method, a comparative study is performed on the influences of different types of atomic and molecular gases at various background gas pressures on the generation of broadband and intense Terahertz (THz) radiation via the application of two-color laser pulses. These two modes are focused into Argon (Ar), Xenon (Xe), Nitrogen (N2), Oxygen (O2), and air as the background gaseous media and the plasma channel is created. It is observed that the THz radiation emission dramatically changes due to the propagation effects. A wider THz pulse is emitted from the formed plasma channel at the higher gas pressures. The significant effects of the propagation features of the emitted THz pulse on its energy at the longer lengths of the plasma channel are observed.

Parametric study of broadband terahertz radiation generation based on interaction of two-color ultra-short laser pulses

In this work, using a two-dimensional kinetic model based on particle in cell-Monte Carlo collision simulation method, the influence of different parameters on the broadband intense Terahertz (THz) radiation generation via application of two-color laser fields, i.e., the fundamental and second harmonic modes, is studied. These two modes are focused into the molecular oxygen (O2) with uniform density background gaseous media and the plasma channels are created. Thus, a broadband THz pulse that is around the plasma frequency is emitted from the formed plasma channel and co-propagates with the laser pulse. For different laser pulse shapes, the THz electric field and its spectrum are both calculated. The effects of laser pulse and medium parameters, i.e., positive and negative chirp pulse, number of laser cycles in the pulse, laser pulse shape, background gas pressure, and exerted DC electric field on THz spectrum are verified. Application of a negatively chirped femtosecond (40 fs) laser pulse results in four times enhancement of the THz pulse energy (2 times in THz electric field). The emission of THz radiation is mostly observed in the forward direction.

A hybrid model for simulation of secondary electron emission in plasma immersion ion implantation under different pulse rise time

A hybrid fluid Particle in Cell–Monte Carlo Collision (PiC–MCC) model is presented to study the effect of secondary electron emission on the plasma immersion ion implantation process under different pulse rise time. The model describes the temporal evolution of various parameters of plasma such as ion density, ion velocity, secondary electron density, and secondary electron current for different rise times. A 3D–3 V PiC–MCC model is developed to simulate the secondary electrons which are emitted from the sample surface while the plasma ions and electrons are treated using a 1D fluid model. The simulation results indicate that the secondary electron density and secondary electron current increase as the rise time decreases. The main differences between the results for different rise times are found during the initial phase of the pulse. The results are explained through studying the fundamental parameters of plasma.

Theoretical study on the stream formation in the nitrogen switch

The stream formation in a 1-atm nitrogen gas switch is investigated by the two-dimensional and three-velocity (2D3V) particles through the cell-Monte Carlo collision (PIC-MCC) simulation and theoretical analysis. For simplicity, two parallel plane electrodes of 0.6 mm width are separated by a distance of 1.6 mm. It is found that the analytical solution of the electron density equation can be used to study the evolution of the plasma before the stream breaks down, for the ionization frequency, mean electron energy and electron drift velocity are all constant. After the breakdown of the stream, random collisions destroy the symmetry of the plasma region and cause plasma to branch. As plasma density increases, the electric field inside the plasma region decreases due to the shielding effect. However, charge densities at both ends of the plasma region increase and the density at the anode end is larger than that at the cathode end, for the plasma exponentially grows as electrons move from the cathode toward the anode. This causes the electric field at the end of plasma near the anode to be larger than that near the cathode. It is found that the electrons can achieve their stable mean energy in several picoseconds due to the high transfer frequency (1011-1012 Hz) of the electron energy in the nitrogen plasma. After the breakdown of the stream, the mean electron energy decreases due to the decrease of the electron energies inside the plasma. By increasing the electrode voltage, it is found that the mean electron energy increases, the electron drift velocity increases linearly, and the variation rate of ionization frequency with electric field is in a range between E4 and E5. Therefore, the time taking for breaking down the stream decreases with the increase of the electrode voltage.

Numerical Simulation of Characteristics of CEX Ions in Ion Thruster Optical System

Charge exchange(CEX) ions could inflict severe damages on the ion thruster optical system. This article is aimed at investigating the characteristics of the CEX ions and their influences upon the optical system by means of particle-in- cell(PIC) ion simulation and Monte Carlo collision(MCC) methods. The results from numerical simulation indicate that despite the fact that CEX ions appear in the entire beamlet region near the ion optical system, the ones that present themselves downstream of the accelerator grid have good reason for attracting more attention. As their trajectories are significantly affected by the local electric field, a great number of CEX ions are accelerated toward grids resulting in sputtering erosion. When the influences of the CEX ions are considered in the numerical simulation, there could hardly be observed augments in the screen grid current, but the accelerator grid current increases from zero to 1. 4% of the beamlet current. It can be understood from the numerical simulation that the CEX ions formed in the region far downstream of the accelerator grid should be blamed for the erosion on the downstream surface of the accelerator grid.