Particle‐in‐cell simulation of an optimized high‐efficiency multistage plasma thruster

Electric propulsion attracts increasing attention in contemporary space missions as an interesting alternative to chemical propulsion because of the high efficiency it offers. The High‐Efficiency Multistage Plasma thruster, a class of cusped field thruster, is able to operate at different anode voltages and operation points and thereby generate different levels of thrust in a stable and efficient way. Since experiments of such thrusters are inherently expensive, multi‐objective design optimization (MDO) is of great interest. Several optimized thruster designs have resulted from a MDO model based on a zero‐dimensional (0D) power balance model. However, the MDO solutions do not warrant self‐consistency due to their dependency on estimation from empirical modelling based on former experimental studies. In this study, one of the optimized thruster designs is investigated by means of particle‐in‐cell (PIC) analysis to examine the predicted performance characteristics with self‐consistent simulations. The 0D power balance model is used to develop additional diagnostics for the PIC simulations to improve the physics analysis. Using input parameters for the 0D power balance model from the PIC simulations allows further improvement for the design optimization.     

Particle‐in‐cell simulation of the effect of curved magnetic field on wall bombardment and erosion in a hall thruster

A curved, convex towards the channel bottom magnetic field is an important feature of an advanced Hall thruster that allows confining the plasma flow in the channel center, reducing the divergence angle of the ejected ion beam, and improving the discharge performance. In this article, the discharge behaviour of a Hall thruster in magnetic fields with different degrees of curvature is simulated with a particle‐in‐cell numerical method, and the effect of curved magnetic field on the ion bombardment and wall erosion and the associated mechanisms are studied and analysed. The results show that, as the curvature of the magnetic field increases, the propellant ionization becomes more confined at the channel center, the potential drop inside the channel decreases, and the acceleration region shifts outside the channel, which lead to the attenuation of the ion energy bombarding the wall and the deviation of the bombardment angle from the optimal sputtering angle. Conversely, the ion flux bombarding the wall near the channel exit increases. Nevertheless, the bombardment energy and angle are the dominant factors for the wall erosion, and the wall erosion rate clearly decreases with the increasing curvature of the magnetic field. These findings are closely related to the behaviour of electron conduction under a curved magnetic field; the relevant mechanisms are clarified in this article.     

2 cm电子回旋共振离子推力器离子源中磁场对等离子体特性与壁面电流影响的数值模拟

电子回旋共振离子推力器(electron cyclotron resonance ion thruster,ECRIT)离子源内等离子体分布会影响束流引出,而磁场结构决定的ECR区与天线的相对位置共同影响了等离子体分布.在鞘层作用下,等离子体中的离子或电子被加速对壁面产生溅射,形成壁面离子或电子电流,造成壁面磨损和等离子体损失,因此研究壁面电流与等离子体特征十分重要.为此本文建立2 cm ECRIT的粒子PIC/MCC(particle-in-cell with Monte Carlo collision)仿真模型,数值模拟研究磁场结构对离子源内等离子体与壁面电流特性的影响.计算表明,当ECR区位于天线上游时,等离子体集中在天线上游和内外磁环间,栅极前离子密度最低,故离子源引出束流、磁环端面电流和天线壁面电流较低.ECR区位于天线下游时,天线和栅极上游附近的等离子体密度较高,故离子源引出束流、天线壁面电流和磁环端面电流较高.腔体壁面等离子体分布与电流受磁场影响最小.     

Study on the Relation Between Discharge Voltage and Magnetic Field Topography in a Hall Thruster Discharge Channel

The relation between magnetic field topography and operating voltage is investigated in a 1kW Hall thruster discharge channel in order to focus the ion beam effectively and optimize the performance. The curvature of magnetic field line (α) is introduced to characterize the differences of topologies. The optimized magnetic field distribution under each operating voltage is obtained by experiment. Through the curvature transformation, we find that the area of (α > 1) in the channel gradually decreases with the increase of the operating voltage. In response to the results above, two dimensional plasma flows are simulated employing Particle‐in‐Cell method. The distributions of the electric potential, ion density and ion radial velocity are calculated to understand the important influence of the relation above on ion beam focusing. The numerical results indicate that magnetic field curvature and thermal electric field control the ion beam in the ionization and acceleration zone, respectively. The magnetic field topography and discharge voltage interact with each other and together form the focusing electric field. The ion radial mobility is suppressed effectively and the ion beam is focused to the channel centerline. In addition, for a given voltages, when the area of (α > 1) is larger than the optimal scope, the electric potential lines excessively bend to the anode causing ion over focus; contrarily, the electric potential lines will bend to the exit and defocus ions. All these results suggest the relation between magnetic field topography and discharge voltage is important to the ion radial flow control and performance optimization of the Hall thruster (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stopping of Heavy Ions in Magnetized Strongly Coupled Plasmas: High‐Velocity Limit

The energy loss of a heavy ion moving in a magnetized strongly coupled electron plasma is considered within the linear response treatment and in high‐velocity regime. The analytical expressions for the stopping power have been found for the arbitrary ion incidence angle. The obtained general expression for the stopping power is analyzed for the ion which moves parallel or perpendicular to the magnetic field. It is found that in general the magnetic field and the Coulomb coupling reduce the stopping power as well as the dynamical screening length at high velocities. The influence of the magnetic field and the Coulomb coupling on the high‐velocity stopping power is discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dust‐ion‐acoustic solitary waves in a magnetized dusty pair‐ion plasma with Cairns‐Gurevich electrons and opposite polarity dust particles

The nonlinear dust‐ion‐acoustic (DIA) solitary structures have been studied in a dusty plasma, including the Cairns‐Gurevich distribution for electrons, both negative and positive ions, and immobile opposite polarity dust grains. The external magnetic field directed along the z‐axis is considered. By using the standard reductive perturbation technique and the hydrodynamics model for the ion fluid, the modified Zakharov–Kuznetsov equation was derived for small but finite amplitude waves and was provided the solitary wave solution for the parameters relevant. Using the appropriate independent variable, we could find the modified Korteweg–de Vries equation. By plotting some figures, we have discussed and emphasized how the different plasma values, such as the trapping parameter, the positive (or negative) dust number density, the non‐thermal electron parameter, and the ion cyclotron frequency, can influence the solitary wave structures. In addition, using the bifurcation theory of planar dynamical systems, we have extracted the centre and saddle points and illustrated the phase portrait of such a system for some particular plasma parameters. Finally, we have graphically investigated the behaviour of the solitary energy wave by changing the plasma values as well as by calculating the instability criterion; we have also discussed the growth rate of the solitary waves. The results could be useful for studying the physical mechanism of nonlinear propagation of DIA solitary waves in laboratory and space plasmas where non‐thermal electrons, pair‐ions, and dust particles can exist.     

Laser‐Triggered Proton Acceleration From Micro‐Structured thin Targets

By using relativistic massively parallel PIC code MANDOR, which features arbitrary target design including 3D micro‐structuring, a study of ion acceleration in short laser pulse interaction with different thin targets has been performed. Based on 3D simulation results it has been shown that micro‐structures on the front surface of thin plane targets increase a number and energy of hot electrons in comparison with that for the case of pure plain foils of optimal thickness. As a result, the energy of accelerated ions also increases up to 50%. However, the efficiency of ion acceleration from structured target reduces with laser pulse intensity increase, so that for laser pulses of ultra‐relativistic intensity a positive role of surface micro‐structuring diminishes. We have also studied to which extent a sub‐ps imperfection of the laser pulse shape, which smoothes the surface micro‐structures suppresses high‐energy ion generation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

霍尔推进器中磁化二次电子对鞘层特性的影响

为进一步研究霍尔推进器壁面二次电子发射对推进器性能的影响,采用流体模型数值模拟了二次电子磁化效应的等离子体鞘层特性.得到二次电子磁化鞘层的玻姆判据.讨论了不同的磁场强度和方向、二次电子发射系数以及不同种类等离子体推进器的鞘层结构.结果表明:随器壁二次电子发射系数的增大,鞘层中粒子密度增加,器壁电势升高,鞘层厚度减小;鞘层电势及粒子密度随着磁场强度和方位角的增加而增加;而对于不同种类的等离子体,壁面电势和鞘层厚度也不同.这为霍尔推进器的磁安特性实验提供了理论解释. 关键词： 霍尔推进器 磁鞘 二次电子     

Ion‐ and positron‐acoustic solitons in magnetized dusty plasma with q‐non‐extensive electron and positron velocity distribution

In this study, the properties of ion‐ and positron‐acoustic solitons are investigated in a magnetized multi‐component plasma system consisting of warm fluid ions, warm fluid positrons, q‐non‐extensive distributed positrons, q‐non‐extensive distributed electrons, and immobile dust particles. To drive the Korteweg–de Vries (KdV) equation, the reductive perturbation method is used. The effects of the ratio of the density of positrons to ions, the temperature of the positrons, and ions to electrons, the non‐extensivity parameters q_e and q_p , and the angle of the propagation of the wave with the magnetic field on the potential of ion‐ and positron‐acoustic solitons are also studied. The present investigation is applicable to solitons in fusion plasmas in the edge of tokamak.     

On the Effect of the Surface State and Geometry of the Discharge- Chamber Edges and Sizes of Morozov’s Stationary Plasma Thruster upon its Long-Term Operation

The results of long-term tests of Morozov’s stationary plasma thrusters are presented. It is revealed how the surface state and geometry of the discharge chamber’s edges influence the thruster’s parameters. It is shown that, during the ground tests of thrusters with cylindrical geometry of the acceleration channel under initial stage of operation, material sputtered from the discharge chambers’ walls is deposited onto the nearanode segment of the walls. Films of deposited material fail during thruster operation causing fragment formation, which jut out towards the discharge volume and disturb the motion of drifting electrons in the area of their acceleration. As a result the thruster reactive force and specific impulse decrease. The way in which the forming fragments influence thruster performance and operation is examined. It is shown that it decreases under long-term operation and significant channel widening since the ion flux to the wall and the quantity of the sputtered material decrease, and since the profile of the walls changes due to their wear and cleaning effect of the discharge. As a result the thruster’s parameters are restored to a level close to the initial one. It is shown that the dynamics of thruster parameters variation in space and during ground tests is different. This means that it is necessary to simulate more properly the conditions of thruster operation in space when conducting ground development tests. Thrusters with a long lifetime should be designed with widening of the acceleration channel beyond the loop which surrounds the magnetic system so that areas of acceleration and the erosion of walls are located in the widened part of the channel.     

Ion‐Acoustic Waves Instability in a Three Components Magneto‐Plasma with Nonthermal Electrons

A theoretical investigation has been made on obliquely propagating ion‐acoustic (IA) solitary structures in a three components magneto‐plasma containing cold inertial ions, Boltzmann distributed positrons, and hot non‐thermal electrons. The Zakharov‐Kuznetsov equation has been derived by the reductive perturbation method, and its solitary wave solution has been analyzed. Multi‐dimensional instability has also studied by the small‐k (long wave‐length plane wave) perturbation expansion technique, which is found to exist in such a plasma. The effects of the external magnetic field, nonthermal electrons, obliqueness and temperature ratio have significantly modified the basic properties of small but finite‐amplitude IA solitary waves, such as amplitude, width, instability criterion and the growth rate. The present investigation contributes to the physics of the nonlinear IA waves in space and laboratory electron‐positron‐ion magneto‐plasmas in which wave damping produces an electron tail. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dissipative ion‐acoustic waves in collisional electron‐positron‐ion plasmas with Kappa distribution

In this work, linear and non‐linear structures of ion‐acoustic waves (IAWs) are investigated in a collisional plasma consisting of warm ions, superthermal electrons, and positrons. A dissipative effect is assumed due to ion‐neutral collisions. The linear properties of IAWs are investigated. It is shown that the dynamics of the IAWs is governed by the damped Korteweg‐de Vries (K‐dV) equation. It is seen that the ion‐neutral collisions modify the basic features of ion‐acoustic solitary waves significantly. Also, the effect of the plasma parameters on the dissipative IAWs is discussed in detail.     

单链表在离子发动机光学系统粒子模拟中的应用

 目前离子发动机光学系统数值模拟大多采用PIC方法，由于该方法需要跟踪单个粒子的运动，因此需要存储每个粒子的位置信息与运动信息。传统的粒子模拟程序中，保存粒子信息大多采用数组的方式，但是采用这种方式存在弊端，例如对粒子总数变化的自适应不好。基于此开发了一种使用链式存储结构的粒子模拟程序，该程序使用带头节点的单向链表存储粒子信息。使用基于链式存储结构的PIC方法对离子发动机光学系统进行了粒子模拟，验证了链式粒子信息存储方法在粒子模拟中的可用性。模拟表明：（1）在粒子模拟中采用链式存储结构存储粒子信息，无需预先指定最大粒子总数，程序可自适应粒子总数的变化，因此无需进行试算，节省了计算时间；（2）在粒子模拟中采用链式存储结构，由于不存在内存资源的浪费，因而可显著提高程序的存储效率与计算效率。     

Near‐3‐MeV protons from target‐normal‐sheath‐acceleration femtosecond laser irradiating advanced targets

Advanced targets based on graphene oxide and gold thin film were irradiated at high laser intensity (10¹⁸–10¹⁹ W/cm²) with 50‐fs laser pulses and high contrast (10⁸) to investigate ion acceleration in the target‐normal‐sheath‐acceleration regime. Time‐of‐flight technique was employed with SiC semiconductor detectors and ion collectors in order to measure the ion kinetic energy and to control the properties of the generated plasma. It was found that, at the optimized laser focus position with respect to the target, maximum proton acceleration up to about 3 MeV energy and low angular divergence could be generated. The high proton energy is explained as due to the high electrical and thermal conductivity of the reduced graphene oxide structure. Dependence of the maximum proton energy on the target focal position and thickness is presented and discussed.     

Quantum regime of a plasma‐wave‐pumped free‐electron laser in the presence of an axial magnetic field

The quantum regime of a plasma‐whistler‐wave‐pumped free‐electron laser (FEL) in the presence of an axial‐guide magnetic field is presented. By quantizing both the plasma whistler field and axial magnetic field, an N‐particle three‐dimensional Hamiltonian of quantum‐FEL (QFEL) has been derived. Employing Heisenberg evolution equations and introducing a new collective operator which controls the vertical motion of electrons, a quantum dispersion relation of the plasma whistler wiggler has been obtained analytically. Numerical results indicate that, by increasing the intrinsic quantum momentum spread and/or increasing the axial magnetic field strength, the bunching and the radiation fields grow exponentially. In addition, a spiking behavior of the spectrum was observed with increasing cyclotron frequency which provides an enormous improvement in the coherence of QFEL radiation even in a limit close‐to‐classical regime, where an overlapping of these spikes is observed. Also, an upper limit of the intrinsic quantum momentum spread which depends on the value of the cyclotron frequency was found.     

Plasma-wall interactions in DC discharges and sheath of Langmuir probes

The plasma-wall interactions in various DC discharges and sheath of Langmuir probe are analyzed and discussed. The methods of their investigations are discussed including fluid and PIC MC. Various assumptions used in fluid models e.g. plasma neutrality, Bohm criterion, Boltzmann electrons approximation, etc. are analyzed. Ion heating and electron cooling effect at the DC plasma wall is discussed and explained. Langmuir probes measurements in high-temperature and ion thruster plasma are analyzed. The secondary electron emission influences the IV characteristic of Langmuir probe especially at positive voltages. However, only elastic reflection processes really contribute significantly to the probe current. The elastic SEE processes reflect electrons from probe with the same relatively high speed. It was observed that the axial magnetic field influences probe characteristics and floating potential more significantly than radial field. The axial field deflects all electrons approaching probe.     

Influence of bias magnetic field on the vacuum arc in double‐break vacuum circuit breakers

When double‐break vacuum circuit breakers (VCBs) interrupt the fault current, the series arc will generate their individual magnetic fields in different breaks. The magnetic field in one break will influence the arc in another break if the magnetic field is strong enough or the two breaks are very close. In this case, an interactive magnetic field effect happens. This field is also called the bias magnetic field (BMF). BMF can cause anode erosion and affect the performance at current zero. The distribution of BMF and the optimal configuration of the double‐break VCBs were obtained by the electromagnetic field simulation using the Ansoft Maxwell software. Based on the simulated magnetic field data, in the experiments, the interaction between the series vacuum arcs in double‐break VCBs was equivalent to the interaction between a single vacuum arc and the magnetic field generated by a Helmholtz coil. A high‐speed CMOS camera was used to record the trajectory of the vacuum arc plasma under different BMFs with different types of contacts. The results show the BMF can increase the arc voltage, and the arc becomes unstable. When the BMF becomes stronger, the arc voltage increases, and the arc becomes more unstable. In addition, for different types of contacts, the development process of the arc and the influence level are different under the same BMF. For a Wan‐type transverse magnetic field (TMF) contact or strong BMF, metal sputtering is evident and anode erosion becomes serious. For a cup‐type axial magnetic field (AMF) contact, the influence of BMF on the series arc plasma in double‐break VCBs is less than that of the Wan‐type TMF contact. The results of this work may be helpful for the design of compact double‐break VCBs.     

Study of low‐frequency oscillations in a double‐stage Hall thruster

The characteristics of low‐frequency oscillations of double‐stage Hall thrusters are quite different from those of conventional single‐stage Hall thrusters. In this paper, the effects of double‐stage discharge on the low‐frequency oscillations are experimentally investigated. The results indicate that the amplitude significantly decreases with increasing magnetic field strength and voltage during the ionization stage. Meanwhile, data analysis reveals that ionization occurs in both stages and that the transport ion current between the two stages is the key factor that affects the amplitude and main frequency of the oscillations. Two new processes, namely the ion transport and ion recombination caused by double‐stage discharge, are found to be relevant to the change in the current oscillations. To summarize, the ionization stage relieves the oscillations in the acceleration stage, leading to a reduction in the amplitude of the discharge current.