Effect of Segmented Electrode Length on the Performances of an Aton-Type Hall Thruster

The influences of the low-emissive graphite segmented electrode placed near the channel exit on the discharge characteristics of a Hall thruster are studied using the particlein-cell method.A two-dimensional physical model is established according to the Hall thruster discharge channel configuration.The effects of electrode length on the potential,ion density,electron temperature,ionization rate and discharge current are investigated.It is found that,with the increasing of the segmented electrode length,the equipotential lines bend towards the channel exit,and approximately parallel to the wall at the channel surface,the radial velocity and radial flow of ions are increased,and the electron temperature is also enhanced.Due to the conductive characteristic of electrodes,the radial electric field and the axial electron conductivity near the wall are enhanced,and the probability of the electron-atom ionization is reduced,which leads to the degradation of the ionization rate in the discharge channel.However,the interaction between electrons and the wall enhances the near wall conductivity,therefore the discharge current grows along with the segmented electrode length,and the performance of the thruster is also affected.     

Numerical simulation of the plasma acceleration process in a magnetically enhanced micro-cathode vacuum arc thruster

A particle-in-cell simulation is conducted to investigate the plasma acceleration process in a micro-cathode vacuum arc thruster. A coaxial electrode structure thruster with an applied magnetic field configuration is used to investigate the effects of the distribution of the magnetic field on the acceleration process and the mechanism of electrons and ions. The modeling results show that due to the small Larmor radius of electrons, they are magnetized and bound by the magnetic field lines to form a narrow electron channel. Heavy ions with a large Larmor radius take a long time to keep up with the electron movement. The presence of a magnetic field strengthens the charge separation phenomenon. The electric field caused by the charge separation is mainly responsible for the ion acceleration downstream of the computation. The impact of variations in the distribution of the magnetic field on the acceleration of the plasma is also investigated in this study, and it is found that the position of the magnetic coil relative to the thruster exit has an important impact on the acceleration of ions. In order to increase the axial velocity of heavy ions, the design should be considered to reduce the confinement of the magnetic field on the electrons in the downstream divergent part of the applied magnetic field.     

Study on the characteristics of non-Maxwellian magnetized sheath in Hall thruster acceleration region

The secondary electron emission (SEE) and inclined magnetic field are typical features at the channel wall of the Hall thruster acceleration region (AR), and the characteristics of the magnetized sheath have a significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this work, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with SEE in the AR of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameter and magnetic field on the AR sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the $x$ direction acting as a resistance decelerating ions becomes larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential, secondary electron number density and sheath thickness.     

Study on the influences of ionization region material arrangement on Hall thruster channel discharge characteristics

There exists strong interaction between the plasma and channel wall in the Hall thruster,which greatly affects the discharge performance of the thruster.In this paper,a two-dimensional physical model is established based on the actual size of an Aton P70 Hall thruster discharge channel.The particle-in-cell simulation method is applied to study the influences of segmented low emissive graphite electrode biased with anode voltage on the discharge characteristics of the Hall thruster channel.The influences of segmented electrode placed at the ionization region on electric potential,ion number density,electron temperature,ionization rate,discharge current and specific impulse are discussed.The results show that,when segmented electrode is placed at the ionization region,the axial length of the acceleration region is shortened,the equipotential lines tend to be vertical with wall at the acceleration region,thus radial velocity of ions is reduced along with the wall corrosion.The axial position of the maximal electron temperature moves towards the exit with the expansion of ionization region.Furthermore,the electron-wall collision frequency and ionization rate also increase,the discharge current decreases and the specific impulse of the Hall thruster is slightly enhanced.     

Simulation study on electron heating characteristics in magnetic enhancement capacitively coupled plasmas with a longitudinal magnetic field

The electron heating characteristics of magnetic enhancement capacitively coupled argon plasmas in presence of both longitudinal and transverse uniform magnetic field have been explored through both theoretical and numerical calculations. It is found that the longitudinal magnetic field can affect the heating by changing the level of the pressure heating along the longitudinal direction and that of the Ohmic heating along the direction which is perpendicular to both driving electric field and the applied transverse magnetic field, and a continuously increased longitudinal magnetic field can induce pressure heating to become dominant. Moreover, the electron temperature as well as proportion of some low energy electrons will increase if a small longitudinal magnetic field is introduced, which is attributed to the increased average electron energy. We believe that the research will provide guidance for optimizing the magnetic field configuration of some discharge systems having both transverse and longitudinal magnetic field.     

空间电荷透镜电子云状态研究

本文分析了新型空间电荷透镜磁镜的轴对称磁场(z,r)平面的磁感应强度的变化曲线,由电子的磁流体运动方程组,推导了处于动态平衡状态下电子云中电子密度的波尔兹曼分布。结合磁镜场的磁感应强度分布和电子的波尔兹曼分布,采用四阶龙格 库塔公式对泊松方程进行二阶微分方程数值求解,得到了理想空间透镜中电子云的电势分布。在磁镜场的磁场基础上分析了空间电荷透镜中心轴线的磁约束因子沿z轴方向变化的趋势和数值模拟中心电子密度、磁约束因子、电子温度对电子云状态的影响。磁约束因子越大,对电子云的约束能力越强,电子集中在空间电荷透镜中心轴附近区域的现象越明显;电子温度越高,电子运动越活跃,弥散空间越大,电子集中在空间电荷透镜中心轴附近区域的现象越不明显。     

The effect of anode axial position on the performance of a miniaturized cylindrical Hall thruster with a cusp-type magnetic field

A 200 W cylindrical Hall thruster with a cusp-type magnetic field was proposed, manifesting convergent plume and high specific impulse. In this paper, a series of ring-shaped anodes are designed and the influence of anode axial position on the performance of CHT with a cusp-type magnetic field is studied. The experimental results indicate that the thruster keeps stable operation at the condition of 140–270 W discharge power. When the anode moves axially towards the upstream cusp field, the thrust enhances from 6.5 mN to 7.6 mN and specific impulse enhances from 1658 s to 1939 s significantly. These improvements of thruster performance should be attributed to the enhancement of current utilization, propellant utilization and acceleration efficiency. According to the analyses on the discharge characteristics, it is revealed that as the anode moves upstream, the electron transport path could be extended, the magnetic field in this extended path could impede electron cross-field transport and facilitate the ionization intensity, yielding to the enhancement of current utilization and propellant utilization efficiency. Moreover, along with this enhancement of upstream ionization at the given anode flow rate, the main ionization region is thought to move upstream and then separate more apparently from the acceleration region, which has been demonstrated by the narrowing of ion energy distribution function shape. This change in acceleration region could decrease the ion energy loss and enhance acceleration efficiency. This work is beneficial for optimizing the electrode structure of thruster and recognize the ionization and acceleration process under the cusp magnetic field.     

Effects of trapped electrons on the ion temperature gradient mode in tokamak plasmas with hollow density profiles

The ion temperature gradient(ITG) mode in the presence of impurity ions and trapped electrons(TEs) is numerically investigated in tokamak plasmas with hollow density profiles, using the gyrokinetic integral eigenmode equation. It is found that in the inverted density plasma, the increase of the ITG enhances the growth rate and frequency of the ITG, and the density gradient plays an important role in the ITG modes. For weak density gradient situations, the trapped electron effects increase the instability of the ITG, while the impurity has an obviously stabilizing effect. For the strong density gradient cases, both the impurities and trapped electrons enhance the ITG instabilities. In addition, it is shown that the growth rate of the ITG decreases with positive magnetic shear s while the real frequency increases with positive magnetic shear.The growth rate of the ITG increases with negative magnetic shear s while the real frequency decreases with negative magnetic shear. The length of the calculated mode structure in the positive and negative magnetic shear intervals is also presented.     

Study of a new cusp field for an 18 GHz ECR ion source

A feasibility study was performed to generate new sufficient mirror cusp magnetic field (CMF) by using the coils of the existing room temperature traditional 18 GHz electron cyclotron resonance ion source (ECRIS) at RIKEN. The CMF configuration was chosen because it contains plasma superbly and no multipole magnet is needed to make the contained plasma quiescent with no magneto-hydrodynamic (MHD) instability and to make the system cost-effective. The least magnetic field, 13 kG is achieved at the interior wall of the plasma chamber including the point cusps (PC) on the central axis and the ring cusp (RC) on the mid-plane. The mirror ratio calculation and electron simulation were done in the computed CMF. It was found to contain the electrons for longer time than in traditional field. It is proposed that a powerful CMF ECRIS can be constructed, which is capable of producing intense highly charged ion (HCI) beam for light and heavy elements.     

Simulation study on the influence of magnetic field in the near-anode region on anode power deposition of ATON-type Hall thruster

The highest deposition of power and temperature is always near the cusp of the ATON-type Hall thruster. This shows that when there are electrons gathering at the cusp, the distribution of heat load will be uniform, which will potentially damage the reliability. Therefore, we optimize the magnetic field near the anode. We changed the magnetic field characteristics in the near-anode region with an additional magnetic screen, and performed numerical simulation with particle-incell simulation. The simulation results show that the magnetic field of the thruster with the additional magnetic screen can alleviate the over-concentration of power deposition on the anode and reduce the power deposition in the anode by 20%, while ensuring that the overall magnetic field characteristics do not change significantly.     

Design Studies of a Multicusp Ion Source with FEMLAB Simulation

Femlab simulations have been used to arrive at the first step in the design of an ion source. The goal is to optimize Magnetic multipole plasma confinement geometries, the increased area of constant magnetic field in the central region of the plasma volume and the increase in number of electrons which have stationary orbits within this region of the field. The confinement of electrons is essential for Multicusp ion source to produce intense beams of negative hydrogen ions (H⁻). A higher electron temperature and density given by a better plasma confinement leads to the higher efficiencies of the ionization and the production of highly charged ions. We have performed Femlab simulations of the magnetic flux density from permanent magnet used for a Multicusp ion source, plasma confinement and trapping of fast electrons by the magnetic field.     

Study on plasma sheath and plasma transport properties in the azimuthator

A physical model of transport in an azimuthator channel with the sheath effect resulting from the interaction between the plasma and insulation wall is established in this paper.Particle in cell simulation is carded out by the model and results show that,besides the transport due to classical and Bohm diffusions,the sheath effect can significantly influences the transport in the channel.As a result,the ion density is larger than the electron density at the exit of azimuthator,and the non-neutral plasma jet is divergent,which is unfavorable for mass separation.Then,in order to improve performance of the azimuthator,a cathode is designed to emit electrons.Experiment results have demonstrated that the auxiliary cathode can obviously compensate the space charge in the plasma.     

直流电弧等离子体点火器射流特性研究

采用光谱仪测量了等离子体点火器出口射流的发射光谱,利用玻尔兹曼曲线斜率法计算了射流的电子温度,并通过电离平衡方程计算了射流气体温度,获得点火器出口射流长度、射流速度、电子温度和射流温度随弧电流及进口氩气流量的变化规律。并分析了航空等离子体电弧射流中是否可使用电子温度来代替射流气体温度。实验表明：弧电流随着进口氩气流量的增大而减小;出口射流长度和速度随弧电流的增大而增大,随进口氩气流量的增大先增大后减小;出口电子温度、电子密度和射流温度随弧电流的增大而升高,随氩气流量的增大而降低。     

Shubnikov-De Haas Oscillations Caused by a Two-Dimensional Leakage Channel on Silicon Mosfets Induced by Electron Irradiation

A possible failure mechanism of n-channel MOSFETs exposed to ionizing radiation are leakage currents between source and drain. This leakage begins to occur at relatively low radiation doses, and can seriously degrade the performance of an otherwise hardened CMOS device. In the present paper, we demonstrate by the observation of quantum oscillations in the magnetoresistance that by room temperature irradiation with high energy electrons an inversion layer is generated under the field oxide, which gives rise to leakage currents. Magnetoresistance oscillations may be observed, if in a degenerate electron gas the carrier mobility is sufficiently high to allow quantization of the electron movement in a magnetic field and the thermal energy is much smaller than the cyclotron energy. This oscillatory behaviour of the electrical resistance as a function of a magnetic field - the Shubnikov-de Haas effect - is a valuable tool to investigate the electronic parameters of bulk semiconductors and surface channels. Such experiments not only give firm evidence about the existence of a bypass inversion layer, but also yield information about the carrier density in the radiation induced channel. It turns out that the electrons behave like a two-dimensional free electron gas the density of which is independent of the gate voltage. Moreover, the experiments give knowledge about the influence of trapped charges, caused by the irradiation at the interface.     

Diagnosis and dynamics in a simple low energy medium current electron beam channel

We present a simple experimental setup and an associated method enabling both the non-destructive diagnosis and the calculation of the beam evolution in a low energy medium current electron beam channel, where the space-charge and emittance effects are comparable. The diagnosis makes use of an axially symmetric magnetic lens while a second lens is added to increase the flexibility in the beam processing. The paper emphasizes the three steps involved in the method: the evaluation of the lenses' magnetic field by numerical simulation, the beam diagnosis, and the computation of the beam envelope. The calculation of the magnetic field is based on the finite element method. Subsequently, the beam parameters at the electron source exit – emittance and cross-over radius and position – are found with the modified three gradient method. Finally, the beam dynamics are modeled with the K–V equation adapted for the particular case of axial symmetry. The results obtained in this paper can be used to optimize technological processes, such as welding, hardening, cladding, and surface alloying.     

Investigation of the self-induced magneticfield characteristics in a pulsed plasma thruster with flared electrodes

The self-induced magnetic field in a pulsed plasma thruster(PPT)with flared electrodes is investigated for a better understanding of the working process and the structural design of the thruster.A two-dimensional model of the magnetic field is built and is validated by comparing the simulated results with the experimental results in literature.The magnetic flux density in the discharge channel during the working process is presented and analyzed regarding the electrode structures.The calculated magnetic field flux density decreases from 0.8 T at the upstream to 0.1 T and below at the downstream in the discharge channel(68 J).The peak of the magnetic flux density over time lags behind the current peak,which provides evidence for the existence of a moving plasma sheet in the discharge process.The magnetic field induced by the current in the extra bending part of the anode enhances the Lorentz force,which acts on the charged particles near the propellant.Finally,the geometric study indicates that the electromagnetic impulse bit does not monotonically increase with the flared angle of the electrodes.Instead,it reaches a maximum at a certain flared angle,which could provide significant suggestions for structural optimization.     

PIC simulation of plasma properties in the discharge channel of a pulsed plasma thruster with flared electrodes

Plasma in the discharge channel of a pulsed plasma thruster (PPT) with flared electrodes is simulated by a self-developed two-dimensional code. The fully particle-in-cell method with Monte Carlo collision is employed to model the particle movement and collisions and investigate the plasma properties and acceleration process. Temporal and spatial variations of the electron density distribution and the ion velocity between electrodes are calculated and analyzed in detail. The computational results of the electron number density, which is in the order of 10²³ m⁻³, show good agreements with experimental results of a PPT named ADD SIMP-LEX. The ion velocity distributions along the center line of the channel lead to a comprehensive understanding of ions accelerated by electromagnetic field. The electron distributions of PPT with discharge voltages varying from 1300 to 2000 V are compared. The diffusion of electrons presents strong dependency on discharge voltage and implies higher degree of ionization for higher voltage.     

Study of axial double layer in helicon plasma by optical emission spectroscopy and simple probe

In this work we used a passive measurement method based on a high-impedance electrostatic probe and an optical emission spectroscope (OES) to investigate the characteristics of the double layer (DL) in an argon helicon plasma.The DL can be confirmed by a rapid change in the plasma potential along the axis.The axial potential variation of the passive measurement shows that the DL forms near a region of strong magnetic field gradient when the plasma is operated in wavecoupled mode,and the DL strength increases at higher powers in this experiment.The emission intensity of the argon atom line,which is strongly dependent on the metastable atom concentration,shows a similar spatial distribution to the plasma potential along the axis.The emission intensity of the argon atom line and the argon ion line in the DL suggests the existence of an energetic electron population upstream of the DL.The electron density upstream is much higher than that downstream,which is mainly caused by these energetic electrons.     

Calculating the electron temperature in the lightning channel by continuous spectrum

Based on the theory of plasma continuous radiation,the relationship between the emission intensity of bremsstrahlung and recombination radiation and the plasma electron temperature is obtained.During the development process of a return stroke of ground flash,the intensity of continuous radiation spectrum is separated on the basis of the spectrums with obviously different luminous intensity at two moments.The electron temperature of the lightning discharge channel is obtained through the curve fitting of the continuous spectrum intensity.It is found that electron temperature increases with the increase of wavelength and begins to reduce after the peak.The peak temperature of the two spectra is close to 25 000 K.To be compared with the result of discrete spectrum,the electron temperature is fitted by the O I line and N II line of the spectrum respectively.The comparison shows that the high temperature value is in good agreement with the temperature of the lightning core current channel obtained from the ion line information,and the low temperature at the high band closes to the calculation result of the atomic line,at a low band is lower than the calculation of the atomic line,which reflects the temperature of the luminous channel of the outer corona.     

Measurement of Ion Parameters by Ion Sensitive Probe in ECR Plasma

Ion parameters in electron cyclotron resonance (ECR) microwave plasma were measured by ion sensitive probe and were compared with the electron parameters obtained by double Langmuir probe. The effects of gas pressure and microwave power on the ion temperature and density were analyzed. The spatial distribution of the ion parameters was also investigated by the ion sensitive probes with a tunable radial depth installed on different probe windows along the chamber axis. Results showed that the ion density measured by the ion sensitive probe was in good agreement with the electron density measured by the double Langmuir probe. The influence of gas pressure on the ion parameters was stronger than that of microwave power. With the increase in working pressure, the ion temperature decreased monotonously with a decreasing rate larger than that at higher pressure. The ion density first increased to a peak (42.3?1010cm-3) at 1 Pa and then decreased. The ion temperature and density increased little with the increase in the microwave power from 400 W to 800 W. The plasma far away from the resonant point is found to be radially uniform.