Effects of pulsed magnetic field on density reduction of high flow velocity plasma sheath

A three-dimensional model is proposed in this paper to study the effect of the pulsed magnetic field on the density distribution of high flow velocity plasma sheath. Taking the typical parameters of plasma sheath at the height of 71 km as an example, the distribution characteristics and time evolution characteristics of plasma density in the flow field under the action of pulsed magnetic field, as well as the effect of self-electric field on the distribution of plasma density, are studied. The simulation results show that pulsed magnetic field can effectively reduce the density of plasma sheath. Meanwhile, the simulation results of three-dimensional plasma density distribution show that the size of the density reduction area is large enough to meet the communication requirements of the Global Position System(GPS) signal. Besides, the location of density reduction area provides a reference for the appropriate location of antenna. The time evolution of plasma density shows that the effective density reduction time can reach 62% of the pulse duration, and the maximum reduction of plasma density can reach 55%. Based on the simulation results, the mechanism of the interaction between pulsed magnetic field and plasma flow field is physically analyzed. Furthermore, the simulation results indicate that the density distributions of electrons and ions are consistent under the action of plasma self-electric field.However, the quasi neutral assumption of plasma in the flow field is not appropriate, because the self-electric field of plasma will weaken the effect of the pulsed magnetic field on the reduction of electron density, which cannot be ignored. The calculation results could provide useful information for the mitigation of communication blackout in hypersonic vehicles.     

Improvement of Azimuthator Based on Particle Simulation

The azimuthator is an important part of plasma optical mass separation.The existing design for an azimuthator is based on the single particle orbit theory and focused on the movement of ions.In this paper,the particle simulation method is adopted to study the behavior of plasma crossing an azimuthator.The results show that electrons are bounded at the entrance of the azimuthator and an axial electric field is produced due to the charge separation.In order to better achieve the function of the azimuthator,a cathode is designed to transmit the electrons and to obtain a quasi-neutral plasma beam.     

The Effect of Gas Flow Rate on Radio-Frequency Hollow Cathode Discharge Characteristics

It is known that gas flow rate is a key factor in controlling industrial plasma processing. In this paper, a 2D PIC/MCC model is developed for an rf hollow cathode discharge with an axial nitrogen gas flow. The effects of the gas flow rate on the plasma parameters are calculated and the results show that： with an increasing flow rate, the total ion（N＋2, N＋） density decreases, the mean sheath thickness becomes wider, the radial electric field in the sheath and the axial electric field show an increase, and the energies of both kinds of nitrogen ions increase;and, as the axial ion current density that is moving toward the ground electrode increases, the ion current density near the ground electrode increases. The simulation results will provide a useful reference for plasma jet technology involving rf hollow cathode discharges in N2.     

The structure of an electronegative magnetized plasma sheath with non-extensive electron distribution

In this paper, an electronegative magnetized plasma sheath model with non-extensive electron distribution is established, and the Bohm criterion affected by the non-extensive parameter q is theoretically derived. The ion Mach number varies with q. The numerical simulation results show that compared with electronegative magnetized plasma sheath with Maxwell distribution (q = 1), the sheath structures with super-extensive distribution (q < 1) and sub-extensive distribution (q > 1) are different. The physical quantities including the sheath potential distribution, ion density distribution, the electron density distribution, negative ion density distribution and the net space charge density distribution are discussed. It is shown that the non-extensive parameter q has a significant influence on the structure of the electronegative magnetized plasma sheath. Due to the Lorentz force, both the magnitude and the angle of the magnetic field affect the structure of the sheath, whether the electrons are Maxwell distributed or non-extensively distributed.     

Research on phase shift characteristics of electromagnetic wave in plasma

The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma.     

Numerical study of atmospheric-pressure argon plasma jet propagating into ambient nitrogen

A 2D axial symmetry fluid model is applied to study the features of an atmospheric-pressure argon (Ar) plasma jet propagating into ambient nitrogen (N₂) driven by a pulsed voltage, emphasizing the influence of gas velocity on the dynamic characteristics of the jet. The results show that the Ar jet exhibits a cylindrical-shaped channel and the jet channel gradually shrinks with the increase in propagation length. The jet propagation velocity varies with time. Inside the dielectric tube, the plasma jet accelerates propagation and reaches its maximum value near the nozzle. Exiting the tube, its velocity quickly decreases and when approaching the metal plane, the decrease in jet velocity slows down. The increase in gas speed results in the variation of jet spatial distribution. The electron density presents a solid structure at lower gas flow speeds, whereas an annular structure can be observed under the higher gas flow velocity in the ionization head. The jet length increases with the flow velocity. However, when the flow velocity exceeds a critical value, the increase in the rate of the plasma jet length slows down. In addition, the gas velocity effect on the generation and transport of the reactive particles is also studied and discussed.     

阴极真空弧源磁过滤弯管偏压模式研究

为了改进阴极真空弧等离子体通过磁过滤弯管的传输效率，测定了磁过滤弯管出口离子电流与阴极弧流的关系，结果表明：磁过滤弯管内表面中，靠近大径中心一侧的表面（内侧面）和远离大径中心一侧的表面（外侧面）与等离子体的相互作用是独立的，存在两种向磁过滤弯等内表面运动的离子流：离子碰撞导致的横向扩散离子流和从阴极弧出来的较高能量的惯性离子流。两种离子流通过磁过滤管的传输过程有不同的机制。整个磁过滤弯管偏压较仅仅Bilek板偏压有更高的离子传输效率，Bilek板偏压对磁过滤弯管离子传输起主要作用。     

Three-dimensional non-equilibrium modeling of a DC multi-cathode arc plasma torch

In this paper, a three-dimensional non-equilibrium steady arc model is used to investigate the temperature, velocity and electromagnetic field in multi-cathode arc torch, and the formation mechanism of a large-area, uniform and diffused arc plasma is analyzed. The numerical simulation results show that a large volume plasma region can be formed in the central region of the generator during discharge. During this process, the maximum electron temperature appears near the cathode and in the central convergence region, while the maximum heavy particle temperature only appears in the central convergence region. This phenomenon is consistent with the experimental arc images. Near the cathode tip, the arc column is in a contraction state. In the area slightly away from the cathode, the six arc columns begin to join together. In the plasma generator, there is a large-scale current distribution in all directions of X, Y and Z, forming a stable arc plasma with a wide range of diffusion. The calculated electron temperature distribution is in good agreement with the measured electron temperature. The results suggest that the largearea diffused arc plasma in the multi-cathode arc torch is the combined effect of current distribution, convection heat transfer and heat conduction.     

Exploration of a MgO cathode for improving the intensity of pulsed discharge plasma at atmosphere

With regard to the lower density and energy of electrons in pulsed discharge plasma (PDP) at atmosphere, leading to the lower energy utilization of plasma, we propose a MgO cathode to enhance the plasma intensity according to field emission principle. The MgO cathode is prepared by an electro-depositing MgO film on a stainless steel plate. This way, the positive charges come to the cathode and accumulate on the surface of the MgO film, leading to the enhancement of the electric field intensity between the cathode and MgO film, and result in the strong emission of secondary electrons from the MgO cathode. As a result, the intensity of plasma can be enhanced. Herein, the effect of the MgO cathode on the intensity of PDP is investigated. It was shown that the discharge peak current was improved by 20% compared with that of without the MgO cathode. With increasing the MgO film thickness, discharge intensity, including the peak current, transforming charge and spectrum intensity first increased and then decreased. Higher enhancement of peak current, transforming charge and spectrum intensity were acquired with a higher peak voltage. Compared to a cathode without MgO film, the ozone production is higher with MgO cathode employed. The research proposes a novel approach for improving the intensity of discharge plasma, and also provides a reference for further application of PDP.     

Transmissivity of electromagnetic wave propagating in magnetized plasma sheath using variational method

A variational method is introduced to analyze the transmissivity of an electromagnetic wave propagating in the magnetized plasma sheath. The plasma density is modeled by two parabolic inhomogeneous regions separated by one homogeneous region. The Lagrangian density of the system is constructed based on the fluid energy density and the electromagnetic energy density.The total variation of the Lagrangian density is derived. The fluid and electromagnetic fields are numerically solved by expansion in piecewise polynomial function space. We investigate the effect of an external magnetic field on the transmissivity of the electromagnetic wave. It is found that the transmissivity is increased when an external magnetic field is applied. The dependence of transmissivity on the collision frequency between the electrons and the neutral particles has also been studied. We also show that the external magnetic field causes a shift in the critical frequency of the plasma sheath.     

Simulations of standing wave effect,stop band effect,and skin effect in large-area very high frequency symmetric capacitive discharges

In this paper, Maxwell equations are coupled with a radially localized global model and an analytical sheath model to investigate the electromagnetic effects under various frequencies and electron powers in large-area very high frequency symmetric capacitive argon discharges.Simulation results indicate that both the vacuum wavelength and the sheath width decrease with frequency, leading to the reduced surface wavelength. As a result, the standing wave effect becomes pronounced, causing the fact that the radial profiles of the electron density, radio frequency voltage, and sheath width shift from uniform over center-high to multiple-node. When the frequency is close to or higher than the series resonance frequency, the surface waves cannot propagate to the radial center because of the significant radial damping. Due to the lack of power deposition near the radial center, the electron density is nearly zero there, i.e. the stop band effect. As power increases, the higher electron density leads to the decrease of the skin depth.Therefore, the importance of the skin effect gradually exceeds that of the standing wave effect,giving rise to the transition from the center-high to edge-high electron density profiles. The method proposed in this work could help to predict the plasma distribution under different discharge conditions in a few minutes, which is of significant importance in optimizing the plasma processing.     

Analysis of the influence of sheath positions,flight parameters and incident wave parameters on the wave propagation in plasma sheath

The plasma sheath covering hypersonic vehicles has a significant effect on the propagation of electromagnetic waves. Based on the calculation of the flow field of a conical cylindrical, this work studies the propagation of electromagnetic waves in plasma sheath at L-band and Ku-band, and discusses the propagation characteristics in the head, side and tail of the sheath. The dielectric properties of plasma sheath are related to flight speed and altitude. A flight condition corresponds to a unique distribution of dielectric properties. For the conical cylindrical, the results show that flight speed is generally negatively correlated with the transmissivity of the plasma sheath. The reflection characteristics of electromagnetic waves at the L-band and Ku-band when obliquely incident to the plasma sheath show a downward trend. When the frequency is increased to Ku-band, the propagation characteristics of electromagnetic waves in the plasma sheath are related to the position of the sheath.     

离子初始速度对等离子体鞘层厚度的影响

本文从理论上研究了离子初始速度对等离子体鞘层厚度的影响,建立了理论计算公式和数值模拟方法,考察了离子初始速度对鞘层厚度的影响因子。在离子初始能量为3.2 eV情况下,用粒子模拟程序进行了数值模拟,模拟与理论计算得到的空间电场分布十分接近。     

Adaptive protograph-based BICM-ID relying on the RJ-MCMC algorithm: a reliable and efficient transmission solution for plasma sheath channels

For reentry communication, owing to the influence of the highly dynamic plasma sheath (PS), the parasitic modulation effect can occur and the received phase shift keying (PSK) signal constellation can be severely rotated, leading to unacceptable demodulation performance degradation. In this work, an adaptive non-coherent bit-interleaved coded modulation with iterative decoding (BICM-ID) system with binary PSK (BPSK) modulation and protograph low-density parity-check under the PS channel is proposed. The proposed protograph-based BICM-ID (P-BICM-ID) system can achieve joint processing of demodulation and decoding, where the soft information is adaptively estimated by reversible-jump Markov chain Monte Carlo (RJ-MCMC) algorithms. Simulation results indicate that compared to existing algorithms, the proposed system can adapt well to the dynamic characteristics of the PS channel and can obtain a 5 dB performance improvement at a bit error rate of 10⁻⁶.     

Development and experimental study of large size composite plasma immersion ion implantation device

Plasma immersion ion implantation(PⅢ) overcomes the direct exposure limit of traditional beamline ion implantation, and is suitable for the treatment of complex work-piece with large size. PⅢ technology is often used for surface modification of metal, plastics and ceramics. Based on the requirement of surface modification of large size insulating material, a composite full-directional PⅢ device based on RF plasma source and metal plasma source is developed in this paper. This device can not only realize gas ion implantation, but also can realize metal ion implantation, and can also realize gas ion mixing with metal ions injection. This device has two metal plasma sources and each metal source contains three cathodes. Under the condition of keeping the vacuum unchanged, the cathode can be switched freely. The volume of the vacuum chamber is about 0.94 m~3, and maximum vacuum degree is about 5?×?10~(-4) Pa. The density of RF plasma in homogeneous region is about 10~9 cm~(-3), and plasma density in the ion implantation region is about 10~(10) cm~(-3). This device can be used for large-size sample material PⅢ treatment, the maximum size of the sample diameter up to 400 mm. The experimental results show that the plasma discharge in the device is stable and can run for a long time. It is suitable for surface treatment of insulating materials.     

Characteristics of plasma in a novel laser-assisted pulsed plasma thruster

A novel laser-assisted pulsed plasma thruster (LA-PPT) is proposed as an electric propulsion thruster, which separates laser ablation and electromagnetic acceleration. It aims for a higher specific impulse than that achieved with conventional LA-PPTs. Owing to the short-time discharge and the novel configuration, the physical mechanism of the discharge is unclear. Time and spatial-resolved optical emission spectroscopy was applied to investigate the variation in the plasma properties in the thruster discharge channel. The plasma species, electron temperature, and electron density were obtained and discussed. Our investigation revealed that there were H_α, H_β, H_γ, H_ε atoms, C I, C II, C III, C IV, Cl I, Cl II particles, and a small amount of CH, C₃, C₂, H₂ neutral molecular groups in the plasma. The electron temperature of the discharge channel of the thruster was within 0.6–4.9 eV, and the electron density was within (1.1–3.0) $\times $ 10¹⁸ cm⁻³, which shows that the optical emission spectroscopy method is to measure the electron excitation temperature and electron density in heavy particles. But the Langmuir probe method is to measure the temperature and density of free electrons. The use of laser instead of spark plug as the ignition mode significantly changed the plasma distribution in the discharge channel. Unlike the conventional PPT, which has high electron density near the thruster surface, LA-PPT showed relatively large electron density at the thruster outlet, which increased the thruster specific impulse. In addition, the change in the ignition mode enabled the electron density in the LA-PPT discharge channel to be higher than that in the conventional PPT. This proves that the ignition mode with laser replacing the spark plug effectively optimised the PPT performance.     

The Electrostatic Cylindrical Sheath in a Plasma

The electrostatic sheath with a cylindrical geometry in an ion-electron plasma is investigated. Assuming a Boltzmann response to electrons and cold ions with bulk flow, it is shown that the radius of the cylindrical geometry do not affect the sheath potential significantly. We also found that the sheath potential profile is steeper in the cylindrical sheath compared to the slab sheath. The distinct feature of the cylindrical sheath is that the ion density distribution is not monotonous. The sheath region can be divided into three regions, two ascendant regions and one descendant region.     

Analysis of inverse synthetic aperture radar imaging in the presence of time-varying plasma sheath

The plasma sheath can induce radar signal modulation, causing not only ineffective target detection, but also defocusing in inverse synthetic aperture radar (ISAR) imaging. In this paper, through establishing radar echo models of the reentry object enveloped with time-varying plasma sheath, we simulated the defocusing of ISAR images in typical environment. Simulation results suggested that the ISAR defocusing is caused by false scatterings, upon which the false scatterings’ formation mechanism and distribution property are analyzed and studied. The range of false scattering correlates with the electron density fluctuation frequency. The combined value of the electron density fluctuation and the pulse repetition frequency jointly determines the Doppler of false scattering. Two measurement metrics including peak signal-to-noise ratio and structural similarity are used to evaluate the influence of ISAR imaging     

Influence of the X-point location on edge plasma transport in the J-TEXT tokamak with a high-field-side single-null divertor

High-density experiments in the high-field-side mid-plane single-null divertor configuration have been performed for the first time on J-TEXT.The experiments show an increase in the highest central channel line-averaged density from 2.73 x 1019 m-3 to 6.49 x 1019 m-3,while the X-point moves away from the target by increasing the divertor coil current.The corresponding Greenwald fraction rises from 0.50 to 0.79.For the impurity transport,the density normalized radiation intensity(absolute extreme ultraviolet and soft x-ray)of the central channel density decreased significantly(＞50％)with an increase in the plasma density.To better understand the underlying physics mechanisms,the 3D edge Monte Carlo code coupled with EIRENE(EMC3-EIRENE)has been implemented for the first time on J-TEXT.The simulation results show good agreement with the experimental findings.As the X-point moves away from the target,the divertor power decay length drops and the scrape-off layer impurity screening effect is enhanced.     

托卡马克等离子体杂质密度的光谱法测量研究

本文介绍了基于托卡马克等离子体被动光谱诊断获得杂质密度的方法。通过被动光谱诊断测量获得杂质线辐射的空间多道弦积分强度分布,利用强度标定系数转换为绝对光亮度分布;通过测量弦与等离子体位形,将弦积分的强度分布反演变换为径向体发射率。根据线辐射强度激发截面求出对应电离态的离子密度,最后采用杂质输运程序模拟计算得出总密度分布。以东方超环(Experimental Advanced Superconducting Tokamak,EAST)托卡马克装置上软X射线-极紫外光谱(Soft X-ray and Extreme Ultraviolet Spectrometers,XEUV)诊断测量到的Mo XXIX-Mo XXXII为例,描叙了获得Mo杂质密度分布的过程,获得的总误差小于10%。