Characteristics of the plasma sheath and the charging near the ion engine of the spacecraft

New sheath structures near the ion engine of the spacecraft are investigated by numerical calculation. We show the double sheath potential, the space charge density and the double electric field by the dust grains and secondary electrons. Temporal evolution of the grain charge near the ion engine is simulated by the conservation law of the electric charge–current equation. It turns out that the control of the plasma sheath near the ion engine of the spacecraft is possible.     

The ion density distribution in a magnetized plasma sheath

The ion density distribution of plasma sheath in an oblique magnetic field is investigated with a fluid model. We performed numerical simulations of the sheath. The results reveal that the magnetic field has significant effects on the plasma sheath, including ion density distribution and space charge density distribution. Two cases of ion incidence are considered here. Under suitable conditions, Lorentz force induces fluctuations in the ion density. And the magnetic field parallel to the board is responsible for these changes. The action states of ions are more complicated while the ions enter the sheath with an oblique incidence angle. Ions could gather in some regions, so that it leads to small peaks of the density curve. Also the space charge density in such regions is slightly higher.     

Distributions of dusts in the plasma column sheath

Dust crystal and void were simulated with a two-dimensional fluid model and a three-dimensional molecular dynamic model. The distributions of dust particles are obtained and these distributions are one-layer or two-layer dust crystals, conversely cone-shaped, and formulation of a cylindrical dust void without ion drag is complex. It is due to a gentle potential bump in the central region of the electrode caused by the cylindrical geometry. It can be concluded that it is the potential well of the central region of the electrode bottom established by lower ion density that contributes to the formation of dust voids.     

The effect of the trapping of dust grains on the sheath structure and the charging in a plasma

A model of the trapping of dust grains is shown in a plasma for the first time. The multiple sheath potential, the space charge density and the multiple electric field associated with our model are simulated. Our result explains the confinement of the dust grains observed in the experiment.     

Characteristics of ions impacting grid surfaces in an ion engine

Numerical evaluations of grid erosions in ion engines are required to estimate the lifetimes of long-term space missions. In this study, to investigate the characteristics of ions colliding with neutrals and impacting the accel grid surface, including ions that collide with neutrals more than once, three-dimensional particle analyses using a full-PIC (Particle in Cell) code were performed. The distributions of erosion by charge-exchanged ions show periodic patterns. The multiple-collided ion constitutes 7 and 8% of the accel grid current to the inner and downstream surfaces, respectively. The energy of the multiple-collided ion impacting the downstream surface has a broader distribution than that of the charge-exchanged ion. These results suggest the importance of investigating multiple-collided ions in high-accuracy quantitative evaluations of grid erosion.     

Effect of pressure on plasma sheath in a magnetic field

The effects of collisions on plasma sheath in an external magnetic field have been investigated by considering the collisions between ions and neutral gas atoms. The ion fluid equations containing an external magnetic field and the collisions are solved numerically to study the ion dynamics under various pressures.     

Effect of dust-neutral collisions on the dust characteristics in a magnetized plasma sheath

The characteristics of dust in a plasma sheath are investigated in the presence of an external magnetic field and taking into account the dust-neutral collision force. The continuity and momentum equations of ions and dust particles are solved numerically with various magnitudes of collision force by using the fluid model. The numerical results have revealed that the collision force reduces the dust gyro radius, changes the positions of the extrema of the dust density and the velocity in the depth direction. It is shown that the collision force reduces the dust kinetic energy which has no fluctuation even in a strong external magnetic field.     

Sheath dynamics in a cylindrical PET-film for plasma immersion ion implantation

The characteristics of sheath in a cylindrical Polyethylene terephthalate (PET) film for plasma immersion ion implantation (PIII) have been investigated numerically using a one-dimensional self-consistent fluid model. The time-dependent surface potential and the charge density accumulated on the inner surface of PET-film are obtained. The numerical results demonstrate that the charging effects lead to the reduction of surface potential during pulse on time, the implanted ions thus cannot reach the desirable energy. In order to diminish the influence of charging effects one can shorten the pulse fall time and thin the PET-film.     

The effect of the trapping of dust grains on the sheath structure and the charging in a plasma

A model of the trapping of dust grains is shown in a plasma for the first time. The multiple sheath potential, the space charge density and the multiple electric field associated with our model are simulated. Our result explains the confinement of the dust grains observed in the experiment.     

Controlling the ion flux on substrates of different geometry by sheath-lens focusing effect

It is shown that a three-dimensional plasma sheath lens that forms on biased electrodes interfacing insulators exhibits novel characteristics such as ion focusing on desired locations, controllability of ion flux uniformity, formation of passive surfaces and applicability to plasma diagnostics. The ion flux profile on substrates of different geometry is obtained by three dimensional simulations of potential distribution and ion trajectory, while experiments are realized in electropositive and electronegative DC and ICP discharges.     

Ion dynamics in plasma sheath under the effect of E×B and collisional forces

Using the fluid model, we investigated the velocity, kinetic energy and the density distribution of the ions in collisional and collisionless magnetized plasma sheath. Considering an external magnetic field, the ion movement under the effect of magnetic, electric and collisional forces has been analyzed numerically. The nonexistence of fluctuations in ions kinetic energy in collisionless strong magnetized plasma sheath and increasing the ions velocity in depth direction due to the collisions in some positions in the sheath are shown. The fluctuations of ion velocity in weak magnetized plasma sheath are shown too when ions enter the sheath with oblique incident angle.     

Effects of strong magnetic field on plasma immersion ion implantation of dielectric substrates

In this paper the effects of a strong magnetic field on plasma immersion ion implantation (PHI) of dielectric substrates were investigated. A plasma fluid model and finite difference schemes were used to simulate a one-dimensional system of plasma immersion ion implantation. The effect of secondary electron emission from the electrode on PHI was also taken into consideration. It was found that the magnitude and direction of the magnetic field have slight effects on sheath thickness but have considerable effects on current densities in the y and z directions which are perpendicular to the direction of the electric field (the x direction). The simulations showed that the current densities in the y and z directions increased significantly with increasing magnitude of the magnetic field at a given fixed angle, the reason being attributed to the rotational force exerted on the ions by the magnetic field. With a fixed magnetic field, increasing the angle of the magnetic field, θ, with respect to the electric field produced a continuous increase in current density in the y direction from zero to its maximum at θ = 90° but the current density in the z direction could be described as saddle-shaped being zero at both ends.     

Control of the energy of ion flow affecting electrically insulated surface in plasma processing reactor based on a beam plasma discharge

We compare two ways to control the distribution function of ions on the isolated structure which is treated in a plasma reactor based on beam plasma discharge. In the first case, the periodic pulse voltage is applied to the substrate holder. The calculation of currents and voltages on the surface in contact with the plasma in a simple empirical model has been performed; the comparison of results of calculation and experiment is presented. In the latter case, the pulsed voltage is applied to the discharge collector, thus modulating the plasma potential. The comparison shows that the second method provides more efficient control of the distribution function of ions, acting on the treated substrate.     

Bias frequency dependence of pn junction charging damage induced by plasma processing

Plasma-induced charging damage to pn junction in metal-oxide-semiconductor field-effect transistors (MOSFETs) was studied by using an inductively coupled plasma (ICP) reactor with Ar gas. The junction leakage current (I_leak) was measured for various source/drain to well in MOSFETs and simple diode structures. Two different rf bias frequencies, 13.56 MHz and 400 kHz, were utilized to investigate the effect of frequency on an increase in I_leak. The I_leak of n⁺/p and p⁺/n junctions was found to increase by a plasma treatment. In particular, more severe damage was observed for n⁺/p junction in both n-channel MOSFETs and the diodes. These observed results imply that plasma plays a role primarily as a positive current source. With regard to the rf bias frequency effects, samples exposed to the 400-kHz plasma were found to suffer from larger I_leak than those to the 13.56 MHz. From capacitance-voltage (C-V) measurement of junction capacitance changes, we also clarified that the observed increase in I_leak was attributed to the defect density at pn junction created by the plasma charging damage.     

Steel surface treatment by a dual process of ion nitriding and thermal shock

Samples of AISI 4140 steel were surface treated under two different processes: ion nitriding and high energy pulsed plasma irradiation. Ion nitriding was performed with a 100 Hz square wave glow discharge, in an atmosphere of an 80% N₂ and 20% H₂ mixture, under a total pressure of 5.6 mbar. Pulsed plasma irradiation consisted in the surface irradiation with a predetermined number of pulses of high energy and short duration argon plasmas, accelerated in a Z-Pinch experiment. Each pulse can induce high temperatures in a short time (<200 ns), followed by an also fast (∼10 μs) cooling down. The samples, ion nitrided and post-irradiated with pulsed plasmas showed important surface property improvements with respect to samples subjected only to ion nitriding. Those improvements consisted of an increase in the thickness of the hardened layer, and in a reduction of the micro-hardness gradient. These results show a complex surface layer structure that improves the support base for loads, reducing the probability of surface layer loosening.     

等离子体交叉场调制开关导通过程的模拟与测试

以冷阴极等离子体交叉场调制开关为研究对象,将系统内放电过程中形成的等离子体视为电子与正离子构成的双流体,引入扩散系数与迁移率来表征电子流与离子流的运动,用离子产率来表征与流体密切相关的离子密度随放电时间的进行而发生的变化,耦合泊松方程来描述等离子体与外电场的相互影响,同时,将气体间隙的脉冲放电合理的与直流放电等效,并确定外电路各量之间的函数关系,利用Comsol软件对调制开关的导通过程进行了数值模拟,所得结果与实测结果吻合,证明了模拟脉冲放电中管内阻抗随时间的变化关系是准确的,从而说明本文所采用的模拟方法对气体放电器件的研制具有积极的指导意义。     

Effect of impinging ion energy on the substrates during deposition of SiOx films by radiofrequency plasma enhanced chemical vapor deposition process

Radiofrequency (13.56 MHz) plasma enhanced chemical vapor deposition process is used for deposition of SiO_x films on bell metal substrates using Ar/hexamethyldisiloxane/O₂ glow discharge. The DC self-bias voltage developed on the substrates is observed to be varied from − 35 V to − 115 V depending on the RF power applied to the plasma. Plasma potential measurements during film deposition process are carried out by self-compensated emissive probe. The deposited films are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), nanoindentation, nano-scratch test and thermogravimetric analysis. The characterization results show strong dependency of the SiO_x films properties on the energy of the ions impinging on the substrates during deposition. Analysis of Raman spectra indicates an increase in vitreous silica content and reduction in defective Si-O-Si chemical structure in the deposited SiO_x films with increasing ion energy impinging on the substrates. The increase in inorganic (Si and O) content in the SiO_x films is further confirmed from XPS analysis. The growth of SiO_x films with more inorganic content and defect free chemical structure apparently contribute to the increase in their hardness and scratch resistance behavior. The films show higher thermal stability as the energy of the ions arriving at substrates increases with DC self-bias voltage. The possibility of using SiO_x films for surface protection of bell metal is also explored.     

Combined impurity gettering effects of helium-induced cavities and oxygen precipitates created by plasma immersion ion implantation

Helium bubbles are formed in silicon by high dose implantation using the plasma immersion ion implantation technique. Thermal annealing of the implanted samples at a temperature above 700 °C causes the helium to diffuse out of silicon and form cavities which can be observed by cross-sectional transmission electron microscopy. The well-defined band of helium-induced cavities both with and without oxygen precipitates (formed by subsequent oxygen implantation and annealing) have been studied as gettering layers for copper and gold. Our secondary ion mass spectrometry and Rutherford backscattering spectrometry results demonstrate that the presence of oxygen does not increase significantly the gettering efficiency of these cavities. The combined cavity/oxygen gettering sites also remain stable up to 1200 °C     

Amorphous carbon nitrogenated films prepared by plasma immersion ion implantation and deposition

In this work, an investigation was conducted on amorphous hydrogenated-nitrogenated carbon films prepared by plasma immersion ion implantation and deposition. Glow discharge was excited by radiofrequency power (13.56 MHz, 40 W) whereas the substrate-holder was biased with 25 kV negative pulses. The films were deposited from benzene, nitrogen and argon mixtures. The proportion of nitrogen in the chamber feed (R_N) was varied against that of argon, while keeping the total pressure constant (1.3 Pa). From infrared reflectance-absorbance spectroscopy it was observed that the molecular structure of the benzene is not preserved in the film. Nitrogen was incorporated from the plasma while oxygen arose as a contaminant. X-ray photoelectron spectroscopy revealed that N/C and O/C atomic ratios change slightly with R_N. Water wettability decreased as the proportion of N in the gas phase increased while surface roughness underwent just small changes. Nanoindentation measurements showed that film deposition by means of ion bombardment was beneficial to the mechanical properties of the film-substrate interface. The intensity of the modifications correlates well with the degree of ion bombardment.     

ZR-VV电缆护套和绝缘材料热解动力学研究

采用热重分析（thermogravimetric analysis）的方法研究了阻燃聚氯乙烯电缆（ZR-VV）护套和绝缘材料在空气中的热降解行为，并采用Kissinger法和Flynn-Wall法计算了护套材料的热降解动力学参数．TG曲线表明，阻燃处理的聚氯乙烯电缆护套和绝缘材料热降解均可分为5个阶段，材料的热降解速率随升温速率的增加而增加．由Kissinger法和Flynn-Wall法计算得到阻燃聚氯乙烯电缆护套材料的降解平均活化能依次为102和118kJ／mol，绝缘材料的降解平均活化能为133和134kJ／mol。