Numerical Study on Characteristics of Argon Radio-Frequency Glow Discharge with Varying gas Pressure

A one-dimensional fluid simulation on argon rf glow discharge with varying linearly gas pressure from 1 Torr to 100 Tort is performed. The model based on mass conservation equations for electron and ion under diffusion and mobility approximation, and the electron energy conservation equation is solved numerically by finite volume method. The numerical results show that a uniform plasma with high density can be obtained from rf glow discharge with varying gas pressure, and the density of plasma becomes higher as the gas pressure varies from 1 Tort to 100 Tort. It is also shown that in the range of the gas pressure from 1 Tort to 100 Tort with the slower rate of varying gas pressure, higher density of plasma can be obtained.     

Effects of gas pressure on plasma characteristics in dual frequency argon capacitive glow discharges at low pressure by a self-consistent fluid model

A self-consistent fluid model for dual radio frequency argon capacitive glow discharges at low pressure is established.Numerical results are obtained by using a finite difference method to solve the model numerically, and the results are analyzed to study the effect of gas pressure on the plasma characteristics. It shows that when the gas pressure increases from 0.3 Torr(1 Torr = 1.33322×10~2 Pa) to 1.5 Torr, the cycle-averaged plasma density and the ionization rate increase;the cycle-averaged ion current densities and ion energy densities on the electrodes electrode increase; the cycle-averaged electron temperature decreases. Also, the instantaneous electron density in the powered sheath region is presented and discussed. The cycle-averaged electric field has a complex behavior with the increasing of gas pressure, and its changes take place mainly in the two sheath regions. The cycle-averaged electron pressure heating, electron ohmic heating, electron heating, and electron energy loss are all influenced by the gas pressure. Two peaks of the electron heating appear in the sheath regions and the two peaks become larger and move to electrodes as the gas pressure increases.     

磁场-微波等离子体与大面积金刚石薄膜

在磁场辅助下电子回旋共振(ECR)效应在大的空间范围形成高密度的等离子体,为大面积金刚石薄膜的淀积提供有利条件,以CH₄-H₂为原料气体,在Si基片上,在比其他制备方法更低的压强(3Torr附近)下已制备出直径约为5cm的金刚石薄膜,从空间等离子体范围来看且有可能达更大的面积,在分析等离子体发射光谱的基础上,对0.5到50Torr压强的金刚石气相生长条件作了讨论。 关键词：     

Nonlocal phenomena in the positive column of a medium-pressure glow discharge

Commercial CFDRC software () is used to self-consistently simulate the plasma of the positive column (PC) of a medium-pressure dc discharge in argon. The software allows simulations in an arbitrary 3D geometry by using Poisson’s equation for the electric potential and fluid equations for the heavy components and by solving a nonlocal kinetic equation for electrons. It is shown that, in calculating the electron distribution function, the local approximation is almost always inapplicable not only at relatively low pressures (pR<1 cm Torr), but also at relatively high pressures (pR<10 cm Torr), i.e., under the real conditions of a diffuse PC usually met in practice. The use of the local approximation in solving the kinetic equation for electrons leads to significant errors in determining the main parameters of the PC. A paradoxical effect has been revealed: the peaks of the profiles of the excitation rates shift from the discharge axis toward the periphery as the pressure increases from low to medium values (1 cm Torr<pR<10 cm Torr). It is shown that the effect is related to the nonlocal character of the electron distribution function.     

Role of polaron pair diffusion and surface losses in organic semiconductor devices

By applying Monte Carlo simulations we find that the extraction of bound polaron pairs (PP) at the electrodes is an important loss factor limiting the efficiency of organic optoelectronic and photovoltaic devices. Based upon this finding, we develop a unified analytic model consisting of exact Onsager theory describing the dissociation of PP in organic donor-acceptor heterojunctions, the Sokel-Hughes model for the extraction of free polarons at the electrodes, as well as of PP diffusion leading to the aforementioned loss mechanism, which was not considered previously. Our approach allows us to describe the simulation details on a macroscopic scale and to gain fundamental insights, which is important in view of developing an optimized photovoltaic device configuration.     

稀薄空气中圆柱腔体内系统电磁脉冲的混合模拟

系统电磁脉冲广泛存在于强电离辐射环境中,且难以有效屏蔽.为了评估稀薄空气对系统电磁脉冲的影响,本文基于粒子-流体混合模拟方法,建立了三维非稳态模型,计算并分析了稀薄空气等离子体的特性以及其与电磁场响应的相互作用.结果表明,压力越高,光电子发射面附近的次级电子数密度越高,轴向分布的梯度越大,腔体中部的电子数密度在20 Torr(1 Torr=133 Pa)下出现峰值,而电子温度随压力升高单调递减.腔体内的稀薄空气等离子体阻碍了空间电荷层的产生,电场响应峰值比真空条件下的低了一个数量级,电场脉冲宽度也显著降低.光电子运动特性决定了电流响应的峰值,压力升高,到达腔体末端的电流先增加再减小.而等离子体电流会抑制总电流的上升速率,并使电流响应出现拖尾.最后,将数值模拟结果与电子束模拟系统电磁脉冲的实验结果进行比较,验证了本文混合模拟模型的可靠性.本研究所采用的混合模拟方法相比于粒子云网格-蒙特卡罗碰撞方法,大幅减小了计算消耗.     

稀薄空气中圆柱腔体内系统电磁脉冲的混合模拟

系统电磁脉冲广泛存在于强电离辐射环境中,且难以有效屏蔽.为了评估稀薄空气对系统电磁脉冲的影响,本文基于粒子-流体混合模拟方法,建立了三维非稳态模型,计算并分析了稀薄空气等离子体的特性以及其与电磁场响应的相互作用.结果表明,压力越高,光电子发射面附近的次级电子数密度越高,轴向分布的梯度越大,腔体中部的电子数密度在20 Torr(1 Torr=133 Pa)下出现峰值,而电子温度随压力升高单调递减.腔体内的稀薄空气等离子体阻碍了空间电荷层的产生,电场响应峰值比真空条件下的低了一个数量级,电场脉冲宽度也显著降低.光电子运动特性决定了电流响应的峰值,压力升高,到达腔体末端的电流先增加再减小.而等离子体电流会抑制总电流的上升速率,并使电流响应出现拖尾.最后,将数值模拟结果与电子束模拟系统电磁脉冲的实验结果进行比较,验证了本文混合模拟模型的可靠性.本研究所采用的混合模拟方法相比于粒子云网格-蒙特卡罗碰撞方法,大幅减小了计算消耗.     

Modeling of the nanoparticle coagulation in pulsed radio-frequency capacitively coupled C_2H_2 discharges

The role of pulse parameters on nanoparticle property is investigated self-consistently based on a couple of fluid model and aerosol dynamics model in a capacitively coupled parallel-plate acetylene(C2H2) discharge. In this model, the mass continuity equation, momentum balance equation, and energy balance equation for neutral gas are taken into account.Thus, the thermophoretic force arises when a gas temperature gradient exists. The typical results of this model are positive and negative ion densities, electron impact collisions rates, nanoparticle density, and charge distributions. The simulation is performed for duty ratio 0.4/0.7/1.0, as well as pulse modulation frequency from 40 k Hz to 2.7 MHz for pure C2H2 discharges at a pressure of 500 m Torr. We find that the pulse parameters, especially the duty ratio, have a great affect on the dissociative attachment coefficient and the negative density. More importantly, by decreasing the duty ratio, nanoparticles start to diffuse to the wall. Under the action of gas flow, nanoparticle density peak is created in front of the pulse electrode,where the gas temperature is smaller.     

Theoretical investigations on multiple-reflection and Rayleigh absorption-emission-scattering effects in laser drilling

A fundamental study of laser drilling was performed with computational analysis. High power Q-switched diode-pumped laser drilling simulations were conducted on N-type silicon wafers. In the numerical model, the volume of fluid method was adopted to trace the free surface of the drilled-hole with the governing equations including continuity, momentum, and energy equation. The laser beam was considered as a surface heat flux with near-Gaussian distribution. The simulation model took into account the physical phenomena, taking place during drilling of the silicon wafer, such as the effects of recoil pressure producing a narrow and deep profile, Fresnel absorption transferring the energy from laser beam to workpiece, multiple-reflection raising the absorption of laser energy, and Rayleigh absorption-emission-scattering absorbing and distributing the entrance laser energy. In particular, a novel model for laser drilling was proposed from the view point of the transmitted, absorbed, emitted, and scattered light.     

甚高频电容耦合氢等离子体特性研究

采用高H₂稀释的SiH₄等离子体放电, 特别是甚高频等离子体增强化学气相沉积技术是当前高速制备优质微晶硅薄膜的主流方法. 尽管在实验上取得了很大的突破, 但其沉积机理一直是研究的热点和难点. 本文通过建立二维时变的轴对称模型,在75 MHz放电频率下, 对与微晶硅沉积非常相关的甚高频电容耦合氢等离子体放电进行了数值模拟, 研究了沉积参数对等离子体特性的影响, 并与光发射谱(OES)在线监测结果进行了比较. 结果表明: 电子浓度 n_e在等离子体体层中间区域最大, 而电子温度 T_e及H_α与H_β的数密度在体层和鞘层界面附近取极大值; 当气压从1 Torr (1 Torr=133.322 Pa)增大至5 Torr时, 等离子体电势单调降低, 在体层中间区域 n_e先快速增大然后逐渐减小, T_e先下降后趋于稳定; 随着放电功率从30 W增大到70 W, 电子浓度 n_e及H_α与H_β的数密度均线性增大, 而电子温度 T_e基本保持不变; OES在线分析结果与模拟结果符合得很好.     

Manipulation of micrometer sized particles within a micromachined fluidic device to form two-dimensional patterns using ultrasound

Ultrasonic manipulation, which uses acoustic radiation forces, is a contactless manipulation technique. It allows the simultaneous handling of single or numerous particles (e.g., copolymer beads, biological cells) suspended in a fluid, without the need for prior localization. Here it is reported on a method for two-dimensional arraying based on the superposition of two in-plane orthogonally oriented standing pressure waves. A device has been built and the experimental results have been compared with a qualitative analytical model. A single piezoelectric transducer is used to excite the structure to vibration, which consists of a square chamber etched in silicon sealed with a glass plate. A set of orthogonally aligned electrodes have been defined on one surface of the piezoelectric. This allows either a quasi-one-dimensional standing pressure field to be excited in one of two directions or if both electrodes are activated simultaneously a two-dimensional pressure field to be generated. Two different operational modes are presented: two signals identical in amplitude and frequency were used to trap particles in oval shaped clumps; two signals with slightly different frequencies to trap particles in circular clumps. The transition between the two operational modes is also investigated.     

针板型大气压氦气冷等离子体射流的二维模拟

大气压冷等离子体射流的传播机理一直是人们研究的一个热点. 本文采用自洽的二维等离子体流体模型, 研究了大气压氦气冷等离子体射流在自身环境气体中以及在介质管中的传播问题. 得到了电子密度、电离速率、空间电场以及电子温度等参量的时空分布规律, 分析了介质管大小以及介质管介电常数对射流放电性质的影响, 得到了一种提高电子密度和射流尺寸的新方法. 关键词： 大气压冷等离子体射流 等离子体子弹 数值模拟 流体模型     

Experimental and theoretical analysis of bias ionization by?α-particles in a nitrogen laser

Nitrogen laser performance with TE configuration and wedge electrodes is analyzed with background ionization in the laser discharge channel by α particles at a low exposition rate. With the bias ionization, the laser power presents two peaks as a function of gas pressure, with one at the normal low pressure, without bias ionization, and the other at high pressure generated by bias ionization. A simple theoretical model has been developed in a trial to understand this behavior. This model was first tested in later results for a TE configuration nitrogen laser, with flat electrodes, without and with bias ionization. It has been observed that due to the competition between electrode shielding by positively charged α particles and bulk ionization by impact, the laser energy is suppressed with pressure below 50 Torr and enhanced above it.     

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two diffe1：ent discharge modes has been verified in an rf （radio-frequency） atmospheric pressure glow discharge （APGD） by Shi [J. Appl. Phys. 97, 023306 （2005）]. In the first mode, referred to as a mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathodc under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the a mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.     

Kinetics of charged particles in a high-voltage gas discharge in a nonuniform electrostatic field

A high-voltage gas discharge is of interest as a possible means of generating directed flows of low-temperature plasma in the off-electrode space distinguished by its original features [1–4]. We propose a model for calculating the trajectories of charges particles in a high-voltage gas discharge in nitrogen at a pressure of 0.15 Torr existing in a nonuniform electrostatic field and the strength of this field. Based on the results of our calculations, we supplement and refine the extensive experimental data concerning the investigation of such a discharge published in [1, 2, 5–8]; good agreement between the theory and experiment has been achieved. The discharge burning is initiated and maintained through bulk electron-impact ionization and ion–electron emission. We have determined the sizes of the cathode surface regions responsible for these processes, including the sizes of the axial zone involved in the discharge generation. The main effect determining the kinetics of charged particles consists in a sharp decrease in the strength of the field under consideration outside the interelectrode space, which allows a free motion of charges with specific energies and trajectories to be generated in it. The simulation results confirm that complex electrode systems that allow directed plasma flows to be generated at a discharge current of hundreds or thousands of milliamperes and a voltage on the electrodes of 0.3–1 kV can be implemented in practice [3, 9, 10].     

常压双频驱动Ar/CCl4等离子体射流制备碳薄膜的研究

在大气压下双频Ar/CCl4 等离子体射流的驱动下,固定低频功率,通过改变射频功率在硅基底上制备了非晶态碳薄膜,并且通过程序进行相应的数值模拟计算。结果给出了不同功率下电子与离子的密度、温度、电场、电势、角度分布等参数对碳材料样品形貌的影响;样品变化趋势的预测及其原因的分析,以及与实验结果的对比。结果表明,对于双频大气压等离子体,射频可以独立控制等离子体的能量和反应强度,可以相对地控制产物。这为制备薄膜材料的形貌提供重要的实验基础。     

Spectral dips in ion emission emerging from ultrashort laser-driven plasmas

Deep dips in MeV ion spectra are obtained from water droplet targets irradiated by intense [(0.5-1.2) x 10(19) W/cm(2)] and ultrashort (35 fs) laser pulses. The existence of these dips is ascribed to the generation of a multielectron-temperature plasma, which is confirmed by our experiments. An existing fluid model based on hot-electron components with significantly different temperatures is consistent with the behavior we observe in the ion spectra of the femtosecond laser-driven interaction. The model provides a good simulation of the observed spectral dips and allows us to establish important parameters such as hot- and cold-electron temperatures and the respective electron density ratios. The result may be of interest for spectral tailoring of proton spectra in future applications of laser-generated proton beams.     

Reflexionsversuche mit elektromagnetisch erzeugten Verdichtungsstößen hoher Mach-Zahlen

In a 80 cm long quartz-tube of 33 mm diameter plasma is generated by theta-pinch mechanism with conical coil and accelerated in the direction along the axis of the tube. A luminous front is observed and its velocity has been studied in, in dependence of coil distance and initial pressure. In Helium in a pressure range from 0·3 to 5 Torr it has been found by reflection experiments that before the luminous front a discontinuity in the pressure is moving ahead of the luminous front. The distanceΔx between this discontinuity and luminous front increases with increasing distance from the coil and with increasing initial pressure for instance up to 10 cm at a distance of about 50 cm and an initial pressure of 5 Torr. The results are compared to theoretical considerations.     

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Amorphous carbon films were prepared on silicon substrate by changing RF power under the driving of dual-frequency Ar/CCl4 plasma jet at atmospheric pressure, and the corresponding numerical simulation was carried out by the program. The influence of the density, temperature, electric field, potential and angular distribution of electrons and ions on the morphology of carbon material samples at different powers was obtained. The prediction of the trend of the sample and the analysis of its causes, and the comparison with the experimental results were conducted. The results indicate that for the dual-frequency atmospheric pressure plasma, the radio frequency can control the energy and reaction intensity of the plasma independently, and can control the product relatively, which provides an important experimental basis for preparing the thin film morphology.