不同成分等离子体鞘层的玻姆判据

在一维平板鞘层中采用流体模型分别研究了不同成分无碰撞等离子体鞘层的玻姆判据.通过拟牛顿法数值模拟了含有电子、离子、负离子以及二次电子的等离子体鞘层玻姆判据.结果表明二次电子发射增加了鞘层离子马赫数的临界值,且器壁发射二次电子温度越高,离子马赫数临界值越小.负离子使离子马赫数临界值减小.而在含有二次电子和负离子的等离子体鞘层中,当负离子较少时,二次电子发射对离子马赫数临界值影响较大;当负离子增加时,离子马赫数的临界值则主要受负离子的影响. 关键词： 鞘层 等离子体 玻姆判据     

Heat flow through a plasma sheath in the presence of secondary electron emission from plasma‐wall interaction

The formation of a plasma sheath in front of a negative wall emitting secondary electron is studied by a one‐dimensional fluid model. The model takes into account the effect of the ion temperature. With the secondary electron emission (SEE ) coefficient obtained by integrating over the Maxwellian electron velocity distribution for various materials such as Be, C, Mo, and W, it is found that the wall potential depends strongly on the ion temperature and the wall material. Before the occurrence of the space‐charge‐limited (SCL ) emission, the wall potential decreases with increasing ion temperature. The variation of the sheath potential caused by SEE affects the sheath energy transmission and impurity sputtering yield. If SEE is below SCL emission , the energy transmission coefficient always varies with the wall materials as a result of the effect of SEE , and it increases as the ion temperature is increased. By comparison of with and without SEE , it is found that sputtering yields have pronounced differences for low ion temperatures but are almost the same for high ion temperatures.     

等离子体对相对论返波管工作影响的粒子模拟

金属高频结构的射频击穿是引起功率下降和脉冲缩短的重要原因,是限制高功率微波(HPM)向更高功率、更长脉冲发展的重要因素。射频击穿的物理过程极其复杂,并且开展射频击穿研究对实验条件等要求高,因此粒子模拟是研究射频击穿的重要手段。通过在慢波结构表面设置爆炸发射电子和离子的方式模拟等离子体对一个X波段的相对论返波振荡器(RBWO)和一个Ka波段的RBWO工作的影响。粒子模拟结果表明,对于分段式慢波结构,后段慢波结构产生等离子体会对电子束的调制造成影响,进而影响器件正常工作,引起微波功率下降。当等离子体由质量较轻的正离子和电子组成时,会对束波作用造成更大的影响,引起较大的输出功率下降。相同密度的射频击穿等离子体对Ka波段RBWO工作的影响大于对X波段RBWO的影响。     

高功率微波沿面闪络击穿机制粒子模拟

针对高功率微波介质沿面闪络击穿物理过程,首先建立了理论模型,包括:动力学方程、粒子模拟算法、二次电子发射, 以及电子与气体分子蒙特卡罗碰撞模型、电子碰撞介质表面退吸附气体分子机制;其次,基于理论模型,编制了1D3V PIC-MCC程序,分别针对真空二次电子倍增、高气压体电离击穿和低气压面电离击穿过程,运用该程序仔细研究了电子和离子随时间演化关系、电子运动轨迹、电子及离子密度分布、空间电荷场时空分布、电子平均能量、碰撞电子平均能量、碰撞电子数目随时间演化关系、电子能量分布函数、平均二次电子发射率以及能量转换关系。研究结果表明:真空二次电子倍增引发的介质表面沉积功率只能达到入射微波功率1%左右的水平,不足以击穿;气体碰撞电离主导的高气压体电离击穿,是由低能电子（eV量级）数目指数增长到一定程度导致的,形成位置远离介质表面,形成时间为s量级;低气压下的介质沿面闪络击穿,是在二次电子倍增和气体碰撞电离共同作用下,由于数目持续增长的高能电子（keV量级）碰撞介质沿面导致沉积功率激增而引发的,形成位置贴近介质沿面,形成时间在ns量级。     

二次电子发射和负离子存在时的鞘层结构特性

 建立了包括电子、离子、器壁发射二次电子以及负离子多种成分的等离子体无碰撞鞘层的基本模型,讨论了二次电子发射和负离子对1维稳态等离子体鞘层结构的影响,并且分析了多种成分等离子体鞘层内的二次电子和负离子的相互作用。结果表明：二次电子发射系数的增加和负离子含量的增加,都将导致鞘层的厚度有所减小;二次电子发射系数超过临界发射系数之后,鞘层不再是离子鞘。随着器壁材料二次电子发射系数的增加,鞘层中的负离子密度分布也逐渐增加;负离子的增加,导致二次电子临界发射系数有所增加。另外,在等离子体鞘层中二次电子发射和负离子的存在,也影响着鞘层中电子的放电特性与器壁材料的腐蚀。     

Secondary effects on solid surfaces stimulated by the cathode plasma flux in vacuum arcs

The conducting plasma region between the electrodes of a vacuum arc contains single-and multiple-charged ions of the cathode material. Investigation into the behaviour of this flux when it impinges upon a surface should take into account secondary processes. From among various processes on the surface in vacuum arc, particular attention is paid to the sputtering of the surface and to the secondary electron emission by positive ions emitted from the cathode spots. On the basis of the sputtering yield characteristicsS _r as a function of ion kinetic energyE _i and their energy distribution in the cathode flux, the total sputtering yieldsS _re for atomicaly clean surface have been determined. Experimental verification ofS _re was performed by measuring the ratio of mass deposited on the collectors to mass carried by the particle incident flux. It was found that the calculated value ofS _re is comparable to the evaluated one from the experiments. The emission of secondary electrons from clean metal surfaces due to a bombardment by multiple-charged ions is considered. The secondary electron emission _c depends on excess energyE _ex of incident ion and its average value _av for multiple-charged ions depends both on the fractional distribution of ion flux _zi and on the excess energyE _ex of each ion. It is proposed that the relationship derived is applicable to most combinations of atomic ions and pure metal surfaces.Presented at 17^th Symposium Plasma Physics and Technology, Prague, June 13–16, 1995.This work was supported by the State Committee for Scientific Research within the research project No. 3P40101507.     

高电荷态离子207Pb36+与金属Nb表面作用产生的二次离子产额

本文报道了利用兰州重离子加速器国家实验室的ECR离子源引出的高电荷态离子207Pb36 入射到金属Nb表面产生的二次离子的实验测量结果.实验发现,二次离子产额Y随入射初动能Ek的增加有先增加后减小的关系,在初动能为576 keV时二次离子产额达到最大.通过对实验点做高斯拟合发现,曲线峰值对应的入射初动能为602 keV.分析表明,这是势能沉积作用与线性级联碰撞过程协同作用的结果.高电荷态离子本身携带的高势能沉积在靶表面引起势能溅射,促进了二次离子的发射;而主导二次离子溅射的过程是动能溅射,它与靶表面的动量沉积(核能损)过程密切相关.     

多成分电负性碰撞等离子体的鞘层玻姆判据

采用双流体模型研究了多成分的电负性碰撞等离子体鞘层的玻姆判据,讨论了一维稳态情形下不同的带电粒子对鞘层玻姆判据的影响。采用拟牛顿法,得到了数值解。结果表明：玻姆判据存在上限和下限。二次电子发射系数越大,离子的马赫数越大;负离子的含量越多,离子的马赫数越小。鞘层中离子的温度、离子与中性粒子的碰撞以及离子的带电量对玻姆判据的上下限的取值都有一定的影响。     

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

The secondary alkali ion yield vs. the work function change (Δφ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δφ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.     

The influence of ion bombardment on emission properties of small dust grains

Dust grains that are present in many plasma and vacuum systems and in the space usually carry a non-negligible charge. Their charging significantly depends on surface properties of the grain material. In cold plasma, charging is mainly given by electron attachment, nevertheless, when plasma becomes hot, other processes (secondary electron emission, field emission, etc.) take place. Emission properties of the grain surface could be modified by grain baking or by ion bombardment. Our study is carried out at the dust charging experiment dealing with a single dust grain electro-dynamically levitated in a 3D quadrupole trap. The grain can be exposed to the ion beam in the energy range of 100 eV–5 keV and to the electron beam in the energy range of 100 eV–10 keV. We have chosen He⁺ and Ar⁺ ions for the surface treatment and the observed influence on the surface properties is discussed in terms of secondary emission. A non-negligible shift of the secondary electron emission yield, as well as a change of energy distribution of secondary electrons, were measured after Ar⁺ bombardment. A preliminary study suggests that the effects of He⁺ and Ar⁺ are comparable.     

Fluid Model of a Sheath Formed in Front of an Electron Emitting Electrode Immersed in a Plasma with Two Electron Temperatures

The formation of a sheath in front of a negatively biased electrode (collector) that emits electrons is studied by a one‐dimensional fluid model. Electron and ion emission coefficients are introduced in the model. It is assumed that the electrode is immersed in a plasma that contains energetic electrons. The electron velocity distribution function is assumed to be a sum of two Maxwellian distributions with two different temperatures, while the ions and the emitted electrons are assumed to be monoenergetic. The condition for zero electric field at the collector is derived. Using this equation the dependence of electron and ion critical emission coefficients on various parameters ‐ like the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons ‐ is calculated for a floating collector. A modification of the Bohm criterion due to the presence of hot and emitted electrons is also given. The transition between space charge limited and temperature limited electron emission for a current‐carrying collector is also analyzed. The critical potential, where this transition occurs, is calculated as a function of several parameters like the Richardson emission current, the ratio between the hot and cool electron density, the ratio between hot and cool electron temperature and the initial velocity of secondary electrons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A comparison of ion-induced electron emission and secondary ion yields

Ion-induced electron emission from solid surfaces is studied using a beam of caesium ions. Features of the spectra obtained during depth profiling of layered structures suggest a novel technique for investigating ion-induced Auger processes. Depth profiles are presented in terms of measured secondary ion signals, electron-induced Auger emission, and the intensities of features in the ion-induced electron spectra. It is shown that changes in features of the ion-induced electron spectra can be related to changes of chemical composition and sputtering probability. These help in the interpretation of variations in secondary-ion yields with matrix composition during depth profiling.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

A model of the ion sputtering process

This paper proposes a new model of the ion sputtering process. According to this model: (1) Sputtering atoms or ions originate almost entirely from the surface layer. (2) Atoms initially leave the surface in the same charge state in which they exist in the crystal, and are largely neutralized in traversing a region near the surface. (3) Neutralization can be explained in terms of quantum mechanical electron transmission and reflection phenomena that occur when the atom is within about three lattice spacings from the surface. The model upon which the theory is based pictures a potential well at the metal surface caused by the electric field of the departing ion. This potential well completely cancels the effect of the potential barrier normally present at the surface of the undisturbed metal, and for certain distances of the ion from the surface, results in a very high probability of emission of an electron from the metal surface, which then immediately neutralizes the emitted ion. This high neutralization probability explains the very low (10^?2 to 10^?6) sputtered ion yields observed experimentally. The ion sputtering yields for a number of elements were computed and compared with values obtained experimentally. It is found that the theory gives results compatible with experiment, and also provides an, at least qualitative, explanation for some of the heretofore rather puzzling experimentally observed features of ion sputtering.     

Ion beam sputtering of multicomponent targets: Surface composition change and cluster emission

Preferential sputtering effect, which occurs during irradiation of a multicomponent target by medium energy ions, was under our investigation. A new term characterizing the preferential sputtering, called “surface sputtering yield” and defined as average number of components i sputtered from a top surface layer per one primary ion, was suggested. A direct proportionality between the dimer emission and surface concentration of the component, forming the dimer, was concluded; this let us estimate the preferential sputtering of any target from the composition of the flux of secondary particles, analyzed directly during the ion sputtering process, and define the composition of the ion sputtered surface.     

Origin of Initial Current Peak in High Power Impulse Magnetron Sputtering and Verification by Non-Sputtering Discharge

A non-sputtering discharge is utilized to verify the effect of replacement of gas ions by metallic ions and consequent decrease in the secondary electron emission coefficient in the discharge current curves in high-power impulse magnetron sputtering(HiPIMS).In the non-sputtering discharge involving hydrogen,replacement of ions is avoided while the rarefaction still contributes.The initial peak and ensuing decay disappear and all the discharge current curves show a similar feature as the HiPIMS discharge of materials with low sputtering yields such as carbon.The results demonstrate the key effect of ion replacement during sputtering.     

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建立了包含两种正离子的碰撞等离子体鞘层的流体模型,通过四阶龙格-库塔法模拟了碰撞对含有两种正离子的等离子体鞘层中的离子密度和速度分布产生的影响。结果表明,对于两种正离子的等离子体来说,鞘层中无论哪种离子与中性粒子碰撞频率的增加,该种离子的密度和速度分布都将呈现波动变化,密度是先增加后降低,速度是先降低后增加;而另一种离子的密度和速度呈单调变化。鞘边正离子的含量越少,受自身与中性粒子的碰撞频率增加,鞘层中该种离子密度先增加后降低的变化位置就越远离等离子体的鞘层边界。同时发现该种离子密度分布受自身碰撞频率增加,降低的幅度变化非常小。另外发现电子碰撞器壁产生的二次电子发射系数对质量较轻的离子影响要大一些。     

On Auger electron spectroscopic measurements of ZnCl2 —coal interaction

By combining electron stimulated desorption (ESD) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function change (Δφ) measurements the information content of ESD with regard to surface structure and composition is examined, using the surface systems O/W(100) and O/W(110). Although it is not possible to separate the local interaction from the ion escape phase, the comparison of the ESD results with Information derived from LEED, AES and Δφ and the use of simple models of the local interaction gives a rather detailed picture of the location and environment of adsorbed atoms which provides a reasonably reliable basis for the interpretation of UPS spectra of adsorption layers.ESD is extremely sensitive to adsorbed layers. The fact that the ion signal depends not only on coverage but also on the structure and structure-dependent properties of the adsorbate makes on the one hand coverage determination difficult if not impossible, on the other hand opens the door to structure analysis. The potential for obtaining structure information can be easily assessed by comparison with electron probe results.In comparison with other ion probes such as ion scattering spectroscopy and secondary ion mass spectroscopy, ESD is at present the most promising ion probe method for obtaining information on the location of adsorbed atoms from angular and energy distribution measurement (ESDIAD and ESDIED). This is clearly seen by the comparison with the structural data derived from LEED, AES and Δφ measurements for the complex system O/W(100). The consistency of the data obtained with ESD and electron probe techniques lends strong support to the simple models on which the analysis of the ESD results from chemisorbed layers are based. The comparison of ESD results from the system O/W(100) at high coverage and from O/W(110) with 0⁺ ion emission from oxides shows, however, that caution is in place when assigning ESD features to atoms chemisorbed on the metal surface. Without a careful analysis of the ion energy, threshold and/or cross-section such ions cannot be distinguished from ions produced by dissociation of oxides which may be present on the surface only in small quantity. These ions usually are not related to the chemisorbed species which covers most of the surface and therefore dominates the signals seen with (nearly) all other surface probes.If the consistency of LEED, AES, Δφ and ESD data for O/W(100) is not fortuitous, then ESD has already given some important feed-back to the electron probe techniques: the structural models derived from vibrational ELS spectra have to be revised. Increasing accumulation of experimental data and deepening of the theoretical understanding of the physical processes involved in ion emission will have to show how much further information complementary to that from electron probes can be obtained from ion probes.     

Characterization of a RF/dc-magnetron discharge for the sputter deposition of transparent and highly conductive ITO films

A RF-superimposed dc-magnetron sputter process for coating color filter materials with transparent and conducting ITO films was investigated. In this process, the sputtering cathode is excited simultaneously by dc- and RF-power (at 13.56 MHz). This work summarises the measured properties of the gas discharge. Some basic data of the deposited ITO films are given, also. The dependence of the RF portion of the total sputtering power on the discharge voltage has been monitored for different values of total power and process pressure. The ion energy distribution function of the positively charged ions approaching the substrate surface has been measured using a retarding field plasma analyser probe. It was shown that the mean energy of the ions increases with increasing RF portion of the total power. The electron temperature in the body region of the gas discharge has been derived from measurements of the optical emission of the excited species. Received: 3 November 1998 / Accepted: 8 March 1999 / Published online: 14 July 1999     

相对论全腔轴向提取磁控管阳极释气数值模拟研究

相对论电子轰击阳极产生二次电子和释气以及释气电离产生的击穿现象是限制全腔轴向提取透明阴极磁控管（TCMAC）工作性能的一个重要因素。本研究对TCMAC中轰击产生的阳极二次电子以及释气电离现象进行了物理建模以及三维数值模拟研究,考察了其对TCMAC运行性能的影响。初步计算结果表明,二次电子发射与阳极释气对TCMAC工作都有一定影响,释气后电离产生的正离子数大于系统中电子数时,会导致TCMAC击穿。