共查询到20条相似文献,搜索用时 31 毫秒
1.
Wang S.-B. Wendt A.E. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1999,27(5):1358-1365
The widely used Child-Langmuir law for sheath thickness evaluation in semi-infinite collisionless plasmas makes the assumptions of quasi-neutrality (ne=ni) and zero electric field intensity E=0 at the sheath edge, as well as applying the Bohm criterion for ions entering the sheath. However, through a whole region fluid model, Poisson's equation has been solved numerically for the steady-state solution through the sheath and presheath without these assumptions. With the sheath edge defined, as in the Child-Langmuir law, at the place where the ion velocity is equal to the Bohm velocity, the sheath thickness of a bounded collisionless or weakly collisional plasma has been found with this model in some cases to be much larger than that obtained with the Child-Langmuir Law. The sheath thickness discrepancy is significant under conditions found in low pressure high density plasma (HDP) tools for plasma processing. Results presented indicate that the sheath thickness is very sensitive to the electric field and space charge density at the sheath edge. The electric field and space charge density can be successfully estimated by an intermediate scale matching method, and are used to derive a modified expression for the potential in the sheath that can be solved numerically for sheath thickness. With these results, the matching problem, arising when sheath and plasma are modeled separately, can be overcome 相似文献
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The relation between plasma parameters determining a steady‐state charged sheath at a plasma‐wall boundary was derived and analyzed using a fluid model, which accounted for inertia and partial pressure of both plasma components. The relation generalizes the well‐known Bohm criterion and, in particular, shows that a steadystate sheath may be formed in some cases where the Bohm criterion is not satisfied. Conditions allowing formation of a double sheath structure were formulated and analyzed. It was shown that the structure may be formed only if a) the generalized Bohm criterion is satisfied, and b) the Sagdeev potential has a minimum. The double sheath structure consists of two sub‐sheath of different polarity, which depends on the wall potential polarity and the relation between the plasma component temperatures and inertia. For positive wall potential, the sub‐sheath adjacent to the plasma (the sheath edge) is enriched by negative particles, and the sub‐sheath adjacent to the wall is enriched by positive particles, while the sub‐sheath polarities are reversed for negative wall potential. An analytical theory was formulated and illustrated by numerical solutions of the Poisson equation for special cases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Sternberg N. Poggie J. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2004,32(6):2217-2226
A model for the collisionless plasma-wall problem under the action of an applied magnetic field is developed. The behavior of its solution is examined and found to be qualitatively consistent with experiment. The plasma and the sheath are then modeled separately to obtain the position of the quasi-neutral plasma boundary and the position of the edge of the electron-free sheath. It is shown that the plasma boundary can be specified as the point where the component of the ion velocity normal to the wall reaches the ion sound speed (Bohm criterion), and the sheath edge is specified as the point corresponding to Godyak's condition for the electric field. Studying the behavior near the plasma boundary and the sheath edge, the plasma solution and the solution of the space charge region are patched together to approximate the solution of the plasma-wall problem. 相似文献
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本文对于无内电极放电的射频低压等离子体,采用探针电流调制法,设计并建立了三探针诊断电子能量分布函数的测量系统,测量了压力在10-3—10-1Torr下氮气等离子体的电子能量分布函数。从理论和实验上研究了探针鞘层上射频干扰电压对测量电子能量分布函数的影响,给出了确定射频干扰值以及对射频干扰的影响进行修正的方法,并采用该方法对实验数据进行了修正,得到了正确的电子能量分布函数。根据所得到的电子能量分布函数计算的电子平均能量,与由探针伏安特性计算得到的电子平均能量相差不超过5%,该结果间接证明了本文实验测量系统的可靠性以及对射频干扰影响进行修正的正确性。 相似文献
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Levitated particles are a new powerful diagnostic of the midplasma sheath region. They can reveal features undetectable either to plasma or to surface measurements. The equilibrium position of microparticles suspended in an oxygen plasma sheath, together with a model of the levitation force and Langmuir probe measurements, gives evidence of secondary electropositive plasmas in the already established plasma sheath, in the range of parameters where the modified Bohm criterion breaks down into multiple solutions. 相似文献
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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。 相似文献
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The evolution of the electron energy distribution function (EEDF) over a weak magnetic field range is investigated in magnetized radio-frequency (rf) inductive discharges under a collisionless regime where an anomalous skin effect and electron cyclotron resonance (ECR) can occur. A significant change in the low-energy range of the EEDF is found in the ECR condition during the evolution. The observed result reveals the low-energy electrons are efficiently heated by the rf ECR in the presence of the anomalous skin effect. The calculated result of the electron distribution based on kinetic theory is in good agreement with the experiment. 相似文献
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通过求解一维稳态的尘埃等离子体鞘层模型,得到了等离子体鞘层势、正离子密度、电子密度和尘埃颗粒密度随一维横向的分布,Bohm判据及鞘层边界无量纲Bohm速度随尘埃密度的变化曲线,尘埃颗粒的带电量和尘埃密度的关系,尘埃颗粒的温度对尘埃颗粒自身在鞘层中分布的影响。结果表明,随着尘埃密度的增加,鞘层的厚度在减小,鞘层内的电子密度在下降,而且尘埃颗粒的带电量也在逐渐减少;随着尘埃温度的增加,鞘层的厚度减小,电子密度下降,而且鞘层附近的尘埃颗粒在逐渐增多。 相似文献
10.
Langmuir Probe in Non‐Maxwellian Plasma: Correlation Between the Electron Energy Distribution Function,the Ion Energy at the Sheath Edge and the Floating Voltage
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This work is devoted to the study of the Langmuir probe in non‐Maxwellian plasma, assuming mono‐energetic singly charged ions and a collisionless sheath. Using a general analytical equation for the Electron Energy Distribution Function (EEDF), we study the effect of the EEDF profile on: a) The ion energy at the sheath edge of a negatively biased collector, b) the I‐V probe characteristic and c) the floating voltage (Vp‐Vf). Different methods are used and compared to determine these parameters or characteristics. A correlation is given between the floating voltage, the ion energy at the sheath edge and the EEDF profile. The study is also extended to distribution functions with several components. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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S. F. Masoudi Zh. Ebrahiminejad 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2010,59(3):421-425
The Bohm sheath criterion is modified for collisional plasma containing
Boltzmann electrons and cold fluid ions at the presence of external magnetic
field. Based on fluid model, the effects of the strength and the orientation
of an external magnetic field on the upper and lower limits of Bohm sheath
criterion have been studied by considering the collision frequency between
ions and neutrals. The results show that the sheath criterion depends on the
orientation and magnitude of magnetic field and the ion flow velocity at the
sheath boundary. 相似文献
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This work is devoted to the study of the Bohm criterion in the general case of the electron energy distribution function (EEDF). Investigations are performed by means of a Monte Carlo integration method. We resolve the cold fluid equation system describing the ion motion within the sheath, assuming collisionless conditions, singly charged and mono kinetic incoming ions (BOHM model). Results confirm that the limit ion velocity at the sheath edge to assure a monotone electric field with a positive charge over the entire sheath is vi ≥ (kTe/Mi) or εi ≥ 1/3 <εe> in the case of Maxwellian electrons. We show that in the case of a Druyvesteyn electron energy distribution, this limit is larger, it is εi ≥ 0.6 <εe>. The study is also extended to other distributions functions. Because of the large controversy in recent publications, concerning the boundary conditions at the sheath entrance, we discuss the collisionless conditions at the sheath edge according to the plasma parameters. It is shown that in a collisionless sheath, the condition ni(χ) ≥ ne(χ) can be used to determine the limit ion velocity at the sheath edge of the negatively biased collector (Langmuir probe for instance) (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
A concept of self-organisation is used to derive the structure of dust sheath close to the wall taking into account self-consistently dust charges and dust distribution together with electron, ion distributions and ion drift. It is shown that instead of Bohm criterion the dust sheath is determined by other criteria allowing Mach number to be in restricted value ranges. The total structure of the dust sheath in dimensionless variables is determined by the Mach number only. Dust sheaths can contain definite number of dust particles per unit area which are determined by the value of the Mach number. The dust sheaths change drastically the electrostatic potential distributions and are usually much thicker than the usual plasma sheaths. A theoretical model together with numerical computations gives all dust sheath parameter distributions and the global characteristics such as the total drop of the potential, the thickness of the sheath, the number of dust particles in the sheath and the criterion of dust levitation. The recent observations on “Textor 94” indicating the long time dust survival in edge region of the present tokamaks, the dust levitation and agglomeration in the volume of edge tokamak plasma are discussed. 相似文献
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Mikhail Z. Tokar 《Journal of computational physics》2011,230(7):2696-2705
In plasmas bounded by material surfaces the Bohm criterion has to be satisfied at the entrance of the Debye sheath near the surface. With a single ion species this constraint prescribes a boundary condition for the momentum balance equation governing the ion mass velocity. If, however, several ion species are present a generalization of the Bohm criterion does not provide enough number of boundary conditions. Additional “intermediate” conditions follow from the requirement that spatial derivatives of the ion velocities are finite everywhere within the plasma volume. The amount of such independent conditions is sufficient to determine, in an iterative way, also the position in the plasma where they have to be imposed. A numerical approach to find unique regular solutions of fluid motion equations, satisfying the generalized Bohm criterion at the plasma boundary, is elaborated and realized for the case of two ion species. 相似文献
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为进一步研究霍尔推进器壁面二次电子发射对推进器性能的影响,采用流体模型数值模拟了二次电子磁化效应的等离子体鞘层特性.得到二次电子磁化鞘层的玻姆判据.讨论了不同的磁场强度和方向、二次电子发射系数以及不同种类等离子体推进器的鞘层结构.结果表明:随器壁二次电子发射系数的增大,鞘层中粒子密度增加,器壁电势升高,鞘层厚度减小;鞘层电势及粒子密度随着磁场强度和方位角的增加而增加;而对于不同种类的等离子体,壁面电势和鞘层厚度也不同.这为霍尔推进器的磁安特性实验提供了理论解释.
关键词:
霍尔推进器
磁鞘
二次电子 相似文献
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The formation of the sheath in a dusty plasma is investigated. The Bohm criterion is derived for two different cases: (a)
when electrons are in thermodynamic equilibrium and dust grains provide the immobile, stationary background and (b) when both
electrons and ions are in thermodynamic equilibrium and dust grains are moving. In the first case, Bohm criterion gets modified
due to the fluctuation of the charge on the grain surface. In the second case, the collisional and Coulombic drag play important
role in determining the Bohm criterion. 相似文献
20.
Peng-Sheng Wei Feng-Bin Yeh Ching-Yen Ho 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2000,28(4):1244-1253
In this study, the velocity distribution functions of the ions and electrons in a collisional presheath and collisionless sheath of a plasma near a wall emitting and reflecting ions and electrons are systematically determined. The collisions in the presheath are modeled by a relaxation time approximation (namely, Bhatnagar-Gross-Krook model, or simply BGK model). To find the variation in electrostatic potential with position, the model and analysis from Emmert et al. (1980), are used. Distribution functions of the ions and electrons in a collisionless presheath and sheath on a wall partially reflecting ions and electrons, therefore, can be exactly obtained. The reflections of the ions and electrons by a wall play important roles in studying heat transfer from a plasma sheath to a workpiece surface, and sputter etching and deposition, ion implantation, and ion scattering spectroscopy. Irrespective of ion and electron reflectivities, velocities of the ions in the presheath and sheath are of highly non-Maxwell-Boltzmann distributions. The electrons in the presheath are close to Maxwell-Boltzmann distributions, whereas those in the sheath are non-Maxwell-Boltzmann distributions. Even though the wall partially reflects ions and electrons, the Bohm's criterion is marginally satisfied at the sheath edge. The computed distribution functions for a completely absorbing surface agree with theoretical results provided in the literature. Good comparison of the resulted transport variables with available analytical work is presented in the companion paper 相似文献