Molecular Dynamics Simulations of the Interactions Between Tungsten Dust and Beryllium Plasma-Facing Material

In the present research,molecular dynamics simulation is applied to study the interactions between tungsten dusts and a beryllium plasma-facing material surface.Calculation results show that it is quite difficult for nanometer-size dust particles to damage the plasma-facing material surface,which is different from the micrometer-size ones.The reason may be the size difference between dust and crystal grains.The depth of dust penetration into plasma-facing materials is closely related to the incident velocity,and the impacting angle also plays an important role.Dust and material surface damage is also investigated.Results show that both incident velocity and angle can significantly influence the damage.     

Bootstrap Current in Spherical Tokamaks

Variational principle for the neoclassical theory has been developed by including a momentum restoring term in the electron-electron collisional operator,which gives an additional free parameter maximizing the heat production rate.All transport coefficients are obtained including the bootstrap current.The essential feature of the study is that the aspect ratio affects the function of the electron -electron collison vanishes.The resulting diffusion coefficient is in rough agreement with Hazeltine,When the aspect ratio approaches to infinity,the results are in agreement with Rosenbluth.The formalism gives the two extreme cases a connection.The theory is particularly important for the calculation of bootstrap current in spherical tokamaks and the present tokamaks,in which the square root of the inverse aspect ratio.in general,is not small.     

Internal Kink Instability in Shaped Tokamaks

A criterion of an ideal internal kink mode is derved for a shaped tokamak configuration in which q-profile is very flat in the core region.A combining criterion is obtained including the necessary criterion of Mercier and the sufficient criterion of Lortz.The new criterion makes progress compared with the necessary criterion of Mercier.In the elongated plasma.a polidal beta can cause instability,while the triangularity has a stabilizing effect.The result is applicable for DⅢ-D and SUNIST.     

Effect of Equilibrium Current Profiles on External Kink Modes in Tokamaks

Based on a linearized MHD model, the effect of equilibrium current profiles on external kink modes in tokamaks is studied by MARS code. Three types of equilibrium current profiles are adopted in this work. Firstly, a set of parabolic equilibrium current profiles are chosen. In these profiles the maximum current values in the center of the plasma are fixed, and the currents have different gradient and jump at the plasma boundary. The effects of the current gradient and jump on the growth rate of external kink mode are investigated. It is found that the current jump which causes the q profiles to change plays an important role in the externM kink modes in tokamaks. Secondly, a set of step equilibrium current profiles with different jump positions are chosen. The effect of jump position on external kink modes is discussed. Thirdly, a set of parabolic equilibrium current profiles with current bumps are chosen for the case of off-axis heating. The effects of height~ width and position of the current bumps on external kink modes are analyzed. The fiat equilibrium current profiles are disadvantageous for the MHD stabilities of tokamaks, because of the large current jump at the plasma edge. The peaked equilibrium current profiles and a large and localized current bump near the plasma edge benefit the MHD stabilities of tokamaks.     

The Fusion Science Research Plan for the Major U.S. Tokamaks

This is the May 1996 report of a subpanel of the US Department of Energy Fusion Energy Sciences Advisory Committee (FESAC), charged with conducting a review of the progress, priorities and potential near-term contributions of TFTR, DIII-D and Alcator C-MOD (and other facilities as appropriate) as part of the transition to a Fusion Energy Sciences Program and produce an optimum plan for obtaining the most scientific benefit from them.     

Stationary Potential Formation and Oscillations in Plasma with Immovable Dust Particles

Stationary electrostatic-potential formation in plasma with immovable dust particles was investigated by using one-dimensional kinetic analysis. It is clarified that the density of negatively charged dust particles below the threshold value makes the potential decrease monotonically. When the dust densities are above the threshold, there appears the stationary oscillation in an electrostatic potential due to the streaming plasma. It is found that the wavelength of this mode is of the order of Debye length. These phenomena are different from those of the conventional dust ion-acoustic waves in plasma, where the effect of dust-charging is not taken into account.     

Dynamics of Dust in a Plasma Sheath with Magnetic Field

Dynamics of dust in a plasma sheath with a magnetic field was investigated using a single particle model. The result shows that the radius, initial position, initial velocity of the dust particles and the magnetic field do effect their movement and equilibrium position in the plasma sheath. Generally, the dust particles with the same size, whatever original velocity and position they have, will locate at the same position in the end under the net actions of electrostatic, gravitational, neutral collisional, and Lorentz forces. But the dust particles will not locate in the plasma sheath if their radius is beyond a certain value.     

Induced Charge of Spherical Dust Particle on Plasma-facing Wall in Non-uniform Electric Field

Induced charge of a spherical dust particle on a plasma-facing wall was investigated analytically, where non-uniform electric field was applied externally. The one-dimensional nonuniform electrostatic potential was approximated by the polynomial of the normal coordinate toward the wall. The bipolar coordinate was introduced to solve the Laplace equation of the induced electrostatic potential. The boundary condition at the dust surface determines the unknown coefficients of the general solution of the Laplace equation for the induced potential. From the obtained potential the surface induced charge can be calculated. This result allows estimating the effect of the surrounding plasma, which shields the induced charge.     

Molecular Dynamics Simulations of Deposition and Damage on Tungsten Plasma-Facing Materials by Tungsten Dust

Classical molecular dynamics has been used to study the interactions between tung- sten （W） plasma-facing materials （PFMs） and dust grains. The impact velocity of dust grains is in the range from 324 m/s to 3240 m/s. The main effect of dust grains with low impact velocity is deposition. However, a material surface can be damaged by high velocity dust grains. The cumulative damage of impacting dust grains has also been take into account. When the impact velocity is low, no significant damage is detected but a porous firm forms on the surface. Serious damage can be produced on PFMs if the impact velocity is high.     

高温气冷堆热气导管中石墨粉尘沉积特性分析

高温气冷堆中石墨粉尘的运动规律对反应堆安全具有重要意义。本工作采用计算流体力学方法得到热气导管中的温度场分布,在此基础上分析了热气导管中热泳沉积与湍流沉积的规律。结果表明,随着颗粒粒径的增大,热泳沉积率下降,而湍流沉积率则先减少后增大。通过比较30%FP及100%FP两种反应堆功率下的计算结果发现：反应堆功率为30%FP时对应的热泳沉积率更高,而反应堆功率为100%FP时,湍流沉积率增长更加迅速。当颗粒粒径较小时,热泳沉积与湍流沉积作用相当,颗粒粒径较大时,湍流沉积明显占主导地位。最后,采用最可几分布的粒径计算了热气导管中石墨粉尘总的沉积量,计算结果表明石墨粉尘沉积总量有限。     

The Effect of the Charge Fluctuation of Dust Parti-cles on Ion-acoustic Wave Excited Through Ionization Instability

The effect of the charge fluctuation of dust particles on ion acoustic wave (IAW) excited through ionization instability was investigated. The hydrodynamic equations and linear time-dependent perturbation theory served as the starting point of theory, by which the dispersion relation and growth rate of the IAW were given. By comparing the results with the case of constant dust charges, it was found that the charge fluctuation of dust particles reduces the instability of the wave mode.     

Three-Dimensional Dust Crystal

Dust plasmas have received considerable attention in recent years due to the fact that a kind of crystal could be established, which is a typical example of an ideal material. In this system, the dust particles are often confined upon a three-dimensional sheath of an axissymmetric concave electrode, where the axial electrostatic force balances the gravitation and the radial electrostatic force keeps the dust from running away horizontally. In this paper, a threedimensional sheath structure is simulated through a liquid model, and the movement of the dust particles is simulated via the dynamic method. The crystals are obtained self consistently. The effect of both the electrode size and the mass of dust particles on the crystals are discussed. The simulation result demonstrates that two kinds of dust crystals could be established by varying the sheath diameter, and some proper conditions are necessary for the formation of a double layer dust crystal.     

Vibrational Mode in Two-Dimensional Dust Monolayer with Finite Size Dust Grains

Vibrational mode in a two-dimensional dust monolayer is investigated by considering the finite size of dust grains. Each dust grain is assumed to be a negative point charge and a dipole moment due to the inhomogeneous charge distribution on its surface. The dispersion relation of the vibrational mode is derived. Both the self-excited and externally excited cases are discussed. It is shown that the mode is sensitive to the direction of the dipole moment.     

Charging Effect in Plasma Etching Mask of Hole Array

It has already been found that the round shape of holes can be changed into hexagonal shape during plasma etching processes.This work aims to understand the mechanism behind such a shape change using particle simulation method.The distribution of electric field produced by electrons was calculated for different heights from the mask surface.It is found that the field strength reaches its maximum around a hole edge and becomes the weakest between two holes. The field strength is weakened as moving away from the surface.The spatial distribution of this electric field shows obvious hexagonal shape around a hole edge at some distances from the surface. This charging distribution then affects the trajectories of ions that fall on a mask surface so that the round hole edge is etched to become a hexagonal hole edge.The changing of this hole shape will again alter the spatial distribution of electric field to enhance the charging effect dynamically.     

Effects of Tailed Pulse-Bias on Ion Energy Distributions and Charging Effects on Insulating Substrates

A hybrid sheath model,including a fluid model and a Monte Carlo(MC) method,is proposed to study ion energy distributions(IEDs) driven by a radiofrequency(RF) with a tailed pulse-bias on an insulating substrate,where a charging effect is obviously caused by the ions accumulated.This surface charging effect will significantly affect the IEDs on the insulating substrate.In this paper,a voltage compensation method is employed to eliminate the charging effect by making the pulse-bias waveform have a certain gradient.Furthermore,we investigate the IEDs under the condition of different pulse-bias duty ratios,waveforms,amplitudes,and cycle proportions.It is found that the parameters of the pulsed source can effectively modulate the IEDs on the insulating substrate and the charging effect,and more desired IEDs are obtained by using the voltage compensation method with modulations of pulse parameters.     

航天器内部充电风险评估模型的问题及改进

通过与蒙特卡罗计算结果对比,发现现有内部充电风险评估模型(DICTAT)对辐射电子在材料内深度-剂量关系的表述不正确,本文引入Tabata的一组半经验公式重建了电子输运模型,并开发了新的内部充电风险评估模型。利用新模型、DICTAT和三维仿真工具,分别针对一组不同厚度的背部接地聚酰亚胺平板的充电电场和电位进行了计算,对比和解释了计算结果的差异。新模型与三维仿真工具的结果具有较好的一致性,其可更准确地评估内部充电风险。     

Dust Streaming Effect on Dust Acoustic Solitary Wave in Magnetized Dust-Ion Plasmas

The dust acoustic solitary structures in the presence of dust streaming are obtained through Sagdeev's pseudopotential approach in magnetized dustion plasmas. It is found that dust streaming plays a destructive role in the formation of dust acoustic solitary structures. However, the wave amplitude increases with the increase in the Mach number and obliqueness of the wave in the presence of contant dust streaming along a uniform external magnetic field. The numerical results are applied to Saturn's F-rings for illustrative purposes.     

Cycloid Motions of Aggregates in a Dust Plasma

Hypocycloid and epicycloid motions of aggregates consisted of one large and one small grains are experimentally observed in an rf dust plasma.The cycloid motions are regarded as combination of a primary circle and a secondary circle.Measurements with high spatiotemporal resolution show that the secondary circle is determined by the initial angle velocity of the dropped aggregate.The primary circle originates from the asymmetry of the aggregate.The small grain in the aggregate always leads the large one as they travelling,which results from the difference of the natural frequency of the two grains.Comparative experiments with regular microspheres show that the cycloid motions are distinctive features of aggregates immersed in a plasma.     

压水堆核电站上充泵的去污实践及其理论探讨

本文论述了某核电站在换料大修期间完成的上充泵去污工作,针对马氏体不锈钢在去污过程中出现的特殊现象,分析探讨了马氏体不锈钢的化学特性。通过总结上充泵去污的经验成果,对未来马氏体不锈钢的去污工作提出了建议。     

电子直线加速器串联谐振充电电源的设计与实现

电子直线加速器一般使用线型脉冲调制器作为脉冲电源。在脉冲调制器充电系统中,使用LC串联谐振充电技术为脉冲形成网络（PFN）充电可提高效率并减小体积,是大功率线型脉冲调制器的发展方向之一。本工作基于串联谐振充电技术的基本原理,设计并制造了1台脉冲调制器样机,结合实测波形和工程应用,提出了改善稳定度的方法。测量结果表明,脉冲调制器样机的设计方案正确,实现了设计指标。