Enhanced critical current densities in Cu-sheathed MgB2

Micron-scale mixing of magnesium and boron powders by mechanical alloying allows the MgB₂ formation reaction to proceed as low as 450 °C, with high-quality material formed in an hour at 600 °C. At these low reaction temperatures the reaction can be performed in situ in a copper sheath, and it is found that the presence of copper enhances the superconducting properties of the MgB₂. Self-field critical current densities, calculated from full critical-state simulation of magnetization hysteresis, are up to 7 × 10⁵ A/cm² at 13.6 K and 1 × 10⁵ A/cm² at 30 K.     

粒子束对玻姆鞘层判据的影响

在一维平板鞘层中应用双流体模型在有无碰撞两种情况下分别研究了定向离子束和定向电子束的存在对玻姆鞘层判据的影响.研究所涉及的碰撞主要有电荷交换碰撞、电离碰撞和总能量损失碰撞.工作气体是氩气，气压是常压.通过采用四阶龙格-库塔法，得到了数值解.结果表明定向离子束能减小漂移离子进入鞘层的最小速度，电荷交换碰撞也能减小这一速度；定向电子束使这一速度增加，与定向电子束相关的电离碰撞却使这一速度大幅度降低. 关键词： 双流体模型 玻姆鞘层判据 碰撞     

Self-consistent charge-up simulation for the microscopic feature of SiO2 layer in rf capacitive discharge

The charge-up simulation of the microscopic feature with SiO₂ layer was investigated in various conditions of rf capacitive discharge by using the three-dimensional (3-D) particle-in-cell (PIC) charge-up simulation coupled with the one-dimensional (1-D) particle-in-cell Monte Carlo collision (PIC-MCC) simulation of rf capacitive argon discharge. The result showed that the charge-up effect on the micro-trench was greatly influenced by the conditions of the gas pressure and the discharge voltage in rf capacitive discharge. Based on the analysis of the distributions of electrons and ions arriving at the substrate in various plasma conditions, the charge-up effect and its reduction mechanisms on the micro-trench of capacitive discharge were discussed. This article is expected to provide qualitative and quantitative insight for the understanding of charging and its reduction mechanism on many plasma processes performed by the rf capacitive discharge.     

Laser-induced ion acceleration at ultra-high laser intensities

Ion acceleration by ultrashort laser pulses of very high intensities of the order 10²²?W/cm² is studied by two-dimensional Particle-In-Cell simulations. We show that laser normal incidence is preferred for such high intensities. For linearly polarized laser radiation, higher maximum proton/ion energies are achieved than for circular polarization. For linear polarization, the transition from the target normal sheath acceleration to the acceleration on the target front side by the radiation pressure is analyzed in detail. The transition intensity is increasing with the target thickness. The radiation pressure dominated regime leads to considerably higher number of accelerated protons and thus to a higher acceleration efficiency.     

Variations of Plasma Parameters and Sheath Thickness Due to the Extraction of Ion and Electron Flux in a Double Plasma Device

Influence of charged particle extraction on plasma parameters and on ion sheath has been investigated in a double plasma device. When ions are extracted from the plasma, the plasma density as well as the positive ion flux into the sheath increases. As a result a sheath contraction takes place. Again, in case of electron extraction, it is found that the plasma density as well as the positive ion flux into the sheath decreases. As a result a sheath expansion takes place. Furthermore, it is observed that the floating potential of a plate can be controlled by extracting charged particles from the plasma. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

有序Banach空间正则锥的刻画

本文在序Banach空间中引入了序闭区间套的概念,获得了闭凸锥正则的两个充要条件.     

蒙特卡罗模拟在辉光放电鞘层离子输运研究中的应用

利用蒙特卡罗方法模拟氩气直流辉光放电鞘层内离子的运动过程.模拟基于离子与中性原子的电荷转移和弹性散射两种主要的散射过程，考虑了碰撞截面与能量相关和不相关两种情况，在弹性散射中采用了势场相互作用模型和刚性球碰撞两种模型.通过模拟得到不同气压和不同放电电压下离子入射阴极的能量分布和角度分布，并对几种模型的模拟结果进行了比较和讨论. 关键词： 辉光放电 等离子体鞘层 蒙特卡罗模拟     

碰撞效应对入射到射频偏压电极上离子能量分布和角度分布的影响

考虑了离子与中性粒子的弹性碰撞和电荷交换碰撞效应,建立了一套描述射频等离子体鞘层动力学特性的自洽模型,并利用MonteCarlo模拟方法研究了入射到电极上的离子的能量分布和角度分布.数值结果表明:随着放电气压增加,入射到电极上离子的能量分布逐渐地由双峰分布变成单峰分布,而且低能离子的数目也逐渐地增加.入射到电极上的离子呈小角分布,而且放电气压等参数对角度分布的影响不是太明显. 关键词： 射频放电 等离子体 离子 鞘层     

电子温度各向异性对霍尔推力器BN绝缘壁面鞘层特性的影响

建立多价态多组分等离子体一维流体鞘层模型,引入电子温度各向异性系数并考虑出射电子速度分布,研究了电子温度各向异性对霍尔推力器中的BN绝缘壁面鞘层特性和近壁电子流的影响.分析结果表明,相比于纯一价氙等离子体鞘层参数,推力器中的多价态氙等离子体鞘层电势降略有降低,电子壁面损失增加,临界二次电子发射系数减小.推力器中的电子温度各向异性现象可以显著地加大出射电子能量系数,进而降低鞘层电势降,增强电子壁面相互作用.数值结果表明,空间电荷饱和机制下电子温度各向异性对鞘层空间电势分布影响显著. 关键词： 霍尔推力器 电子温度各向异性 空间电荷饱和鞘层     

霍尔推进器壁面材料二次电子发射及鞘层特性

霍尔推进器放电通道等离子体与壁面相互作用形成鞘层,不同壁面材料的二次电子发射对推进器鞘层特性具有重要影响,本文针对推进器壁面鞘层区域建立二维物理模型,研究了氮化硼(BN)、碳化硅(SiC)和三氧化二铝(Al_2O_3)三种不同壁面材料的二次电子发射特性,在改进SiC材料二次电子发射模型的基础上,采用粒子模拟方法,讨论了壁面二次电子发射系数与电子温度和磁场强度的关系,研究了三种材料(BN,SiC和Al_2O_3)的鞘层特性,结果表明:修正的二次电子发射模型拟合曲线与实验曲线几乎一致;在相同电子温度下,三种材料(BN,SiC和Al_2O_3)的二次电子发射系数和壁面电子数密度依次增大,而鞘层电场和鞘层电势降依次减小,BN材料具有合适的二次电子发生射系数,使得霍尔推进器能在低电流下稳态工作。