Charged dust grain acceleration in tokamak edges

It is shown that the force associated with the normal component of the space charge electric field at the plasma surface, arising from the charge separation, can accelerate a charged dust particle that is sitting in the scrape-off layer (SOL) close to the chamber walls in tokamaks. The acceleration of dust particle is found to be proportional to the strength of the space charge electric field and inversely proportional to the square root of the dust mass density.     

Optimization of permanent magnet plasma confinement

Confinement of plasma by permanent magnets is studied with three magnet geometries. Full line cusps are found to give optimum plasma density. Increase in density is primarily due to confinement of primary electrons.     

Motion of nanodroplets near edges and wedges

Nanodroplets residing near wedges or edges of solid substrates exhibit a disjoining pressure induced dynamics. Our nanoscale hydrodynamic calculations reveal that nonvolatile droplets are attracted or repelled from edges or wedges depending on details of the corresponding laterally varying disjoining pressure generated, e.g., by a possible surface coating.     

同轴相对论速调管周期永磁聚焦系统的初步研究

为了探索相对论速调管放大器(RKA)的小型化技术,开展了同轴RKA周期永磁聚焦的物理与设计技术研究。周期永磁聚焦系统采用Halbach阵列结构,产生的磁场类型为周期性会切磁场。首先给出该系统的磁场各个分量的表达式,分析该系统磁场分布的特点,并推导得出该系统聚焦强流环形电子束的稳定条件。根据该稳定条件,对Ka波段同轴RKA设计了一个周期永磁聚焦系统,并优化了周期磁场参数,确定了磁场系统设计的最佳周期和幅值。研究结果显示,周期永磁(PPM)聚焦系统在周期长度18 mm和磁场幅值0.33 T的条件下可引导500 kV、6 kA的同轴RKA,得到1 GW的微波输出功率,物理分析确定了周期永磁聚焦系统应用于高功率同轴RKA的技术可能性。     

A permanent spherical magnet with inhomogeneous magnetization

Inhomogeneously distributed magnetization that induces a strong magnetic field in a predetermined local area is established in finite permanent magnets. The maximum possible magnetic induction in the vicinity of a magnet’s center is found analytically for a perfect spherical magnet.     

Disordered magnet near the percolation threshold

A disordered Potts magnet containing a random mixture of ferromagnetic exchange constants J_a and J_b (J_a?J_b) near the percolation threshold is considered. The scaling form for the free energy contains two crossover exponents. Duality arguments in two dimensions show that these exponents are equal. They are also shown to be equal to unity in d = 6 ? ? dimensions to order ?.     

永磁同步电动机的自适应混沌控制

首先基于LaSalle不变集定理设计一种自适应控制器,实现了永磁同步电动机中的混沌控制. 然后在控制项中增加一个控制强度因子,给出了一种改进的自适应控制器. 与以往的控制方案相比,该控制方案具有最简形式,控制器可以施加在状态方程的任何一项上. 仿真结果表明了该方法的有效性和对外界随机干扰的鲁棒性. 关键词： 自适应控制 LaSalle不变集定理 永磁同步电动机 混沌控制     

Magnetostriction of the NdCo5 uniaxial permanent magnet

Magnetostriction measurements, between ≈5 and 300 K, of powder magnetically aligned samples of the hexagonal compound NdCo₅, in strong pulsed magnetic fields up to 15 T are reported. The measurement have been performed parallel, perpendicular and at 45° to the alignment c-axis, for the field also applied in those directions. This set of measurements allows us to determine the six irreducible magnetoelastic modes: λ^α₁₁, λ^α₂₁, related to the strain dependence of the isotropic exchange, and λ^α₁₂, λ^α₂₂, λ^γ, λ^ϵ, related to the strain dependence of the crystalline field anisotropy energy. Anomalies on the strains, associated with the spin reorientation (SR) regime are observed, in particular at the temperatures of beginning, T_SR1, and end, T_SR2, of the SR. The thermal variation of the strains is explained by the standard magnetostriction model, including an ingredients: exchange striction (varying as m²_Nd), single-ion anisotropy (varying as m³_Nd), (where m_Nd is the reduc ed Nd sublattice magnetization), as well as an angular dependence of the form {sin²θ(H, T)−sin²θ(0, T)}, where θ(H, T) and θ(0, T), respectively, are the SR angles under field and spontaneous. The irreducible magnetoelastic coupling constants at 0 K have been estimated, although the point charge model is far from agreement with those results.     

永磁体外部磁场的不均匀性研究

从永磁体的分子电流观点、退磁场、工艺等出发, 以矩形永磁体为例, 从理论上分析了影响永磁体外部磁场不均匀性的各种因素.研究结果表明, 永磁体外部磁场宏观不均匀性(好场区均匀度和面积相对大小) 和空间距离及永磁体的外形设计密切相关. 退磁场对永磁体外部磁场微观不均匀性有着复杂影响. 永磁体工艺如粉末颗粒、取向度、烧结凝固、机械加工等将影响永磁体外部磁场的不均匀性, 如磁化偏角、对称性、光滑性等. 关键词： 永磁体 外部磁场 不均匀性 退磁场     

Simultaneous enhancement in coercivity and remanence of Nd2Fe14B permanent magnet by grain boundary diffusion process using NdHx

We have successfully developed a Dy-free grain boundary diffusion process with neodymium hydride (NdH_x) alloy to the permanent magnet Nd₂Fe₁₄B powders using hydrogenation – disproportionation – desorption – recombination (HDDR) method. All the diffusion treatments were performed at 700–800 °C for various annealing time under the high vacuum with rotating diffusion method that effectively control the abnormal grain growth. The coercivities of Dy-treated Nd₂Fe₁₄B powders were varied from 9.5 kOe to 13.2 kOe but the remanence was decreased to 8.1 kG (10% reduction) depending on dysprosium hydride (DyH_x) content and diffusion treated time. However, the coercivity and remanence of Dy-free diffusion treated powder have been increased to 12.2 kOe (28.5% enhancement) and 11.1 kG (22% enhancement) at the optimal diffusion treatment (800 °C for 3 h), respectively. This unique simultaneous enhancement is to isolate the magnetic coupling between Nd₂Fe₁₄B grains by creating non-magnetic Nd grain boundaries and enhance the alignment of the Nd₂Fe₁4B hard magnetic phase, fabricated by optimal diffusion conditions.     

Mössbauer spectroscopy in modern permanent magnet alloys

Intermetallic compounds involving the rare earths and a transition metal, especially iron, aroused great interest in the past twenty five years with particular attention been paid to their magnetic properties, due to the fact that these compounds have been used as a permanent magnet materials. Their study using different techniques has given new information about the mechanisms of the magnetic interactions, which are present in these compounds. Among them Mössbauer Spectroscopy (MS) has been proven to be an indispensable tool, due to the fact that information can be obtained either from the spectra of the iron sublattice or from the spectra of the rare earth sublattice. Thus information on local moments, crystal field effects, single ion anisotropy and exchange interactions can be extracted from such spectra and compared with results from other techniques. Among the best alloys for permanent magnet applications are the ones based on the Nd₂Fe₁₄B type structure. Very interesting magnetic properties are also present in the recently discovered series of RFe_12?x M _x , where M=V, Ti, Mo, Si. We will review their intrinsic and extrinsic magnetic properties, as they have been measured using (MS) and correlate them with the findings from other techniques.     

Nd-Fe-B永磁材料低磁时效机理研究

研究烧结Nd-Fe-B磁体的低磁时效问题.实验显示样品在室温293和353K经920d磁时效后，磁通量的衰减率分别为1.4%和13%，经老化处理后，磁通量的衰减率分别为6‰和6.9%，说明未做老化处理的磁体时间稳定性较差. 假设微观杂质运动临界能E₁、极微小杂质运动临界能E₂和移动原子对键取向转动临界能E₃对时间稳定性的影响最为显著，依据半经典的Boltzmann统计分布，导出的烧结Nd-Fe-B磁体的低磁时效所满足的规律.理论结果与实验相符. 关键词： 烧结Nd-Fe-B 低磁时效 临界能 时间稳定性     

银吸收边附近X射线衰减系数测量

在北京同步辐射装置的4B7A中能X光束线上,光源能区为2.1~6.0 keV,能量分辨大于5000,高次谐波小于0.1%,光源强度大于109光子/s。通过全能区多能点的透过率精确测量Ag样品质量厚度,然后采用Ag薄膜对单能X光子的透过率进行测量,给出了Ag薄膜在吸收边（3.4~3.9 keV）的衰减系数 。建立了Ag样品吸收边附近衰减系数同步辐射测量方法。通过不确定度分析给出衰减系数测量不确定度小于1％,填补了在该区间衰减系数的空白。     

银吸收边附近X射线衰减系数测量

在北京同步辐射装置的4B7A中能X光束线上,光源能区为2.1~6.0keV,能量分辨大于5000,高次谐波小于0.1%,光源强度大于109光子/s。通过全能区多能点的透过率精确测量Ag样品质量厚度,然后采用Ag薄膜对单能X光子的透过率进行测量,给出了Ag薄膜在吸收边(3.4~3.9keV)的衰减系数。建立了Ag样品吸收边附近衰减系数同步辐射测量方法。通过不确定度分析给出衰减系数测量不确定度小于1%,填补了在该区间衰减系数的空白。     

上海光源低温波荡器永磁铁在低温下的磁特性研究

用综合物性测量系统测试了国产Nd₂Fe₁₄B (N50M)永磁铁在低温下(10–300 K)的M-H和M-T. 获得了N50M剩磁B_r和内禀矫顽力H_ci在低温下的变化图, 对其取向度和三维磁化强度进行了分析研究. 结果表明, N50M在80–150 K发生强烈自旋再取向效应, B_r在120–130 K出现峰值, H_ci随温度下降呈线性增加. 在130 K, B_r和H_ci分别比常温(300 K)增加15.6%和220%, 达到1.65 T和3638 kA/m. 在150–300 K, 随温度下降, N50M宏观取向度与外磁场均匀性逐步改善, 但在80–235 K, 微观外磁场均匀性恶化.实验研究发现, 235 K附近, N50M垂直取向方向呈现“剩余磁化强度跳跃”. 研究结果为上海光源Ⅱ期低温波荡器及其他高精度低温永磁仪器与设备的物理设计提供了参考. 关键词： 低温波荡器 2Fe₁₄B永磁铁')" href="#">Nd₂Fe₁₄B永磁铁 低温 磁特性     

X波段永磁包装相对论返波管研制

设计了一个X波段相对论返波管,通过对器件结构的特殊设计,提高了器件的Q值,同时增大了电子束与慢波结构之间的耦合阻抗,从而实现器件的低磁场运行以便对其进行永磁包装;当引导磁场强度0.46 T、电子束束压750 kV、束流约5.5 kA时,得到频率9.1 GHz、功率1.24 GW的微波输出。根据模拟结果设计加工了一个磁场强度为0.46 T的小型化永磁磁体,该磁体长48 cm,最大外半径15 cm,总重量约116 kg。开展了永磁包装返波管的实验研究,得到以20 Hz的频率运行1 s时功率900 MW、单次运行时功率940 MW的X波段微波输出。     

X波段永磁包装相对论返波管研制

设计了一个X波段相对论返波管,通过对器件结构的特殊设计,提高了器件的Q值,同时增大了电子束与慢波结构之间的耦合阻抗,从而实现器件的低磁场运行以便对其进行永磁包装;当引导磁场强度0.46 T、电子束束压750 kV、束流约5.5 kA时,得到频率9.1 GHz、功率1.24 GW的微波输出。根据模拟结果设计加工了一个磁场强度为0.46 T的小型化永磁磁体,该磁体长48 cm,最大外半径15 cm,总重量约116 kg。开展了永磁包装返波管的实验研究,得到以20 Hz的频率运行1 s时功率900 MW、单次运行时功率940 MW的X波段微波输出。     

Chaotic vibrations of a magnet near a superconductor

The forced vibration of a small permanent magnet near the surface of a high temperature superconducting ceramic disc is shown to produce period doubling oscillations and chaos. The source of these anomalous vibrations is the nonlinear, hysteretic force between the magnet and the superconductor. These forces are believed to be related to flux pinning and flux dragging effects in the superconductor in the type II state. A return map, based on the displacement of the magnet, reveals a single hump polynomial function which under iteration exhibits a bifurcation structure similar to the experimental results.     

The dissolution kinetics of free iron in Nd-Fe-B permanent magnet alloys

Three kinds of NdFeB alloys, near the Nd₂Fe₁₄B ( φ phase ) composition of the Fe-(Nd₂B) pseudobinary phase diagram, were used to study the kinetics of free iron dissolution in the matrix, which is extremely important for the attainment of high coercivity. The dissolution equations together with the time exponent, n( , ) and activation energies of rate constants, Q(34–40.9 kcal/mol) were derived. These indicate that the quantity of Nd-rich liquid phase plays an important role in the dissolution kinetics. For the ‘hypo-φ’ composition (Fe-rich), grain boundary diffusion dominates. For φ and ‘hyper-φ’ (Nd-rich) composition, interface reaction dominates. The low activation energies of this system are attributed to the activated effect of φ phase formation and liquid phase assisted grain boundary diffusion, it is found that there will be no free iron left after powder pulverizing and sintering of the commercial Nd₁₅Fe₇₇B₈ alloy.