磁控直流辉光等离子体放电特性

本文利用单探针诊断等离子体参数来研究自行设计的磁控直流辉光等离子体实验装置的放电特性, 从而得出电子密度与气体压强、电子密度分布与磁场位型以及磁场强度等的关系. 另外, 用有限元的方法对线圈通电产生的磁场进行数值计算, 模拟出不同接线方式的两种磁场位型分布. 通过实验得出这两种不同的位型的磁场均对等离子体的状态有一定的“控制”作用, 而且这种“控制”作用与现有理论相符合.     

Measurement of turbulence decorrelation during transport barrier evolution in a high-temperature fusion plasma

A low power polychromatic beam of microwaves is used to diagnose the behavior of turbulent fluctuations in the core of the JT-60U tokamak during the evolution of the internal transport barrier. A continuous reduction in the size of turbulent structures is observed concomitant with the reduction of the density scale length during the evolution of the internal transport barrier. The density correlation length decreases to the order of the ion gyroradius, in contrast with the much longer scale lengths observed earlier in the discharge, while the density fluctuation level remain similar to the level before transport barrier formation.     

Effect of edge transport barrier on required toroidal field for ignition of elongated tokamak

The effect of an edge transport barrier on the toroidal field required for the ignition of an elongated tokamak is studied by modifying an analytic model which was calibrated against a transport code. It is found that the presence of the edge transport barrier will lead to a higher marginal toroidal field needed for ignition. This seemingly counter intuitive result is explained as being due to the equivalent effect of profile broadening by the edge transport barrier. This effect is further traced to its physical origin: in the case close to ignition, the fusion power input is predominantly concentrated in the center of plasma. It is demonstrated that if the fusion power input could be shifted from the center to the edge by a sufficient amount, then the presence of an edge transport barrier would lead to a reduction of the required toroidal field for ignition.     

Analysis of tokamak plasma confinement modes using the fast Fourier transformation

The Fourier analysis is a satisfactory technique for detecting plasma confinement modes in tokamaks. The confinement mode of tokamak plasma was analysed using the fast Fourier transformation (FFT). For this purpose, we used the data of Mirnov coils that is one of the identifying tools in the IR-T1 tokamak, with and without external field (electric biasing), and then compared it with each other. After the Fourier analysis of Mirnov coil data, the diagram of power spectrum density was depicted in different angles of Mirnov coils in the ‘presence of external field’ as well as in the ‘absence of external field’. The power spectrum density (PSD) interprets the manner of power distribution of a signal with frequency. In this article, the number of plasma modes and the safety factor q were obtained by using the mode number of q = m /n (m is the mode number). The maximum MHD activity was obtained in 30–35 kHz frequency, using the density of the energy spectrum. In addition, the number of different modes across 0–35 ms time was compared with each other in the presence and absence of the external field.     

Observation of compressional Alfvén modes during neutral-beam heating on the national spherical torus experiment

Neutral-beam-driven compressional Alfvén eigenmodes at frequencies below the ion cyclotron frequency have been observed and identified for the first time in the National Spherical Torus Experiment. The modes are observed as a broad spectrum of nearly equally spaced peaks in the frequency range from approximately 0.2omega(ci) to approximately 1.2omega(ci). The frequency has a scaling with toroidal field and plasma density consistent with Alfvén waves. The modes have been observed with high bandwidth magnetic pickup coils and with a reflectometer.     

First observation of edge localized modes mitigation with resonant and nonresonant magnetic perturbations in ASDEX Upgrade

First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number n=2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.     

A simple electromagnetic method of cyclic loss measurement for superconducting wires in a combined alternating transverse magnetic field and transport current

We measured cyclic losses in a superconducting wire, carrying alternating transport current, simultaneously exposed to an alternating transverse magnetic field. Samples of Bi-2223 Ag-sheathed tapes have configuration of a double-layer non-inductive coil, which itself is a pickup coil to measure the AC losses. Potential taps were attached to both terminals of the sample coil. The external field was applied along the axis of the sample coil. In this procedure, we can estimate an averaged Poynting's vector on a cylindrical surface between the two layers by means of signals from a pair of the potential taps and from pickup coils for the external magnetic field and the transport current. We can also measure a magnetization and an extended transport-current components of AC losses in addition to a total cyclic loss for a combined alternating external field and transport current. Obtained results are compared with numerical predictions of the critical state model taking into account the magnetic field dependence of critical current density.     

Active-feedback control of the magnetic boundary for magnetohydrodynamic stabilization of a fusion plasma

Stable operation with control on magnetohydrodynamic modes has been obtained in the modified reversed field experiment employing a set of 192 feedback controlled saddle coils. Improvements of plasma temperature, confinement (twofold), and pulse length (threefold) and, as a consequence of the magnetic fluctuation reduction, strong mitigation of plasma-wall interaction and mode locking are reported.     

Globally optimal superconducting magnets part II: symmetric MSE coil arrangement

A globally optimal superconducting magnet coil design procedure based on the Minimum Stored Energy (MSE) current density map is outlined. The method has the ability to arrange coils in a manner that generates a strong and homogeneous axial magnetic field over a predefined region, and ensures the stray field external to the assembly and peak magnetic field at the wires are in acceptable ranges. The outlined strategy of allocating coils within a given domain suggests that coils should be placed around the perimeter of the domain with adjacent coils possessing alternating winding directions for optimum performance. The underlying current density maps from which the coils themselves are derived are unique, and optimized to possess minimal stored energy. Therefore, the method produces magnet designs with the lowest possible overall stored energy. Optimal coil layouts are provided for unshielded and shielded short bore symmetric superconducting magnets.     

A Magnet Field System for a High Density Tokamak Reactor

A design for a magnet system for a high density high field tokamak reactor is described. In this design stress in the trunk of the toroidal field structure is significantly reduced by elimination of the central ohmic heating transformer. In addition all superconducting coils are operated in the steady state and pulsed fields are generated only by copper coils.     

SiC Schottky结反向特性的研究

对Ti/6H-SiC Schottky结的反向特性进行了测试和理论分析，提出了一种综合的包括SiC Schottky结主要反向漏电流产生机理的反向隧穿电流模型，该模型考虑了Schottky势垒不均匀性、Ti/SiC界面层电压降和镜像力对SiC Schottky结反向特性的影响，模拟结果和测量值的相符说明了以上所考虑因素是引起SiC Schottky结反向漏电流高于常规计算值的主要原因.分析结果表明在一般工作条件下SiC Schottky结的反向特性主要是由场发射和热电子场发射电流决定的.     

Multi-objective optimization of gradient coil for benchtop magnetic resonance imaging system with high-resolution

Significant high magnetic gradient field strength is essential to obtaining high-resolution images in a benchtop mag- netic resonance imaging （BT-MRI） system with permanent magnet. Extending minimum wire spacing and maximum wire width of gradient coils is one of the key solutions to minimize the maximum current density so as to reduce the local heating and generate higher magnetic field gradient strength. However, maximum current density is hard to optimize together with field linearity, stored magnetic energy, and power dissipation by the traditional target field method. In this paper, a new multi-objective method is proposed to optimize the maximum current density, field linearity, stored magnetic energy, and power dissipation in MRI gradient coils. The simulation and experimental results show that the minimum wire spacings are improved by 159% and 62% for the transverse and longitudinal gradient coil respectively. The maximum wire width increases from 0.5 mm to 1.5 mm. Maximum gradient field strengths of 157 mT/m and 405 mT/m for transverse and lon- gitudinal coil are achieved, respectively. The experimental results in BT-MRI instrument demonstrate that the MRI images with in-plane resolution of 50 ~tm can be obtained by using the designed coils.     

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

Theoretical basis of the design of magnetic field generator for DCvacuum circuit breakers

The magnetic field generator (MFG) for producing the transverse magnetic field switching off the direct current in the vacuum chamber is presented. The method of calculating generator coils inductance for different geometrical dimensions and various systems of coils connections, assuming the maximization of the magnetic flux density value in the contact gap and the maximization of the vacuum chamber switching ability, is presented. The possibilities of MAG program for designing magnetic field generators is described     

直线感应加速器输运磁场遗传算法优化

以直线感应加速器（LIA）匹配磁场设计和束线调谐为背景,提出解决强流相对论电子束长距离、小波动、多元件磁约束的输运优化问题的数值优化办法,建立基于遗传算法的优化程序。结合束质心轨迹及束包络耦合模型,设计描述束传输半径波动大小的评价函数,采用励磁元件馈入电流为优化对象,解决LIA磁场配置组合爆炸优化问题。计算结果表明:优化程序可依据不同的初始束流,有针对性地快速给出一组符合束输运要求的励磁电流配置。研究成果为在建的LIA装置束线调谐提供一种重要的数值分析工具。     

有机半导体异质界面电荷传输解析模型研究

有机半导体多层薄膜器件的性质很大程度上由有机-有机界面的传输性质所决定，但是现有的关于有机-有机界面的分析模型很难适用于实际器件的模拟.以Miller-Abrahams跳跃传导理论为基础，充分考虑有机-有机界面和金属-有机界面性质的不同，建立了一个新的描述有机-有机异质界面电荷传输的解析模型.结果表明有机异质界面的载流子传输不仅取决于界面的肖特基势垒，而且还取决于界面附近两边的电场强度和载流子浓度.此模型可用于有机半导体多层薄膜器件的电流密度、电场分布和载流子浓度分布的自洽计算. 关键词： 有机半导体 界面 载流子传输     

单层有机器件的电子传输特性的数值模拟

在漂移扩散模型的基础上建立了单层有机器件的模型,包括了电荷注入、传输、空间电荷效应和陷阱的影响.电荷注入考虑了热电子发射电流和隧道电流.模拟得到的结果和文献中报道的实验测试数据一致.模拟研究了各个因素对器件J-V曲线的影响,电流和器件长度成反比,电流随着空穴注入势垒的减小而增加.电子注入势垒从1.7 eV减少到0.5 eV时,电流随着电子注入势垒的减小而减小,这主要是因为有机材料中电子迁移率太小,电子注入电流的增加可以忽略,而电子注入势垒的减小使内建势增加,在同样的电压下,场强 关键词： 有机器件 传输特性 数值模拟     

栅极电势对强光场下石墨烯场效应管中电子隧穿的影响

利用时域有限差分方法研究了强光场下石墨烯场效应管中栅极电势对电子隧穿的影响. 在强光场下由于光学stark效应,石墨烯场效应管的完美手征透射被抑制.这种抑制除了 可以利用光场来调控外,也可以通过改变栅极电势的宽度、势垒高度等来调控. 研究了非方势垒中电子的隧穿. 研究发现,当电势的倾斜较小时,电子隧穿概率变化不大.而当电势倾斜很大时,电子隧穿概率急剧改变.     

Theoretical Optimization of Stellarators

Net current free toroidal ("stellarator") confinement is studied with a combination of several methods: a complete set of analytical vacuum fields for finding favorable vacuum field configurations; three-dimensional MHD codes for finite-?, equilibrium computations; the expansion of a general toroidal equilibrium around its magnetic axis as guideline for the computational search in configurational space and for finite-?, MHD stability; Monte Carlo simulations for particle containment; continuous modular coil systems generating the configurations considered. Results are: vacuum field configurations with sizeable Q = 0, 1, 2, 3 helical fields, substantial twist number (? 1/2), significant reduction of the parallel current density, and vacuum magnetic well exist for a toroidal aspect ratio of 15-20 and can be generated by modular coils whose excursions from meridional planes are small compared to the toroidal period length. In these configurations, the finite-? toroidal shift is strongly reduced, so that a larger ? value (factor 2-4) than in the equivalent Q = 2 stellarator can be achieved. Stability calculations do not exclude the possibility of stable equilibria of this kind with (?) ? 0.05-0.1; transport calculations without electrical field show improvement-as compared to the Q = 2 stellarator-in the collisional and plateau regimes.