HL—1装置的低q放电实验

装置获得的最低qL值是衡最托卡马克磁流体不稳定性的控制水平的重要品质参数.通过精细调节补充送气和电流上升率的方法控制电流密度分布,使用钛吸气方法控制边缘等离子体参数,HL-1装置获得了最低qL值为1.8的稳定等离子体。实验结果表明,若电流上升率与密度上升率之比为(23—40)×10~(-19)kA·m~3的范围内,最利于获得低MHD增长率的稳定放电。预计这与中心q(0)<1峰化的电流密度分布有关。     

CT-6托卡马克电子回旋辐射测量

本文报道了测量CT—6托卡马克二次谐波非寻常模的电子回旋辐射的实验结果及局部等离子体的电子温度Te随时间变化。通过改变装置纵场B_T值得到Te的空间分布。与硬X射线测量比较,确定了CT-6装置在电流上升阶段经常出现的逃逸电子辐射现象。本文也给出了实验中所观察到的等离子体内部MHD不稳定性所引起的辐射锯齿形扰动。     

HL-1M装置上MHD不稳定性磁扰动模的传播

对在HL-1M装置放电实验中发现的宏观MHD不稳定性磁扰动模的传播现象进行了研究。通过对实验中发现的各种极向模数m值的MHD磁扰动模特征的观察,以及在不同放电条件,特别是在偏压H模放电下传播方向不同的分析,深入研究了MHD模传播与等离子体旋转的定性关系。     

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HL-2A和HL-1M装置采用了激光吹气注入高Z杂质来缓减大破裂中的等离子体电流衰竭,并给出了初步实验结果。在HL-2A装置上建立了利用MHD扰动的参量预报放电破裂先兆的报警系统,研制了MHD实时检测与处理系统,实现了放电破裂先兆的预报、快速触发激光吹气、形成阻性高辐射等离子体、消耗热能和磁能,缓减大破裂。实验证明,这是一种使得大型聚变实验装置在放电破裂之前显著减少等离子体中热能和磁能,而且能安全终止放电的简单、快速和有效的途径。     

HT-6A托卡马克平衡系统的实验

本文叙述了HT-6A托卡马克平衡系统及第一期平衡实验结果.讨论了HT-6A上铜壳对平衡的贡献;观察到改变垂直场形态因子n而产生的水平位移及垂直位移不稳定现象;介绍了正常放电时的平衡实验,观察到等离子体位移与MHD扰动是紧密相连的;用沙弗拉诺夫薄壳理论计算了对多大的等离子体电流应施加多大的垂直场,结果与实验完全符合。     

HL—1M等离子体的复合锯齿振荡

用软X射线二极管阵列可以很容易地探测到在等离子体内部形成的软X射线辐射扰动的锯齿振荡波形,根据观测到的锯齿振荡的某些特征可以定性地确定等离子体芯部的磁场位形。在托卡马克等离子体加热过程中,电子加热和电流穿透使得电流分布不断峰化、中心磁轴附近的安全因子下降,当中心安全因子q(0)＜1时在q=1磁面附近形成磁岛,磁岛的增长和重联构成了一个完整的锯齿振荡波形。由于电流分布的峰化而形成的振荡被称为单锯齿振荡,振荡的周期和幅度具有单一的特性,尽管它们会随着放电条件的改变而变化,但振荡的单一性保持不变,破裂位于中心磁轴位置。在特定的条件下,当等离子体电流变成中空分布而且在等离子体内部出现两个q=1磁面时,复合锯齿取代常规的单锯齿,它们或者有规则地连续出现,或者间或地出现在单锯齿之间,这取决于中空电流分布的维持程度。我们在HL－1M装置上通过测量软X坶辐射扰动观测到等离子体的复合锯齿振荡。本文叙述的就是在离轴电子回旋共振加热期间所观测到的复合锯齿振荡及其产生的条件,并对锯齿产生的可能的机制作了定性的描述。     

HL-1工程联调中真空室的放电清洗

HL-1托卡马克装置工程联调时,内真空系统经200℃60小时烘烤和7万次脉冲放电清洗,接着作了124次低参数托卡马克放电。在环向磁场为15kG下获得了60kA的等离子体电流,等离子体存在时达85ms,本文总结分析了HL-1工程联调期间真空系统的烘烤、真空室的放电清洗技术及效果。     

等离子体破裂期间电流猝灭特征研究

等离子体破裂会对托卡马克装置的安全运行造成严重威胁.等离子体破裂期间电流猝灭速率与电磁负载的大小及逃逸电流平台的形成都密切相关.本文对HL-2A装置等离子体破裂进行了统计分析,统计选用等离子电流的两个衰减区间90%-10%和80%-20%.分析结果表明:HL-2A装置等离子体破裂有四种不同的电流猝灭波形,两个衰减区间最小电流猝灭时间的参数区分别为2.6 ms和2.2 ms,并且不同衰减区间下平均电流猝灭时间统计分布明显不同.     

HL—1装置电流上升段辐射与杂质特性

托卡马克等离子体中的杂质会影响托卡马克的放电品质及等离子体特性。许多理论和实验对杂质的产生和输运做了深入详细的研究。等离子体电流起始阶段,由于约束性能不好,会引起大量的杂质产生,辐射损失增大是杂质增加引起的直接后果。杂质辐射是等离子体辐射的主要组成部分之一,等离子体线辐射功率～Z_(eff)~6,复合辐射功率～Z_(eff)~4,轫致辐     

导体壳对电阻磁流体不稳定性的影响

结合HL-1装置的条件,采用撕裂模的准线性理论,研究了托克马克中导体壁对m=2/n=1扰动模的稳定作用。着重研究了导体壁位置,等离子体电流分布,等离子体位形对这种稳定效应的影响。结果表明,共振面的位置与壁的稳定作用有密切关系,存q_a接近于2的位形中,m=2的撕裂模扰动可以被靠近等离子体的导体壁完全抑制。导体壁的稳定效应与等离子体电流分布相联系,在一些现实的电流分布中,只要适当地压低等离子体边界区的电流密度,壁的稳定效应会更加显现出来。     

Three-dimensional structures of MHD flow in a disk generator

Three-dimensional nonuniform plasmas and boundary layers have been studied numerically under an MHD interaction. The nonuniform plasma of “streamer” owing to weak ionization of seed material has a spiral structure in the r-&thetas; plane, and the plasma becomes almost uniform between the walls in the r-z plane. This structure is almost the same as that in our previous paper (1997), where the gas (heavy particle) properties are assumed to be invariant and steady. In addition to the streamer, the nonuniform plasma of “domain” owing to weak ionization of noble gas is revealed. The domain has the structure perpendicular to the streamer. In a strong MHD interaction case, the static pressure considerably increases in the upstream region of a generation channel, and the pseudo-shock waves appear in the generator, but the plasma is almost uniform along the &thetas; direction. The boundary layer in the strong MHD interaction is considerably thick, and in the broad region near the wall the Hall current flows reversely. In the weak MHD interaction case, the plasma forms a nonuniform structure along the &thetas; direction, and the Hall current does not always flow in the opposite direction even on the insulator wall since the azimuthal electric field is not zero     

Measurement of the current sheet during magnetic reconnection in a toroidal plasma

The current and magnetic-field fluctuations associated with magnetic-field-line reconnection have been measured in the reversed field pinch plasma configuration. The current sheet resulting from this reconnection has been measured. The current layer is radially broad, comparable to a magnetic-island width, as may be expected from current transport along magnetic-field lines. It is much larger than that predicted by resistive MHD for linear tearing modes and larger than prediction from two-fluid linear theory.     

A consistent and conservative scheme for incompressible MHD flows at a low magnetic Reynolds number. Part III: On a staggered mesh

The consistent and conservative scheme developed on a rectangular collocated mesh [M.-J. Ni, R. Munipalli, N.B. Morley, P. Huang, M.A. Abdou, A current density conservative scheme for incompressible MHD flows at a low magnetic Reynolds number. Part I: on a rectangular collocated grid system, Journal of Computational Physics 227 (2007) 174–204] and on an arbitrary collocated mesh [M.-J. Ni, R. Munipalli, P. Huang, N.B. Morley, M.A. Abdou, A current density conservative scheme for incompressible MHD flows at a low magnetic Reynolds number. Part II: on an arbitrary collocated mesh, Journal of Computational Physics 227 (2007) 205–228] has been extended and specially designed for calculation of the Lorentz force on a staggered grid system (Part III) by solving the electrical potential equation for magnetohydrodynamics (MHD) at a low magnetic Reynolds number. In a staggered mesh, pressure (p) and electrical potential (φ) are located in the cell center, while velocities and current fluxes are located on the cell faces of a main control volume. The scheme numerically meets the physical conservation laws, charge conservation law and momentum conservation law. Physically, the Lorentz force conserves the momentum when the magnetic field is constant or spatial coordinate independent. The calculation of current density fluxes on cell faces is conducted using a scheme consistent with the discretization for solution of the electrical potential Poisson equation, which can ensure the calculated current density conserves the charge. A divergence formula of the Lorentz force is used to calculate the Lorentz force at the cell center of a main control volume, which can numerically conserve the momentum at constant or spatial coordinate independent magnetic field. The calculated cell-center Lorentz forces are then interpolated to the cell faces, which are used to obtain the corresponding velocity fluxes by solving the momentum equations. The “conservative” is an important property of the scheme, which can guarantee computational accuracy of MHD flows at high Hartmann number with a strongly non-uniform mesh employed to resolve the Hartmann layers and side layers. 2D fully developed MHD flows with analytical solutions available have been conducted to validate the scheme at a staggered mesh. 3D MHD flows, with the experimental data available, at a constant magnetic field in a rectangular duct with sudden expansion and at a varying magnetic field in a rectangular duct are conducted on a staggered mesh to verify the computational accuracy of the scheme. It is expected that the scheme for the Lorentz force can be employed together with a fully conservative scheme for the convective term and the pressure term [Y. Morinishi, T.S. Lund, O.V. Vasilyev, P. Moin, Fully conservative higher order finite difference schemes for incompressible flow, Journal of Computational Physics 143 (1998) 90–124] for direct simulation of MHD turbulence and MHD instability with good accuracy at a staggered mesh.     

MHD modeling of conductors at ultrahigh current density

In conjunction with ongoing high-current experiments on Sandia National Laboratories' Z accelerator (Albuquerque, NM) we have revisited a problem first described in detail by Heinz Knoepfel (1970). Unlike the 1-Tesla MITL's of pulsed power accelerators used to produce intense particle beams, Z's disk, transmission line (downstream of the current addition) is in a 100-1200-Tesla regime; so its conductors cannot be modeled simply as static infinite conductivity boundaries. Using the MHD code MACH2 we have been investigating the conductor hydrodynamics, characterizing the joule heating, magnetic field diffusion, and material deformation, pressure, and velocity over a range of current densities, current rise-times, and conductor materials. The three purposes of this work are 1) to quantify power flow losses owing to ultrahigh magnetic fields, 2) to model the response of VISAR diagnostic samples in various configurations on Z, and 3) to incorporate the most appropriate equation of state and conductivity models into our magnetohydrodynamics (MHD) computations. Certain features are strongly dependent on the details of the conductivity model     

Investigation of electron-distribution function and dynamo mechanisms in a reversed-field pinch by analysis of hydrogen-pellet deflection

In reversed-field pinches, two different mechanisms have been proposed to explain the dynamo process which drives the poloidal current needed to sustain the magnetic configuration: the kinetic dynamo theory and the magnetohydrodynamic (MHD) dynamo theory. Experimentally, they can be distinguished by the radial behavior of the electron distribution function. In this Letter the trajectory deflection of frozen hydrogen pellets has been used as a diagnostic of suprathermal electrons in the plasma. The classical Spitzer-Harm distortion of the electron distribution function consistent with the MHD dynamo electric field is found to give a better modeling of the pellet trajectory.     

Experimental Studies on the Sustainment of Splieromak Plasmas by an Inductive Drive of the Toroidal Current

Sustainment of spheromak plasmas produced in an external equilibrium field has been demonstrated with a center current transformer (ohmic heating (OH) coil) which is used to inductively drive the toroidal current of the plasma. The OH coil is covered by a cylindrical metal liner. It provides the stability against the tilt and shift motions of spheromaks at the expense of the simple connection of its geometry. Since the spheromak is characterized by the elimination of external toroidal fields in association with nonconservation of a toroidal flux during magnetic relaxation, the metal liner was made electrically disconnected from the main vacuum vessel (spheromak mode). In the experiments, existense of the dynamo effect, meaning automatic generation of toroidal flux similar to that of a reversed field pinch (RFP), is observed. Measured MHD activity consists of multihelicity helical modes with toroidal mode numbers N = 1-3. In order to investigate the difference between spheromaks and RFP's in the MHD activity during sustainment, experiments have also been made with the metal liner of the OH coil connected with the vessel (RFP mode). The dynamics of the MHD activities observed are compared with those obtained from the three-dimensional MHD simulations by Katayama and Katsurai [18], and their implication in the dynamo effect is discussed.     

Numerical modelling of magnetogasdynamic processes in the pulsed setup duct

A computational model for processes in the duct of an experimental setup consisting of a shock tube and an MHD duct is presented. The one-dimensional model is used for determining the flow characteristics in the entire setup duct, and the three-dimensional model is used for studying the current layer dynamics in the MHD channel. Computations have enabled the elucidation of flow structure and of the peculiarities of current layer formation.     

Modeling two-phase flows in channels and nozzles

The gas dynamics of the flow of a two-phase mixture, consisting of a gaseous phase and polydisperse liquid particles of oxides suspended in it, plays a major role in determining the thrust parameters of rocket engines and the power characteristics of MHD stations based on metallized fuels. A number of monographs and reviews have now been published that reflect the current state in this branch of continuum mechanics. Here we give the results of a numerical investigation of certain features of two-phase streams in combustion chambers, Laval nozzles, and MHD generators.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 71–81, August, 1992.     

Characterization of the three-dimensional supersonic flow for the MHD generator

A numerical procedure based on a five-wave MHD model associated with non-ideal, low magnetic Reynolds number MHD flows was developed in the present study for analyzing the flow fields in the MHD generator of a MHD bypass scramjet. The numerical procedure is composed of an entropy conditioned scheme for solving the non-homogeneous Navier-Stokes equations, in conjunction with an SOR method for solving the elliptic equation governing the electrical potential. It was found that a separation would take place near the downstream edge of the second electrode, where the local adverse pressure gradient is large, and the core of the flow field is characterized as a 2-D flow due to the Hartmann effects along the direction of the magnetic field. The electric current lines would be increasingly distorted as the magnetic interactive parameter increases, and even induce an eddy current. Induced eddy current was also found in the different cross-sections along the axial direction, all of these would definitely deteriorate the performance of the MHD generator. The cross-sectional M-shape velocity profile found along the axial direction between the insulating walls is responsible for the formation of the vortex flow at the corner of the insulator cross-section, which, in turn, induces the corner eddy current at the corner. A numerical parametric study was also performed, and the computed performance parameters for the MHD generator suggest that, in order to enhance the performance of MHD generator, the magnetic interaction parameter should be elevated.     

Induced electric current-based formulation in computations of low magnetic Reynolds number magnetohydrodynamic flows

We use the induced electric current as the main electromagnetic variable to compute low magnetic Reynolds number magnetohydrodynamic (MHD) flows. The equation for the induced electric current is derived by taking the curl of the induction equation and using Ampère’s law. Boundary conditions on the induced electric current are derived at the interface between the liquid and the thin conducting wall by considering the current loop closing in the wall and the adjacent liquid. These boundary conditions at the liquid–solid interface include the Robin boundary condition for the wall-normal component of the current and an additional equation for the wall potential to compute the tangential current component. The suggested formulation (denominated j-formulation) is applied to three common types of MHD wall-bounded flows by implementing the finite-difference technique: (i) high Hartmann number fully developed flows in a rectangular duct with conducting walls; (ii) quasi-two-dimensional duct flow in the entry into a magnet; and (iii) flow past a magnetic obstacle. Comparisons have been performed against the traditional formulation based on the induced magnetic field (B-formulation), demonstrating very good agreement.