Temperature and density measurements in a non-axisymmetric,continuously operating fusion experiment using a heavy ion beam probe

Two-dimensional electron temperature and density data have been obtained in the midplane of the non-axisymmetric magnetic confinement device ELMO Bumpy Torus (EBT) through the use of a heavy ion beam probe. This beam probe differs from others operated on toroidal or open magnetic geometries in its combination of complete computer control with the steady-state nature of EBT which allows, under normal operating conditions, for extensive calibration of the system in situ, minimizing both alignment and acquisition errors, along with the use of synchronous detection to dramatically improve the quality of the detected signal over what is typically possible in fast pulse devices. These techniques are important and applicable to long pulse devices where the beam probe may be an ideal diagnostic to measure, for example, parameters of the edge plasma. While the EBT beam probe was implemented to obtain profiles of plasma space potential, we have found that it can also be used effectively to measure the temperature and density profiles in the midplane between magnetic field coils. The data obtained support the contention that the formation of a hollow temperature profile in the T-Mode sufficiently inverts the plasma pressure such that stability of the core plasma would be expected even without diamagnetic effects from the hot electron rings which have previously been considered essential     

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.     

The Interchangeable Module Stellarator

The Interchangeable Module Stellarator (IMS) is a new toroidal stellarator-type device under design and construction at the University of Wisconsin Torsatron/Stellarator Laboratory. A new design strategy for constructing stellarator magnetic fields from discrete modular coils has been developed, utilizing orthogonal toroidal coordinates. Application of this method has resulted in a modular coil system (IMS), whose magnetic structure closely approximates that of the Proto-Cleo Q = 3 7-field period device (i. e., rotational transform, shear, flux volume, etc.).     

New approaches on application of multipole moments for determination of toroidal plasma shift

An application of multipole moments for the determination of toroidal plasma column shift is presented. First, we present analytical details for using this technique. Then, the principle of different models based on this technique for design and fabrication of a six coils will be presented: four modified Rogowski coils (two cosine coils and two sine coils) and two saddle coils (saddle sine coil and saddle cosine coil). As continuous measurements of magnetic field distribution around the toroidal plasma can be made using these coils, this technique is a good method for the determination of toroidal plasma column shift.     

Role of flow shear in the ballooning stability of tokamak transport barriers

A tokamak's confinement time is greatly increased by a transport barrier (TB), a region having a high pressure gradient and usually also a strongly sheared plasma flow. The pressure gradient in a TB can be limited by ideal magnetohydrodynamic instabilities with a high toroidal mode number n ("ballooning modes"). Previous studies in the limit n--> infinity showed that arbitrarily small (but nonzero) flow shears have a large stabilizing influence. In contrast, the more realistic finite n ballooning modes studied here are found to be insensitive to sub-Alfvénic flow shears, provided the magnetic shear s approximately 1 (typical for TBs near the plasma's edge). However, for the lower magnetic shears that are associated with internal transport barriers, significantly lower flow shears will influence ballooning mode stability, and flow shear should be retained in the analysis of their stability.     

Toroidal current generation by lower hybrid waves in a WT-1 device

A toroidal current driven by a lower hybrid wave (LHW) is obtained by applying RF power to a toroidal plasma produced by electron cyclotron resonance. The direction as well as the magnitude of the current depend on the supplementary vertical and horizontal magnetic fields, which is explained by confinement of current-carrying electrons in a toroidal device.     

反场构形的传输过程

介于惯性约束聚变与磁约束聚变之间的磁化靶聚变技术, 可能是一种实现纯聚变更低廉更有效的途径. 磁化靶聚变一般分为三个过程: 形成过程、传输过程和内爆压缩过程. 利用二维磁流体力学模拟程序MPF-2D, 对反场构形的传输过程进行了理论研究. 结果显示, 反场构形在传输过程中必须外加适当的磁场, 使得其内外磁压平衡, 才能维持其拓扑结构并进行稳定的传输. 还对初始磁压、传输磁场以及线圈间隙对反场构形传输过程的影响进行了详细的分析.     

识别TdeV托卡马克等离子体的边界和中心位置

用虚壳原理可以得到TdeV托卡马克等离子体边界、电流中心和X点,用最小二乘法可以确定Glad-Shafranov方程解中的待定参数。用探针测量极向磁场,初始时等离子体电流被看作是线电流,然后被虚壳电流代替。这个虚壳电流在等离子体内部产生负的约束磁场,在等离子体外部产生的磁场与等离子体电流产生的磁场一致,所用的叠代法充分快,可以在两次放电间给出等离子体图象,重建的位形与TV成像非常一致。     

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.     

CT-6托卡马克研究(Ⅰ)——实验装置的研制和调试

托卡马克是一种产生高温环形等离子体的聚变实验装置,也是一类复杂的现代化的电物理装备。CT-6是我国首先投入运行的小型托卡马克装置,其主要参数为:大半径45cm;小半径9.2cm;环向磁场2TL;等离子电流30kA;经过调试阶段,达到了预期目标,产生了平衡、稳定的电子温度为250eV左右的环形高温等离子体。CT-6由电磁系统(包括环向场、涡旋场、平衡场)、超高真空系统(包括环形真空室和抽气机组)、电源控制系统和诊断测量系统等部份组成,它是由中国科学院物理研究所、电工研究所及其他许多单位协作研制成功的。本文描述CT-6装置的设计、结构、工程研制和调试过程,以及有关的试验结果。 关键词：     

HL-2M 装置环向场线圈的工程研制与调试

介绍了 HL-2M 装置环向场(TF)线圈的工程设计、关键制造工艺及其调试结果。目前，TF 线圈最大通电电流达到 42kA，对应 TF 线圈产生的磁场为 0.66T，满足 HL-2M 装置初始放电需求。      

Status of Stellarator Experiments?1981

During the last years growing interest in stellarators as toroidal magnetic confinement devices becomes evident. In spite of many restrictions, e.g., small minor radius a ~0.1 m, plasma parameters comparable to those in tokamaks have been achieved. The deleterious effect of the plasma current on stability, equilibrium, and transport could be clarified. The progress of heating by the use of neutral injection and RF, allows to substitute ohmic heating. In the currentless phase significant improvement of the confinement at reasonable parameters has been observed. The substantial reduction of low-frequency and MHD fluctuations seems related to a decreased electron heat conduction.     

Measurement of pressure-gradient-driven currents in tokamak edge plasmas

Localized currents driven by pressure gradients play a pivotal role in the magnetohydrodynamic stability of toroidal plasma confinement devices. We have measured the currents generated in the edge of L- (low) and H- (high confinement) mode discharges on the DIII-D tokamak, utilizing the Zeeman effect in an injected lithium beam to obtain high resolution profiles of the poloidal magnetic field. We find current densities in excess of 1 MA/m2 in a 1 to 2 cm region near the peak of the edge pressure gradient. These values are sufficient to challenge edge stability theories based on specific current formation models.     

Role of pressure gradient on intrinsic toroidal rotation in tokamak plasmas

The toroidal plasma rotation generated by the external momentum input and by the plasma itself (intrinsic rotation) has been separated through a novel momentum transport analysis in the JT-60U tokamak device. The toroidal rotation, which is not determined by the momentum transport coefficients and the external momentum input, has been observed. It is found that this intrinsic rotation is locally determined by the local pressure gradient and increases with increasing pressure gradient. This trend is almost the same for various plasmas: low and high confinement mode, co and counterrotating plasmas.     

Resistive-wall-mode active rotation in the RFX-mod device

The fundamental question of how the flow velocity of the background plasma can influence the motion of magnetohydrodynamics instabilities and, in the ultimate analysis, their stability is addressed. The growth of resistive-wall-mode instabilities in toroidal confinement devices well represents one example of such a problem. In this Letter, we illustrate a new strategy that allowed, for the first time in a reversed field pinch experiment, a fully controlled rotation of a nonresonant instability by means of a set of active coils and how the new findings compare with numerical modeling.     

Current-carrying element based on second-generation high-temperature superconductor for the magnet system of a fusion neutron source

Application of current-carrying elements (CCEs) made of second-generation high-temperature superconductor (2G HTS) in magnet systems of a fusion neutron source (FNS) and other fusion devices will allow their magnetic field and thermodynamic stability to be increased substantially in comparison with those of low-temperature superconductor (LTS) magnets. For a toroidal magnet of the FNS, a design of a helical (partially transposed) CCE made of 2G HTS is under development with forced-flow cooling by helium gas, a current of 20–30 kA, an operating temperature of 10–20 K, and a magnetic field on the winding of 12–15 T (prospectively ~20 T). Short-sized samples of the helical flexible heavy-current CCE are being fabricated and investigated; a pilot-line unit for production of long-sized CCE pieces is under construction. The applied fabrication technique allows the CCE to be produced which combines a high operating current, thermal and mechanical stability, manufacturability, and low losses in the alternating modes. The possibility of fabricating the CCE with the outer dimensions and values of the operating parameter required for the FNS (and with a significant margin) using already available serial 2G HTS tapes is substantiated. The maximum field of toroidal magnets with CCEs made of 2G HTS will be limited only by mechanical properties of the magnet’s casing and structure, while the thermal stability will be approximately two orders of magnitude higher than that of toroidal magnets with LTS-based CCEs. The helical CCE made of 2G HTS is very promising for fusion and hybrid electric power plants, and its design and technologies of production, as well as the prototype coils made of it for the FNS and other tokamaks, are worth developing now.     

托卡马克极向场线圈的优化方法

托卡马克等离子体的三角形变和拉长比对约束和磁流体稳定性有很强的影响，因此在托卡马克装置极向场设计中，在基于物理和工程考虑所预先选定的等离子体平衡位形几何参数下，如何优化确定外部极向场线圈位置和电流，是一个具有重要实际意义的研究课题.为优化确定托卡马克极向场线圈，给出了一个有效的多变量平衡优化方法，能以事先规定的等离子体平衡位形的一些几何参数为目标函数，优化确定极向场线圈位置和电流.并应用它于HT-7U平衡位形计算，得到了所需的结果. 关键词： 等离子体平衡 极向场线圈 优化     

Reduced critical rotation for resistive-wall mode stabilization in a near-axisymmetric configuration

Recent DIII-D experiments with reduced neutral beam torque and minimum nonaxisymmetric perturbations of the magnetic field show a significant reduction of the toroidal plasma rotation required for the stabilization of the resistive-wall mode (RWM) below the threshold values observed in experiments that apply nonaxisymmetric magnetic fields to slow the plasma rotation. A toroidal rotation frequency of less than 10 krad/s at the q=2 surface (measured with charge exchange recombination spectroscopy using C VI) corresponding to 0.3% of the inverse of the toroidal Alfvén time is sufficient to sustain the plasma pressure above the ideal MHD no-wall stability limit. The low-rotation threshold is found to be consistent with predictions by a kinetic model of RWM damping.     

Flux conversion and evidence of relaxation in a high-beta plasma formed by high-speed injection into a mirror confinement structure

High-beta plasmoids can survive the violent dynamics of supersonic reflection off mirror structures, producing a stable high-beta field-reversed configuration (FRC). This shows both the robustness of FRCs and their tendency to assume a preferred plasma state, possibly conforming to a relaxation principle. The key observations are (1) approximate preservation of the magnetic helicity, (2) substantial conversion from toroidal to poloidal magnetic flux, (3) substantial toroidal flow, and (4) a high-beta quiescent final state. These results are from the Translation, Confinement, and Sustainment experiment where a disorganized plasmoid is injected at super-Alfvenic speed into a confinement chamber. After successive reflections from end mirrors, the plasmoid settled into a near-FRC state with high beta and low toroidal magnetic field. The flux conversion and helicity preservation are inferred by an interpretive model.     

Diffusive Losses and Scaling Law for Large Tokamaks

Under plasma conditions aimed for in the next generation of tokamak devices, it is necessary to consider a multi-parameter model for diffusive energy losses which would limit the temperatures and confinement times attainable. In this paper, we study a model for plasma diffusion in a reactor size tokamak considering various plasma parameters (like density, temperature, spatial gradients, collisional effects, etc.) and also topological elements of the magnetic configuration (like magnetic shear, magnetic curvature, etc.).