Observations of Edge Plasma on HL-2A Tokamak

The edge plasma characteristics are studied by a movable array of Mach/Reynolds stress/Langmuir 10-probes and fast reciprocating 4-probes in the boundary region. These probes can measure edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles obtained by changing the radial positions of the probe array in different discharges. Measurement resuits are used to analyze plasma confinement, turbulent fluctuations and their correlation. The fixed flush 3-probe arrays are mounted on the 4-divertor neutralization plates at the same toroidal cross-section in divertor chamber. These probes are used to measure the profiles of the electron temperature, density and float potential in divertor chamber.     

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通过中平面活动10探针组、往复快速扫描4探针和低杂波天线口的固定4探针测量了主等离子体边缘的温度、密度、悬浮电位、径向和极向电场、雷诺协强、径向和极向等离子体流速及其径向分布。用偏滤器靶板上的14组嵌入式静电3探针阵列测量了同一环向截面的内外中性化板上的电子温度、密度、悬浮电位及其分布。比较了在孔栏位形和偏滤器位形下边缘等离子体特性的差异,特别是两种位形下边缘温度和密度衰减长度的变化。分析了在多脉冲超声分子束加料和低杂波注入条件下的边界等离子体特性,以及雷诺协强的径向梯度与极向流和径向电场梯度与湍流损失的相互关系。     

Diagnostics for radial electric field measurements in hot magnetized plasmas

Measurements of the radial electric field profile in magnetically confined plasmas have yielded important new insights in the physics of L-H transitions, edge biasing and/or the active control of Internal and Edge Transport Barriers. The radial electric field is not an easy plasma parameter to diagnose. Techniques to measure the radial electric field in the plasma core are the Heavy Ion Beam Probe and the Motional Stark Effect. An indirect method that is quite often applied is to derive the electric field from measurements of the poloidal and toroidal rotation velocities via the radial ion force balance. This paper will first briefly explain the need for detailed measurements of the radial electric field profile. Subsequently, the various diagnostics to measure this parameter will be reviewed. The emphasis will be especially put on recent trends, rather than on an exhaustive overview. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002. Partner in the Trilateral Euregio Cluster     

Observation of peak neoclassical toroidal viscous force in the DIII-D tokamak

Observation of a theoretically predicted peak in the neoclassical toroidal viscosity (NTV) force as a function of toroidal plasma rotation rate Ω is reported. The NTV was generated by applying n=3 magnetic fields from internal coils to low Ω plasmas produced with nearly balanced neutral beam injection. Locally, the peak corresponds to a toroidal rotation rate Ω(0) where the radial electric field E(r) is near zero as determined by radial ion force balance.     

HL—1装置的金属杂质沉积实验

采用立体探针与二次离子质谱计（ＳＩＭＳ）分析相结合，对ＨＬ－１装置刮削层空间的杂质沉积特性和分布规律进行了实验研究。测量了在石墨活动孔栏条件下，立体针表面杂质沉积特性和分布规律进行了实验研究。测量了在石墨活动孔栏条件下，立体探针表面杂质沉积的径向分布，纵向分布，极向分布和Ｈ＾＋剖面分布。并讨论了实验结果。     

Equilibrium profiles for RF-plasma sources with ponderomotiveforces

An analysis is given of the influence of the electron ponderomotive force on the equilibrium plasma profiles of partially ionized, radio frequency discharge sources, The ponderomotive force can be written as a gradient of a potential varying with the square of the RF field in the plasma and is largest for electrons, The impact of this electron ponderomotive force on density and electrostatic potential profiles is demonstrated using a one-dimensional analytic model with supporting numerical solutions and a two dimensional fluid simulation. For nearly collisionless plasmas the ponderomotive force is valid when ω_ce^h/ω<1 where ω_ce ^h is the electron cyclotron frequency due to the RF magnetic field and ω is the RF driving frequency, In processing plasmas with parameters that satisfy this validity criteria, the equilibrium density profiles are weakly modified, For nearly collisionless processing plasmas with parameters such that ω_ce^h /ω>1, the ponderomotive force, is modified by other nonlinear force terms that need to be evaluated     

Effect of poloidal density variation of neutral atoms on the tokamak edge

Neutral atoms in the tokamak edge affect the electric field and plasma flow through charge exchange. Here, it is shown that these effects depend sensitively on the poloidal location of the atoms. It is found that the radial electric field and toroidal flow velocity in a collisional edge plasma are largest if the atoms are concentrated on the inboard side. This feature may explain MAST and COMPASS-D observations indicating easier H-mode access for inboard gas puffing. The results suggest an external means of controlling the toroidal flow and radial electric field and their shear, and have motivated further MAST measurements appearing to support the predictions.     

Observations of an ion-driven instability in non-neutral plasmas confined on magnetic surfaces

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number m=1, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the E x B flow of the electron plasma. The frequency does not, however, scale exactly with E/B, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.     

EDGE REYNOLDS STRESS STRUCTURE OF LOWER HYBRID WAVE INJECTION IN THE HL—1M PLASMA

The radial profiles of electrostatic Reynolds stress,plasma poloidal rotations,radial and poloidal electric fields have been measured in the plasma boundary region on the HL-1M tokamak using a multi-array of Mach/Langmuir probes.During experiments of lower hybrid wave current drive,the variations in LHW drive power xill cause changes in the edge electric field,poloidal rotation veloity and Reynolds stress.The results indicate that sheared poloidal flow can be generated in the edge plasma due to radially varied Reynolds stress.     

Poloidal spin up and electric-field generation related to displacement current and neoclassical transport

Summary In accordance with the conventional orderings of neoclassical theory, poloidal and toroidal accelerations with constant parallel flow can be driven by heat transport in the absence of external momentum input and with vanishing parallel viscous stress. In a transient phase in which the heat transport is the primary source of the time dependence, the torque generating the rotation is provided at third order in the adiabatic expansion by the surface-averaged (non-ambipolar) displacement current, which is also responsible for charge build-up and for the radial electric field. The heat transport equation has been solved in a narrow layer interfaced with the intensely heated plasma core through heat flux continuity, assuming neoclassical multicollisional coefficients with self-consistent suppression mechanism of anomalous transport. Starting from low temperature in the edge layer, a strong temperature gradient, a mass poloidal rotation in the ion direction and a strongly negative sheared radial electric field can be generated, in agreement with the observations, and reach a stationary state after a displacement current-dominated triggering phase (intrinsically non-ambipolar) lasting few milliseconds. Momentum input becomes important on longer time scale and is responsible for the toroidal rotation, decoupled from temperature gradient and for a further development of the radial electric field. The results show the ability of edge transport processes to adapt flexibly to a high temperature imposed on the inner side of the edge layer and support the view that the edge processes are an integral part of a more fundamental global process involving possibly an internal bifurcation of state.     

Dynamic transport simulation code including plasma rotation and radial electric field

A new one-dimensional transport code named TASK/TX, which is able to describe dynamic behavior of tokamak plasmas, has been developed. It solves simultaneously a set of flux-surface averaged equations composed of Maxwell’s equations, continuity equations, equations of motion, heat transport equations, fast-particle slowing-down equations and two-group neutral diffusion equations. The set of equations describes plasma rotations in both toroidal and poloidal directions through momentum transfer and evaluates the radial electric field self-consistently. The finite element method with a piecewise linear interpolation function is employed with a fine radial mesh near the plasma surface. The Streamline Upwind Petrov–Galerkin method is also used for robust calculation. We have confirmed that the neoclassical properties are well described by the poloidal neoclassical viscous force. The modification of density profile during neutral beam injection is presented. In the presence of ion orbit loss, the generation of the inward radial electric field and torque due to radial current is self-consistently calculated.     

Effect of subsonic toroidal flows on plasma poloidal rotation and ion heat conductivity. In collisional plasmas of elongated tokamaks

The ion poloidal rotation and heat conductivity in collisional plasmas of axially-symmetric tokamaks with elongated cross-sections and with subsonic toroidal plasma flows are considered. It is shown that subsonic toroidal plasma flows, induced by neutral beam injection or radio frequency waves, can strongly affect the poloidal plasma velocity and ion heat conductivity in collisional plasmas of tokamaks. The transport coefficients also depend on the tokamak ellipticity parameter which, in combination with the Mach number, allows to operate transport processes at smaller values of the toroidal Mach number. The importance of taking into account the ion-electron heat exchange and electron temperature toroidal perturbations to find ion temperature toroidal perturbations is demonstrated. This work was partially supported by the State University of Rio de Janeiro (UERJ) and the Rio de Janeiro Research Foundation (FAPERJ).     

HT-7超导托卡马克边界等离子体参量及其涨落的实验研究

利用径向可移动朗缪尔三探针和马赫探针对HT-7超导托卡马克边界等离子体参量及其涨落进行了空时空分辨测量.给出了欧姆放电及其与低杂波电流驱动共同作用下边界等离子体电位φ_p、电子温度T_e和电子密度n_e及其涨落的径向分布.实验表明,在限制器附近,存在一由E_r×B确定的极向旋转速度剪切层.在剪切层内,T_e和n_e分布较陡,且φ_p,T_e和n_e的相对涨落水平下降明显.这说明剪切层对边界区的等离子体涨落具有抑制作用.低杂波驱动使径向电场梯度变陡,从而使剪切程度加深但对剪切层宽度无影响.此外,测量表明等离子体环向速度马赫数M_φ存在径向梯度.环向流的这种径向梯度可能是形成径向电场所需的平均极向流的一种重要驱动源 关键词： 托卡马克 边界涨落 低杂波驱动     

HL-2A 装置的边缘参数测量

HL-2A 装置中平面边缘的等离子体特性通过磁力传动的马赫/ 雷诺协强/ 朗缪尔10 探针组进行了研究。10 探针组安装在可径向向里和向外移动, 并可绕轴旋转360^o 的传动杆上, 用于测量主等离子体边缘的温度、密度、悬浮电位、空间电位、径向和极向电场、湍流的雷诺协强、径向和极向等离子体流速及其径向分布。HL- 2A 装置的实验结果表明, 边缘等离子体扰动诱发的雷诺协强产生了边缘极向流; 雷诺协强的径向梯度驱动带状流抑制了湍流输运。     

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利用往返式朗缪尔探针组在HL-2A装置等离子体边缘首次同时观测到明显的低频(ƒ=0~5kHz)和测地声模(ƒ=16kHz)带状流的极向和环向对称性(m~0,n~0),径向结构及其传播特征。并同时推算出流的径向波矢(Kr-LF=0.6 cm-1,Kr-GAM=2cm-1)。双谱分析的结果表明低频和测地声模带状流的形成可能都是由于高频湍流和这种流之间的非线性三波耦合引起的。初步研究了电子回旋加热功率和边界安全因子引起带状流幅度的变化。此外,也观测了带状流幅度在径向方向的改变。     

Nonlinear Resonance Cone Surfaces

The nonlinear modification of the equation describing the surface (the resonance cone) on which the maximum electric field occurs in a magnetized plasma excited by an arbitrary, axially symmetric, localized RF source is derived. The nonlinearity is assumed to be due to the ponderomotive force. The nonlinear modification of the slope of the resonance cone is most pronounced near the plasma and upper hybrid frequencies, and is inversely proportional to the distance from the exciter. For frequencies of the lower branch the slope is decreased, whereas for upper branch frequencies the slope is increased.     

Investigation of Plasma Equilibrium in the Saskatchewan Torus‐Modified (STOR‐M) during Alternating Current Operation

An experiment to investigate the source of plasma equilibrium of the residual plasma at zero cross over of plasma current is presented. The role of limiter in providing this equilibrium by short circuiting the electric field developed due to the toroidal drifts is examined by measuring the current flowing through the limiter. The role of a fast poloidal rotation observed by a set of Mach probes, during zero cross over is also examined in providing the equilibrium. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

低杂波有质动力的非线性二维效应

本文在窄谱条件下讨论了大功率低杂波(|E|～1.5kV/cm)对等离子体边界密度分布的影响,发现沿磁场方向有明显的非线性密度不均匀,它足以影响N_‖～1的长波的传播,在垂直磁场方向,当等离子体内部(比如托卡马克隔板附近)存在反射时,沿低杂波通道将出现与驻波场相应的串接的密度空穴,等离子体整体平移的图象不再适用。 关键词：     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

Experimental studies on E×B drift rotational transform andconfinement in the bumpy torus NBT-1M

Experimental studies on E×B drift rotational transform for toroidal plasma confinement were carried out using a bumpy torus NBT-1M. An inward radial electric field was observed, which induced E×B poloidal precession and formed closed drift surfaces in a conventional ECH configuration. However, the confinement was degraded by the plasma convective loss due to the asymmetric potential profile and core electrostatic fluctuations. By the use of a combination of two frequencies for ECH (18 GHz and 8.5 GHz), we could reduce both the potential asymmetry and the fluctuation. In this operation, the plasma confinement by E×B drift rotational transform was demonstrated more clearly than that in the conventional single frequency operation