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The high speed CCD visible-light camera system has been installed to detect the dynamic characteristics of ELMs filaments on EAST tokamak. In order to convert the CCD video imaging coordinates into real space coordinates, the CCD system has calibrated using Tsai’s two-stage calibration technique. A raised spike in the grayscale profile, drawn by selecting the grayscale value of the image in the vertical direction of the ELMs filament in the CCD image, represents an ELMs filament structure whose positional coordinates are determined by the apex of the spike. The Sobel edge detection operator was used to examine the edge profile of the filament to obtain the width of the filament. By analyzing the characteristics of the filament width of the ELMs, it is found that the width of ELMs filaments continuously decreases outward from the outermost magnetic surface. This indicates that the filaments continue to dissipate energy and particles as they propagate outward. The width of filaments of Type-I ELMs is greater than that of Type-III ELMs.     

EAST 装置边缘局域模细丝宽度的高速CCD可见光相机系统诊断

利用高速CCD 可见光相机系统研究了EAST 上的边缘局域模(ELMs)细丝动力学特性。采用Tsai 两步标定法对CCD 相机进行标定,以实现将CCD 图像坐标转换为真实空间坐标。选取CCD 图像中与边缘局域模细丝垂直方向上的图像灰度值,绘制了灰度分布图。灰度分布图中的一个凸起的尖峰代表了一条边缘局域模细丝结构,细丝的位置坐标由尖峰顶点确定。运用Sobel 边缘检测算子检测细丝边缘轮廓,从而得到细丝的宽度。通过对边缘局域模细丝宽度特性的分析,发现边缘局域模细丝在从最外层封闭磁面向外传播的过程中宽度不断减小。这表明细丝在向外传播的过程中其能量和粒子不断耗散。I-型边缘局域模的细丝宽度比III-型边缘局域模细丝的宽度更大。     

Spatial and temporal structure of edge-localized modes

This Letter provides information on the spatial and temporal structure of periodic eruptions observed in magnetically confined laboratory fusion plasmas, called edge-localized modes (ELMs), and highlights similarities with solar eruptions. Taken together, the observations presented in this Letter provide strong evidence for ELMs being associated with a filamentlike structure. These filaments are extended along a field line, are generated on a 100 micros time scale, erupt from the outboard side, and connect back into the plasma. Such structures are predicted by a theoretical model based on the "ballooning" instability, developed for both solar and tokamak applications.     

Scrape-off-layer current model for filament structure observed during edge-localized modes in the DIII-D tokamak

The plasma in tokamaks often exhibits a relaxation oscillation called the edge-localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX.     

Filamentary plasma eruptions: Results using the non‐linear ballooning model

This paper provides an overview of recent results on two distinct studies exploiting the non‐linear model for ideal ballooning modes with potential applications to edge‐localized modes (ELMs). The non‐linear model for tokamak geometries was developed by Wilson and Cowley in 2004 and consists of two differential equations that characterize the temporal and spatial evolution of the plasma displacement. The variation of the radial displacement along the magnetic field line is described by the first equation, which is identical to the linear ballooning equation. The second differential equation is a two‐dimensional non‐linear ballooning‐like equation, which is often second order in time but can involve a fractional time derivative depending on the geometry. In the first study, the interaction of multiple filamentary eruptions is addressed in a magnetized plasma in a slab geometry. Equally sized filaments evolve independently in both the linear and non‐linear regimes. However, if filaments are initiated with slightly different heights from the reference flux surface, they interact with each other in the non‐linear regime: lower filaments are slowed down and are eventually completely suppressed, while the higher filaments grow faster because of the non‐linear interaction. In the second study, this model of non‐linear ballooning modes is examined quantitatively against experimental observations of ELMs in Mega Amp Spherical Tokamak (MAST) geometries. The results suggest that experimentally relevant results can only be obtained using modified equilibria.     

Two-dimensional visualization of growth and burst of the edge-localized filaments in KSTAR H-mode plasmas

The filamentary nature and dynamics of edge-localized modes (ELMs) in the KSTAR high-confinement mode plasmas have been visualized in 2D via electron cyclotron emission imaging. The ELM filaments rotating with a net poloidal velocity are observed to evolve in three distinctive stages: initial linear growth, interim quasisteady state, and final crash. The crash is initiated by a narrow fingerlike perturbation growing radially from a poloidally elongated filament. The filament bursts through this finger, leading to fast and collective heat convection from the edge region into the scrape-off layer, i.e., ELM crash.     

Model for current-driven edge-localized modes

Edge-localized modes (ELMs) are cyclic disturbances in the outer region of tokamak plasmas that are influential in determining present and future tokamak performance. In this Letter, we outline an approach to modeling ELMs in which we envisage toroidal peeling modes initiating a Taylor relaxation [Phys. Rev. Lett. 33, 1139 (1974)10.1103/PhysRevLett.33.1139] of a tokamak outer region plasma. Relaxation produces a peeling destabilizing flattened edge current profile and a stabilizing plasma-vacuum current sheet; the balance between the two determines the radial extent of the relaxed region. The model can be used to predict the energy losses due to an ELM and reproduces experimentally observed variations with edge safety factor and plasma collisionality. There is an intrinsic "deterministic scatter" in the model that also accords with observation.     

Sandpile model with tokamaklike enhanced confinement phenomenology

Confinement phenomenology characteristic of magnetically confined plasmas emerges naturally from a simple sandpile algorithm when the parameter controlling redistribution scale length is varied. Close analogs are found for enhanced confinement, edge pedestals, and edge localized modes (ELMs), and for the qualitative correlations between them. These results suggest that tokamak observations of avalanching transport are deeply linked to the existence of enhanced confinement and ELMs.     

Simulations of filamentary structure in an edge-localized mode burst on EAST using BOUT++ code

In this paper, simulation on the filamentary structure for the low-hybrid wave heating H-mode on experimental advanced superconducting tokamak (EAST) is carried out for the first time using BOUT++, and the speed and width of edge localized mode (ELM) filaments have been evaluated during the simulation. The evolutions of the ELM filaments are illustrated temporally and spatially. Then the results are compared with the experimental observations. It is found that at the peak gradient region in the outer midplane, the radial speed of the filaments is changed frequently and varied from 0.016 to 0.38 km/s. For the contrast, the poloidal speed oscillates in a narrower range of 0.68–0.88 km/s. The calculation results show the width of ELM filaments oscillates during the simulation, the minimum width is around 8.3 mm, and the maximum value is 41.24 mm. The distributions of the ELM filament widths with the radial position indicate the width is decreased with the radial position. Furthermore, by tracing one single filament, the radial width is decreased gradually when the filament is moving outward, which is also consistent with experiment conclusion. The consistencies also indicate the availability and practicability of the six-field two-fluid model of BOUT++ on the study of ELMs.     

Quiescent double barrier regime in the DIII-D tokamak

A new sustained high-performance regime, combining discrete edge and core transport barriers, has been discovered in the DIII-D tokamak. Edge localized modes (ELMs) are replaced by a steady oscillation that increases edge particle transport, thereby allowing particle control with no ELM-induced pulsed divertor heat load. The core barrier resembles those usually seen with a low (L) mode edge, without the degradation often associated with ELMs. The barriers are separated by a narrow region of high transport associated with a zero crossing in the E x B shearing rate.     

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.     

Features of the repetition frequency of edge localized modes in EAST

This paper presents the features of the edge localized modes (ELMs) observed in the 2010 experimental campaign on the Experimental Advanced Superconducting Tokamak (EAST). The first high-confinement mode (H-mode) at an H-factor of H_{IPB98(y, 2)}～ 1 has been obtained with about 1 MW lower hybrid wave (LHW) heating power on the EAST tokamak. The ELMs in EAST are Type III ELMs with high frequency (several hundred Hertz) and low amplitude. ELM features for the following two types of EAST discharges are investigated: discharges with only LHW and those with both LHW and ion cyclotron resonance frequency (ICRF) wave. The ELMs are observed in a wide q ₉₅ window ranging from 2.6 to 5.0, and the ELM frequencies decrease with the safety factor, q ₉₅. In contrast to most observations, the ELM frequency in EAST appears to be weakly proportional to the power through the separatrix. When the ICRF wave is injected, the ELM frequency increases. The control of the ELM frequency and amplitude has been achieved by puffing impure argon gas into the EAST discharge.     

HL-2A 装置超声分子束注入缓解偏滤器靶板上边缘局域模热通量研究

在HL-2A 装置上优化和发展了偏滤器靶板上的红外测温系统,并利用该系统分析了高约束模放电期间边缘局域模的热沉积分布特性。在高约束模式放电期间,超声分子束注入使边缘局域模所引起的偏滤器靶板上瞬间热通量峰值下降了~60%,并伴随着边缘局域模爆发频率增加了2~3 倍,而等离子体储能仅下降了~8%。分析结果表明,大幅度的丝状结构在超声分子束注入之后得到了有效抑制,沉积到偏滤器靶板上的瞬间热通量峰值也随之下降。此外,在超声分子束注入之后偏滤器室内的热辐射损失大幅度增加,从而耗散了热输运所携带的部分能量,进一步分散了沉积到偏滤器靶板上的能量,有效地保护了偏滤器靶板。     

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在HL-2A 装置上优化和发展了偏滤器靶板上的红外测温系统,并利用该系统分析了高约束模放电期间边缘局域模的热沉积分布特性。在高约束模式放电期间,超声分子束注入使边缘局域模所引起的偏滤器靶板上瞬间热通量峰值下降了~60%,并伴随着边缘局域模爆发频率增加了2~3 倍,而等离子体储能仅下降了~8%。分析结果表明,大幅度的丝状结构在超声分子束注入之后得到了有效抑制,沉积到偏滤器靶板上的瞬间热通量峰值也随之下降。此外,在超声分子束注入之后偏滤器室内的热辐射损失大幅度增加,从而耗散了热输运所携带的部分能量,进一步分散了沉积到偏滤器靶板上的能量,有效地保护了偏滤器靶板。     

Nuclear fusion: bringing a star down to Earth

Nuclear fusion offers the potential for being a near limitless energy source by fusing together deuterium and tritium nuclei to form helium inside a plasma burning at 100 million K. However, scientific and engineering challenges remain. This paper describes how such a plasma can be confined on Earth, and discusses the similarities and differences with fusion in stars. It focuses on the magnetic confinement technique and, in particular, the method used in a tokamak. The confinement achieved in the equilibrium state is reviewed and it is shown how the confinement can be too good, leading to explosive instabilities at the plasma edge called edge localised modes (ELMs). It is shown how the impact of ELMs can be minimised by the application of magnetic perturbations and discusses the physics behind the penetration of these perturbations into what is ideally a perfect conducting plasma.     

ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A new model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. The model for the pedestal and ELMs is used in the ASTRA integrated transport code to follow the time evolution of tokamak discharges from L-mode through the transition from L-mode to H-mode, with the formation of the H-mode pedestal, and, subsequently, to the triggering of ELMs. Turbulence driven by the ion temperature gradient mode, resistive ballooning mode, trapped electron mode, and electron temperature gradient mode contributes to the anomalous thermal transport at the plasma edge in this model. Formation of the pedestal and the L-H transition is the direct result of flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by MHD instabilities. Two mechanisms for triggering ELMs are considered: ELMs are triggered by ballooning modes if the pressure gradient exceeds the ballooning threshold or by peeling modes if the edge current density exceeds the peeling mode threshold. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Direct observation of current in type-I edge-localized-mode filaments on the ASDEX Upgrade tokamak

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number (n?10-15) structures (filaments) localized in the perpendicular plane and extended along the field lines. By detailed localized measurements of the magnetic field perturbation associated to type-I ELM filaments, it is shown that these filaments carry a substantial current.     

Edge-localized modes explained as the amplification of scrape-off-layer current coupling

It is shown that the edge-localized modes (ELMs) observed in tokamak H mode discharges can be explained as external magnetohydrodynamic (MHD) mode amplification due to coupling with scrape-off-layer current. The proposed model offers a new ELM mechanism that produces a sharp onset and initial fast growth of magnetic perturbations even when the underlying equilibrium is only marginally unstable for a MHD mode and also a quick quenching after the bursting peak. The theory also reproduces various other ELM features.     

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。