SAMI (Synthetic Aperture Microwave Imaging): Measuring tokamak plasma current using electron Bernstein wave emission

SAMI（合成孔径微波成像）：使用电子伯恩斯坦波发射测量托卡马克等离子体电流

• 批准号: EP/H016732/1
• 负责人: Roderick Vann
• 金额: $ 12.88万
• 依托单位: University of York
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FH016732%2F1
• 关键词: SAMI; Synthetic; Aperture; Microwave; Imaging

This research project will apply aperture synthesis, a diagnostic technique used routinely in radio astronomy, to make the first time-resolved measurements of the current density in the tokamak plasma edge. This measurement is crucial for understanding violent eruptions known as ELMs which could be extremely damaging for ITER, the next generation fusion device. At EUR10Bn, ITER is one of the largest international science projects on Earth.Fusion involves making two positively-charged nuclei collide to produce a heavier nucleus, releasing energy in the process. This can only occur at temperatures of about 100 million degrees. The fundamental challenge to performing fusion is to confine the hot ionised gas (plasma) sufficiently well. The principle behind the leading candidate design for a fusion power plant (called a tokamak) is to use the fact that the charged particles of the plasma state respond to electromagnetic fields, which can be used to confine them away from the material walls of the device. If sufficient heating power is injected into a tokamak plasma, then it enters a high-confinement mode. In this mode, the thermal energy of the plasma increases by about a factor of two due to the creation of a highly insulating layer near the plasma edge, which is typically only a few centimetres thick, compared to the body of the plasma which can be a metre or so across. The pressure gradient in this edge layer is extremely high, so there is a vulnerability to instabilities. The plasma experiences a repetitive series of violent plasma eruptions called Edge Localised Modes, or ELMs, which expel large amounts of energy typically within about a hundred millionths of a second. These are an interesting scientific phenomenon on today's tokamaks but on ITER, where the ejected power in an ELM is expected to be an order of magnitude larger, they could cause serious damage if not controlled. There are ideas for how to control ELMs that work on existing tokamaks, but to extrapolate them reliably to ITER requires a more detailed understanding of the physics. In order to test and constrain theoretical models for ELMs, we need to be able to measure the current density and pressure gradient in the thin edge layer. While a number of tokamaks have a good measurement of the pressure gradient, the current density is much more challenging, and the role of the current density in ELMs remains unconfirmed experimentally.This project will develop a novel diagnostic technique to measure the edge current density on the MAST tokamak routinely (in the sense that in principle the process could be automated). Our diagnostic technique will also have good time resolution, being able to make several measurements of the edge current density through an ELM and address the intriguing question of how (or whether) the current density is flushed out of the plasma edge region within the ELM time-scale (ie about 100 microseconds).The physical basis for this diagnostic technique is the directional emission of electron Bernstein wave (EBW) radiation, which is an example of electron cyclotron emission (ECE). Bernstein waves are electrostatic plasma waves generated in the plasma core at frequencies typically around tens of gigahertz. Most of these outgoing waves are reflected back into the core from a cut-off layer, but waves travelling at a particular angle with respect to the equilibrium magnetic field undergo a mode conversion to an electromagnetic wave that enables them to travel to the plasma edge and to be observed. The EBW emission profile allows us to measure both the direction of the magnetic field, and the rate at which it is changing. Since we know the absolute value of the toroidal magnetic field (it varies inversely proportionally with the distance from the centre of the device), we can use the rate of change of direction of the field to calculate the current density.

该研究项目将应用孔径合成（射电天文学中常用的一种诊断技术）对托卡马克等离子体边缘的电流密度进行首次时间分辨测量。这种测量对于理解被称为 ELM 的剧烈喷发至关重要，这种喷发可能对下一代聚变装置 ITER 造成极大的破坏。 ITER 耗资 100 亿欧元，是地球上最大的国际科学项目之一。聚变涉及使两个带正电的原子核碰撞，产生一个更重的原子核，并在此过程中释放能量。这只能发生在大约一亿度的温度下。进行聚变的根本挑战是充分限制热电离气体（等离子体）。聚变发电厂（称为托卡马克）的主要候选设计背后的原理是利用等离子体状态的带电粒子对电磁场做出响应的事实，该电磁场可用于将它们限制在远离设备材料壁的位置。如果将足够的加热功率注入托卡马克等离子体，那么它就会进入高约束模式。在这种模式下，由于在等离子体边缘附近形成了高度绝缘层，等离子体的热能增加了大约两倍，与宽度约为一米的等离子体主体相比，该绝缘层通常只有几厘米厚。该边缘层的压力梯度极高，因此容易不稳定。等离子体会经历一系列重复的剧烈等离子体喷发，称为边缘局域模式（ELM），通常会在大约百万分之一秒内释放出大量能量。对于今天的托卡马克来说，这是一个有趣的科学现象，但在 ITER 上，ELM 中的喷射功率预计会大一个数量级，如果不加以控制，它们可能会造成严重损害。对于如何控制在现有托卡马克上工作的 ELM 有一些想法，但要将它们可靠地推断到 ITER 需要对物理学有更详细的了解。为了测试和约束 ELM 的理论模型，我们需要能够测量薄边缘层中的电流密度和压力梯度。虽然许多托卡马克能够很好地测量压力梯度，但电流密度更具挑战性，并且电流密度在 ELM 中的作用仍未得到实验证实。该项目将开发一种新颖的诊断技术来常规测量 MAST 托卡马克的边缘电流密度（从某种意义上说，原则上该过程可以自动化）。我们的诊断技术还将具有良好的时间分辨率，能够通过 ELM 对边缘电流密度进行多次测量，并解决电流密度如何（或是否）在 ELM 时间尺度（即约 100 微秒）内从等离子体边缘区域冲出的有趣问题。这种诊断技术的物理基础是电子伯恩斯坦波 (EBW) 辐射的定向发射，这是 电子回旋发射（ECE）。伯恩斯坦波是在等离子体核心中产生的静电等离子体波，频率通常约为数十千兆赫。大多数这些出射波从截止层反射回核心，但相对于平衡磁场以特定角度传播的波会经历模式转换为电磁波，使它们能够传播到等离子体边缘并被观察到。电子束发射剖面使我们能够测量磁场的方向及其变化的速率。由于我们知道环形磁场的绝对值（它与距设备中心的距离成反比变化），因此我们可以使用磁场方向的变化率来计算电流密度。

项目成果

Electron kinetics inferred from observations of microwave bursts during edge localized modes in the mega-amp spherical tokamak.

• DOI: 10.1103/physrevlett.114.125004
• 发表时间: 2014-11
• 期刊: Physical review letters
• 影响因子: 8.6
• 作者: S. J. Freethy;K. McClements;S. C. Chapman;R. Dendy;W. Lai;S. J. P. Pamela-S. J. P.-Pamela-2251507688;V. Shevchenko;R. Vann

Particle acceleration during merging-compression plasma start-up in the Mega Amp Spherical Tokamak

兆安球形托卡马克合并压缩等离子体启动过程中的粒子加速

• DOI: 10.1088/1361-6587/aa98fa
• 发表时间: 2018
• 期刊: Plasma Physics and Controlled Fusion
• 影响因子: 2.2
• 作者: McClements K

GPU-Based Data Processing for 2-D Microwave Imaging on MAST

• DOI: 10.13182/fst15-188
• 发表时间: 2016-05
• 期刊: Fusion Science and Technology
• 影响因子: 0.9
• 作者: J. Chorley;R. J Akers;K. J Brunner;N. A Dipper;S. J. Freethy;R. Sharples;V. F Shevchenko;D. Thomas;R. Vann

Modifications to the synthetic aperture microwave imaging diagnostic.

• DOI: 10.1063/1.4961283
• 发表时间: 2016-11
• 期刊: The Review of scientific instruments
• 作者: K. Brunner;J. Chorley;N. Dipper;G. Naylor;R. Sharples;G. Taylor;D. Thomas;R. Vann

Localised Microwave Bursts During ELMs on MAST

MAST 上 ELM 期间的局部微波爆发

• DOI: 10.1051/epjconf/20158703008
• 发表时间: 2015
• 期刊: EPJ Web of Conferences
• 作者: Freethy S