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

该研究项目将应用射电天文学中常用的诊断技术——孔径合成，首次对托卡马克等离子体边缘的电流密度进行时间分辨测量。这种测量对于理解被称为elm的剧烈喷发至关重要，elm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。核聚变包括使两个带正电的原子核相互碰撞，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1亿度的温度下。进行核聚变的基本挑战是充分限制热电离气体（等离子体）。核聚变发电厂（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的响应这一事实，这可以用来将它们限制在远离设备材料壁的地方。如果给托卡马克等离子体注入足够的加热功率，它就会进入高约束模式。在这种模式下，等离子体的热能增加了大约两倍，这是由于在等离子体边缘附近形成了一层高度绝缘层，通常只有几厘米厚，而等离子体的厚度可以达到一米左右。在这个边缘层的压力梯度是非常高的，所以有一个脆弱的不稳定。等离子体经历一系列重复的剧烈等离子体爆发，称为边缘局部模式，或elm，通常在大约一亿分之一秒内释放大量能量。在今天的托卡马克上，这是一个有趣的科学现象，但在ITER上，如果不加以控制，它们可能会造成严重的破坏，在ITER上，ELM的喷射功率预计会大一个数量级。关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。虽然许多托卡马克可以很好地测量压力梯度，但电流密度更具挑战性，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的诊断技术，用于常规测量MAST托卡马克上的边缘电流密度（从原则上讲，该过程可以自动化）。我们的诊断技术也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，并解决在ELM时间尺度（即大约100微秒）内电流密度如何（或是否）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）的一个例子。伯恩斯坦波是在等离子体核心产生的静电等离子体波，频率通常在几十千兆赫兹左右。这些出射波中的大多数从一个截止层反射回核心，但是以相对于平衡磁场的特定角度传播的波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。EBW发射剖面允许我们测量磁场的方向，以及磁场变化的速率。既然我们知道环向磁场的绝对值（它与离装置中心的距离成反比），我们就可以用磁场方向的变化率来计算电流密度。

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.