SAMI-2: two-dimensional Doppler imaging of tokamak plasmas

核聚变是为太阳和所有恒星提供能量的过程。如果能在地球上成功利用，它将提供安全、充足和碳中性的电力供应。被称为托卡马克的地面聚变实验利用磁场限制等离子体（电离气体）燃料。众所周知，等离子体最外缘的电流密度对托卡马克的性能和稳定性起着至关重要的作用，但这个量还不能常规测量。当人们考虑到当前的实验被用来推断未来反应堆的设计和性能时，这种知识差距就显得尤为重要。这里提出的研究目标是从零开始建立一个新的微波诊断系统，被称为SAMI-2，它可以对托卡马克等离子体边缘的电流密度进行常规测量。这是具有挑战性的，因为携带电流的层很薄，等离子体很热（通常为1000万度）。SAMI-2的工作原理是用广角微波光束照射等离子体表面。等离子体表面呈波纹状，与磁场平行，因为等离子体沿着磁场线的速度比穿过磁场线的速度快得多。照明光束，其波长与波纹之间的距离相当（大约）。根据一个众所周知的条件，即布拉格定律，1cm)，被散射回优先垂直于磁场的SAMI-2。由于等离子体是旋转的，这种反向散射信号被多普勒移位，也就是说，根据等离子体是朝着诊断设备旋转还是远离诊断设备旋转，其频率会比照明光束的频率高或低。以几兆赫兹为间隔的频率进行扫描，对应于选取间隔几毫米的后向散射表面。如果我们能分辨出后向散射信号中峰的来源，那么我们就可以推断出磁场的方向；如果我们在两个位置（对应于扫描光束的两个频率）这样做，那么我们就可以根据安培定律计算出边缘电流密度。SAMI-2利用32个接收天线阵列对反向散射的微波进行成像。信号到达每个天线所需的时间取决于天线和信号源之间的距离；通过测量每对天线（“基线”）信号之间的时间差（技术上是相位差），我们可以重建发射模式。(这与环绕立体声电影的原理是一样的，环绕感兴趣的主题使用多个麦克风进行录制。到不同麦克风的时间延迟取决于源到每个麦克风的距离；当这些相位延迟的信号通过扬声器回放时，在特定位置的源的感觉就会重新产生。我们首次用EPSRC支持的合成孔径微波成像仪（SAMI）证明了这种成像方法的可行性。然而，SAMI最初并不是为多普勒后向散射而设计的，无法以足够的精度测量磁场方向来推导电流密度。相比之下，SAMI-2是专门为二维多普勒后向散射设计的，几乎没有与原始SAMI相同的组件。SAMI-2将使用巧妙的弯曲天线类型，可以测量水平和垂直极化的微波；它有32个天线（是SAMI的四倍，将基线数量从28个增加到496个）；它将同时在两个频率上成像。SAMI-2的天线阵列和数据传输方法本身在技术上就很有趣。SAMI-2将部署在英国国家聚变实验室Culham聚变能源中心的英国MAST-U托卡马克上，及时参加2019年MAST-U的首次实验活动。

Fusion is the process that powers our Sun and indeed all stars. If it could be successfully harnessed on Earth, it would provide a safe, plentiful and carbon-neutral supply of electricity. Terrestrial fusion experiments known as tokamaks confine the plasma (ionised gas) fuel using magnetic fields. It is known that the electric current density in the very outer edge of the plasma critically determines the tokamak's performance & stability, yet this quantity cannot yet be routinely measured. This knowledge gap is particularly important when one considers that current experiments are being used to extrapolate to the design and performance of future reactors.The objective of the research proposed here is to build from scratch a novel microwave diagnostic, to be known as SAMI-2, that can make routine measurements of the electric current density in the edge of a tokamak plasma. This is challenging because the layer in which the current is carried is thin and the plasma is hot (typically 10 million degrees).SAMI-2 works by illuminating the plasma surface with a wide-angled microwave beam. The plasma surface is corrugated parallel to the magnetic field because plasma travels much faster along magnetic field lines than across them. The illuminating beam, whose wavelength is comparable to the distance between the corrugations (approx. 1cm), is scattered back towards SAMI-2 preferentially perpendicular to the magnetic field according according to a well-understood condition known as Bragg's Law. Because the plasma is rotating, this back-scattered signal is Doppler shifted i.e. the frequency is shifted above or below the frequency of the illuminating beam depending whether the plasma is rotating towards or away from the diagnostic, respectively. Scanning at frequencies spaced by a few gigahertz corresponds to picking back-scattering surfaces that are a few millimetres apart. If we can resolve the origin of the peaks in the back-scattered signal, then we can deduce the orientation of the magnetic field; if we do this at two locations (corresponding to two frequencies of scanning beam), then we can calculate the edge current density from Ampère's Law.SAMI-2 images the back-scattered microwaves using an array of 32 receiving antennas. The time taken for the signal to reach each antenna depends on the distance between the antenna and the source; by measuring the time difference (technically, phase difference) between the signals at each pair of antennas ("baseline"), we can reconstruct the emission pattern. (This is the same principle by which surround sound films are recorded using multiple microphones positioned around the subject of interest. The time delay to different microphones depends on the distance of the source to each microphone; the sensation of a source at a particular location is recreated when these phase-delayed signals are played back through loudspeakers.)We demonstrated the feasibility of this imaging methodology for the first time with the Synthetic Aperture Microwave Imager (SAMI), supported 2009-11 by EPSRC. However SAMI was not originally designed for Doppler back-scattering and could not measure the magnetic field direction with sufficient accuracy to derive the current density. In contrast, SAMI-2 is specifically designed for 2-D Doppler backscattering and shares hardly a component in common with the original SAMI. SAMI-2 will use the ingenious sinuous antenna type which can measure both horizontally and vertically polarised microwaves; it has 32 antennas (four times as many as SAMI, increasing number of baselines from 28 to 496); it will image at two frequencies simultaneously. SAMI-2's antenna array and data transmission method are technologically interesting in their own right.SAMI-2 will be deployed at the UK's MAST-U tokamak at the Culham Centre for Fusion Energy, the UK's national fusion laboratory, in time for MAST-U's first experimental campaign in 2019.