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

核聚变是为我们的太阳乃至所有恒星提供能量的过程。如果它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球聚变实验使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决定了托卡马克的性能和稳定性，但这个量还不能常规测量。这种知识差距是特别重要的，当一个人认为，目前的实验正在被用来推断未来reactor.The目标的设计和性能的研究建议是从头开始建立一个新的微波诊断，被称为SAMI-2，可以在托卡马克等离子体的边缘电流密度的常规测量。这是具有挑战性的，因为其中携带电流的层是薄的，并且等离子体是热的（通常为1000万度）。SAMI-2通过用广角微波束照射等离子体表面来工作。等离子体表面是平行于磁场的波纹状，因为等离子体沿沿着磁场线的速度比穿过磁场线的速度快得多。照明光束的波长与波纹之间的距离相当（约为100米）。1厘米），被散射回SAMI-2优先垂直于磁场根据众所周知的条件称为布拉格定律。因为等离子体正在旋转，所以该反向散射信号是多普勒频移的，即，取决于等离子体是朝向诊断器旋转还是远离诊断器旋转，频率分别被偏移到照射光束的频率之上或之下。以几千兆赫的频率间隔扫描对应于拾取相隔几毫米的后向散射表面。如果我们能够分辨出后向散射信号中峰值的来源，那么我们就可以推断出磁场的方向;如果我们在两个位置（对应于扫描波束的两个频率）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使用32个接收天线阵列对后向散射微波进行成像。信号到达每个天线所需的时间取决于天线和源之间的距离;通过测量每对天线（“基线”）上信号之间的时间差（技术上，相位差），我们可以重建发射模式。(This使用位于感兴趣对象周围的多个麦克风来记录环绕声电影的原理是相同的。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，在特定位置处的源的感觉被重新创建。我们证明了这种成像方法的可行性首次与合成孔径微波成像仪（SAMI），支持2009-11年由EPSRC。然而，SAMI最初并不是为多普勒反向散射而设计的，并且不能以足够的精度测量磁场方向以导出电流密度。相比之下，SAMI-2是专门设计的2-D多普勒后向散射和股票几乎没有一个组件与原来的SAMI。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，基线数量从28增加到496）;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将被部署在英国国家聚变实验室卡勒姆聚变能源中心的MAST-U托卡马克上，以赶上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.