Physics of Ignition: Collaboration with the National Ignition Facility: Diagnosing Hot-Spot Mix via X-Ray Spectroscopy

轻核的聚变是为太阳提供能量的能源。如果在地球上利用，它可以提供无限的低碳能源。重氢的基本燃料--重氢的重氢(D&T)形式--既可以在海水中随时获得，也可以从丰富的元素锂(手机电池中的元素)中“培育”出来。主要的核废料是无害的--主要是氦(一种阿尔法粒子)，这是一种在派对气球中发现的惰性气体。这一切听起来好得令人难以置信--从某种意义上说是真的--因为让反应发生是极其困难的--因为推动D和T靠近，使强大的力使它们结合需要大量能量(它们相互排斥，因为它们是带正电的原子核)。让它们移动得足够快，这样当它们碰巧发生迎头相撞并接近到足够近的时候，就相当于将它们加热到1亿K。将如此热的等离子体限制足够长的时间，以使碰撞发生，并不是一件容易的事情。有两种方法：第一种方法是使用磁瓶使低密度气体远离容器的壁。由于密度较低，碰撞需要几秒钟的时间--这就是磁融合方法。第二个想法是用激光照射一个含有重氢的小球形气球。激光从不同的方向加热气球的外部，产生热等离子体，然后膨胀到真空中，然后，就像球形火箭一样，外壳向中心移动，将重氢压缩到比普通液体高出百分之一的高温和密度。不需要磁场，因为由于密度高，碰撞非常迅速，尽管压缩的微型太阳会再次膨胀(如果发生核聚变，爆炸会更快)，但反应发生得比爆炸本身更快--材料受到其自身惯性的限制。这被称为惯性约束聚变。在加利福尼亚州国家点火设施目前的研究中，这一目标即将实现。但是，目前还存在一些问题需要克服。其中一个主要的原因是壳的压缩不均匀，众所周知，如果内爆不是接近完美的球形，那么任何涟漪都会生长，在内爆的峰值之前打破壳的壁。然后气球的外壳混合到燃料中，由于高温而开始发光，并冷却系统，阻止熔化。因此，需要解决两个相互关联的问题--首先，我们需要找出有多少壳层混合到重氢核中--其次，我们需要研究如何防止这种情况发生(要么制造更好的靶子，要么更均匀地照亮球体)。这项研究拨款解决了第一个衡量问题。由于各种物理原因，气球外壳含有一些重元素(特别是锗)，如果它们混合到热核中，就会被点燃，并发出特有的X射线线。通过对这些线的绝对亮度和相对亮度的研究，有可能获得关于材料温度、密度以及混合到地核中的壳层的数量的信息。其中一些工作已经由我们的美国同事完成。然而，目前这项技术还不够准确，无法判断混入的量是否真的足以消除反应。英国牛津大学和约克大学的研究小组在这里提出了几个新的想法，以改进理论和实验技术，我们相信我们将能够说出混合水平是否可以接受。这些想法基于一种新的高分辨率X射线仪器，新的光谱理论观察来自不同元素的X射线的亮度，并通过对发射过程进行复杂的全三维模拟。

The fusion of light nuclei is the energy source that powers the sun. If harnessed on earth, it could provide limitless low-carbon energy. The basic fuel - the Deuterium and Tritium (D&T) forms of heavy hydrogen, are either readily available in sea-water, or can be 'bred' from the abundant element Lithium (the element in a mobile phone battery). The primary nuclear waste products are harmless - the main being helium (an alpha particle), an inert gas found in party balloons. This all sounds too good to be true - and in a sense it is - because getting the reaction to occur is incredibly difficult - because pushing the D and T close together such that the strong force causes them to bind takes a lot of energy (they repel as they are positively charged nuclei). Getting them to move fast enough so that when by chance they have a head-on collision and get close enough to fuse corresponds to heating them to 100 million K. Confining such a hot plasma for long enough for the collisions to occur is no mean feat. There are two approaches: the first uses a magnetic bottle to keep a low density gas away from the walls of a container. As the density is low, collisions take several seconds - this is the magnetic fusion approach. The second idea uses lasers irradiating a small spherical balloon containing the heavy hydrogen. The laser heats the outside of the balloon from different directions, creating a hot plasma that expands into the vacuum, and then, like a spherical rocket, the shell moves towards the centre, compressing the heavy hydrogen to high temperatures and densities 100s of times denser than ordinary liquid. No magnetic fields are needed, because owing to the high density, the collisions are very rapid, and although the compressed miniature sun will expand again (and blow up more quickly if fusion takes place), the reaction occurs faster than the explosion itself - the material is confined by its own inertia. This is called inertial confinement fusion. In current studies at the National Ignition Facility in California, this goal is close to being realised. However, at present there are still problems to be overcome. One of the major ones is that the shell does not compress uniformly, and it is known that if the implosion is not close to being perfectly spherical, then any ripples will grow, breaking up the wall of the shell before the peak of the implosion. The shell of the balloon then mixes into the fuel, and starts to 'glow' due to the high temperatures, and cools the system, preventing fusion. Therefore, two interlinked problems need to be tackled - firstly, we need to find out how much of the shell is mixing into the heavy hydrogen core - and secondly we need to work out how to prevent this happening (either by making better targets, or illuminating the sphere more uniformly). This research grant addresses the first measurement problem. For various physics reasons the shell of the balloon contains some heavy elements (particularly Germanium) which, if they mix into the hot core, 'light-up' and emit characteristic X-ray lines. From a study of the absolute and relative brightness of these lines, it is possible to gain information on the temperature of the material, and of the density, and also, of the amount of the shell that has mixed into the core. Some of this work has already been performed by our US colleagues. However, at present the technique is not quite accurate enough to say if the amount that has mixed in is really enough to extinguish the reaction. The Oxford and York groups in the UK here put forward several new ideas to improve the theory and experimental technique to a point where we believe we will be able to say if the mix level is acceptable. These ideas are based on a new high resolution x-ray instrument, novel spectroscopic theory looking at the brightness of X-rays from different elements, and by performing sophisticated full 3 dimensional simulations of the emission process.