Measuring the microscopic properties of warm dense matter and driven solids

在这个项目中，学生将发展非弹性x射线散射（IXRS）技术来诊断热致密物质和激光冲击和斜坡压缩固体。IXRS是一种技术，通过该技术，入射的硬x射线从致密系统散射，产生或破坏声学模式的量子。在致密等离子体中，这种散射来自所谓的离子声模式，而在固体中，这种散射来自系统内的声子。由于声波模式的频率很小（声子的频率为几十兆电子伏，离子声波的频率为100兆电子伏），x射线（10千电子伏左右）经历的能量增益或损失很小。这就需要使用高度单色的x射线（带宽为百万分之一），如果要研究瞬态物质，这反过来又需要一个非常明亮的x射线源——也就是说，我们必须使用第四代光源——x射线激光器。该项目由几个部分组成。(1)利用致密等离子体中离子声波的x射线散射来提供压力和状态方程的信息。在这些实验中，通过测量入射到致密激光等离子体上的x射线的频率和波矢量增益/损失，记录了离子声波的频率/k矢量关系。这提供了系统中的声速，而声速又与压力有关。此外，光谱的形状本身包含了丰富的附加信息。等离子体共振的频宽与粘度有关，而波散射光强度与随机移动离子散射光强度之比是热导率的测量指标。(2)使用相同的技术在斜坡压缩和冲击材料中直接测量温度——这是冲击物理学中一个长期存在的问题。在这种情况下，向上和向下移动的x射线（斯托克斯和反斯托克斯）的强度根据详细平衡定律与玻尔兹曼因子有关。坡道压缩材料的典型温度为德拜温度，因此两个散射峰的强度有很好的可测量差异。(3)如上所述，该技术为测量传输系数（即粘度、导热系数和声速）提供了一种独特的工具。众所周知，这些物质很难在压缩物质中获得，而且它们会影响压缩物质的演化。行星的演化模型在很大程度上取决于输运系数的选择，因此我们的实验可以提供有用的实验约束。此外，这些系数在热致密物质条件下的值被认为达到了弦理论方法设定的边界，即AdS/CFT对应。这种对应假设在保角变换下，强耦合流体总是可以作为弱相互作用流体重新映射到黑洞的视界上。这些技术在我们的领域是新的，但这里提出的工作可以提供不同的途径来探索。上述实验将在位于汉堡的欧洲XFEL的HED端站进行。我们设想项目的第一年还将涉及详细的实验设计，评估光谱分辨率和收集效率之间的最佳权衡（LCLS的原理证明实验使用了70兆电子伏特的分辨率，其中接近15 -20兆电子伏特的数字可能更适合直接温度测量）。在项目的第一部分，学生还将评估哪些材料将是早期实验的最佳候选材料。我们将申请时间在仅寄生x射线模式下使用XFEL的HED端站（以测试光学器件），然后从2019年中期开始，我们将申请时间将其与偶极子激光器一起使用。

Under this project a student will develop the technique of inelastic x-ray scattering (IXRS) to diagnose both warm dense matter and laser-shocked and ramp-compressed solids. IXRS is a technique whereby an incoming hard x-ray scatters from a dense system, creating or destroying a quantum of an acoustic mode. In the case of a dense plasma, this is scattering from the so-called ion-acoustic modes, whereas in the case of a solid, one scatters from the phonons within the system. As the frequency of the acoustic modes is small (a few tens of meV for phonons, and unto a few 100 meV for ion acoustic waves) the energy gain or loss experienced by the x-ray (which is of order 10 keV or so) is small. This necessitates the use of highly monochromatic x-rays (with a bandwidth of order 1 part in a million), which in turn requires an extremely bright x-ray source if transient matter is to be interrogated-that is to say we must use a 4th generation light source- an x-ray laser.The project comprises several components. (1) Using x-ray scattering from ion acoustic waves in dense plasmas to provide information on the pressure and equation of state. In these experiments the frequency/k-vector relationship of the ion acoustic waves are recorded by measuring the frequency and wave-vector gain/loss of the x-ray incident on a dense laser-produced plasma. This provides the speed of sound in the system, which in turn is related to the pressure. Moreover, the shape of the spectrum itself contains a wealth of additional information. The frequency broadening of the plasmon resonances is related to the viscosity, and the ratio between the intensity of the light scattered by waves to that scattered by randomly moving ions is a measurement of the thermal conductivity. (2) Using the same technique in ramp compressed and shocked materials to directly measure the temperature-this being a long-standing problem in shock physics. In this case the up and down-shifted x-rays (Stokes and anti-Stokes) are related in their intensity by the Boltzmann factor according to the law of detailed balance. For ramp compressed materials typical temperatures are of order the Debye temperature, and thus there is a good measurable difference in the intensity of the two scattered peaks.(3) The technique, as discussed above, provides a unique tool for the measurement of transport coefficients - namely viscosity, thermal conductivity and sound speed. These are notoriously difficult to obtain in compressed matter and they affect how the compressed matter evolves. Evolutionary models for planets strongly depend on the choice of the transport coefficients and so our experiments can give useful experimental constraints.In addition, the values of such coefficients under warm dense matter conditions are believed to reach the bounds set by string theory methods-that is, the AdS/CFT correspondence. Such correspondence postulate that under a conformal transformation, a strongly coupled fluid can always be remapped onto as a weakly interacting fluid onto the event horizon of a black hole. These techniques are new in our field, but the work proposed here could offer different avenues to be explored.The experiments described above will be performed on the HED end-station at the European XFEL in Hamburg. We envisage the first year of the project will also involve detailed experimental design, assessing the best trade off between spectral resolution and collection efficiency (the proof of principle experiments at LCLS have used a resolution of order 70 meV, where a figure closer to 15 -20 meV may be more suitable for direct temperature measurements).Within the first part of the project the student will also asses which materials will be the best candidates for early experiments. We will apply for time to use the HED end-station at XFEL in both parasitic x-ray only mode (to test the optics), and then from mid 2019 onwards we will request time to use it in conjunction with the DIPOLE laser.