Measuring the microscopic properties of warm dense matter and driven solids

在这个项目中，一名学生将开发非弹性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，通过这种技术，入射的硬X射线从密集的系统散射，产生或摧毁声学模式的量子。在稠密等离子体的情况下，这是所谓的离子-声学模式的散射，而在固体的情况下，这是从系统内的声子散射。由于声学模式的频率很小(声子的频率为几十meV，离子声波的频率为100 meV)，X射线(大约10keV左右)所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(带宽为百万分之一的数量级)，这反过来又需要一个极其明亮的X射线源，如果要询问瞬变物质--也就是说，我们必须使用第四代光源--X射线激光。该项目由几个部件组成。(1)利用密集等离子体中离子声波的X射线散射来提供有关压强和状态方程的信息。在这些实验中，通过测量入射到密集激光产生的等离子体上的X射线的频率和波矢量增益/损耗，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。等离子体激元共振的频率展宽与粘性有关，波散射光强度与随机运动离子散射光强度之比是导热系数的量度。(2)在斜坡压缩和冲击材料中使用相同的技术来直接测量温度--这是冲击物理学中的一个长期问题。在这种情况下，根据详细平衡定律，上下移动的X射线(斯托克斯和反斯托克斯)的强度与玻尔兹曼因子有关。对于斜坡压缩材料，典型温度约为德拜温度，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--提供了一个独特的工具。众所周知，这些物质很难在压缩物质中获得，它们会影响压缩物质的演化方式。行星演化模型强烈依赖于输运系数的选择，因此我们的实验可以给出有用的实验约束。此外，在热致密物质条件下，这些系数的值被认为达到了弦理论方法设定的界限--即ADS/CFT对应。这种对应假设是，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探索途径。上述实验将在汉堡欧洲XFEL的HED终端站上进行。我们预计项目的第一年还将包括详细的实验设计，评估光谱分辨率和收集效率之间的最佳权衡(LCLS的原理实验证明使用了70 MeV的分辨率，其中接近15-20 MeV的数值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者。我们将申请时间在两种寄生X射线模式下使用XFEL的HED终端站(以测试光学器件)，然后从2019年年中开始，我们将申请时间将其与偶极激光器一起使用。

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.