Temperature in laser compressed high pressure solids: measurement and control

高压物质(我们这里指的是压力超过1兆巴的固体物质)显示出一系列有趣的特征。处于这种状态的固体由于在这些极端密度的机械能下的不寻常的化学反应而显示出显著的结构和电子的复杂性。这可能导致生产具有新的和潜在有价值的性能的材料，例如极端的机械性能或不寻常的电子结构。这种高压物质也是大多数行星内部的主要成分，因此在宇宙中广泛存在。此外，这些不寻常的高压结构中的一些预计在释放回环境条件时是稳定的，这可能允许它们被回收用于实验室的进一步研究和应用。因此，在不断增加的压力下，一系列科学学科对固体材料的产生和诊断越来越感兴趣。当然，在实验室创造这种条件的挑战是相当大的。一种成功的高压途径是通过激光照射样品进行瞬时压缩，其中固体中的压力已达到10Mbar以上。然而，在诊断这些实验中产生的材料方面仍然存在挑战。脉冲X射线衍射仪的发展使密度和结构的原位测定成为可能，从而极大地提高了我们的诊断能力。这项建议旨在扩大这些现有的、非常成功的衍射诊断的用途，以允许确定材料温度，这是迄今为止实验中最不受约束的基本热力学参数。具体地说，这项工作的目的是研究由于热无序(德拜-沃勒效应)导致的X射线衍射信号的修改，并从理论和实验上开发在高度变形的固体的复杂环境中利用这一点的方法。这种方法与目前X射线衍射的使用完全兼容，这意味着它可以在各种国际设施的现有实验平台上进行开发。这将为快速扩张的社区带来重大的新能力。具体地说，为了获得上面提到的新的高压状态，人们必须经常在压力-温度空间中通过精心选择的路径来驱动材料。这一过程需要控制材料在压缩过程中的行为，因此需要能够在到达目标条件的过程中对材料的状态进行随时间变化的测量。拟议的工作将通过提供手段来确认材料在变形期间的温度轨迹，从而实现这一点。这将使我们第一次能够通过动态压缩来重复和准确地定位特定感兴趣的状态。作为这种现场温度诊断开发和测试的一部分，我们还将研究一种新的目标类型的响应，该类型的目标旨在通过显著改变样品对压缩的响应性质来获得新的压力-温度状态。这些靶材很容易以低成本大量生产，这将使它们成为下一代高重复率设施(如x射线自由电子激光)的理想实施方案。这些靶子的设计和响应将通过计算和实验相结合的方法进行改进，并将评估它们在高压科学应用中的有效性。这项工作将包括在英国猎户座激光器以及其他领先的国际设施进行的实验活动。此外，还将进行德拜-沃勒测温方法的理论和计算研究，以及新型靶材的设计和实现。

High pressure material (by which we mean here solid matter at pressures exceeding one megabar) exhibits a range of interesting features. Solids in such states display remarkable structural and electronic complexity due to unusual chemical response at these extreme densities of mechanical energy. This can lead to the production of materials with novel and potentially valuable properties such as extreme mechanical properties, or unusual electronic structure. This high pressure material is also a major constituent of the majority of planetary interiors, and as such is widely found within the universe. Moreover, some of these unusual high pressure structures are predicted to be stable on release back to ambient conditions, which may allow for them to be recovered for further study and application in the laboratory. As such, there is a growing interest from a range of scientific disciplines in the generation and diagnosis of solid material at ever increasing pressure. The challenge of creating such conditions in the laboratory is of course considerable. One successful route to high pressure is via transient compression via laser irradiation of samples, where pressures in excess of 10 Mbar have been attained in solids. However, there remain challenges in diagnosing the material produced in these experiments. The development of pulsed x-ray diffraction has allowed for the in-situ determination of density and structure, and thus greatly increased our diagnostic capabilities. This proposal aims to expand the utility of these existing, and highly successful diffraction diagnostics to allow for the determination of material temperature, by far the most poorly constrained fundamental thermodynamic quantity in experiments.Specifically, this work will aim to investigate the modification of x-ray diffraction signals due to thermal disorder (the Debye-Waller effect) and to theoretically and experimentally develop methods to exploit this in the complex environment of a highly deformed solid. This approach is entirely compatible with current uses of x-ray diffraction, meaning it can be exploited on existing experimental platforms at various international facilities. This will bring a significant new capability to a rapidly expanding community. Specifically, in order to access the novel high pressure states referenced above, one must often drive the material through a carefully chosen path in pressure-temperature space. This process requires control of the material's behaviour during compression, and therefore, the ability to perform time-dependent measurement of the material's state en-route to the target conditions. The work proposed will enable this by providing the means to confirm the temperature track of the material during deformation. This will allow us, for the first time, to repeatably and accurately target states of specific interest via dynamic compression. As part of the development and testing of this in-situ temperature diagnostic, we will also investigate the response of a novel target type which aims to access novel pressure-temperature states by significantly altering the nature of sample response to compression. These targets are potentially simple to manufacture in large quantities at low cost, which would make them ideal for implementation at next generation, high repetition rate facilities such as x-ray free electron lasers. The design and response of these targets will be refined by a combination of computational and experimental approaches, and their utility for high pressure science applications will be assessed. This work will consist of an experimental campaign at the UK's Orion laser, as well as other leading international facilities. In addition, theoretical and computational studies of the Debye-Waller approach to temperature measurement, and the design and implementation of novel targets will be conducted.