Femtosecond Diffraction Studies of Shock and Ramp-Compressed Matter

该项目旨在使用硬x射线自由电子激光器（SLAC的直线相干光源- LCLS）的飞秒脉冲来测量压缩到兆巴压力的物质冲击或斜坡的变形和结构，进入金刚石砧细胞无法研究的相空间区域。实验将增加几百万原子分子动力学模拟和衍射模拟。该研究既有应用方面的，也有基础方面的。从基本观点来看，可见宇宙中的大量物质存在于许多实验室实验无法达到的压力和温度状态下-例如，我们太阳系中大行星的内部，以及那些已被发现围绕其他恒星运行的（系外行星）。重建和了解这些地区的现状具有根本意义。从应用的角度来看，目前还不清楚哪些材料一旦在高温高压下生产出来，在环境条件下可能是亚稳的（并具有商业用途）。例如，众所周知，金刚石是亚稳态的（它不是自由能最低的状态），但它与石墨相之间的焓垒很大，因此在环境条件下存在，具有相当大的工业影响。因此，在新条件下探索物质的整个领域，学习其结构，然后将其带回（如果可能的话）环境条件可能会产生相当大的影响。该项目的目的是在晶格水平上更好地理解高应变率塑性和多晶相变的物理学。这将通过理论研究（分子动力学模拟）和利用飞秒x射线衍射对高功率激光烧蚀压缩的材料进行冲击或斜坡实验来进行。目的是为了更好地理解在激光烧蚀施加的单轴应变条件下，材料如何释放剪切应力以流向流体静力。我们知道，根据材料的不同，这可以通过位错的产生和流动，通过孪晶和多晶相变来发生。然而，在晶格水平和单个颗粒的中尺度上究竟发生了什么，还有待于详细探索。在这个项目中，学生将研究简单的金属和更复杂的目标，以开始解开这一复杂的现象。该项目与EPSRC的激光等离子体和聚变主题直接相关。例如，物理学包含在EPSRC资助EP/J017256/1多兆帕压力下固态物质的产生和诊断。其中50%的资金由AWE提供，我们还将与美国的LLNL密切合作。

The project aims to use the femtosecond pulses from a hard x-ray free electron laser (the Linac Coherent Light Source - LCLS at SLAC) to measure deformation and structure of matter shock or ramp compressed to multi-megabar pressures, accessing regions of phase space inaccessible to study with diamond anvil cells. The experiments will be augmented with multi-million atom molecular dynamics simulations coupled with diffraction simulations. The research has both applied and fundamental aspects. From the fundamental point of view a significant amount of matter in the visible universe exists in states of pressure and temperature inaccessible to many laboratory experiments - e.g. the interior of the large planets in our own solar system, and those that have been discovered orbiting other stars (exoplanets). Recreating and understanding the conditions existing in these regions is of fundamental interest. From the applied point of view it is as yet unknown what materials, once produced at high temperature and pressure, may be metastable (and of commercial use) at ambient conditions. For example, it is well known that diamond is metastable (it is not the state with the lowest free energy), however, the enthalpy barrier between it and the graphite phase is huge, and it thus exists under ambient conditions, and has considerable industrial impact. Thus the overall field of explore matter under novel conditions, learning its structure, and then bringing it back (if possible) to ambient conditions could be of considerable impact.The aims of the project are to better understand, at the lattice level, the physics of high-strain rate plasticity and polymorphic phase transformations. This will be undertaken both by theoretical investigation (molecular dynamics simulations) and via experiments employing femtosecond x-ray diffraction of materials shock or ramp compressed by high power laser ablation. The aim is, for several classes of material, to better understand how, under the uniaxial strain conditions applied by laser ablation, the material relieves the shear stresses to flow towards the hydrostatic. We know that, depending upon the material, this can happen via dislocation generation and flow, via twinning, and via polymorphic phase transitions. However, what actually happens at the lattice level, and at the mesoscale of individual grains, has yet to be explored in any detail. In this project the student will study both simple metals and more complex targets to start to unravel this complicated set of phenomena. This project is directly related to the laser-plasmas and fusion EPSRC theme. For example the physics is encompassed under EPSRC grant EP/J017256/1 The Creation and Diagnosis of Solid-State Matter at Multi-TeraPascal Pressures. This is 50% funded by AWE, and we will also collaborate closely with LLNL in the USA.