Direct measurements of Temperature and Transport Properties at Extreme Conditions

了解物质在极端压力和温度下的结构和行为对于许多基础物理应用至关重要，包括地球物理、行星科学和天体物理、激波和等离子体物理，以及寻找新的材料相和非平衡热力学。特别令人感兴趣的是压力-温度空间区域，它定义了热致密物质(WDM)，这是一种有趣的物质状态，太热，因此太高电离，不能用凝聚态物质理论来描述，并且太强的耦合和关联，以至于经典等离子体物理学无法提供准确的描述。因此，在这样的条件下直接描述物质是至关重要的。从实验上讲，WDM所在的极端温度-密度空间很难创造出来。通常，这样的状态是通过使用纳秒激光的强烈冲击压缩[1，2]，通过飞秒激光激发的等容加热[3-5]，或者通过激光产生的质子束[6-8]来实现的。这些状态是极其短暂的，因此对其进行研究是非常具有挑战性的。硬X射线自由电子激光(XFELs)，如美国的LCLS、德国的欧洲XFEL和拟议中的英国XFEL[9]，能够发射脉冲长度小于100 fs的极明亮的硬X射线，允许人们捕捉这些短暂的状态。这些XFELs与激光压缩驱动器相结合，通过测量静态结构因子[4，10-12]，提供了对极端压力下物质结构的前所未有的洞察。然而，到目前为止，人们对开发测量温度或传输性质(如声速或粘度)的方法关注很少，这些方法可以通过非弹性X射线散射(IXS)测量动态结构因子来获得。IXS是一种光子输入-光子输出过程，涵盖了与所选择测量的能量传递范围相关的广泛的物理现象。到目前为止，使用激光压缩物质的IXS的开创性研究只集中在电子子系统上，测量范围是从eV到keV的能量转移。离子子系统的性质仍然无法测量。在这个研究方案中，我将开发利用非弹性X射线散射来直接测量毫电子伏能量传递范围内的动态结构因子的技术。根据这些测量结果，我将从极端条件下的物质中确定温度。此外，我还将测量粘度和热扩散率等性质，这对于建立行星形成和演化的准确模型是必不可少的。9 274(2015)[2]米洛特等人，科学，347 418(2015)[3]Cho等人，PRL，106 167601(2011)[4]Engelhorn等人，PRB 91,214305(2015)[5]Ping等人，HEDP，4 246(2010年)[6]Gauthier等人，PRL，110 125003(2013)[7]Patel et al.，PRL，91 125004(2003)[8]Mancic et al.，PRL.，035002(2010年)[9]英国自由电子激光科学案例(2020年)，(https://stfc.ukri.org/news/uk-xfel-draft-science-case-consultation/)[10]Kraus等人，NAT。最终，1 606(2017)[11]Gorman等人，PRL。095701(2015年)[12]麦克布莱德等人，NAT。太棒了。15(2019)

Understanding the structure and behaviour of matter at extreme pressures and temperatures is of critical importance to many fundamental physics applications, including geophysics, planetary science and astrophysics, shock and plasma physics, and the search for novel phases of materials and non-equilibrium thermodynamics. Of particular interest is the area of pressure-temperature space that defines Warm Dense Matter (WDM), an intriguing state of matter too hot, and hence too highly-ionised, to be described by condensed matter theories, and too strongly coupled and correlated for classical plasma physics to provide an accurate description. A direct characterisation of matter at such conditions is therefore essential.Experimentally, the extreme temperature-density space where WDM resides is difficult to create. Often, such states are made by strong shock compression using nanosecond lasers [1, 2], by isochoric heating via femtosecond laser excitation [3-5], or by laser-generated proton beams [6-8]. These states are extremely short-lived and are hence very challenging to probe. Hard X-ray Free Electron Lasers (XFELs), such as the LCLS, USA; European XFEL, Germany; and the proposed UK XFEL[9], have the ability to emit extremely bright, hard X-rays with a pulse length of less than 100 fs, allowing one to capture these short-lived states.These XFELs, combined with laser-compression drivers, have provided unprecedented insight into the structure of matter at extreme pressures through measurement of the static structure factor [4, 10-12]. To-date, however, remarkably little attention has been paid to developing methods to measure temperature, or transport properties such as sound speed or viscosity, which would be accessible through measurement of the dynamic structure factor by inelastic X-ray scattering (IXS). IXS is a photon in - photon out process covering a broad range of physical phenomena relating to the energy transfer range one chooses to measure. To date, the pioneering studies that have used IXS from laser-compressed matter have focused only on the electron subsystem, measuring in the eV to keV energy transfer range. The properties of the ionic subsystem remain unmeasured.In this research proposal I will develop techniques using inelastic X-ray scattering to directly measure the dynamic structure factor with the milli-electronvolt energy transfer range. From these measurements I will determine temperature from matter at extreme conditions. In addition I will measure properties such as viscosity and thermal diffusivity, essential for creating accurate models of planetary formation and evolution.References[1] Fletcher et al., Nat. Phot., 9 274 (2015)[2] Millot et al., Science, 347 418 (2015)[3] Cho et al., PRL, 106 167601 (2011)[4] Engelhorn et al., PRB 91, 214305 (2015)[5] Ping et al., HEDP, 4 246 (2010)[6] Gauthier et al., PRL, 110 125003 (2013)[7] Patel et al., PRL., 91 125004 (2003)[8] Mancic et al., PRL., 104 035002 (2010)[9] UK FEL Science Case (2020) (https://stfc.ukri.org/news/uk-xfel-draft-science-case-consultation/)[10] Kraus et al., Nat. Ast., 1 606 (2017)[11] Gorman et al., PRL. 115 095701 (2015)[12] McBride et al., Nat. Phys. 15 89 (2019)