High Energy Density Plasmas Generated and Probed with Fourth Generation Light Sources

XUV体制下的自由电子激光器现在已经存在了——DESY的FLASH激光器。这是硬x射线（1埃）系统XFEL的前身，该系统将于今年开始建设，英国已承诺对此进行重大投资。这种光源的光谱亮度将比任何同步加速器都要高10个数量级，并有望在物理和生命科学的许多领域带来革命性的变化。英国必须建立一个可行的用户群来利用这些系统。牛津小组是英国使用皮秒和飞秒x射线技术研究各种问题的领先小组。在这个项目中，我们建议使用FLASH自由电子激光器的短时间XUV辐射，在固体密度物质有时间膨胀之前将其加热到数万开尔文。这样，我们就创造了一个由热而致密的材料组成的薄板，它随后会膨胀。通过使用称为傅里叶域干涉测量法的光学技术来监测表面如何膨胀，我们可以获得有关热、致密物质中原始压力的信息。这是很重要的，因为对于给定的能量密度，压力的理论模型变化可达两倍——离子之间的静电势与它们的热能相当，这使得压力的计算极其困难。因此，这种温暖的致密物质很难建模，尽管它与各种物理系统有很大的相关性，比如巨行星和惯性约束聚变研究。因此，在这一领域可靠的实验数据是非常需要的。迄今为止，这类数据一直难以捉摸，因为能够在短时间内均匀加热物质的辐射源还不存在。新一代自由电子激光器有可能克服这些问题。

Free electron lasers in the XUV regime now exist - the FLASH laser at DESY. This is the precursor to the hard x-ray (1 Angstrom) system XFEL, which should begin construction this year, and to which the UK has committed a significant investment. Such sources will have spectral brightnesses ten orders of magnitude greater than any synchrotron, and hold out the promise of revolutionizing many areas of the physical and life sciences. It is imperative that the UK build a viable user-base to exploit such systems. The Oxford group is the leading group in the UK using picosecond and femtosecond x-ray techniques to study a variety of problems. In this project we propose to use the short duration XUV radiation from the FLASH free electron laser to heat solid density matter to tens of thousands of Kelvin before it has time to expand. We thus create a thin slab of hot, dense, material, that subsequently expands. By monitoring how the surface expands by using an optical technique known as Fourier Domain Interferometry, we can obtain information about the original pressure in the hot, dense, matter. This is important, as theoretical models of the pressure, for a given energy density, vary by up to a factor of two - the fact that the electrostatic potential between the ions is comparable to their thermal energy makes calculations of the pressure extremely difficult. Such warm dense matter is thus hard to model, even though it is of great relevance to a variety of physical systems, such as the giant planets and inertial confinement fusion research. Therefore reliable experimental data in this area is highly desirable. To date, such data has been elusive, as radiation sources capable of heating matter uniformly in short timescales have not existed. The new generation of free electron lasers can potentially overcome these problems.