Experimental characterisation of shock waves launched by high intensity laser interaction with over-dense media

我提出了一个方案的实验，开发，然后利用一种新的诊断技术，以更好地了解冲击波的形成在附近的高强度激光与过密等离子体相互作用 *。超高强度激光器能够为地球上的科学家提供一些最极端的条件。然而，在实验上诊断最极端的行为可能是具有挑战性的：在几皮秒和几微米的相互作用内。这对于由这种相互作用产生的流体动力学行为尤其如此，因为这种观察通常依赖于对流体的宏观运动的检测，而流体的宏观运动可能需要几十皮秒才变得明显。在这里，我们建议使用多普勒频移的短波长探测激光询问的扩展冲击波，因为它的激光焦点附近形成。这种诊断依赖于受冲击流体的高速多普勒频移反射的激光，而不是依赖于它检查流体中的扰动已经行进了多远。因此，它能够在冲击波形成时并且在流体的运动能够通过更常规的X射线诊断技术检测到之前分辨冲击波，所述更常规的X射线诊断技术通常具有15-20微米量级的分辨率。一个初步的实验已经完成，结果发表在2010年9月的物理评论快报。然而，该实验使用了固体密度目标，极大地限制了提取冲击波行为信息的可能性，因为大部分目标对探测激光来说是不透明的。在这里，我们计划采用低密度的泡沫目标，这是过于密集的泵脉冲，但只有成为过于密集的探头时，强烈冲击。使用这种靶允许从所有角度观察冲击波演变，并且还允许确定是否存在多个冲击形成区域的可能性，因为在某些情况下，冲击波很可能既通过激光的直接作用又通过激光产生的高能电子间接发射，所述高能电子将其能量存款在靶的深处。所获得的数据对于理解诸如超新星等极端天体物理场景中发生的极强冲击波的行为是有意义的;它也与惯性聚变能量的快速点火方法有着巨大的相关性，在这种方法中，类似的强激光脉冲被用来加热聚变燃料。在这种情况下，负责发射冲击波的物理学还没有得到很好的理解。更清楚地了解各种机制的相对重要性对这一领域的进展至关重要，因为快速点火目标中冲击波的行为直接影响点火过程，并在确定点火所需的激光技术规格方面发挥作用。这些实验将提供关键的数据，从而能够对复杂的计算机模拟代码进行基准测试，然后将其应用于快速点火聚变靶的设计。过密等离子体是其中给定频率的激光束由于电子气的密度足够高而不再能够传播，使得振荡电子运动容易在激光器的频率下建立;具体地，它指的是其中电子等离子体频率超过激光辐射的频率的等离子体。在非常高的激光强度下，等离子体满足该标准的密度由于电子的有效质量增加而增加，因为它们在激光场中被加速到相对论速度。

I propose a programme of experiments to develop and then utilise a novel diagnostic technique to better understand the formation of shock waves in the immediate vicinity of a high intensity laser interaction with over-dense plasma*. Ultra-high intensity lasers offer the ability to generate some of the most extreme conditions available to scientists here on earth. However it can be challenging to experimentally diagnose the behaviour where it is at its most extreme: within a few picoseconds and a few microns of the interaction. This is particularly true of the hydrodynamic behaviour that results from such interactions, since such observations typically rely upon detection of macroscopic motion of fluid which may take many tens of picoseconds to become apparent. Here we propose to use the Doppler shift of a short wavelength probe laser to interrogate the expanding shock wave as it forms near the laser focus. This diagnostic relies upon the high velocity of the shocked fluid Doppler-shifting the reflected laser light, rather than upon it examining how far perturbations in the fluid have travelled. Therefore it is capable of resolving shock waves when they are forming, and well before the motion of the fluid would be detectable by more conventional x-ray diagnostic techniques, which typically have resolutions on the order of 15-20 microns. A preliminary experiment has been performed, and the results published in Physical Review Letters in September 2010. This experiment used a solid density target, however, greatly limiting the possibilities for extracting information on shock wave behaviour, since most of the target was opaque to the probe laser. Here we plan to employ low density foam targets which are over-dense to the pump pulse, but only become over-dense to the probe when strongly shocked. The use of such targets allows for the shock wave evolution to be observed from all angles, and also for the possibility of determining if there are multiple regions of shock formation, since it is likely that, in some cases, shock waves will be launched both by the direct action of the laser and indirectly by laser generated energetic electrons that deposit their energy deep within the target. The data obtained is of interest for understanding the behaviour of extremely strong shock waves such as occur in extreme astrophysical scenarios like supernovae; it is also of tremendous relevance to the fast ignition approach to inertial fusion energy, where similarly intense laser pulses are used to heat fusion fuel. The physics responsible for launching shock waves in this scenario is not well understood. Gaining a clearer insight into the relative importance of the various mechanisms responsible is critical to progress in this field, since the behaviour of the shock waves in a fast ignition target directly affects the ignition process and plays a role in determining the required laser specifications for ignition. These experiments will provide critical data that will enable the benchmarking of sophisticated computer simulation codes that can then be applied to the design of fast ignition fusion targets.* Over-dense plasma is that in which a laser beam of a given frequency can no longer propagate due to the density of the electron gas being sufficiently high that oscillatory electron motion is readily established at the frequency of the laser; specifically it refers to plasma in which the electron plasma frequency exceeds the frequency of the laser radiation. At very high laser intensities the density at which the plasma meets this criterion increases due to the effective mass of the electrons increasing, as they are accelerated to relativistic velocities in the field of the laser.