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

点击翻译按钮获取中文摘要

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.