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

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.

This grant is intended to support studies in the field of Inertial Fusion Energy (IFE). IFE is one of two mainstream approaches to achieving fusion energy for the purposes of electrical power production here on earth. Fusion energy is a greenhouse-gas-emission-free source of nuclear power which is expected to produce around 1/100000th the quantity of radioactive material created by nuclear-fission-based power production for comparable levels of output. It is hoped that fusion energy will be harnessed in coming decades to power the development of civilisation. It is critical to the future viability of our energy supply chain that UK plc invests heavily in such new forms of energy that are not dependent upon fossil fuel reserves. This grant will advance our knowledge of the science underpinning IFE, and therefore, in cohort with many other ongoing scientific endeavours in this field, may have a dramatic impact on the UK's future. Specifically we will look at the formation of strong shock waves near the focus of intense laser-plasma interactions with dense plasma, with picosecond resolution. The formation of such shock waves on these time scales plays a key role in determining the laser requirements in the fast ignition approach to IFE. The evolution of the shock waves in the dense blob of fusion fuel, launched by the interaction with the ultra-high intensity heater laser beam and consequent relativistic electron heating, determines the time available for delivering the required ignition energy to the fuel, and therefore the required laser power and pulse duration. We will exploit our close ties with the Rutherford Appleton Laboratory (RAL) and Lawrence Livermore National Laboratory (LLNL), as well as the press office at the University of York, to ensure maximum impact. Both RAL and LLNL have extensive ties to both relevant specialist as well as non-academic publication channels. Several recent developments in the field of fast ignition have been publicised through National and International news media. Our work will also be disseminated by publication in high-quality peer reviewed international scientific journals, and presented at conferences both at home and abroad. In addition to the funding requested as part of this grant application, funding for student conference attendance is already in place through existing arrangements, and more will be applied for from other channels such as the Institute of Physics Research Student Conference Fund. As well as advancing our progress toward clean energy, fusion energy experiments are particularly effective in enlivening public interest in science, particularly amongst young people. The University of York, and the PI, are closely associated with the UK's premier public outreach communicator in the field of Inertial Fusion Energy, Dr Kate Lancaster, who has recently taken up a visiting lectureship in our department. In association with Dr Lancaster, we promote public understanding of science and fusion energy in particular. Such activities are only possible due to the strong support our research receives from EPSRC. Increasing public understanding and interest in science is critical to ensuring that sufficient science graduates are produced within the UK to continue our long tradition of scientific innovation, which is so critical to the maintenance of a healthy modern economy. In addition to hosting public science lectures in the department at the University of York, we engage actively with Dr Lancaster's programme of public scientific lectures and interviews. These ventures are already funded by other arrangements including support from the University of York and STFC, but we highlight them here since they serve as useful avenues for ensuring that the research proposed herein would deliver maximum impact.