We aim to exploit our expertise in laser-induced dynamic compression and x-ray diffraction to make the first ever structural studies of solid matter above 1 TPa (10 megabars) using the JANUS, OMEGA, and National Ignition Facility (NIF) laser platforms in the US. At such pressures, the compression energy is sufficient to break all chemical bonds, providing a regime where new physics and chemistry are predicted to occur. By developing optimised target designs and x-ray diffraction facilities, we will collect high-quality diffraction data on nano-second timescales, and, aided by theory and computation, will determine the structures and phase transitions in a number of fundamental materials to an upper pressure of 3 TPa - almost 10 times higher than the maximum pressure attainable using static compression techniques. We plan to apply these developments to (1) studies of the structures and transitions in carbon (diamond) to 3 TPa, searching for transitions to the metallic BC8 phase, and the creation of super-hard metastable phases of carbon at ambient pressure; (2) studies of the 'simple' metals Na and Li to 3 TPa, searching for metal-insulator and insulator-metal transitions, and the appearance of electride structure-types as valence electrons and cores on neighbouring atoms are forced to overlap; and (3) studies of the onset of "cold-melting" and a liquid ground-state in lithium as a result of the relative enhancement of the zero-point energy at high compression.

The fusion of light nuclei is the energy source that powers the sun. If harnessed on earth, it could provide limitless low-carbon energy. The basic fuel - the Deuterium and Tritium (D&T) forms of heavy hydrogen, are either readily available in sea-water, or can be 'bred' from the abundant element Lithium (the element in a mobile phone battery). The primary nuclear waste products are harmless - the main being helium (an alpha particle), an inert gas found in party balloons. This all sounds too good to be true - and in a sense it is - because getting the reaction to occur is incredibly difficult - because pushing the D and T close together such that the strong force causes them to bind takes a lot of energy (they repel as they are positively charged nuclei). Getting them to move fast enough so that when by chance they have a head-on collision and get close enough to fuse corresponds to heating them to 100 million K. Confining such a hot plasma for long enough for the collisions to occur is no mean feat. There are two approaches: the first uses a magnetic bottle to keep a low density gas away from the walls of a container. As the density is low, collisions take several seconds - this is the magnetic fusion approach. The second idea uses lasers irradiating a small spherical balloon containing the heavy hydrogen. The laser heats the outside of the balloon from different directions, creating a hot plasma that expands into the vacuum, and then, like a spherical rocket, the shell moves towards the centre, compressing the heavy hydrogen to high temperatures and densities 100s of times denser than ordinary liquid. No magnetic fields are needed, because owing to the high density, the collisions are very rapid, and although the compressed miniature sun will expand again (and blow up more quickly if fusion takes place), the reaction occurs faster than the explosion itself - the material is confined by its own inertia. This is called inertial confinement fusion. In current studies at the National Ignition Facility in California, this goal is close to being realised. However, at present there are still problems to be overcome. One of the major ones is that the shell does not compress uniformly, and it is known that if the implosion is not close to being perfectly spherical, then any ripples will grow, breaking up the wall of the shell before the peak of the implosion. The shell of the balloon then mixes into the fuel, and starts to 'glow' due to the high temperatures, and cools the system, preventing fusion. Therefore, two interlinked problems need to be tackled - firstly, we need to find out how much of the shell is mixing into the heavy hydrogen core - and secondly we need to work out how to prevent this happening (either by making better targets, or illuminating the sphere more uniformly). This research grant addresses the first measurement problem. For various physics reasons the shell of the balloon contains some heavy elements (particularly Germanium) which, if they mix into the hot core, 'light-up' and emit characteristic X-ray lines. From a study of the absolute and relative brightness of these lines, it is possible to gain information on the temperature of the material, and of the density, and also, of the amount of the shell that has mixed into the core. Some of this work has already been performed by our US colleagues. However, at present the technique is not quite accurate enough to say if the amount that has mixed in is really enough to extinguish the reaction. The Oxford and York groups in the UK here put forward several new ideas to improve the theory and experimental technique to a point where we believe we will be able to say if the mix level is acceptable. These ideas are based on a new high resolution x-ray instrument, novel spectroscopic theory looking at the brightness of X-rays from different elements, and by performing sophisticated full 3 dimensional simulations of the emission process.

Accretion is the dominant energy conversion process in the Universe, and involves the flow of material down the deep gravitational potential well of a massive compact object, such as a neutron star or black hole. Potential energy is converted into kinetic and radiant energy, and since the radiation is generated in a compact region, its effective temperature can be as high as several million degrees, thereby leading to strong X-ray emission. This general scenario describes the class of astronomical objects known as accretion-powered X-ray sources. The most intense accretion-powered sources known are the active galactic nuclei (AGN), which are powered by the accretion of material onto a supermassive black hole. In particular, quasars (QSOs) - very high luminosity AGN which appear as point sources due to their large distances - may have black holes with masses of a billion times that of the Sun.Surprisingly, in spite of decades of research, the mechanism of AGN accretion is still poorly understood, and remains one of the outstanding problems in astrophysics today. However this is being addressed by two recently launched satellite missions, Chandra and XMM-Newton, which are obtaining high spectral resolution X-ray observations of AGN. Given adequate interpretive tools, these satellites will finally begin to shed light on this important class of objects. Quasars are of particular importance, as the most distant of these are around 10 billion light-years away, and as a result are observed as they were when the Universe was less than a billion years old. Hence the analysis of these objects provides vital information about the early Universe, such as its chemical composition.Highly sophisticated modelling codes are used by the astrophysics community to analyse the X-ray spectra of AGN, the most popular probably being CLOUDY, which is employed in over 100 papers per year. However, how can one ensure that the modelling code employed to analyse astronomical observations is providing accurate results? This is only possible by `benchmarking' the code against well-diagnosed laboratory experimental data.Unfortunately, until recently it has not been possible to mimic an accretion-powered astronomical source in the laboratory, where one requires a plasma in which the excitation and ionization are dominated by the ambient radiation field (the so-called photoionization-dominated regime). However, we are part of a major international consortium - the OMEGA-QSO Consortium - comprised of scientists from Queen's University Belfast, Lawrence Livermore National Laboratory (LLNL), Laboratory for Laser Energetics (LLE) and the University of Kentucky. This Consortium has designed experiments on the OMEGA laser at the LLE, due to start in late 2006, which will re-create the physics of an AGN, specifically the effect of a high ambient radiation field on the excitation and ionization of a plasma. The experiments will hence allow, for the first time, the benchmarking of the CLOUDY code under the extreme conditions found in AGN, including quasars. Just as importantly, we will be able to benchmark the FLYCHK and GALAXY modelling codes, which are widely employed by the laboratory plasma physics community in situations where the radiation field is intense.In this proposal we seek funds from EPSRC to fulfil our responsibilities to the OMEGA-QSO Consortium project, including the provision of high quality atomic physics calculations required for input to the modelling codes. Support from EPSRC will be vital for full and continuing UK participation in this exciting, unique project.

X-ray free-electron lasers (XFELs) are the most brilliant sources of x-rays on Earth. The highly coherent, near-monochromatic, sub-picosecond bursts of radiation they deliver make them the ultimate 'high-speed camera', capable of capturing extremely fast, atomic-level phenomena as they unfold in unprecedented detail. XFELs are therefore ideally suited to probing matter undergoing laser-based dynamic compression, whereby one or more high-power optical lasers rapidly vaporise the surface of a solid target, launching into it a compression wave that generates internal stresses many millions of times greater than atmospheric pressure. During the few billionths of a second for which they survive before being disintegrated, these targets reach extreme pressures of the kind ordinarily encountered only in planetary interiors, and experience rates of deformation rivalling those of meteoric impact events. By illuminating these short-lived samples with extremely bright XFEL pulses, we can generate x-ray diffraction or absorption spectra rich with information about their atomic arrangement, structure, and dynamics in the moments before their destruction. This ability to diagnose the dynamic response of matter under extraordinary thermodynamic conditions is transforming experimental high-pressure physics, allowing us to better understand not only the internal structure, formation, and collision dynamics of planetary bodies, but how to synthesise and recover exotic high-pressure phases of matter, and how engineering alloys and ceramics respond to the huge dynamic stresses created by hypervelocity impacts. In this project, we aim to leverage the diagnostic power of the recently commissioned European XFEL (EuXFEL), an international XFEL facility backed by a consortium of twelve countries to which the UK has committed approximately £30M in capital to date. We will exploit the EuXFEL to shed new light on the plasticity and strength of model metals dynamically deforming at extreme pressures and strain rates. Our aim is to take the 'ordinary' physical processes controlling plastic deformation that materials scientists have studied for over a century, and to examine them under the 'extraordinary' thermodynamic conditions accessible via dynamic compression. Using the UK-built, high-repetition-rate, £8M DiPOLE-100 laser recently installed at EuXFEL, we will laser-compress a range of metals and alloys to planetary pressures over nanosecond timescales at an unprecedented shot rate. We will use femtosecond x-ray diffraction to measure the ultrafast rotation experienced by our samples' microstructure, and use it to identify the plasticity mechanisms that relieve the colossal shear stresses accumulated during compression. From these same diffraction measurements, we will extract the strain state of our metallic samples, allowing us to measure their dynamic strength at extreme strain rates. We will also use EuXFEL to study these samples' x-ray absorption properties under extreme loading, with which we can track their temperature dynamics in situ. Together, these XFEL-enabled experimental measurements of plasticity mechanisms, strength, and temperature evolution have the potential to transform our understanding of material deformation physics under extreme loading conditions.

Thermonuclear fusion is the mechanism by which energy is generated in the Sun. For decades scientists have been attempting to harness fusion for electrical power production because of the huge advantages it offers as a safe, clean and almost inexhaustible supply of energy. In laboratory experiments, fusion is normally studied by heating the heavy isotopes of hydrogen to very high temperatures forming a plasma, in which the rapid motion of the positively charged ions is sufficient to overcome their electrostatic repulsion and allow them to undergo nuclear reactions. One of the main approaches to extracting energy from these reactions is Inertial Confinement Fusion. This involves assembly of the thermonuclear fuel to ultra-high density (over 1000 times the density of water) inside a mm-scale capsule through a spherical implosion driven by high-power lasers. Central to this method is the process of ignition in which the energetic alpha particles emerging from the reactions are themselves used to further heat the fuel, resulting in a self-sustaining burn wave which releases copious amounts of energy. This is a very exciting time for fusion research because with the completion of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), the first laboratory facility with the capacity to demonstrate ignition is now operational. Early results from the NIF however have highlighted differences between the predictions of computer models and the behaviour observed. Most importantly the number of nuclear reactions has remained too low to initiate ignition. The PI and Co-I on this grant have worked extensively with scientists at LLNL to understand the origins of these discrepancies and participated in the Science of Ignition workshop which identified priority research directions to address these issues. For this proposal we wish to capitalise upon our experience with plasmas of extremely high density and temperature to address key uncertainties in the design of inertial confinement fusion experiments and the physics of ignition and work to provide an explanation of why the current design does not achieve ignition and burn. Key areas of research will include understanding the way in which the radiation used to drive the implosion is absorbed in the surface of the capsule, the susceptibility of the imploding capsule to hydrodynamic instabilities which cause the fuel to disintegrate before it is fully compressed and the tendency of the high temperature and low temperature regions of the fuel to stir and mix together which quenches the burn. We will also investigate the physics of the ignition process itself, evaluating whether the energetic alpha particles are able to escape the fuel before depositing their energy and the role of spontaneously generated magnetic fields which provide a form of thermal insulation and serve to keep the heat within the fuel. Part of the work will involve developing a number of advanced computer modelling capabilities. In addition to their use in fusion research, these capabilities can also be used to exploit large scale laser facilities for fundamental research in plasma physics, nuclear physics and laboratory astrophysics. Using these computer models to simulate a fusion capsule in which we deliberately introduce an imperfection, we can calculate what characteristic signatures of this defect are embedded within the flux of energetic neutrons and X-rays emanating from the reacting fuel. Comparing synthetic diagnostic data with that obtained in experiment then allows us to isolate which physical processes are responsible for limiting fusion performance. The same computer models can then be used to design improvements which mitigate these effects and allow us to make progress towards achieving ignition. The work described in this proposal therefore represents an opportunity for UK science to make a significant contribution to what would be a major scientific achievement.