X-ray Studies of Exotic Novel States of Solid-Density Matter Created with 4th Generation Light Sources

Over the past year or so there has been a revolution in X-ray science, in that new sources of soft and hard X-rays have been developed that are ten billion times brighter than those produced by synchrotrons. These novel sources emit extremely short (sub 100fsec) pulses of x-rays, and can be focussed to very small spots. As the pulses are so short, the power in the light is enormous - for the brief duration of the emission the power in the light is equivalent to that in a fair-sized electrical power station. When all this power is focussed to a small spot, enormous intensities of x-rays impinge upon the target in its path - intensities that have hitherto never been produced in the X-ray regime. In the last few months we have performed some of the first experiments aimed at understanding how matter reacts to such intense X-ray light, and the aim of this proposal is to vastly further that understanding. What we have already found is that the intensity is so great that an electron from every atom in the target can be knocked out by the X-rays, and this can alter the X-ray properties of the material itself - indeed, by this method we have made a so-called saturable absorber. What is of fundamental interest to us is that as the electrons re-fill the core holes, they provide information about the electronic structure of this exotic and highly-ionized state, providing completely new insight into the physics of very dense, yet very hot material. This material (warm dense matter) is of interest in that the thermal energies and electronic energies (the coulomb potential) are comparable, making its properties extremely difficult to calclulate. This situation - where the thermal and coulombic energies compete - also occurs in the initial stages of inertial confinement fusion, and is also part of the physics that is relevant to the understanding of the interior of the giant planets - thus there are many reasons for wishing to understand it better. The intense X-rays give a unique opportunity to understand such matter, as within femtoseconds they make a particular state - very hot electrons but cold ions, at a well defined density. Watching this state evolve ( by looking at the fluorescence, and monitoring the absorption as a function of time) gives detailed information on the electronic structure. For example, with highly ionized aluminium, we have an unusual situation at the highest intensities where a particular aluminium ion that undergoes recombination is now doing so with neighbours that themselves are still ionized. This drastically alters the shape of the fluorescence emission in a way which has much to do with how the fluorescence signal from an alloy is altered as the compound composition changes. Thus this research will provide unique insight into the electronic structure of matter at hundreds of thousands of degrees kelvin, yet still at solid density.

There can be no doubt that the exploitation of advanced light sources in general have had a significant impact on our society. As a simple yet telling example, synchrotrons, throughout their so-called 3 generations have been the main tool by which protein structures have been ascertained (leading to innumerable advances in drug design, and consequent improvement of health on a global scale). It is not unreasonable to expect that so-called 4th generation sources -- that is to say the XUV and X-ray free-electron-laser sources that we intend to use in the research presented here -- will in the future have similar beneficial impact on society as a whole. Yet, at present, the number of UK scientists who have used such a source can be counted on the fingers of one hand. If the UK scientific community is to manouver itself such that it is in a position to exploit the unique capabilities of these novel sources -- more than ten orders of brighter than any insertion device on any synchrotron -- then it is essential that we start to train a new generation of scientists who are aware of their characteristics, and well-placed to exploit their unique characteristics. We believe it is important to view the impact of this proposal from this viewpoint -- that is to say it is salient to note that this proposal, if funded, would result in the training of more young UK scientists in the use of XUV and X-ray FELs than have hitherto been produced. Given the huge step-change in technology that such incredible sources represent, we strongly believe that the UK will start to lag behind other countries if we do not start to invest in the training of the next generation of scientists who will be the users of these novel sources. The training of such young scientists via this proposal (and hopefully others in the area of 4th generation science) will be the main tool by which such a pool of talent is generated. Given the unique capabilities of 4th generation sources, it is also clear that this research will have a significant impact on the careers of the young researchers themselves. The postdoc and two students funded under this proposal will have a unique set of skills, right at the time when several new sources around the world will be coming on line. For example, although the work we plan to do will be performed at the FLASH and LCLS facilities, at the end of this research the European facility XFEL in Hamburg, which started construction earlier this year, will be close to operations. This research will thus open up many new opportunities for these young researchers, as they will be some of the first UK scientists to be trained in this burgeoning area of research and technology, and the unique set of skills that they will have acquired by using these ultra-fast and ultra-bright x-ray sources will provide them with significant opportunities for the advancement of their own scientific careers.