A common perception is that laboratory tests of fundamental physics necessarily require large particle colliders. However, thanks to the development of ultra high-intensity optical lasers and 4th generation light sources, new approaches are now possible that exploit the simultaneous interactions of multiple photons with matter and vacua via quantum field fluctuations. In this proposal, we will employ these high-field non-perturbative quantum optics processes to search for new fundamental particles. Since accelerator-based searches have not yet found new physics at high energies, ultra high-intensity optical lasers and 4th generation light sources offer a novel complementary approach for searches at optical and X-ray energies. This proposal addresses an important question in fundamental physics by developing a laboratory search for new particles beyond the Standard Model called axions. Our work will be able to probe axion masses bigger than a few eV up to a keV - a region that is currently inaccessible to laboratory searches. In the eV-keV mass range. the searches proposed here are the only model-independent ones, meaning that the experiments have full control over both the production and reconversion of axions within the same apparatus - without the need to assume that axions are produced by astrophysical objects (such as the Sun) or constitute a large fraction of the dark matter.

Understanding how the structure and physical properties of materials change under extremes of pressure and temperature is essential if we are to develop predictive capabilities on how materials work under such conditions, thereby driving innovation in material design and engineering for the improved materials of tomorrow. Much progress has been made in the last 20 years, to the extent that our understanding of how the crystallographic and electronic structure of matter changes when it is compressed to very high pressures has transformed completely in that time. However, the lack of suitable technologies has severely limited our ability to tackle two key "known unknowns": how do pressure-induced structural changes occur in elements, and how are the microstructure and physical properties of more complex materials, such as key binary alloys, affected by extreme pressures and temperatures. We will exploit our team's expertise in experimental high-pressure physics, combined with recent advances in high repetition rate lasers, and the unprecedented brightness and spatial coherence of next generation synchrotron and x-ray free electron laser facilities, to make definitive studies of phase transitions, transition mechanisms, microstructure, and material strength in key elemental and alloy systems using x-ray diffraction and imaging. In collaboration with our Project Partners, we will then use electronic structure calculations to understand the physics behind the observed material response, and thereby develop new understanding and improved predictive capabilities in the behaviour of matter at extreme conditions.

Scientific breakthroughs are often strongly associated with technological developments, which enable the measurement of matter to an increased level of detail. A modern revolution is underway in X-ray spectroscopy (XS), driven by the transformative effect of next-generation, high-brilliance light sources e.g. Diamond Light Source and the European X-ray Free Electron Laser and the emergence of laboratory-based X-ray spectrometers. Alongside instrumental and methodological developments, the advances enabled in X-ray absorption (XAS) and (non-)resonant emission (XES and RXES/RIXS) spectroscopies are having far-reaching effects across the natural sciences. However, these new kinds of experiments, and their ever-higher resolution and data acquisition rates, have brought acutely into focus a new challenge: How do we efficiently and accurately analyse these data to ensure that valuable quantitative information encoded in each spectrum can be extracted? The high information content of an XS, demands detailed theoretical treatments to link the spectroscopic observables to the underlying geometric, electronic and spin structure. However, this is a far from trivial task. A prime example is found in the XS of disordered systems, e.g. in operando catalysts, in which the spectrum represents an average signal recorded from many inequivalent absorption sites. The disorder of the system must be modelled for a quantitative analysis, but to treat theoretically every possible chemically inequivalent absorption site (or even to sample a meaningful number of such sites) is computationally challenging, resource-intensive, and time-consuming. It is presently out of reach for the majority of XS end-users and, for the most complex systems, even expert theoreticians. To add to this, it is not always apparent to end-users: a) how to apply the most appropriate theoretical treatments, or b) where more insight might be attainable from the data by their application. Consequently, the status quo is to rely heavily on empirical rules, e.g. the scaling of absorption edge position with oxidation state, or to collect reference spectra and use linear combinations of these to fit the absorption profile. As long as this status quo is unchallenged, the many XS experiments remain useful for little more than fingerprinting, and a wealth of valuable quantitative information is left unexploited, ultimately limiting our understanding. The objective of this fellowship proposal is to develop and subsequently equip researchers with easy-to-use, computationally inexpensive, and accessible tools for the fast and automated analysis and prediction of XS. We will optimize and deploy deep neural networks (DNNs) capable of providing instantaneous predictions of XS for arbitrary absorption sites, introducing a step change in ease and accuracy of the XS data analysis workflow. Using DNNs, it is possible to reduce the time taken to predict XS data from hours/days to seconds, democratise data analysis, open the door to the development of new high-throughput XS experiments, and allow end users to plan and utilise better their beamtime allocations by facilitating on-the-fly 'real-time' analysis/diagnostics for XS data.

Fusion is the process that powers the Sun, and if it can be reproduced here on Earth it would solve one of the biggest challenges facing humanity - plentiful, safe, sustainable power to the grid. For fusion to occur requires the deuterium and tritium (DT) mix of fuels to be heated to ten times the temperature at the centre of the Sun, and confined for sufficient time at sufficient density. The fuel is then in the plasma state - a form of ionised gas. Our CDT explores two approaches to creating the fusion conditions in the plasma: (1) magnetic confinement fusion which holds the fuel by magnetic fields at relatively low density for relatively long times in a chamber called a tokamak, and (2) inertial confinement fusion which holds the fuel for a very short time related to the plasma inertia but at huge densities which are achieved by powerful lasers focused onto a solid DT pellet. A main driver for our CDT is the people that are required as we approach the final stages towards the commercialisation of fusion energy. This requires high calibre researchers to be internationally competitive and win time on the new generation of fusion facilities such as the 15Bn Euro ITER international tokamak under construction in the South of France, and the range of new high power laser facilities across Europe and beyond (e.g. NIF in the US). ITER, for example, will produce ten times more fusion power than that used to heat the plasma to fusion conditions, to answer the final physics questions and most technology questions to enable the design of the first demonstration reactors. Fusion integrates many research areas. Our CDT trains across plasma physics and materials strands, giving students depth of knowledge in their chosen strand, but also breadth across both to instil an understanding of how the two are closely coupled in a fusion device. Training in advanced instrumentation and microscopy is required to understand how materials and plasmas behave (and interact) in the extreme fusion conditions. Advanced computing cuts across materials science and plasma physics, so high performance computing is embedded in our taught programme and several PhD research projects. Fusion requires advances in technology as well as scientific research. We focus on areas that link to our core interests of materials and plasmas, such as the negative ion sources required for the large neutral beam heating systems or the design of the divertor components to handle high heat loads. Our students have access to world-class facilities that enhance the local infrastructure of the partner universities. The Central Laser Facility and Orion laser at AWE, for example, provide an important UK capability, while LMJ, XFEL and the ELI suite of laser facilities offer opportunities for high impact research to establish track records. In materials, we have access to the National Ion Beam Centre, including Dalton Cumbria Facility; the Materials Research Facility at Culham for studying radioactive samples; the emerging capability of the Royce institute, and the Jules Horowitz reactor for neutron irradiation experiments in the near future. The JET and MAST-U tokamaks at Culham are key for plasma physics and materials science. MAST-U is returning to experiments following a £55M upgrade, while JET is preparing for record- breaking fusion experiments with DT. Overseas, we have an MoU with the Korean national fusion institute (NFRI) to collaborate on materials research and on their superconducting tokamak, KSTAR. The latter provides important experience for our students as both the JT-60SA tokamak (under construction in Japan as an EU-Japan collaboration) and ITER will have superconducting magnets, and plays to the strengths of our superconducting materials capability at Durham and Oxford. These opportunities together provide an excellent training environment and create a high impact arena with strong international visibility for our students.