Temperature in laser compressed high pressure solids: measurement and control

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High pressure material (by which we mean here solid matter at pressures exceeding one megabar) exhibits a range of interesting features. Solids in such states display remarkable structural and electronic complexity due to unusual chemical response at these extreme densities of mechanical energy. This can lead to the production of materials with novel and potentially valuable properties such as extreme mechanical properties, or unusual electronic structure. This high pressure material is also a major constituent of the majority of planetary interiors, and as such is widely found within the universe. Moreover, some of these unusual high pressure structures are predicted to be stable on release back to ambient conditions, which may allow for them to be recovered for further study and application in the laboratory. As such, there is a growing interest from a range of scientific disciplines in the generation and diagnosis of solid material at ever increasing pressure. The challenge of creating such conditions in the laboratory is of course considerable. One successful route to high pressure is via transient compression via laser irradiation of samples, where pressures in excess of 10 Mbar have been attained in solids. However, there remain challenges in diagnosing the material produced in these experiments. The development of pulsed x-ray diffraction has allowed for the in-situ determination of density and structure, and thus greatly increased our diagnostic capabilities. This proposal aims to expand the utility of these existing, and highly successful diffraction diagnostics to allow for the determination of material temperature, by far the most poorly constrained fundamental thermodynamic quantity in experiments.Specifically, this work will aim to investigate the modification of x-ray diffraction signals due to thermal disorder (the Debye-Waller effect) and to theoretically and experimentally develop methods to exploit this in the complex environment of a highly deformed solid. This approach is entirely compatible with current uses of x-ray diffraction, meaning it can be exploited on existing experimental platforms at various international facilities. This will bring a significant new capability to a rapidly expanding community. Specifically, in order to access the novel high pressure states referenced above, one must often drive the material through a carefully chosen path in pressure-temperature space. This process requires control of the material's behaviour during compression, and therefore, the ability to perform time-dependent measurement of the material's state en-route to the target conditions. The work proposed will enable this by providing the means to confirm the temperature track of the material during deformation. This will allow us, for the first time, to repeatably and accurately target states of specific interest via dynamic compression. As part of the development and testing of this in-situ temperature diagnostic, we will also investigate the response of a novel target type which aims to access novel pressure-temperature states by significantly altering the nature of sample response to compression. These targets are potentially simple to manufacture in large quantities at low cost, which would make them ideal for implementation at next generation, high repetition rate facilities such as x-ray free electron lasers. The design and response of these targets will be refined by a combination of computational and experimental approaches, and their utility for high pressure science applications will be assessed. This work will consist of an experimental campaign at the UK's Orion laser, as well as other leading international facilities. In addition, theoretical and computational studies of the Debye-Waller approach to temperature measurement, and the design and implementation of novel targets will be conducted.