Temperature in laser compressed high pressure solids: measurement and control

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.

This proposal aims to increase our capability in both the creation and diagnosis of high pressure solids via laser based dynamic compression techniques. These approaches allow us to reach unprecedented pressures in solid materials and to diagnose their properties. Such conditions are of interest across a range of disciplines due to the complex mechanical and electronic response of samples in the regime where mechanical energy densities are comparable to chemical energy. One important example is the determination of the melt curves of materials at high pressure, which is key to understanding the structure of planets.

As such, the research proposed here will strengthen and supplement fundamental science investigations by academics both in the UK and the wider international community. Moreover, dynamic compression may have future technological applications due to new research avenues which investigate metastable high pressure phases of materials (indeed, one commercially important route for the generation of artificial diamonds is based on dynamic compression techniques). All of this research requires a greatly improved ability to diagnose and control the temperature of samples during compression, which is the aim of this proposal.

The UK has a strong capability in high power laser technology and exploitation. For example, the STFC's diode pumped laser technology (DiPOLE) is being used as the basis for high energy, high repetition rate research laser applications across a number of major European projects. As a leader in high repetition rate optical laser science, the UK is uniquely positioned to take a lead in developing advanced capabilities for dynamic compression. This proposal forms a key pillar of the efforts to move from a mode of operation suited to basic science research, where short campaigns aim to understand the fundamental, microstructural sample response, to a more advanced, high volume approach which will allow for a wider survey of materials and conditions.

Specifically, as a project partner, the UK's Central Laser Facility Target Fabrication team, and their commercial spinout, Scitech Precision, stand to benefit from the target development portion of this proposal. A successful low cost, versatile target type capable of reaching novel pressure-temperature states, such as the one proposed here, would likely see significant adoption across the community. This proposal will allow for the UK's (already world class) laser target fabrication to take a lead on the development and production of these targets, ensuring that the UK can benefit from their widespread adoption, particularly in the route from small batch production required for this proposal, to the high volume manufacture required for future facilities such as the European XFEL.