Microscopic dynamics of warm dense matter

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

In this proposal we seek to explore fundamental issues of dense plasmas and warm dense matter. These states of matter are important as they occur in the natural world, for example, in the cores of large planets and brown dwarfs as well as in laser-fusion pellets and solids heated to plasma temperatures during laser ablation.There are many fundamental questions to be answered and theoretical modelling is difficult because strong coupling makes classical plasma treatments unsuitable and the temperatures are too high to make the ground state approximations of condensed matter. As a result there is much uncertaintly about the equation of state under many relevant conditions. In fact, apparently equally vaild theoretical approaches can easily differ by more than 50% in the predicted pressure for a given temperature and density. Some fundamental questions we wish to address are:'Is there really a plasma phase transition i.e., a flipping over from a low to high ionisation state as has been predicted by theory and can we observe it?''In experiments, how fast do the electrons and ion equilibrate when one species is preferentially heated by shocks or x-rays?' 'What effect does unequal ion and electron temperatures have on the structrure factors?''How does the microscopic spatial and temporal structure of the plasma evolve and can we moniotor it in careful experiments?'One of the key diagnostics will be x-ray scattering, both spectrally and angularly resolved to access respectively the temporal and spatial structure of the dense plasmas. The PI's have played leading roles in the development of these diagnostics. Other diagnostics that will be used to validate experiments are shock and particle speed measurements using techniques such as VISAR and streaked optical emission measurements.The experimental program will make use not only of the Central Laser Facility, but we will continue to access major international laser facilities such as LULI (Paris), TITAN (Livermore) and, the free electron laser facilities (FLASH and LCLS X-FEL).The experimental program will be backed up by a rigorous theoretical effort involving experienced plasma scientists at the Univ. Rostock as collaborators.

Publications

10 25 50

publication icon
Brown CR (2014) Evidence for a glassy state in strongly driven carbon. in Scientific reports

publication icon
Cho BI (2012) Resonant Ka spectroscopy of solid-density aluminum plasmas. in Physical review letters

publication icon
Crowley B (2013) X-ray scattering by many-particle systems in New Journal of Physics

publication icon
Denoeud A (2016) Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa. in Proceedings of the National Academy of Sciences of the United States of America

publication icon
Falk K (2013) Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium. in Physical review. E, Statistical, nonlinear, and soft matter physics

 
Description We have studied matter at extreme conditions - whose found in the interior of stars or planets, and in white dwarfs. This matter is dense and ionized, which means there are strong interactions between different atoms and change the way they arrange among each other. We have found the following important results:
- The transition from solid state to liquid state of dense carbon occurs in a way that cannot be described by simple chemical models, instead it occurs via a large coexistence region. This could have impact in the modeling of dynamo processes and magnetic field generation in carbon rich planets.
- The energy equilibration between electrons and ions in carbon occurs at a much slower rate than previously thought. On the other hand, metals such gold, have a much faster energy equilibration. The energy equilibration affects the equation of state, and it has important consequences in inertial confinement fusion research.
- Our experiments seems to suggests that the crystallization rate in dense carbon is very slow. This has also impact in white dwarf models and the predicted luminosity of the star.
Exploitation Route Our research and findings might be of interest for the astrophysical and plasma physics communities. As discussed above, our finding show that the current equation of state model for carbon should be revised in order to account for slower crystallization and equilibration rates, thus possibly impacting prediction of white dwarf luminosity, planetary magnetic fields, etc. Our work may also have an important practical application in fusion energy. Since schemes for inertial confinement fusion research relies of the energy coupling between a driver laser into first electrons and then ions, the way the two subsystem interact should be properly modeled.
Sectors Aerospace, Defence and Marine,Education,Energy

 
Description In this project we have been able to address several outstanding issues in warm dense matter research. We have demonstrated via X-ray scattering that we can probe the degree of melting in a proton heated warm dense carbon sample. In this work a sample of solid carbon was irradiated by a beam of protons created by a short pulse high intensity laser. By use of X-ray scatter we were able to measure the fraction of carbon that was in the liquid phase as function of energy deposition. This is significant as equations of state tables often treat the matter in question as being in one phase or another, whereas in reality in some important regimes there is likely to be a mixture of phases. We have made the first measurements of X-ray scatter from shock compressed Fe. This was technically a difficult material to work with, since it is highly absorbing for keV x-rays and has a low optical emission on shock break-out, thus making the scattering data and the shock calibration data difficult to obtain. Nevertheless, after several attempts, we were successful. This data is in the initial phases of analysis but already an interesting preliminary finding is that the slope of the ion-ion structure factor as a function of scatter wave-vector is steep at low values of wave-vector. In fact this is in accord with preliminary hyper-netted chain calculations made prior to the experiment. Iron is of course a very important material as it is the principal component of the Earth's core, in particular, the outer liquid core responsible for the Earth's magnetic field. The structure factor is a key parameter in deciding both the thermal and electrical conductivity. In addition to the work on X-ray scattering, we have used the FLASH and LCLS XUV and X-ray free electron lasers to create new, exotic states of matter. In particular, we have demonstrated via soft x-ray emission spectroscopy, the creation of transient crystalline states of matter at temperatures of many thousands of degrees. In this research, we have attempted to explore the microscopic behaviour of warm dense matter. That is to say, matter that is at temperatures ranging from about 1 eV to 100 eV (10,000 to a million degrees) and at high density similar to solids. We have acccessed large laser facilities such as the Vulcan Laser at the Rutherford Appleton Laboratory and the Titan laser in LLNL as well as new light sources such as the X-ray laser at Stanford in order to create and probe such states of matter. We have done so, principally by X-ray scattering / diffraction but also via other techniques such as emission spectroscopy. In doing so, we have explored the melt fraction for carbon under warm dense matter conditions as well as the behaviour of Fe under high density and pressure at temperatures of over 20,000 degrees. Both these elements are of extreme interest in planetary sciences.
Sector Education,Energy
Impact Types Cultural