Warm Dense Matter


Justin Wark
Gianluca Gregori

Dense, warm plasmas are prevalent in nature, but notoriously difficult to model as they are very far from being ideal. An 'ideal' plasma is one where the thermal energy of the constituent particles is much greater than the mean Coulomb potential between them - which is equivalent to saying that there are many particles present within a sphere of radius the Debye length (the characteristic shielding length). As the thermal energy vastly exceeds the coulomb energy, many approximations can be made for such ideal plasmas (just as, for example, the interatomic forces in a Van de Waal's gas can be seen as a small perturbation to the ideal gas situation). However, in warm dense matter this approximation cannot be made. For example, in solid density matter the coulomb potential between electrons and ions are of order eV, or ten of eV, so even very hot solid-density matter is far from ideal (recall 1 eV is about 11,000 Kelvin). Also, for WDM, we can't make the opposite approximation that the coulomb energy exceeds the thermal energy (which is what we do when we model solid crystals, which have eV binding energies, but room temperature is only a perturbation of 1/40th eV). Hence WDM is in a 'non-man's land' where the thermal energies and coulomb energies are comparable, and no perturbation approach is valid. However, this sort of material is found in all measure of astrophysical situations - within the stars and the giant planets - yet it is very poorly understood in terms of its equation of state and transport properties.

Within the Oxford group and the OxCHEDS consortium we aim to create and diagnose such matter under laboratory conditions, as well as develop the fundamental theory of its properties. This type of matter, being dense and hot, is typically at very high pressures - certainly many millions of atmospheres, and as soon as it is created thus has a tendency to rapidly expand. For this reason we create the matter with very short burst (down to a few femtoseconds) of optical or X-ray lasers, and then interrogate the conditions and properties before the sample has time to expand. Current projects are investigating the equation of state, the ionization and optical properties, and the transport properties of this fascinating regime of the phase space of matter.