Quantum high energy density physics

Abstract:

We present experimental observations of K_β emission from highly charged Mg ions at solid density, driven by intense x rays from a free electron laser. The presence of K_β emission indicates the n = 3 atomic shell is relocalized for high charge states, providing an upper constraint on the depression of the ionization potential. We explore the process of state relocalization in dense plasmas from first principles using finite-temperature density functional theory alongside a wave-function localization metric, and find excellent agreement with experimental results.

Abstract:

An extension to the wave packet description of quantum plasmas is presented, where the wave packet can be elongated in arbitrary directions. A generalised Ewald summation is constructed for the wave packet models accounting for long-range Coulomb interactions and fermionic effects are approximated by purpose-built Pauli potentials, self-consistent with the wave packets used. We demonstrate its numerical implementation with good parallel support and close to linear scaling in particle number, used for comparisons with the more common wave packet employing isotropic states. Ground state and thermal properties are compared between the models with differences occurring primarily in the electronic subsystem. Especially, the electrical conductivity of dense hydrogen is investigated where a 15% increase in DC conductivity can be seen in our wave packet model compared to other models.

Abstract:

We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.

Abstract:

The capability for molecular dynamics simulations to treat relativistic dynamics is extended by the inclusion of relativistic kinetic energy. In particular, relativistic corrections to the diffusion coefficient are considered for an argon gas modeled with a Lennard-Jones interaction. Forces are transmitted instantaneously without being retarded, an approximation that is allowed due to the short-range nature of the Lennard-Jones interaction. At a mass density of 1.4g/cm3, significant deviations from classical results are observed at temperatures above kBT≈0.05mc2, corresponding to an average thermal velocity of 32% of the speed of light. For temperatures approaching kBT≈mc2, the semirelativistic simulations agree with analytical results for hard spheres, which is seen to be a good approximation as far as diffusion effects are concerned.