Gianluca Gregori

Abstract:

In this paper we present an analysis of Thomson scattered light from an arc plasma jet. Our approach goes beyond the standard random-phase approximation (RPA) and provides more consistent data for the electron temperature and density in plasmas that are weakly nonideal and collisional. The theory is based on a memory function formalism for the spectral density function with the use of the three lowest-order frequency-moment sum rules. These moments are then corrected for temperature inhomogeneities in the scattering volume. The proposed interpretation of scattering data is compared with the RPA result and with the standard Bhatnagar-Gross-Krook collisional model for the dynamic structure factor. It is shown that the obtained electron temperature values are closer but not equal to local thermodynamic equilibrium temperature values extracted from spectroscopic measurements.

Abstract:

We present analytical expressions for the dynamic structure factor, or form factor S(k, omega), which is the quantity describing the inelastic x-ray cross section from a dense plasma or a simple liquid. Our results, based on the random phase approximation (RPA) for the treatment on the charged particle coupling, can be applied to describe scattering from either weakly coupled classical plasmas or degenerate electron liquids. Our form factor correctly reproduces the Compton energy downshift and the usual Fermi-Dirac electron velocity distribution for S(k, omega) in the case of a cold degenerate plasma. The results shown in this work can be applied to interpreting x-ray scattering in warm dense plasmas occurring in inertial confinement fusion experiments. We show that electron density, electron temperature and ionization state can be directly inferred from such measurements. Specifically, we present as an example, use the results of experiments performed at the Vulcan laser facility at the Rutherford Appleton Laboratories (UK) on a LiH target.

Abstract:

We present results from three-dimensional numerical simulations investigating the magnetohydrodynamics of cloud-wind interactions. The initial cloud is spherical, while the magnetic field is uniform and transverse to the cloud motion. A simplified analytical model that describes the magnetic energy evolution in front of the cloud is developed and compared with simulation results. In addition, it is found that the interaction of the cloud with a magnetized interstellar medium results in the formation of a highly structured magnetotail. The magnetic flux in the wake of the cloud organizes into flux ropes, and a reconnection current sheet is developed as field lines of opposite polarity are brought close together near the symmetry axis. At the same time magnetic pressure is strongly enhanced at the leading edge of the cloud from the stretching of the field lines that occurs there. This has an important dynamical effect on the subsequent evolution of the cloud, since some unstable modes tend to be strongly enhanced.

Abstract:

We present results from the first three-dimensional numerical simulations of moderately supersonic cloud motion through a tenuous, magnetized medium. We show that the interaction of the cloud with a magnetic field perpendicular to its motion has a great dynamical impact on the development of instabilities at the cloud surface. Even for initially spherical clouds, magnetic field lines become trapped in surface deformations and undergo stretching. The consequent field amplification that occurs there and, in particular, its variation across the cloud face then dramatically enhance the growth rate of Rayleigh-Taylor unstable modes, hastening the cloud disruption.