Transport in laser-produced plasmas

Heat-flow is of fundamental interest in plasmas with extreme temperature gradients. Transport effects such as heat-flow are driven by anisotropy within the distribution function in plasma systems. When these plasmas are magnetised, there is a fundamental coupling between the anisotropic transport and magnetic field dynamics. As anisotropy increases, the system is driven from local thermodynamic equilibrium (LTE). Fluid models, which often assume the anisotropy leads to only a small perturbation away from a Maxwellian distribution, can then fail to accurately describe transport effects far from LTE. This thesis examines how magnetisation suppresses anisotropic transport perpendicular to the magnetic field axis and restores local thermodynamic equilibrium.

Kinetic modelling of electron transport is performed using a Vlasov-Fokker-Planck (VFP) code. Magnetisation of a plasma most significantly affects the relatively collisionless heat-carrying electron population, reducing the heat-carrying velocity. Suppression of anisotropic transport is demonstrated by increasing the magnetisation, which can be done by both increasing the magnetic field strength and decreasing the collision frequency. On decreasing the collision frequency in the weakly magnetised regime anisotropy initially increases. However, once the heat-carrying electrons are strongly magnetised transport becomes increasingly local as the collision frequency decreases further. This result is attributed to a reduction in the heat-carrying electron mobility relative to the thermal electron population.

An experiment is then described that provides the first direct measurement of the Nernst effect, the transport of magnetic fields driven by heat flow. Proton radiography and interferometry measurements were used to demonstrate a decoupling of the magnetic field advection from the bulk flow. Despite the demagnetisation of the hot plasma, the transport was found to be kept in a relatively local regime due to the formation of a magnetic transport barrier at the edge of the heat-front. On decreasing the gas density used in the experiment suppression of the Nernst effect was observed.