Computational studies of high power nanosecond laser propagation in magnetised plasmas

Abstract

The effects of magnetic fields on long-pulse (nanosecond) laser-plasma interactions have been a subject of research interest in recent years. Applied fields have been used for the formation and control of plasma waveguides (Froula 2009), for improving energy coupling under conditions relevant to indirect-drive ICF (Montgomery 2015) and have been observed to arise naturally in the gas-fill of hohlraums due to field generation by the Biermann battery mechanism at the wall (Li 2009). These systems are complicated by the range of coupled magnetised electron transport phenomena which can occur. For example, heat-flow across field lines is suppressed in a magnetised plasma and magnetic fields can rapidly advect along temperature gradients due to Nernst advection, an effect which is predominant at moderate magnetisations (wt ~ 1).

This thesis addresses the question of how these phenomena, coupled with inverse bremsstrahlung heating, affect the hydrodynamic evolution of the plasma and in turn change laser self-focusing. This problem is investigated by means of theoretical and computational modelling. A paraxial wave solver has been developed and used in conjunction with the existing 2D plasma codes, CTC, an MHD code including a detailed model of Braginskii electron transport, and IMPACT, a Vlasov-Fokker-Planck code with fully implicit magnetic fields.

Simulations of moderate intensity (~ 10^14 W/cm^2), 10 micron width infrared laser pulses propagating through under-dense (ne = 10^18 - 10^19 cm^-3) plasmas in the presence of 0 - 12 T applied fields demonstrate an inhibition to beam self-focusing and thermal pressure driven density channel formation resulting from Nernst advection over time-scales greater than ~ 200 ps. VFP simulations accounting for non-locality indicate that heat-flow and Nernst advection can be over-estimated however and result in a re-emergence of channeling phenomena under these conditions. Finally, the magnetothermal instability - the result of feedback between the Nernst effect and Righi-Leduc heat-flow - frequently arises, affecting temperature and field profiles and is considered in the context of such conditions.