Irradiation nonuniformity is a major source of degrading target performance at ongoing spherical- direct-drive experiments at OMEGA [2, 3] and polar-direct-drive experiments at the National Ignition Facility [4]. Laser energy is deposited in the underdense plasma and transported, through the conduction zone, towards the cold target surface. The damage of irradiation nonuniformity is mitigated by thermal smoothing in this conduction zone. Heat flow, tangen- tial to the target surface, attenuates perturbation amplitudes before they imprint themselves onto the target surface, whereupon they seed hydrodynamic instabilities.
Epperlein [5] showed that nonlocal suppression of the heat flux tangential to the target surface may severely reduce the thermal smoothing efficiency relative to local models. However, in this transverse direction, nonlocal suppression of the heat flux also competes with the magnetised Righi-Leduc heat flow [6]. Furthermore, nonlocality modifies the thermal response time of the plasma [7], which becomes pertinent when considering perturbations seeded by rapidly varying laser speckles of an optically smoothed beam [8]. In this thesis, I study the effects of how nonlocal modification of the lateral transport, the temporal response and magnetised transport may combine and interplay to influence thermal smoothing.
Two dimensional Vlasov-Fokker-Planck simulations of the conduction zone of planar ablating targets are performed. These simulations are novel in the inclusion of both ablation and mag- netic field within a kinetic study of thermal smoothing. The key result is that magnetic fields may significantly reduce thermal smoothing efficiency. A novel phase inversion is discovered, caused by the magnetised Righi-Leduc heat flow. It is shown that magnetised transport effects become important even for small Hall parameters (ωτ <0.05).
The thermal smoothing of a time evolving speckle-like perturbation is investigated. Self- generated magnetic fields are found to significantly affect the plasma temporal response. The perturbation correlation time may grow to more than double the laser heating value as a result of magnetic field build-up within the conduction zone.Open Acces
