Hot Electron Generation and Transport in Fast Ignition Relevant Plasmas

This thesis presents a mixture of theoretical work and experimental results relating to the generation and transport of relativistic electrons in fast ignition inertial confinement fusion.

First the theoretical work is presented, which focuses on the effect that a fast electron beam has on a background plasma. The fast electron beam drives a resistive return current in the plasma, which causes Ohmic heating, leading to a pressure gradient, and a $J \times B$ force. Both of these would be expected to cause cavitation in the background plasma. In this work an analytic model has been developed which shows that the pressure gradient is the dominant force, and predicts the significance of cavitation over a range of parameters relevant to fast ignition fusion. In addition to this the timescale on which shocks can form is considered. This work was verified by the development of a one dimensional fluid code which included the effects of a resistive return current, and was used to model shock formation when the cavitation in the plasma is strong. Some results from a two dimensional version of the code are also presented.

The experimental work in this thesis focuses on an experiment which looked at the interaction of a high-powered laser with gold cone targets, similar to those that would be used in cone-guided fast ignition schemes. In this experiment, the effect of defocusing the laser upon the production of hot electrons was investigated. A copper wire was attached to the cones to act as a diagnostic for the hot electrons. A ray-tracing code was developed to better understand the change in intensity inside the cone when the laser is defocused. The results of this experiment demonstrate that the energy coupling of the laser into hot electrons is maintained when defocusing, while the spectrum of the hot electrons softens.