Fast Electron Transport and Spatial Energy Deposition in Cu-doped Fast Ignition Plasmas

Fast Ignition (FI) is a form of Inertial Confinement Fusion where the compression phase and the ignition phase are separated. In this scheme, a radially symmetric configuration of driver beams composed of either direct laser illumination or laser produced x-ray radiation are used to isochorically compress the fuel shell to ̃300g/cc. Once the fuel is assembled, a high-intensity ignition beam ("short pulse'') is used to generate relativistic electrons which then transport to the assembled fuel and deposit their energy which then causes it to ignite, initiating a thermonuclear burn wave to propagate throughout the rest of the assembled fuel. Understanding relativistic electron generation and transport is extremely important for the development and success of the FI scheme. Previous integrated FI experiments measured neutron yield enhancements to infer increased energy delivered to the compressed core by relativistic electrons generated from the short-pulse laser. While this method has demonstrated enhancements in core heating, the exact location of neutron generation continued to be model dependent and the trajectory of these relativistic electrons was not investigated, leaving questions about the processes that influence energy coupling. In this work, first-ever experimental observations of the spatial energy deposition of cone-guided relativistic electrons into an imploded FI plasma core are reported. Utilizing a new experimental platform that has been developed on the OMEGA Laser Facility, the spatial energy deposition of these relativistic electrons was characterized via relativistic electron induced K-alpha fluorescence from a copper tracer added to a deuterated plastic FI shell. Two-dimensional images of the copper K-alpha fluorescence were obtained using a spherically bent Bragg crystal. The data show Cu K -alpha emission from a 300 micron region surrounding the cone tip, correlating well with the predicted core size. Also, copper K-alpha emission was seen to be produced away from the cone tip, along the cone walls, indicative of a large pre-formed plasma within the cone as a result of the OMEGA-EP pedestal pulse. To validate experimental findings, relativistic transport simulations were carried out utilizing a retrograde analysis of the relativistic electron conversion efficiency, divergence, generation position and temperature. The simulated copper K-alpha spatial distribution was then compared with experimental findings to examine the sensitivity each of these parameters on the short pulse energy coupling efficiency to the assembled core. These findings helped facilitate new target designs and implosion dynamics which resulted in a factor of four improvement to the energy coupling and also defines a clear path towards high coupling efficiencies on large-scale laser facilitie