Implosion of cone-in-shell targets for direct-drive fast ignition

The laser direct-drive fast ignition scheme is a candidate for inertial fusion energy reactors. The scheme promises higher fusion gains (fusion energy divided by laser energy) than with conventional central hot-spot ignition. For a particle beam (electrons or ions) to reach and heat the compressed fuel, a reentrant cone is mounted in the fuel shell. The cone, which is made from higher Z material, has a significant impact on the implosion of the fuel shell. This work presents two-dimensional radiation hydrodynamic simulations with the flash code of the laser direct-drive compression of reactor-scale cone-in-shell targets with 2.7mg of deuterium-tritium fuel. The simulations highlight the different stages of the implosion leading to a nonspherical fuel shape at stagnation. The implosion also significantly affects the cone tip and the extreme high pressure in the hot spot creates a strong shock wave that crushes the cone tip. A comprehensive parameter study is presented with different cone geometries, different cone materials (gold, copper, and aluminum) and laser drive asymmetry to study the fuel assembly in front of the cone tip and the shock formation in the cone tip material. For the reactor-scale capsule design we analyzed, we find that a 30^{∘} half cone angle made from copper, and a wall thickness of about 200µm provides a good trade-off in terms of fuel assembly, cone tip survivability, and minimization of travel distance for the particle beam through the dense plasma. The results indicate that it is feasible to compress fuel to the required density of 400g/cm^{3} using a laser pulse energy of approximately 1.5MJ. This opens the potential for achieving high energy gains with moderate pulse energies, envisioning massive energy production through laser fusion.