Optimized Driver-to-Target Coupling in Direct Drive Heavy Ion Fusion

Recent theoretical work on fusion energy targets driven directly by heavy ion beams has indicated a very efficient coupling of beam energy into the kinetic energy of the ingoing fuel. As ablation proceeds and the areal density of the plasma corona rises, increasing the range of the ion beam particles can nevertheless keep the energy deposition close to the ablation front. This should increase the strengths of the shocks compressing and accelerating the fuel mass, resulting in superior coupling for a given integrated drive energy. We have performed 1-D implosion calculations with the arbitrary Lagrangian Eulerian code HYDRA that explore the effects of tuning the pulse shapes in ion energy and current. These sweeps in parameter space were conducted using a single reactor-grade (gain ~ 100 at 50 MJ yield), pure-DT spherical target driven by mercury ion beams. With a payload fraction of 1/3, this low-aspect ratio target should operate near the peak of rocket efficiency and may be robust to hydrodynamic instabilities. The pulse profiles optimizing coupling efficiency will be presented.