Energetics and Symmetry of Hohlraum Targets Driven by Ion Beam Pulses with Simple Time Shape

The energetics of hohlraum targets for inertial fusion are studied by means of one-dimensional radiation hydrodynamics simulations, assuming that a pulse of thermal X-rays with a simple time shape is fed into the cavity. A fusion yield Efus = 160–250 MJ is released by a capsule with fuel mass mDT = 3.3 mg, driven by a two-step pulse. The required input energy is Ex ≈ 3.4 MJ for a hohlraum area ratio a = 9 and 6.6 MJ for a = 20, corresponding to gains of Gx = Efus/Ex = 50–73 and 25–35 respectively. Higher gains are obtained by three-step pulses. Targets with mDT = 0.4 mg require better-shaped pulses, with at least three steps. Driven by Ex = 0.85−1.7 MJ, they release Efus = 8–10 MJ. Symmetry aspects of axially symmetric hohlraums driven by heavy ion beams are studied by a viewfactor code, employing wall motion and re-emissivities provided by the one-dimensional hydro-simulations. The dependence of the capsule irradiation asymmetry on the hohlraum aspect ratio, area ratio and fill density is analyzed. Reductions of wall motion and converter expansion, and the use of shields appear necessary to allow for the use of a moderate area ratio a ≈ 10–15.