Heavy-Ion Fusion Targets with 'Diffuse' Spherical Radiation Converter

Spherically symmetric targets for indirect-drive heavy-ion fusion are studied, in which the fusion capsule is enclosed in a low-density thick spherical shell, where the ion beams are stopped and their energy is converted into thermal radiation. The thermal radiation then drives the implosion of the fusion capsule, with mininum hydrodynamic coupling between the energy deposition region and the ablation layer. The conditions for effective hydrodynamic decoupling have been derived. It is found that with the use of heavy ions with energy about or below 8 GeV, the beam-to-fuel energy coupling efficiency can be as large as in foreseen conventional hohlraums. On the other hand, these targets only allow for a low dynamic range of pulse shaping, which results in rather poor entropy shaping and modest fuel gain. Robust targets have been designed, which achieve energy gain G approximate to 30, when driven by shaped pulses of 12.5 MJ of 8.5 GeV Bi ions. Inclusion of a high-density pusher, which increases the fuel compression leads to higher gain at lower beam energy, but two-dimensional simulations demonstrate the extremely violent instability of the fuel-pusher interface.