Two dimensional simulations of a radiation driven target with two sided illumination for heavy ion fusion

The conventional configuration of a radiation driven target for heavy ion fusion is a quasi-cylindrical hohlraum containing a fusion capsule with radiation converters placed at opposite ends of the hohlraum. Ion beams enter each converter from opposite directions and are stopped by the material inside the converters. The first comprehensive two dimensional (2-D) simulations are presented for this configuration using the radiation hydrodynamics code LASNEX. The fusion capsule inside the hohlraum absorbs 1 MJ of radiation energy and can give a yield of 430 MJ if the illumination symmetry condition and the pulse shape requirements are satisfied. To drive this capsule, the total ion beam energy input into the hohlraum is about 22 MJ, which results in a target gain of only about 20. This yield is substantially lower than the previous estimate of greater than 80. The reason is that the initial length of the converters has to be reasonably long to prevent the hot beam stopping material from leaking into the hohlraum and causing unacceptable implosion asymmetry, without other target modifications. Furthermore, as the beam stopping material inside the converters is being heated, ii; expands substantially in the axial direction. This stretches the length of the converters, and, consequently, the ratio of opening area to volume of the converters decreases. These factors make it difficult for the radiation to escape from the converters and substantially reduce the efficiency of converting ion beam energy into radiation. It is concluded that the gain of this type of target is too low for commercial fusion and an alternative hohlraum configuration that can provide higher gain should therefore be sought.