EFFICIENCY AND SYMMETRY OF HEAVY-ION DRIVEN INERTIAL FUSION HOHLRAUM TARGETS

High conversion efficiency, a high degree of radiation symmetry and a large transfer efficiency onto the capsule surface are among the most important issues associated with an indirect drive inertial fusion sch eme. These problems are interrelated and it is possible that when we t ry to improve one of these factors, the others might be affected in an adverse manner. For example, by using a sufficiently large hohlraum-t o-capsule radii ratio (of the order of 4-5), or by keeping this ratio reasonably small (of the order of 3) and introducing an appropriate nu mber of radiation shields at suitable locations in the hohlraum, a ver y high degree of symmetry (of the order of 99%) may be achieved, but t he transfer efficiency may become as low as 15% or even less. It is th erefore extremely important to optimize these effects simultaneously t o design an efficient indirect drive target. In this paper we study th e above problems in a self-consistent manner using two-dimensional num erical simulation. The problem considered is three-dimensional with an axial symmetry. The hohlraum is an ellipsoid made of solid gold with a spherical capsule at the centre. Two identical cylindrical converter s made of low-density solid gold are placed symmetrically around the c apsule. In addition, a number of radiation shields have been placed at appropriate places inside the hohlraum. It has been found that this d esign leads to an 80-90% conversion efficiency, 15-20% transfer effici ency and similar to 98% symmetry of the radiation field.