Efficiency and symmetry of heavy ion driven inertial fusion hohlraum targets

Abstract

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 scheme. These problems are interrelated and it is possible that when we try to improve one of these factors, the others might be affected in an adverse manner. For example, by using a sufficiently large hohlraum-to-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 number of radiation shields at suitable locations in the hohlraum, a very high degree of symmetry (of the order of 99%) may be achieved, but the transfer efficiency may become as low as 15% or even less. It is therefore extremely important to optimize these effects simultaneously to design an efficient indirect drive target. In this paper we study the above problems in a self-consistent manner using two-dimensional numerical 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 converters made of low-density solid gold are placed symmetrically around the capsule. In addition, a number of radiation shields have been placed at appropriate places inside the hohlraum. It has been found that this design leads to an 80 - 90% conversion efficiency, 15 - 20% transfer efficiency and symmetry of the radiation field.