Lessons to be learned from the HIDIF-study for a full driver

The scope of the European Bi+ HIDIF study with its 3 MJ total driver energy was to demonstrate the feasibility of an RF linac and storage based scheme for ignition with an indirectly driven low gain target. More recent upgrade scenarios with at least 4.5 MJ total driver energy are focused on energy production and high gain targets. Keeping the linac beam current constant and increasing instead the number of storage rings from 12 to 18 will give the required 4.5 MJ total driver energy. Aiming for high total driver efficiency was not a topic for the ignition facility, however it is a very crucial item for a heavy ion fusion power plant. All RF linac components should be feasible for up to 50 Hz rep. rate in order to be cost effective for energy production. A total driver efficiency of 20% is only achievable at 50 Hz rep. rate. However, it is preferred to have the 20% driver efficiency at 10 Hz rep. rate, which is expected to be the maximal achievable rep. rate for a single target station. egant way to get the wanted 20% driver efficiency at 10 Hz rep. rate with 4.5 MJ total driver energy and 12 storage rings only, is to change the HIDIF layout from single charged Bi+ to double charged Bi2+ ions. The layout of a cost effective and less complex Bi2+ separated section main linac from 10 MeV/n on is outlined. Reduced numbers of large apertures, high gradient external quadrupoles, feasible up to 50 Hz rep. rate are used for focusing the high intensity beam. Monte Carlo simulations for a Bi2+ beam with 600 mA particle current are presented, demonstrating the feasibility of such a Bi2+ linac as injector for the storage rings. By adding more target stations later on, the full potential of a 50 Hz rep. rate Bi2+ fusion power plant can be realized.