Recirculating induction accelerators for inertial fusion: prospects and status

The USA is developing the physics and technology of induction accelerators for heavy ion beam-driven inertial fusion. The recirculating induction accelerator repeatedly passes beams through the same set of accelerating and focusing elements, thereby reducing both the length and gradient of the accelerator structure. This promises an attractive driver cost if the technical challenges associated with recirculation can be met. Point designs for recirculator drivers were developed in multiyear study by LLNL, LBNL and FM Technologies and that work is briefly reviewed here. To validate major elements of the recirculator concept, we are developing a small (4.5 m diameter) prototype recirculator which will accelerate a space-charge-dominated beam of K+ ions through 15 laps from 80 to 320 keV and from 2 to 8 mA. Transverse beam confinement is effected via permanent magnet quadrupoles; bending is via electric dipoles. This ‘small recirculator’ is being developed through a sequence of experiments. An injector, matching section and linear magnetic channel using seven half-lattice periods of permanent magnet quadrupole lenses are operational. A prototype recirculator half-lattice period is being fabricated. This paper outlines the research program and presents initial experimental results.