Split and Segmented-Type Helical Coils for the Heliotron Fusion Energy Reactor

Abstract

Configuration optimization is carried out for the heliotron-type fusion energy reactor FFHR. One of the important issues is to find sufficient clearances between the ergodic region outside the nested magnetic surfaces and blankets at the inboard side of the torus so that direct losses of alpha particles are minimized and the heat flux on the first walls is reduced. The latest design has a fairly large major radius Rc ∼ 17 m of the helical coils in order to satisfy this condition. It has been found, as an alternative design, that equivalent clearances are obtained with Rc = 15 m by employing a lower helical pitch parameter and splitting the helical coils in the poloidal cross-section at the outboard side. Furthermore, splitting the helical coils provides another modified configuration at Rc ∼ 17 m that ensures magnetic well formation in the fairly large nested magnetic surfaces with outward shifted configurations. From the engineering viewpoint, we propose that such helical coils be constructed by prefabricating half-pitch segments using high-temperature superconductors; the segments are then to be assembled on site with joints.