Importance of helical pitch parameter in LHD-type heliotron reactor designs

Abstract

One of the key issues in the design of LHD-type heliotron reactors is to secure sufficient blanket spaces. In this respect, helical pitch parameter γ is quite important because it significantly affects both the coil and plasma shapes. For a quantitative understanding of the effect of helical pitch parameter on a design window, a system design code for LHD-type heliotron reactors HELIOSCOPE (HELIOtron System design COde for reactor Performance Evaluation) is developed and parametric scans are carried out with three cases of γ = 1.15, 1.20 and 1.25. It became clear that design windows of heliotron reactors depend significantly on the engineering constraints: the stored magnetic energy of the coil system, the inboard minimum blanket space and the averaged neutron wall load. In the case of a fusion power of 3 GW, γ = 1.20 is optimum for relaxing physics requirements. But γ = 1.15 is also a possible selection if a lower fusion power or a higher neutron wall load is accepted. Since design windows are quite sensitive to the engineering constraints and physics conditions, a further detailed study on design feasibility of advanced engineering components and the effect of γ on the physics conditions is expected to optimize the value of γ.