Heat flux reduction by helical divertor coils in the heliotron fusion energy reactor

To best utilize the built-in helical divertors in the heliotron-type fusion energy reactor, we propose a new divertor sweeping scheme that reduces both the divertor heat flux and erosion of the divertor plates. This scheme employs a small set of helical coils, which we term helical divertor coils. The divertor legs can be moved by modulating the current amplitude of helical divertor coils by a few per cent of the current amplitude of the main helical coils. Despite the movement of the divertor legs, this scheme changes the magnetic surfaces very little. The strike point width is increased to ∼800 mm and rapid sweeping reduces the time-averaged heat flux to a <1 MW m-2 level with a total power flow of ∼600 MW to the divertor regions for a fusion power of 3 GW. Divertor plate erosion is reduced, enabling the replacement cycle to be significantly prolonged. We propose that the helical divertor coils be fabricated using YBCO high-temperature superconductors and be constructed in prefabricated segments that are joined on site.