Separatrix sweeping by in-vessel coils on a fusion reactor

Abstract

The heat flux onto divertor plates has increased with the progress of tokamak fusion research and development. In the International Thermonuclear Experimental Reactor (ITER) operating conditions, the peak heat fluxes onto the divertor plate have been estimated to be 15–30 MW/m2. It has been evaluated that it is marginal to operate the divertor plate with a peak heat flux of 15 MW/m2 and very difficult with higher heat fluxes under static conditions. Although a separatrix sweeping by superconducting (SC) poloidal field (PF) coils located outside toroidal field (TF) coils has been introduced during Conceptual Design Activities (CDA), the operation to keep the temperature under the maximum limit of the armor temperature, i.e., 1000°C in terms of erosion due to sputtering and sublimation, is still difficult for peak heat fluxes exceeding about 20 MW/m2, because the sweeping condition such as the moving range of null point and sweeping frequency is restricted to be below about ±0.75 Hz·cm by induced AC losses in cryogenic structures. Therefore, we have newly proposed an in-vessel sweeping coil system for faster sweeping frequency. A higher sweeping frequency of 1 Hz is able to keep the maximum armor temperature below 1000°C for a heat flux on the divertor plate of 30 MW/m2, where the induced AC loss is only half of the acceptable level. When the operational plasma current is substantially below 22 MA, a sweeping frequency of 4 Hz may be allowed within the acceptable AC loss. The structural feasibility of the in-vessel sweeping coil can be obtained by introducing a saddle-loop coil.