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Description
A limiter is a plasma-facing component in a fusion reactor that protects blanket modules from excessive surface heat load and high-energy particles interaction. It achieves this by protruding from the first wall (FW), effectively shadowing the blanket. The limiter extends continuously in the poloidal direction, except near the divertor, and is installed in each 90-degree toroidal segment. To withstand high neutron irradiation, its plasma-facing surface uses tungsten monoblocks (W-MBs) with reduced activation ferritic/martensitic (RAFM) steel cooling pipes. However, RAFM’s low thermal conductivity limits the heat removal to $4.6\ MW/m^2$, significantly lower than ITER divertor T-MBs with copper alloy pipes.
FW surface heat load is a critical design factor. Excess localized heat must be avoided, making heat load evaluation and surface shape optimization essential. The incident angle of magnetic field lines intersect the FW strongly influences charged particle heat flux. A simple constant-curvature surface can cause localized high heat flux. To address this, a simulation code was developed using a decay length approach to evaluate heat flux along magnetic field lines, incorporating connection length for particle shadowing. Based on plasma operation scenario, simulations indicate that a 30 mm limiter protrusion effectively shadows the blanket. The limiter surface shape was optimized theoretically to achieve a uniform and wide heat load distribution. The peak heat flux on the limiter during flat-top phase is calculated to be $2.4 MW/m^2$, fulfilling the cooling capability.
Further design development included coolant flow path and remote maintenance strategies. Component lifetime is another key factor, requiring erosion analysis of the plasma-facing surface. We picked up a set of reference parameters: electron temperature 5 eV, argon density $ 3 \times 10^{15} /m^3 $ by extrapolating a divertor transport simulation result of JA DEMO. The estimated eroded depth per year was not significant; however, it can easily change due to the high sensitivity of sputtering yield to the electron temperature and the impurity transport. A sensitivity study on heat load and erosion by varying plasma parameters will be presented in detail. This work highlights the correlation between limiter shape, surface heat load and erosion characteristics.