Structure design of poloidal horseshoe limiter for pulse operation heat load in JA DEMO

In JA DEMO, a poloidal horseshoe limiter has been developed to protect the breeding blanket from excessive heat loads during plasma operation. The limiter protrudes from the first wall and intercepts charged particle heat load along magnetic field lines. Because this heat load is highly localized and sensitive to magnetic equilibrium, the heat load on limiter was evaluated at representative time points using magnetic field line tracing with the APPLE code. During ramp-up, plasma first touch the inboard limiter, and the peak heat flux reaches 2.8 MW/m² at 27 s after plasma initiation. Owing to the transition from limiter to divertor configuration of the plasma, the heat flux shows spike-like temporal behaviour in this phase. During flat-top, the heat flux evolves more gradually and reaches a peak heat flux of 1.9 MW/m² at 800 s. Based on this heat load scenario, time-dependent coupled thermal-structural analyses were performed for tungsten-monoblock plasma-facing units with copper or titanium interlayers and an F82H reduced activation ferritic/martensitic steel cooling pipe. The results showed that the interlayer material strongly affects temperature, stress, and plastic deformation. Although copper provides high thermal conductivity, its large thermal expansion causes higher von Mises stress and greater plastic deformation, particularly at the inner surface of the F82H pipe. In contrast, titanium reduces the stress by 52%, maintains elastic deformation in the pipe, and improves the structural integrity of the pressure boundary, indicating its promise for JA DEMO limiter applications.