Impact of material-dependent energy reflection coefficient on divertor heat flux in the Large Helical Device

A divertor module comprising actively-cooled tungsten (W) and carbon (C) materials placed side-by-side was simultaneously exposed to the divertor plasma in the Large Helical Device (LHD) to experimentally observe the difference in energy reflection coefficients, R iE . The divertor temperatures were measured using infrared (IR) thermography and embedded thermocouples (TCs) and were converted into heat fluxes using the finite element method (FEM). The emissivity of the W surface was evaluated using a calibration chamber. This study demonstrated that combining TC measurements and IR thermography with FEM analysis enabled accurate evaluation of the heat flux on the W even in low-temperature regions, where IR thermography is susceptible to reflection-induced overestimation. The experiment showed that the peak heat flux on the C surface reached approximately 10 MWm − 2 , while that on the W surface was ∼ 75% of this value. Furthermore, the theoretical heat fluxes were estimated using plasma parameters obtained from Langmuir probes, combined with R iE from an empirical formula. The observed ratio of heat fluxes between W and C was in good agreement with theoretical predictions based on material-dependent R iE . These results quantitatively demonstrate that the higher R iE of W can effectively reduce the absorbed heat flux by 20 to 30% compared to that with C. This finding offers important insights for the selection of plasma-facing materials and the optimization of cooling system designs. • In LHD, actively cooled W and C divertors were simultaneously exposed to divertor plasma. • IR thermography and FEM show W absorbs ∼ 75% of heat load on C, matching sheath theory. • Higher R iE of W cuts absorbed heat by 20–30%, guiding future divertor material selection.