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Preceding study (Homma Y, Hoshino K, Tokunaga S, et al., 2018 Contrib. Plasma Phys. 58, 629-637) presented an advanced model for thermal force whose applicable condition is extended from collisional to relatively low collisional plasma. According to this extended model, the thermal force reduces when plasma collisionality decreases. In the present study, the extended thermal force model has been implemented into a SOL-divertor integrated simulation code SONIC, in order to study impact on impurity transport in DEMO-relevant Scrape-off layer (SOL) plasma, due to thermal force reduction. A set of test simulation has been carried out supposing a reference steady- state operation scenario of Japanese DEMO fusion reactor concept (JA DEMO). The thermal force has reduced by as much as 20-70%, reflecting relatively lower collisionality in DEMO SOL plasma. The simulation results have demonstrated that introduction of the collisionality dependence of thermal force leads to as much as 20-80% of effective decrease in impurity density and its content ratio widely over the SOL upstream area under DEMO relevant condition. Compared to the case with conventional thermal force model, relative change rate of impurity content at representative poloidal positions are as follows: low-eld side(LFS) X-point -59%; poloidal top area -22%; HFS upstream area up to -80%; no major impact around LFS upstream.

Power exhaust to the divertor and the conceptual design have been investigated for a steady-state DEMO in Japan with 1.5 GW-level fusion power and the major radius of 8.5 m, where the plasma parameters were revised appropriate for the impurity seeding scenario. A system code survey for the Ar impurity seeding suggested the volume-averaged density, impurity concentration and exhaust power from the main plasma of = 205–285 MW. The divertor plasma simulation (SONIC) was performed in the divertor leg length of 1.6 m with the fixed exhaust power to the edge of = 250 MW and the total radiation fraction at the edge, SOL and divertor ( = 0.8), as a first step to investigate appropriate design of the divertor size and geometry. At the outer target, partial detachment was produced near the strike-point, and the peak heat load () at the attached region was reduced to ~5 MW m−2 with appropriate fuel and impurity puff rates. At the inner divertor target, full detachment of ion flux was produced and the peak was less than 10 MW m−2 mostly due to the surface-recombination. These results showed a power exhaust scenario and the divertor design concept. An integrated design of the water-cooling heat sink for the long leg divertor was proposed. Cu-ally (CuCrZr) cooling pipe was applicable as the heat sink to handle the high heat flux near the strike-point, where displacements per atom rate was estimated to be 0.5–1.5 per year by neutronics calculation. An arrangement of the coolant rooting for Cu-alloy and Reduced Activation Ferritic Martensitic (RAFM) steel (F82H) pipes in a divertor cassette was investigated, and the heat transport analysis of the W-monoblock and Cu-alloy pipe under the peak of 10 MWm−2 and nuclear heating was performed. The maximum temperatures on the W-surface and Cu-alloy pipe were 1021 and 331 °C. Heat flux of 16 MW m−2 was distributed in the major part of the coolant pipe. These results were acceptable for the plasma facing and structural materials.

Power exhaust for a 3?GW class fusion reactor with an ITER-sized plasma was investigated by enhancing the radiation loss from seeding impurity. The impurity transport and plasma detachment were simulated under the Demo divertor condition using an integrated divertor code SONIC, in which the impurity Monte-Carlo code, IMPMC, can handle most kinetic effects on the impurity ions in the original formula. The simulation results of impurity species from low Z (neon) to high Z (krypton) and divertor length with a plasma exhausted power of 500?MW and radiation loss of 460?MW, and a fixed core?edge boundary of 7???1019?m?3 were investigated at the first stage for the Demo divertor operation scenario and the geometry design. Results for the different seeding impurities showed that the total heat load, including the plasma transport and radiation , was reduced from 15?16?MW?m?2 (Ne and Ar) to 11?MW?m?2 for the higher Z (Kr), and extended over a wide area accompanied by increasing impurity recycling. The geometry effect of the long-leg divertor showed that full detachment was obtained, and the peak qtarget value was decreased to 12?MW?m?2, where neutral heat load became comparable to and due to smaller flux expansion. Fuel dilution was reduced but was still at a high level. These results showed that a divertor design with a long leg with higher Z seeding such as Ar and Kr is not fulfilled, but will be appropriate to obtain the divertor scenario for the Demo divertor. Finally, influences of ? and D? enhancement were seen significantly in the divertor, i.e. the radiation and density profiles became wider, leading to full detachment. Both qtarget near the separatrix and Te at the outer flux surfaces were decreased to a level for the conventional technology design. On the other hand, the problem of fuel dilution became worse. Extrapolation of the plasma transport coefficients to ITER and Demo, where density and temperature will be higher than ITER and edge-localized modes are mitigated, is a key issue for the divertor design.

The mechanism of internal transport barrier (ITB) collapse in the reversed magnetic shear configuration is investigated using a global ion temperature gradient (ITG) driven drift wave turbulence code. A heating source and a toroidal momentum source are introduced to follow the self-consistent evolution of the ion temperature and flow profiles. A scenario of transport barrier collapse driven by a meso-scale mode excited in the barrier region is suggested. The importance of the quasi-linear effect due to profile modification as well as three-wave coupling is clearly shown by means of energy transfer analysis. The effect of the toroidal flow shear (TFS) profile on the dynamics of ITB evolution is investigated. It is found that the decorrelation between meso-scale modes and ITG driven modes due to the TFS can prevent global relaxation.