In the future fusion reactor, huge power has to be exhausted through the divertor due to the high fusion power. Therefore, it is critical to find an appropriate way to reduce the heat load onto divertor target, which has an engineering limit of 10 MW/m2. In snowflake configuration , second order null is introduced to increase the flux expansion as well as connection length, which benefits the wetted area and the radiation volume. However, due to the limit on the ability of the coils, in China Fusion Engineering Test Reactor (CFETR), it is hard to achieve the exact/near-exact snowflake configuration. Instead, by introducing additional divertor coil, a quasi-snowflake configuration can be achieved, where the flux expansion can be increased by a factor of ~2 compared with the single-null configuration, even the second null is still at a distance from the X point . On the other hand, it is also an effective way to dissipate the heat power in the scrape-off layer (SOL) by radiation. For CFETR, due to the consideration of tritium retention and wall sputtering issues, full tungsten wall is proposed, which means impurities of high radiation efficiency have to be seeded to form a radiative divertor. It is natural to combine the quasi-snowflake configuration and impurity radiation together to find the effective solution of power exhaust. However, it is still not well understood the influence of flux expansion on the divertor physics, such as detachment, impurity screening, and so on. Simulation study is performed for the radiative quasi-snowflake divertor using SOLPS . By varying the deuterium and impurity puffing rate, the boundary plasma with different radiation fraction is simulated for different upstream density. It is found, for the low density case, due to the influence of flux expansion in quasi-snowflake divertor, the outer divertor is detached earlier than the inner divertor, together with a strong impurity compression in the outer divertor. For the high density case, the detachment is more symmetry for inner and outer divertor.
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 M.Y. Ye et al., Nucl. Fusion, doi: 10.1088/1741-4326/ab2bd0, 2019.
|Country or International Organization||China, People’s Republic|