Impacts of sawtooth crashes on tokamak plasmas in DEMOs

Self-consistent simulations of sawtooth oscillations on four DEMO (demonstration fusion power plant) designs, proposed by European, Indian, Chinese, and Korean teams, are carried out using the Porcelli sawtooth triggering model and the modified version of the Kadomtsev magnetic reconnection model coupled with the BALDUR integrated code. The simulation results suggest that all sawtooth crashes are triggered by the driving force for the internal kink instability overcoming the fast ion stabilization found in all DEMOs. Conversely, different expansions of the pre-crash helical flux are found, causing different mixing radii on each design. The mixing radius of the European DEMO is found to be the largest, resulting in the largest reduction of the core plasma temperature after the sawtooth crash. It is also observed that for all DEMOs except the Chinese DEMO, regular sawtooth crashes lead to an increase in the core plasma density and radiative power density in different levels, which are due to the steepness of the pre-crash hollow density profile. Furthermore, while helium is found to be outwardly transported by sawtooth crashes for all DEMOs, the sawteeth yield a penetration of the seeded impurities, remarkably found in the European design. Interestingly, based on the European design, a decrease in the atomic mass of the plasma fuels and seeded impurities are found to yield greater flatness of the pre-crash density profile and thus a smaller impact of sawtooth oscillations on central density. Additionally, the sawteeth degrade the plasma performance in the Chinese DEMO, but demonstrate positive impacts on the performance in the European, Indian, and Korean DEMOs, which results from the increase of the central ion density caused by the sawteeth. Furthermore, variation in neutral beam injection power exhibits both positive and negative correlations of sawtooth oscillation impacts on central ion density. The sensitivity of the mixing region of DEMOs is also investigated for scans where the main plasma parameters are varied.