Theoretical Study On The Stability Of Resistive Wall Modes In Advanced Tokamak Scenarios

The advanced tokamak(AT)scenarios are now considered to be the most promising for future reactor-scale tokamaks,such as ITER(under construction),CFETR and DEMO(under design).By improving the current profile control,strong reversed magnetic field configuration is obtained together with high bootstrap current fraction and low extra current drive requirement.It is for these reasons good energy confinement and economical high fusion performance steady-state operations are achieved in AT scenarios.However,highly peaked pressure gradients and large off-axis bootstrap currents,could bring new physics issues to be investigated and understood to the ideal MHD stability of resistive wall modes(RWMs).Complementing the ITER science and engineering technology and bridging the gap to the future fusion demonstration power plant DEMO,the Chinese Fusion Engineering Testing Reactor(CFETR)is the next generation device in the Chinese Magnetic Confinement Fusion(MCF)roadmap.The designed fusion power of CFETR is up to 2GW.The research and analysis on RWMs for CFETR AT scenarios are of great importance to the physics design of CFETR.Provided that RWMs are controlled,the plasma parameter regime can be significantly improved and thus higher economic efficiency for CFETR.The ideal-magnetohydrodynamics(MHD)eigenvalue code AEGIS and nonideal MHD initial-value code NIMROD are employed in this study.The main points are as follows.First,the low n(toroidal number)ideal MHD instabilities in the CFETR Phase I 200MW baseline scenario are investigated using both AEGIS and NIMROD codes,along with the code benchmarking.Despite the distinctly different approaches in modeling the scrape off layer region,the dominant growth and mode structure in each of the low-n modes are consistent between the two codes.The higher-n modes are dominated by ballooning modes and localized in the pedestal region,while the lower-n modes have more prominent external kink components and broader mode profiles.The mode structure changes and the growth rate becomes larger as the toroidal number n increases.In addition,the effect of parameter(inculuding the density,resistivity and viscosity profiles and the wall model)fineness on the calculation are investigated and the results are summarized.For the resistive wall,the n=1 mode is found to be unstable.The toroidal rotation required for full suppression of RWM is determined to be 2.9%Alfvénic speed.Second,the RWM stability in the CFETR Phase Ⅱ 1GW steady-state operation scenario is studied using AEGIS code,along with a new physics issue,i.e.the infernal mode,coupled.Whereas in earlier advanced tokamak studies,more attention is drawn to the influence of the central safety factor reversal to lead to decrease in RWM βN limits,it is found that the pedestal flattening on the safety-factor profile(flat-q),caused by the unavoidable high fusion power and high bootstrap current fraction,would cause larger impact on RWM stability.The infernal components develop in both plasma central and edge regions.However,the edge infernal components take the dominate place.The βN limits become lower and consequently the CFETR 1GW SSO scenario is unstable to the n=1 RWM.In addition,the stabilization effects from wall are almost lost if the safety factor value at edge flat-q is close to a rational number.The infernal mode is also found to weaken the stabilizing effects of toroidal plasma rotation by testing different types of uniform and non-uniform rotation profiles.Due to the influence of the edge infernal components,the edge rotation become critical for the stabilization effects while the central rotation is less important.To fully suppress such edge-infernal-type RWM in the CFETR 1GW SSO scenario,the edge rotation speed needs to be maintained at above 1.5%Alfvenic speed.Third,the NIMROD simulations on RWMs are developed.The analysis on external kink stability for a circular cross-section cylinder plasma using NIMROD is compared with analytical solutions.Mode growth from NIMROD calculation is found sensitive to location and profile of plasma-vacuum interface.Employing the ideal-like MHD model,the NIMROD results are in good agreement with analytical theory.The RWM simulations are then implemented in toroidal geometry.In toroidal geometry,the GS shift in equilibrium lead to in-and out-board asymmetry of flux surfaces,which are also found in the calculated RWM mode structures.RWM growth becomes enhanced for small aspect ratio,while the enhancement gradually reduces as aspect ratio increases.The NIMROD simulation further verifies the presence of edge infernal components in the n=1 external kink mode for the CFETR 1GW SSO scenario.This work prepares the foundation for future global MHD simulations on the CFETR RWMs.