时间：周四，12月5日，14:00
地点：海纳苑8幢323教室
报告人：孟祥地
题目：External Kink Modes and Edge Localized Modes with SOL in Diverted Tokamaks
摘要：This paper mainly investigates external kink modes in a diverted tokamak using the initial-value code CLTx and the free-boundary equilibrium solver CLT-EQ. We initially focus on the low-n (2≤n≤5) peeling mode, analyzing its characteristics in the presence of the X-point and the scrape-off layer (SOL). The simulation results exhibit a certain degree of consistency with the ideal external kink theory in the limiter configuration. Furthermore, the presence of the X-point can trigger the n=1 peeling-tearing mode when considering a non-zero edge current profile and the Spitzer resistivity model. Simulation results exhibit a favorable agreement with the experimental observations of external kink modes as the current in a tokamak is ramped up, and q_95 decreases through rational values. Remarkably, our simulation results demonstrate that the presence of the scrape-off layer (SOL), characterized by high resistivity and low-density properties, effectively absorbs perturbations close to the separatrix. This observation raises the potential for stabilizing the unstable external mode. In addition to external kink modes, we investigated the nonlinear stages of edge-localized modes (ELMs) in the HL-2A and HL-3 configurations. The radial transport of edge density and energy is consistent with previous simulations, where small-scale bubble structures are observed. Magnetic field stochasticity appears near the X-point, and strong parallel particle and energy transport leads to heat loads in the divertor region, aligning with experimental observations. Additionally, we found that compared to simulations with discrete toroidal mode numbers (n=5, 10, 15), nonlinear simulations with continuous multimode coupling drive ELMs into a strong turbulent phase, significantly enhancing the radial transport of particles and energy.
Finally, we investigated the effects of toroidal field ripple on the linear phase of high-n instability modes in the EAST configuration. The presence of ripple significantly alters the toroidal magnetic field and poloidal current, potentially disrupting force balance and reconstructing the two-dimensional equilibrium. Periodic structures emerge in the scrape-off layer (SOL) on the outer side of the pedestal region, which may open radial transport channels. A scan over the toroidal mode number n demonstrates that ripple has a pronounced impact on the growth rate of high-n modes, while their influence on medium- and low-n modes is negligible. Additionally, we observed that under different ripple field periodicities, mode structures and ELMs exhibit two distinct patterns: in-phase and out-of-phase. It is shown that out-of-phase coupling enhances instability, whereas in-phase coupling exerts a stabilizing effect.
