Edge-Localized Modes Explained as the Amplification of Scrape-Off-Layer Current Coupling

It is shown that the edge-localized modes (ELMs) observed in tokamak $H$ mode discharges can be explained as external magnetohydrodynamic (MHD) mode amplification due to coupling with scrape-off-layer current. The proposed model offers a new ELM mechanism that produces a sharp onset and initial fast growth of magnetic perturbations even when the underlying equilibrium is only marginally unstable for a MHD mode and also a quick quenching after the bursting peak. The theory also reproduces various other ELM features.

Figure 1

Numerical results for ELMs. The first row describes the pedestal temperature evolution with time, the second and third give, respectively, the ion temperature and the ion or electron density in the hot divertor sheath, the forth is the SOL current, and the last gives the radial magnetic field $\delta B_r$. The right column is the high-time-resolution redisplay of the first ELM pulse in the left column.