Parameter dependences of small edge localized modes (ELMs)

The development of small edge localized mode (ELM) scenarios is important in order to reduce the strain on plasma facing components. One such scenario can be found at high densities, in highly shaped, close to double-null plasmas in ASDEX Upgrade, showing small ELMs characterized by a frequency fELM > 300 Hz and a low power loss. Changing from gas fuelling to pellet fuelling results in edge profiles in which the collisionality of pedestal top and separatrix are decoupled. While the pedestal top values remain unchanged, only the phases with low separatrix and scrape-off layer (SOL) density show large ELMs with small ELMs in between. In phases with high separatrix density the small ELMs increase in amplitude and large ELMs do not occur. Similarly, a change in vertical plasma position by only ∼2 cm downwards, at constant ne,sep reduces the size of small ELMs while the large ELMs appear more intense. A possible explanation of this behaviour could be the influence of the drive and the stabilization of modes positioned close to the separatrix. When these small ELM modes cause enough transport, they flatten the gradient region around the separatrix and thereby consequently narrow the effective pedestal width. Because a narrower pedestal is more stable against global PB modes, the stability boundary is shifted towards higher pressure gradients and type-I ELMs do not occur. It is shown that a higher ne,sep increases the amplitude of small ELMs and, in agreement with basic ballooning mode theory, a higher local magnetic shear reduces their amplitude.