Energy and particle losses during type-I ELMy H-mode in ASDEX Upgrade

Abstract

Characteristics of edge-localized mode (ELM) energy and particle losses in type-I ELMy H-mode plasmas on ASDEX Upgrade are investigated by a large data set with as much as possible independent variations in engineering parameters. A statistically unbiased estimate for ELM energy and particle losses has been employed to remove the diagnostic noise as much as possible. The data set shows that the ELM energy loss decreases with the pedestal density or collisionality. However, the role of plasma shape, especially, triangularity, as an explicit parameter is revealed. Elevated triangularity leads to lower ELM frequency and larger ELM energy loss significantly exceeding that simply expected from the increased pedestal pressure in high triangularity plasmas at a fixed power. The observed larger ELM energy drop at higher triangularity involves the ELM perturbations of the electron temperature profile across an ELM that extend radially more inward, suggesting that there is a direct effect of plasma shape on ELM energy losses. It is found that the fraction of ELM loss power does not remain constant but the increased pedestal collisionality enhances the transport level between ELMs and reduces the ELM loss power. The ELM particle flux at fixed power rises with increasing gas fuelling rate. On the other hand, at fixed gas puff, an inverse proportionality between ELM frequency and ELM particle loss roughly holds despite of a large variation of power. When the particle flux near the separatrix is enhanced, the increase of ELM frequency and the reduction of ELM loss power caused by the increased collisionality lead to the reduction of ELM energy loss so that the energy balance could be sustained.