Effect of ELMs on rotation and momentum confinement in H-mode discharges in JET

The loss of plasma toroidal angular momentum and thermal energy by edge localized modes (ELMs) has been studied in JET. The analysis shows a consistently larger drop in momentum in comparison with the energy loss associated with the ELMs. This difference originates from the large reduction in angular frequency at the plasma edge, observed to penetrate into the plasma up to r/a ~ 0.65 during large type-I ELMs. As a result, the time averaged angular frequency is lowered near the top of the pedestal with increasing ELM frequency, resulting in a significant drop in thermal Mach number at the edge. An increase in profile peaking of ion temperature and angular frequency is observed. At the same time the plasma confinement is reduced while the ratio of confinement times (R_τ = τ_E/τ) increases noticeably with ELM frequency. This change could be explained by the relatively larger ELM induced losses for momentum in combination with the observed longer build-up time for the momentum density at the plasma edge.