Effect of ELMs on Rotation and Momentum Confinement in H-mode Discharges at JET
Momentum transport is an active area of research in many tokamaks due to the role of rotational shear in turbulence suppression and global plasma confinement. However, at present it is not well understood how plasma rotation profiles are affected by the operating conditions. A raw linear scaling between global energy and momentum confinement has been observed in many devices, including ASDEX, DIII-D and JET, although the presence of a large scatter suggests a difference in their local behaviour. A major influence on the confinement in H-mode discharges is observed during Edge Localised Modes (ELMs) in which the repetitive collapse of the pedestal causes an ejection of energetic particles from the plasma. In this process, both kinetic energy and angular momentum can be lost. In database analysis at JET it has been observed that the rotation velocity is reduced with higher ELM frequency. Furthermore, studies of ELM associated energy losses in multiple devices show energy losses in the order of 5-20%. However, the magnitude of the momentum losses are not well known and might not necessarily be of similar magnitude. For the better understanding of the observed momentum confinement it is important to investigate the momentum loss and the influence on the plasma rotation profiles. In this paper the measured changes in plasma profiles of angular frequency (w = vj/R), temperature (Ti, Te) and density (ne) will be discussed in order to assess the momentum and thermal energy losses. The confinement characteristics of several type-I ELMy H-modes are presented. The ELM induced losses are estimated while taking the measurement capabilities into consideration, in order to quantify the observed variations in confinement.