I-mode plasma confinement improvement by real-time lithium injection and its classification on EAST tokamak

Abstract

I-mode is a promising regime for future fusion reactors due to the high energy confinement and the moderate particle confinement. However, the effect of lithium, which has been widely applied for particle recycling and impurity control, on I-mode plasma is still unclear. Recently, experiments of real-time lithium powder injection on I-mode plasma have been carried out in EAST Tokamak. It was found that the confinement performance of the I-mode can be improved by the lithium powder injection, which can strongly reduce electron turbulence (ET) and then trigger ion turbulence (IT). And it was observed that the ET intensity is inversely proportional to the velocity shear, which suggests that the injection of lithium powder leads to a gradual enhancement of the shear flow, whereby the turbulence is reduced and consequently the confinement is improved. Four different regimes of I-mode have been identified in EAST. The Type I I-mode plasma is characterized by the weakly coherent mode (WCM) and the geodesic-acoustic mode (GAM). The Type II I-mode is featured as the WCM and the edge temperature ring oscillation (ETRO). The Type III I-mode corresponds to the plasma with the co-existence of ETRO, GAM, and WCM. The Type IV I-mode denotes the plasma with only WCM but without ETRO and GAM. It was observed that the WCM intensity is increased with lithium powder injection by the confinement improvement/pedestal temperature increase. EAST experiments demonstrate that lithium powder injection is an effective tool for real-time control and confinement improvement of I-mode plasma.

Keywords

• confinement improvement
• I-mode classification
• lithium injection
• turbulence