Speaker
Description
Plasma-wall interaction (PWI) during steady phases and transient events such as disruptions and edge localized modes (ELMs) is a critical issue in the magnetically confined plasmas, mainly because the particles and heat fluxes from plasmas need to be controlled to prevent damage to the plasma facing components (PFCs), as well as the impact of impurity influx on the main plasma performance should be mitigated. An extreme-wide angle view diagnostic with field of view (FOV) of 〖120〗^°, allowing us to observe the inner and outer walls, the divertor region, the lower hybrid current drive (LHCD) antenna and neutral beam injection (NBI) port in the visible range, was installed on the HL-3 tokamak for PWI investigation. New insights into the physical processes in the plasma edge are gained with this diagnostic by measuring the 3D emission profiles in the main chamber as well as in the divertor region. In the paper, hot spots during the PWI induced by the bulges on the divertor graphite tiles and exposed tile edges were experimentally observed, which causes the increase of impurity influxes. Bright objects (UFOs) moving through the plasma and occasionally inducing major disruption are recorded, together with the magnetic field lines development by their luminous trajectories. The online boron powder injection from the top of vacuum vessel was implemented in order to improve the wall conditions, where the interactions between the plasma edge and puffed boron powders are observed. That the boron gathering and glowing in plasma edge probably by the impurity screening mechanism and the almost simultaneous radiation (mainly the bremsstrahlung) enhancement in the plasma center because of the impurity pollution occurs. Actually, a low recycling level and high confinement phase was concurrently achieved by the online boron powder injection. The high spatial and temporal resolution imaging capability also leads to advances in the characterization of ELMs along with their impact on PFCs. During type-I ELMs, main interaction is on the lower divertor and upper baffle, indicating the enhanced particle recycling during the ELM pulse energy deposition. Zeff during one ELM event is increased, indicating the transiently increased impurity influx because of PWI. Nonetheless, Zeff is basically decreased during the H mode period, which demonstrates the impurities are expelled from the plasma by the type-I ELMs. The more the ELMs frequency is, the more impurity is expelled from the plasma.