Study on ELM burn-through induced by small ELMs with divertor detachment in EAST

Abstract

In the H-mode radiative divertor experiments, the transient heat flux induced by edge localized modes (ELMs) could cause the detached divertor plasma to re-attach in a few milliseconds, which is so-called ELM burn-through. The periodic hot plasma burning through the neutral front could bring significant damages to the divertor target and thus has attracted increasing attention. A systematic study on the burn-through of small ELMs during radiative divertor experiments has been conducted in the EAST tokamak. Firstly, the affected area of small ELMs on the divertor target has been evaluated, and the results show that the radial extension of small ELMs is restricted to a range of less than the particle flux fall-off width (λjs) along the divertor target. Secondly, it has been found that the occurrence of small ELM burn-through is highly correlated with the degree of divertor detachment and ELM frequency. When the detached area is larger than λjs, the small ELMs in the main plasma could be mitigated or even suppressed, avoiding the burn-through of small ELMs. When the divertor detachment only occurs near the strike point while the ELM frequency is high enough (fELM ≳ 800 Hz), there is also no burn-through of small ELMs. When the degree of divertor detachment is low and ELM frequency is not high simultaneously, the ELM burn-through induced by small ELMs could be observed and is correlated with the competition between ionization and recombination process of the divertor plasma. Further analysis reveals that the burn-through is triggered by the accumulated particles and energy of multiple small ELMs, not by that of a single small ELM under this condition. Finally, statistical analysis indicates that the small ELM burn-through occurs when the detached area is limited to a region of less than 0.4λjs and simultaneously ELM frequency is smaller than 800 Hz.