Plasma Convection Near the Magnetic Null of a Snowflake Divertor During an ELM Event

Abstract

AbstractA snowflake magnetic configuration is created in a tokamak when the poloidal magnetic field and its first spatial derivatives become zero at a certain point. The separatrix then acquires a characteristic hexagonal shape reminiscent of a snowflake. We study new features of the plasma macroscopic equilibrium and stability in the vicinity of the snowflake null. We note that, compared to the standard X‐point divertor, the zone of weak poloidal magnetic field is much larger. The weak poloidal field leads to development of intense plasma convection over the expanded area around the null‐point during the ejection phase of an edge localized mode (ELM) event when the plasma pressure in the scrape‐off layer increases compared to its inter‐ELM value. Intense convection may lead to a roughly‐equal splitting of the heat flux between the 4 snowflake divertor legs and to a broadening of the plasma wetted area in each leg, thereby mitigating damage to divertor plates (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)