“Snowflake” H Mode in a Tokamak Plasma

Abstract

An edge-localized mode (ELM) H-mode regime, supported by electron cyclotron heating, has been successfully established in a ``snowflake'' (second-order null) divertor configuration for the first time in the TCV tokamak. This regime exhibits 2 to 3 times lower ELM frequency and 20%--30% increased normalized ELM energy ($\ensuremath{\Delta}{W}_{\mathrm{ELM}}/{W}_{p}$) compared to an identically shaped, conventional single-null diverted H mode. Enhanced stability of mid- to high-toroidal-mode-number ideal modes is consistent with the different snowflake ELM phenomenology. The capability of the snowflake to redistribute the edge power on the additional strike points has been confirmed experimentally.