Characterization and interpretation of the Edge Snake in between Type-I ELMs at ASDEX Upgrade

A new magneto-hydro-dynamic instability called ’Edge Snake’, which was found in 2006 at the Tokamak ASDEX Upgrade during type-I ELMy H-modes, is investigated. It is located within the separatrix in the region of high temperature and density gradients and has a toroidal mode number of n = 1. The Edge Snake consists of a radially and poloidally strongly localized current wire, in which the temperature and density profiles flatten. This significant reduction of pressure gradient leads to a reduction of the neoclassical Bootstrap current and can plausibly explain the drive of the instability. The experimental observations point towards a magnetic island with a defect current inside the O-point of the island. The Edge Snake is compared to similar instabilities at JET, DIII-D and ASDEX Upgrade.