Experimental investigation of marfes and the density limit in the ASDEX upgrade

In the ASDEX Upgrade, X-point marfe formation and its behaviour up to the density limit is investigated in gas-fuelled ohmically-heated single-null discharges over a wide range of parameters: Ip=0.6-1.2 MA, BTF=1.35-2.4 T, a plasma elongation of 1.6 and Zeff<2. The standard-ion Del B drift is directed towards the X-point. At medium electron densities inevitably a marfe develops in the vicinity of the active X-point. The marfe formation is consistent with a model of thermal instabilities in the radiating edge plasma. Moreover, stable steady-state operation is demonstrated with marfes which can extend significantly into the bulk plasma. The density limit is always connected with quick marfe expansion and movement followed by mode-locking leading to a major disruption. The limit scales linearly with Ip and is in good agreement with the Greenwald density limit scaling. The resulting experimental Hugill limit is neRq95/BTF=2.8*1020) m-2 T-1. Reversal of the ion Del B drift direction away from the target has a detrimental effect on the density limit.