A decrease in the L-H transition power threshold under the lithium wall conditioning has been observed since 2010 in EAST. The physical mechanism responsible for the reduced is found to be related to three phases of wall recycling as determined by the accumulation of lithium in the vessel. In phase I (a few rounds of wall conditioning) the wall recycling is low, while no clear variation of L-H transition power threshold is observed. In phase II (after another few rounds of wall conditioning) a clear increase in the wall recycling is observed, leading to decreased L-H transition power threshold. In phase III the wall recycling is higher and the hydrogen concentration is lower compared to phases I and II. In this third phase the L-H transition power threshold is mainly reduced by the decreased . During phase III of the wall recycling, the quantitative relationship between the hydrogen content and normalized L-H transition power threshold is studied for campaigns in 2010, 2012, 2015 and 2016. Data scaling reveals that in the hydrogen concentration range of 3–45%, i.e. increases significantly for , while it increases mildly for .
On the role of the edge density profile for the L–H transition power threshold in ASDEX Upgrade