On the role of the hydrogen concentration in the L-H transition power threshold in EAST

Abstract A decrease in the L-H transition power threshold P L-H under the lithium wall conditioning has been observed since 2010 in EAST. The physical mechanism responsible for the reduced P L-H 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 n H / ( n H + n D ) is lower compared to phases I and II. In this third phase the L-H transition power threshold is mainly reduced by the decreased n H / ( n H + n D ) . During phase III of the wall recycling, the quantitative relationship between the hydrogen content and normalized L-H transition power threshold P L-H / P s c a l e is studied for campaigns in 2010, 2012, 2015 and 2016. Data scaling reveals that P L − H / P s c a l e = ( − 1.513 ± 0.135 ) × e ( − 6.989 ± 0.362 ) × n H / ( n H + n D ) + ( 1.698 ± 0.050 ) in the hydrogen concentration range of 3–45%, i.e. P L-H / P s c a l e increases significantly for 3 % < n H / ( n H + n D ) < 20 % , while it increases mildly for 20 % < n H / ( n H + n D ) < 45 % .