L to H Mode Transition: On the Role of Zeff

In this paper, the nature of the primary instability present in the pedestal forming region prior to the transition into H mode will be analyzed by a gyrokinetic code GENE on JET ITER Like Wall (ILW) profiles. It will be shown that the primary instability is of resistive nature, and can therefore be stabilized by increased temperature, hence power. Its growth rate reaches a minimum for a temperature of the order of the experimentally measured temperature at the transition. Its resistive nature leads to a reduced growth rate as the effective charge Zeff is increased. This dependence will be shown to be in qualitative agreement with recent experimental observations. The impact of the ILW shows a L to H mode power threshold reduced by ~ 40% in ILW with respect to similar experiments in C wall. The experiments were carried out with slow power ramps and matched plasma shapes, divertor configurations and Ip/BT pairs. In ASDEX Upgrade as well, a reduction of the power threshold, linked to the metalization of the machine, has been observed when compared to the ITPA-2008 scaling law. A common feature of JET ILWand ASDEX Upgrade is an observed significant reduction of the Zeff when switching from C walls to metallic ones.
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