Recent observations of the impact of the metallic ITER-like Wall (ILW) in European tokamak JET (UK) show that the transition to high confinement mode  (L-H threshold)  is reduced by 40% with respect to similar experiments in the carbon wall. In the german tokamak ASDEX Upgrade, a similar reduction of the threshold when comparing carbon wall to metallic wall was also observed despite quite different divertor configurations, geometries and wall materials. A common feature of both JET-ILW and ASDEX Upgrade is a significant reduction of the plasma effective charge from C walls to metallic ones.

The nature of the primary instability present in the region of the plasma where the confinement is enhanced (pedestal region) prior to the L-H transition is analyzed using a so-called gyro-kinetic code on JET-ILW using temperature and density profile measurements. The linear analysis shows that the primary instability is of resistive nature and can be stabilized when the temperature is increased, hence when the power is increased. The unstable modes are identified as being Resistive Ballooning Modes (RBM). Their growth rates decrease for temperatures increasing towards the experimentally measured temperature at the L-H transition. As the temperature raises further Ion temperature Gradient (ITG) and Trapped Electron Modes (TEM) are destabilized. The low temperature RBM have lower growth rates for lower effective charge Z_eff as illustrated by the figure below. This dependence is shown to be in qualitative agreement with recent and past experimental observations of reduced Z_eff associated with lower L-H power thresholds.

Clarisse Bourdelle, et al, Letter in Nucl. Fusion 54 (2014) 022001