Mode conversion heating of JET plasmas with multiple mode conversion layers

Mode conversion heating has become one of the standard tools to do transport analysis and is often used in rotation experiments (see e.g. [1, 2]). It relies on the mode conversion, at the ion-ion hybrid (IIH) resonance, of the fast wave (FW) launched by standard RF antennas, to shorter wavelength waves that are efficiently damped on electrons. The interference effect described by Fuchs et al. [3] allows to significantly enhance the mode conversion and thereby the overall RF heating efficiency when the machine and plasma parameters are chosen such that an integer number of FW wavelengths can be folded in between the high field side (HFS) FW cutoff and the IIH layer. This effect was already experimentally identified in (3He)-D plasmas [4] and was recently tested in (3He)-H JET plasmas. In the latter case, commonly referred to as an ‘inverted scenario’, the ion-ion hybrid layer is positioned between the antenna on the low field side (LFS) and the ion-cyclotron layer of the minority 3He ions while in standard – e.g. (3He)-D - scenarios the ion-cyclotron layer is in between the IIH layer and the LFS. As shown in the past [5], the (3He)-H scenarios require much lower 3He concentrations, X[3He], to reach the mode-conversion heating regime and their RF wave behavior critically depends on the plasma composition