Enhancing the mode conversion efficiency in JET plasmas with multiple mode conversion layers

The constructive interference effect described by Fuchs et al. [1] shows that the mode conversion and thereby the overall heating efficiency can be enhanced significantly when an integer number of fast wave wavelengths can be folded in between the high field side fast wave cutoff and the ion-ion hybrid layer(s) at which the ion Bernstein or ion cyclotron waves are excited. This effect was already experimentally identified in ( ³He)-D plasmas [2] and was recently tested in ( ³He)-H JET plasmas. The latter is an 'inverted' scenario, which differs significantly from the ( ³He)-D scenarios since the mode-conversion layer is positioned between the low field side edge of the plasma and the ion-cyclotron layer of the minority ³He ions (whereas the order in which a wave entering the plasma from the low field side encounters these layers is inverted in a 'regular' scenario), and because much lower ³He concentrations are needed to achieve the mode-conversion heating regime. The presence of small amounts of ⁴He and D in the discharges gave rise to an additional mode conversion layer on top of the expected one associated with ³He-H, which made the interpretation of the results more complex but also more interesting: Three different regimes could be distinguished as a function of X[ ³He], and the differing dynamics at the various concentrations could be traced back to the presence of these two mode conversion layers and their associated fast wave cutoffs. Whereas (1-D and 2-D) numerical modeling yields quantitative information on the RF absorptivity, recent analytical work by Kazakov [3] permits to grasp the dominant underlying wave interaction physics.