Theoretical and experimental analysis of the LHCD power coupling in JET optimised shear plasmas

In recent LHCD experiments in JET, a large effort has been placed on improving the coupling of Lower Hybrid waves in Optimised Shear plasmas, i.e. in conditions with internal transport barriers (ITB) or H-modes. Good coupling conditions have been obtained by using gas injection of CD4 near the LHCD launcher. This has resulted in a reflection coefficient as low as 5% during the ITB phase, at a plasma-launcher distance of 6 cm. Results from the coupling experiments have been compared with the SWAN coupling code, which has recently been extended to take into account the poloidal cut of the launcher mouth. The launcher mouth is composed of twelve rows of waveguides, all with different electrical lengths. By introducing the scattering matrices for all rows and calculating the resulting power splitting in the multijunctions, a more realistic model for the coupling has been obtained.