ITER relevant coupling of lower hybrid waves in JET

Lower Hybrid (LH) is a highly desirable tool for ITER, particularly for the steady state and hybrid scenarios that rely on forming and maintaining a specific q profile. However, coupling of the LH waves in ITER represents a challenge because the launcher, flush with the outer wall, will be at least 12cm away from the Last Closed Flux Surface (LCFS), where the electron density, ne, is predicted to be below the cut-off density. Also, the plasma will be in H-mode with ELMs that could trigger trips of the protection systems, resulting in reduction of the averaged power. To study solutions to improve coupling of LH waves in these conditions, experiments were performed in JET using plasmas with Internal Transport Barrier (ITB) and H-mode, with distance between the LCFS and the limiter, DPL, up to 10cm. To increase ne in front of the launcher, ne,grill, CD4 or D2 is puffed from a specially designed pipe, recently modified to optimise the gas flow. When no gas is puffed, the LH coupling is poor during the high power phase, indicating that ne,grill is below the cut-off density. With CD4 puffing, the coupling improves dramatically, and 2.5MW of LH power is coupled with DPL= 9cm and type I ELMs.