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, n(e), 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, reducing 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, D(PL), up to 10 cm. To increase n(e) in front of the launcher, n(e,grill), gas is puffed from a specially designed pipe, recently modified to optimise the gas flow. When no gas is puffed, the coupling is poor, indicating that n(e,grill) < the cut-off density. With CD(4) puffing, the coupling improves dramatically and 2.5 MW of LH power is coupled with DPL= 9cm and type I ELMs. With D(2), preferable to CD(4) for ITER because of concerns about T co-deposition, 3MW was coupled with D(PL) up to 10cm. The ELMs are smaller with D(2), but the improvement in coupling is not due only to this. The use of D(2) or CD(4) was compared in other plasmas with ITB and H-mode. In all cases, the coupling is better with D(2) than with CD(4). Moreover, the ITB existence and performance are not affected by the D(2). To help understand the processes leading to the n(e,grill) increase, C(2)H(6) and C(3)H(8) were compared to CD(4) and D(2) in the same scenario. n(e) scales with the ionisation cross-section in the case of the hydro carbon gases, but is higher than expected for the D(2), possibly because of higher D(2) recycling from the walls. In a few shots, bright spots on components magnetically connected to the LH launcher have been observed with a CCD camera. They are probably caused by fast particles created parasitically in the plasma in front of the launcher, as seen in other machines. LH counter current drive was studied in plasmas with reversed magnetic field and plasma current. Simulations indicate a current drive efficiency of the same order of magnitude than in the co current drive case.