Effect of gas injection during LHCD experiments in ITER-relevant coupling conditions

LHCD experiments were carried out on the JET tokamak in ITER-relevant conditions which include: low (δ ∼ 0.22) and high (δ ∼ 0.45) triangularity ELMy plasmas, high safety factor (q₉₅ = 4.0 - 6.8) and gaps between the separatrix and the antenna varying between 0.08 m and 0.16 m. Good coupling conditions, throughout the entire antenna, are found when the power injection is assisted by local D2 gas feed. The beneficial modification of the SOL in the flux tubes passing in front of the antenna is documented by measurements of a reciprocating Langmuir probe. This allowed to couple up to 3.1 MW, with a gap of 0.15 m, corresponding to a power density of 15 MW/m² , but 22 MW/m² on half of the antenna which is close to the power density required at 3.7 GHz for ITER. modelling of the modification of the SOL by LH power absorption was performed with the EDGE2D fluid code. Flat j_sat /density profiles similar to experimental ones are obtained when a small fraction of the LH power is supposed to be absorbed in a ∼ 20 mm thick layer in front of the antenna. The parallel heat flux (F_|) on a plasma-facing component due to LH power dissipation in the SOL is computed from infra-red data. The strong dependence of F_parallel with the injected LH power gives further evidence of the LH-induced density modification. It is concluded that with optimized gas injection F_parallel in H mode should not exceed 5 MW/m² at maximum power density (25 MW/m²). The effect of large gap and gas injection on LH current drive efficiency was studied in L-mode discharges for which the real-time control was used in order to keep. (author)