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 (d ~ 0.22) and high (d ~ 0.45) triangularity ELMy plasmas, high safety factor (q95 = 4.0-6.8) and gaps between the separatrix and the antenna varying between 0.08m and 0.16m. 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 3.1MW, with a gap of 0.15m, corresponding to a power density of 15MW/m2, but 22 MW/m2 on half of the antenna which is close to the power density required at 3.7GHz for ITER. Modeling of the modification of the SOL by LH power absorption was performed with the EDGE2D fluid code. Flat jsat/density profiles similar to experimental ones are obtained when a small fraction of the LH power is supposed to be absorbed in a ~20mm thick layer in front of the antenna. The parallel heat flux (F//) on a plasma-facing component due to LH power dissipation in th SOL is computed from infra-red data. The strong dependence of F// with the injected LH power gives further evidence of the LH-induced density modification. It is concluded that with optimized gas injection F// in H mode should not exceed 5MW/m2 at maximum power density (25MW/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 the loop voltage constant (0.2V) and leave the plasma current floating. The plasma current of the large gap/gas injection case is lower than that the small gap/no injection case by 5%. At the same time a significant increase of density fluctuations measured by reflectometry at the plasma periphery is measured for the large gap/gas injection case. The effect of the gas injection on the energy confinement is investigated. No effect on the global confinement or on the neutron rate was observed in the advanced tokamak scenario, Only a slight modification of the type-I ELMs amplitude and frequency was observed when the connected pipe puffed gas. The confinement degrades with the line-averaged density normalized to the Greenwald density, identically for pulses for which the connected gas injection is used or not.
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