Recent Results of LHCD Coupling Experiments with Near Gas Injection in JET

Dedicated gas injection in the proximity of the Lower Hybrid Current Drive (LHCD) launcher is used routinely in JET for obtaining a good and stable coupling of the LH waves in severe conditions: H-modes, ITB (Internal Transport Barrier) plasmas and presence of strong Edge Localized Modes (ELMs). Moreover this technique is one of the candidates for allowing the coupling of LHCD power in ITER, where the distance between the Last Closed Flux Surface (LCFS) and the wall is expected to be more than 12cm. The dedicated gas pipe (GIM6) was installed in JET in 1995, at a distance of 1.2m from the LH launcher, in the equatorial plane. It has been used in the past to inject deuterium with different results, but has become an essential tool for LH coupling in high power phase of advanced scenario plasmas, injecting CD4. The CD4 was introduced for its larger ionization cross-section (s) and lower ionization potential (12.5eV against the 13.9eV of D2). This can lead to the proper densification of the SOL plasma with lower gas flow with less influence on the quality of confinement. Nevertheless the analysis of the results obtained with CD4 evidenced that the simple local ionization of the gas alone cannot explain the amplitude of the effects (the increase of the SOL density) and some mechanism of modification of transport in the plasma edge has to be involved.
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