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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, 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 10 cm. To increase n(e) in front of the launcher, n(e,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 n(e,grill) is below the cut-off density. With CD4 puffing, the coupling improves dramatically, and 2.5 MW of LH power is coupled with DPL= 9cm and type I ELMs. Measurements with a reciprocating Langmuir probe show that n(e,grill) is at, or slightly above, the cut-off density, in this case. With D2 puffing, preferable to CD4 for ITER because of concerns about T co-deposition, 3MW was coupled successfully with DPL up to 10cm. The ELMs are smaller with D2, but the improvement in coupling can not be attributed only to this. The use of D2 or CD4 was explored further in other plasmas with ITB and H-mode. In all the scenarios, the coupling is better with D2 than with CD4.

Additional information

Authors: MAILLOUX J et al, EURATOM-UKAEA Fusion Association, Culham Science Centre, Abingdon (GB)
Bibliographic Reference: An oral paper given at: 20th IAEA Fusion Energy Conference Organised by: International Atomic Energy Agency Held at: Centrtro de Fusao Nuclear, Vilamoura (PT)
Availability: Available from Association EURATOM-CEA, Département de Recherches sur la Fusion Contrôlée, CEA Cadarache, F-13108 St Paul-Lez-Durance, France Tel: (+33) 4 42 25 70 01; Fax: (+33) 4 42 25 64 21 E-mail:
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