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(EX8/2) High Power Lower Hybrid Current Drive Experiment in TORE SUPRA Tokamak

Y. Peysson1) and the TORE SUPRA Team1)
1) Association Euratom-CEA pour la Fusion Contrôlée, CEA Cadarache, France

Abstract.  A review of the Lower Hybrid (LH) current drive experiments carried out on the TORE SUPRA tokamak is presented. This work highlights the issues for an effective application of the LH wave at high power in reactor relevant conditions. Very promising performances have been obtained with the new launcher that is designed to couple up to 4 MW during 1000 s at a power density of 25 MWm- 2. The heat load on the guard limiter of the antenna and the fast electron acceleration in the near electric field of the grill mouth remain at a low level, while the mean reflection coefficient never exceeds 10%. The powerful diagnosis capabilities of the hard x-ray (HXR) fast electron bremsstrahlung tomography has led to significant progresses in the understanding of the LH wave dynamics. The role of the fastest electrons driven by the LH wave is clearly identified. From HXR measurements, an increase of the LH current drive efficiency with the plasma current is predicted and confirmed by a direct determination at zero loop voltage. LH power absorption is observed to be off-axis in almost all plasma conditions, and its radial width clearly depends of antenna phasing conditions. A correlation between the HXR profiles and the onset of an improved core confinement is identified in fully non-inductive discharges. This regime ascribed to some vanishing of the magnetic shear is found to be transient and usually ends when the minimum of the safety factor becomes very close to 2, leading to a large MHD activity. Experimental observations and numerical simulations suggest that LH power is absorbed in a few number of passes. However, besides toroidal mode coupling, additional mechanisms may likely contribute to a spectral broadening to the LH wave.

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IAEA 2001