High power lower hybrid current drive experiments in the Tore Supra tokamak

A review of the LH current drive experiments carried out on the Tore Supra tokamak is presented. The issues for an effective application of the LH wave at high power in reactor relevant conditions are highlighted. A promising performance has been obtained with the new launcher that is designed to couple up to 4 MW during 1000 s at a power density of 25 MW m(-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 hard X ray (HXR) fast electron bremsstrahlung tomography has led to significant progress in the understanding of LH wave dynamics. The role of the fastest electrons driven by the LH wave is clearly identified. From HXR measurements, an increase of LH current drive efficiency with plasma current is predicted and confirmed by a direct determination at zero loop voltage. Lower hybrid power absorption is observed to be off-axis under almost all plasma conditions, and its radial width clearly depends on the 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 after a few transits across the plasma. However, besides toroidal mode coupling, additional mechanisms probably contribute to a spectral broadening of the LH wave.