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

Abstract

A review of the LH current drive experiments carried out on the Tore Supra tokamak ispresented. 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 newlauncher that is designed to couple up to 4 MW during 1000 s at apower density of 25 MW m-2. The heat load on the guard limiter of theantenna and the fast electron acceleration in the near electric fieldof the grill mouth remain at a low level, while the mean reflectioncoefficient never exceeds 10%. The powerful diagnosis capabilities ofhard X ray (HXR) fast electron bremsstrahlung tomography has ledto significant progress in the understanding of LH wavedynamics. The role of the fastest electrons driven by the LH wave isclearly identified. From HXR measurements, an increase of LHcurrent drive efficiency with plasma current is predicted andconfirmed by a direct determination at zero loop voltage. Lower hybrid powerabsorption is observed to be off-axis under almost all plasma conditions,and its radial width clearly depends on the antenna phasing conditions. Acorrelation between the HXR profiles and the onset of an improved coreconfinement is identified in fully non-inductive discharges. Thisregime ascribed to some vanishing of the magnetic shear is found to betransient and usually ends when the minimum of the safety factorbecomes very close to 2, leading to a large MHD activity. Experimentalobservations and numerical simulations suggest that LH power isabsorbed after a few transits across the plasma. However, besidestoroidal mode coupling, additional mechanisms probably contribute toa spectral broadening of the LH wave.