Progress towards high-power lower hybrid current drive in TORE SUPRA

With a new lower hybrid (LH) antenna that will allow us to couple up to 4 MW for 1000 s at a power density of less than 25 MW m-2, and improved diagnosis capabilities provided by the hard x-ray (HXR) fast electron bremsstrahlung tomography, significant progress has been made towards the achievement of fully non-inductive current driven plasmas in TORE SUPRA. Very promising performances have been obtained with the new launcher successfully tested up to 3 MW. At equivalent power levels, coupling properties behave like those of the former antennas (mean reflection coefficient ≤10%), but the heat load on the guard limiter as well as the fast electron acceleration in the near electric field of the grill mouth are considerably reduced. Using the two launchers, a full non-inductive discharge has been sustained without MHD activity for 9.5 s by 4.7 MW of LH power, at a plasma current Ip of 0.8 MA. The mean LH current drive efficiency at zero-loop voltage and for plasma currents lower than 0.8 MA is found to be 0.65×10+19 A W-1 m-2. HXR measurements suggest a significant improvement of the current drive efficiency with Ip, as a consequence of a widening of the quasilinear plateau towards higher velocities, a tendency which is well confirmed experimentally. Profiles of the HXR emission indicate that LH power is absorbed in a very few number of passes at Bt = 3.9 T, the plasma equilibrium playing in this case an important role in the wave dynamics, in agreement with ray-tracing and Fokker-Planck calculations. However, besides toroidal mode coupling, additional mechanisms are likely to contribute to a spectral broadening of the LH wave. When LH power absorption is broad and off-axis, an improved electron core confinement is observed in fully non-inductive discharges. This regime, which is ascribed to some vanishing of the magnetic shear, is found to be transient and usually ends when the safety factor becomes very close to 2, leading to the onset of large MHD activity.