Progress towards high-power lower hybrid current drive in TORESUPRA

Abstract

With a new lower hybrid (LH) antenna that will allow us to couple up to 4 MWfor 1000 s at a power density of less than 25 MW m-2, and improveddiagnosis capabilities provided by the hard x-ray (HXR) fast electronbremsstrahlung tomography, significant progress has been made towards theachievement of fully non-inductive current driven plasmas in TORE SUPRA. Verypromising performances have been obtained with the new launcher successfullytested up to 3 MW. At equivalent power levels, coupling properties behave likethose of the former antennas (mean reflection coefficient ⩽10%), but the heat load on the guard limiter as well as thefast electron acceleration in the near electric field of the grillmouth are considerably reduced. Using the two launchers, a full non-inductive discharge has beensustained without MHD activity for 9.5 s by 4.7 MW of LH power, at a plasmacurrent Ip of 0.8 MA. The mean LH current drive efficiency atzero-loop voltage and for plasma currents lower than 0.8 MA is found to be0.65×10+19 A W-1 m-2. HXR measurements suggest asignificant improvement of the current drive efficiency with Ip, as aconsequence of a widening of the quasilinear plateau towards highervelocities, a tendency which is well confirmed experimentally. Profiles of theHXR emission indicate that LH power is absorbed in a very few number of passesat Bt = 3.9 T, the plasma equilibrium playing in this case an importantrole in the wave dynamics, in agreement with ray-tracing andFokker-Planck calculations. However, besides toroidal mode coupling,additional mechanisms are likely to contribute to a spectral broadening of theLH wave. When LH power absorption is broad and off-axis, an improved electroncore confinement is observed in fully non-inductive discharges. This regime,which is ascribed to some vanishing of the magnetic shear, is found to betransient and usually ends when the safety factor becomes very close to 2,leading to the onset of large MHD activity.