High Plasma Density Lower-Hybrid Current Drive in the FTU Tokamak

Abstract

Studies on the current-drive (CD) efficiency have been carried out in the FTU tokamak with 8 GHz lower-hybrid waves up to line-averaged plasma density ${\overline{n}}_{e}>1\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{m}}^{\ensuremath{-}3}$. High efficiencies, larger than $0.2\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{m}}^{\ensuremath{-}2}\mathrm{A}/\mathrm{W}$, are obtained for clean plasma conditions, with no significant degradation as the density is increased up to the accessibility limit. The electron temperature affects favorably the CD efficiency. Impurity influx never limits the machine operations up to the maximum coupled power of 1.7 MW.