Parametric instability and lower hybrid current drive at plasma densities required for thermonuclear reactors

The lower hybrid current drive (LHCD), produced by externally launched lower hybrid (LH) wave, potentially provides the most suitable tool for driving current at large radii of fusion relevant plasmas, as required for the advanced scenarios of operation foreseen in the International Tokamak Experiment Reactor (ITER). In this regard, a critical problem exists due to the circumstance that the LH waves do not penetrate into the plasma cores neither of experiments aimed at heating the plasma ions nor of those aimed at driving current in presence of plasma densities approaching those required for ITER. We present here results of the analysis of parametric instability (PI) of LH waves showing that high growth rates occur for ion-sound quasi-mode-driven PIs, considering the plasma parameters of the plasma periphery of the FTU tokamak (Frascati Tokamak Upgrade). The non-linear PI phenomenon can broaden the launched antenna spectra in frequency and parallel wavenumber (n//), and, consequently, produce via quasi-linear effect a deposition at the very periphery of high-density plasmas. As result, higher electron temperatures at the plasma edge have the effect of diminishing the PI growth rate and, potentially, the broadening of the launched spectrum, thus enabling the penetration of the coupled LH power in the core of high density reactor grade plasmas. © 2010 IOP Publishing Ltd.