Current drive at plasma densities required for thermonuclear reactors.

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

Dynamics of tearing modes during strong electron cyclotron heating on the FTU tokamak.

The localized electron cyclotron resonance heating power that can suppress sawteeth reconnection often drives m = 2 tearing modes in a tokamak operating at constant current. The dynamics of mode onset and coupled mode evolution is described in detail and compared with a nonlinear theoretical model that identifies the effects of mode coupling, finite inertia of the rotating islands, and wall braking.