On the bootstrap effect in tokamaks with lower hybrid driven currents of an opposite sign near the plasma boundary than in the plasma interior

The bootstrap effect was observed in both the TFTR and the JET tokamaks. The effect was found to have potentially a considerable influence on the amplification of the driven current in case of the lower hybrid current drive. The question is discussed whether the bootstrap effect can be the cause of the failure in observing the so-called opposite current. Part of the power radiated by the waveguide structures used for launching the lower hybrid waves could drive the current near the plasma boundary in the direction opposite to that of the useful lower hybrid driven current closer to the plasma centre. It is shown using calculations that the bootstrap effect, and even a rather weak bootstrap effect can lead to the disappearance of the opposite current near the plasma boundary. The results are also shown graphically. (M.D.). 2 figs., 7 refs