Role of runaway electrons in LHCD regimes with improved confinement on CASTOR tokamak

Recent lower hybrid current drive (LHCD) experiments indicate that the reduction of edge plasma electrostatic fluctuations is responsible for the observed improvement of particle confinement in tokamaks at moderate lower hybrid powers. A possible explanation of this reduction is an enhanced population and non-ambipolar losses of runaway electrons during LHCD. This mechanism is demonstrated on the basis of a model which makes it possible to calculate the population and losses of runaway electrons. The model equations are solved using the 1-D Astra code. The experimental data obtained at the Castor tokamak, in particular the variations of the fluctuation level with LH power, are in good agreement with the theory at a reasonable choice of free model parameters. (J.U.) 3 figs., 7 refs