ELECTRON ACCELERATION BY A LOCALIZED RF FIELD AND SPECTRUM GAP PROBLEM IN LOWER-HYBRID CURRENT DRIVE

Electron acceleration by a localized r.f. field is considered as a mec hanism to fill the spectrum gap in lower hybrid current drive (LHCD). A simple one-dimensional model along the magnetic lines of force has b een employed. The electron dynamics have been studied by numerically s olving the equation of motion in a model localized r.f. field for a gi ven initial distribution of test electrons. Special attention is focus ed on analyses for the interaction between the r.f. field and the elec trons, whose initial velocity is well below the phase velocity of the wave to understand the acceleration mechanism. Moreover, model calcula tions have been performed using realistic wave parameters (localized w idth, wave number etc.) for typical LHCD experimental conditions in bo th medium and large tokamaks to examine whether or not the mechanism c onsidered here is a possible solution to fill the spectrum gap.