Theory of lower-hybrid-current drive at low densities

Efficient toroidal current sustainment by LH waves has been demonstrated experimentally on a number of tokamaks at low densities. A theoretical model invoking the nonlinear excitation of magnetized plasma waves has been proposed to explain many interesting features observed in these experiments which include enhanced SXR, cyclotron emissions and electrostatic fluctattions. Analysis of a set of moment equations deduced from this analytical mode predicts the existence of relaxation oscillations in perpendicular temperature and plasma wave energy which have subsequently been observed experimentally in certain situations. Due to the excessive stiffness of the moment equations, it is not possible to ascertain the conditions under which relaxation oscillations will occur. It is thus necessary to resort to a detailed kinetic analysis. We have constructed a Fokker-Planck wave kinetic code which takes into account the nonlinear excitation of the plasma waves and follows the temporal evolution of these waves as well as the electron distribution during the RF current drive. Studies with the code show either relaxation oscillations or a steady-state with significantly higher plasma wave energy depending on rf power level. The details of this and other dependences and comparison with experiments will be reported