ON THE ORIGIN OF PARASITIC MODES IN NUMERICAL MODELING OF WAVE-PROPAGATION IN PLASMAS

The mode mixing encountered in solving the boundary value problem arising in studies of Wave propagation in hot one-dimensional plasmas is investigated. The wave differential equations, second order in ion Larmor radius, are shown to contain close to the ion cyclotron resonance an unwanted coupling mechanism, closely related to mode conversion driven by the equilibrium gradients. Consequently, the solution mixes with parasitic modes which carry power out of the resonance zone. In the case of strong coupling, this may lead to a generation of excess energy flux for complex valued coefficients of the wave equations. The importance and effects of this phenomenon are demonstrated in the special cass of simulations of minority ion cyclotron heating in tokamaks. It is shown that the mode mixing can be avoided by adjusting the elements of the coefficient matrix of the wave equations, which makes the second-order system amenable to the problems with high minority concentration which is important for studies of fusion physiCS. (C) 1994 Academic Press, Inc.