REDUCED-ORDER METHODS FOR FULL-WAVE ICRF CALCULATIONS IN TOKAMAKS

Reduced order methods have been used extensively to eliminate ion Bern stein waves (IBWs) from full wave calculations in the ion cyclotron ra nge of frequencies (ICRF). These reduced order methods replace higher order derivatives in the finite Larmor radius (FLR) expansion of the p lasma current with algebraic terms depending on the local perpendicula r wavenumber k(perpendicular to) determined from the fourth order plas ma dispersion relation. However, the dispersion relation determines on ly the magnitude of k(perpendicular to), not the direction. If k(perpe ndicular to) is assumed to be perpendicular to the flux surface, energ y conservation is violated near the magnetic axis. One method of deali ng with this problem is to solve algebraically for the parallel electr ic field E-parallel to, in which case the direction of k(perpendicular to) drops out of the problem completely. Another method, which allows for the complete differential solution of E-parallel to, assumes a di rection for k(perpendicular to). Both methods are limited by assumptio ns. A better approach is to apply the reduced order algorithm to the i on FLR current alone, while treating the electron current differential ly. Since the direction of k(perpendicular to) enters only through ele ctron terms, this method resolves the ambiguity in the direction of k( perpendicular to) while still suppressing the IBW, which originates in the ion current. Comparison with the simpler models suggests that it is necessary to include the differential electron current for an accur ate description of direct electron heating and fast wave current drive near the ion second harmonic.