Ion absorption effects in high-harmonic fast wave ray tracing theory

Abstract

Effects of finite ion temperature on the propagation and absorption characteristics of high-harmonic fast waves (HHFW) are investigated theoretically using a hot electron (cold ion) ray tracing code in combination with solutions of the full hot plasma dispersion relation. Ray tracing is performed on numerical solutions of the Grad-Shafranov equation and the hot plasma dispersion relation is solved along the resultant ray trajectory using the cold ion n∥. As was observed previously (see Ref. [1]), for typical expected plasma parameters in the National Spherical Torus Experiment (NSTX) [2], ion absorption begins to appear between 0.5 and 1.0 keV local ion temperature at high deuterium cyclotron harmonics. Further, the ion power absorption rate is predicted to depend strongly on the launched parallel wavenumber [3,4]. Ray tracing on the full hot plasma dispersion relation has been attempted, but generally fails at high ion temperature near cyclotron harmonics primarily because the group velocity is ill-behaved. Such behavior usually suggests mode conversion to the ion-Bernstein wave (IBW). However, at sufficiently high n∥, mode conversion becomes negligible and the total power flux (Poynting+kinetic) is positive definite, while ray tracing still fails. The underlying cause of this apparent paradox is discussed.