Weakly Relativistic and Nonrelativistic Estimates of EBW Heating in the TJ-II Stellarator

The relativistic effects on electron Bernstein wave (EBW) heating of plasmas confined in the TJ-II stellarator are presented in this work. The Ordinary-eXtraordinary-Bernstein mode conversion at the fundamental electron cyclotron harmonic (f = 28 GHz for the TJ-II central magnetic field) is chosen as the scenario for these estimates. This heating scheme presents high absorbed power for central densities above 1.2 × 10¹⁹ m^-3 and has no upper density limit. Relativistic and nonrelativistic calculations have been performed using the TRUBA beam/ray-tracing code. For this purpose, the weakly relativistic dispersion relation valid for any values of the parallel and perpendicular refractive indexes, thus suitable for EBW, has been obtained. This dispersion relation has been introduced in TRUBA to estimate the ray trajectories and the power absorption to all orders of Larmor radius in the weakly relativistic regime. The result of our comparison is that the relativistic effects are not negligible and must be taken into account both on the ray trajectories and in the power absorption estimations. We also show that the relativistic absorption coefficient is lower than the nonrelativistic one, for the values of parallel refractive index that happen in TJ-II, and the power deposition profile is more centered.