Description
Modelling the electric field of Gaussian beams near the cutoff is important for interpreting diagnostics, such as Doppler backscattering (DBS), and calculating the O-X mode conversion in O-X-B heating and current drive. In slab geometry, beam tracing can overestimate the size of the electric field near the cutoff [1]. Recent work showed that this divergence is integrable and that the beam tracing solution provides an adequate approximation for interpreting DBS [2]. In this work, we compare the electric fields calculated by full-wave (ERMES) and beam-tracing (Scotty) simulations in 2D and 3D tokamak geometry, including for finite toroidal launch angles. We find that at larger poloidal launch angles, such as 7°, 3D beam-tracing is better than 2D full-wave at approximating the 3D full-wave solution. At smaller launch angles, beam-tracing is unable to capture the Airy fringes, but nonetheless provides an estimation of the mean electric field.
[1] O. Maj, G.V. Pereverzev & E. Poli (2009). Validation of the paraxial beam-tracing method in critical cases. Physics of Plasmas, 16(6).
[2] J. Ruiz Ruiz, F.I. Parra, V.H. Hall-Chen, et al. (2025). Beam focusing and consequences for Doppler backscattering measurements. Journal of Plasma Physics, 91(2), E60.