Theoretical and numerical studies of wave-packet propagation in tokamak plasmas

摘要

Theoretical and numerical studies of wave-packet propagation are presented toanalyze the time varying 2D mode structures of electrostatic fluctuations intokamak plasmas, using general flux coordinates. Instead of solving the 2D waveequations directly, the solution of the initial value problem is used to obtainthe 2D mode structure, following the propagation of wave-packets generated by asource and reconstructing the time varying field. As application, the 2D WKBmethod is applied to investigate the shaping effects (elongation andtriangularity) of tokamak geometry on the lower hybrid wave propagation andabsorbtion. Meanwhile, the Mode Structure Decomposition (MSD) method is used tohandle the boundary conditions and simplify the 2D problem to two nested 1Dproblems. The MSD method is related to that discussed earlier by Zonca and Chen[Phys. Fluids B 5, 3668 (1993)], and reduces to the well-known "ballooningformalism" [J. W. Connor, R. J. Hastie, and J. B. Taylor, Phys. Rev. Lett. 40,396 (1978)], when spatial scale separation applies. This method is used toinvestigate the time varying 2D electrostatic ITG mode structure with a mixedWKB-full-wave technique. The time varying field pattern is reconstructed andthe time asymptotic structure of the wave-packet propagation gives the 2Deigenmode and the corresponding eigenvalue. As a general approach toinvestigate 2D mode structures in tokamak plasmas, our method also applies forelectromagnetic waves with general source/sink terms, either by aninternal/external antenna or nonlinear wave interaction with zonal structures.