Theory of waveguide antennas for plasma heating and current drive

摘要

A computational model is developed that allows one to estimate in a real 3-D geometry the electrodynamic characteristics of complicated waveguide antennas ('grills') generating plasma waves in the lower hybrid frequency band. The antenna coupling efficiency and the shape of the wavenumber spectrum are found as a solution of a self-consistent problem taking into account a complete set of waveguide eigenmodes and finite waveguide dimensions in both directions and an arbitrary orientation to the plasma magnetic field. Electric and magnetic fields inside the waveguides are equated to the outside fields represented as a Fourier sum over wavenumbers Ny, Nz in the plane parallel to the plasma surface. The fields at the plasma edge are determined by the 2*2 plasma surface impedance matrix found as a numerical solution of the wave equation in the cold plasma approximation using the finite element (Galerkin) method. The solution is found on a 1-D mesh, i=1, 2, ..., N, in the form E(xi,Ny,Nz) exp(i(Nyy+Nzz- omega t)) so that the fourth order differential equation with appropriate boundary conditions is reduced to a set of 4N+2 algebraic equations. The developed model is applied to modelling a quite new antenna design for generating the fast H waves, which can be used for plasma heating and current drive in tokamaks of ITER scale. The antenna parameters are optimized to obtain the best coupling efficiency. The structure of waves excited in the plasma at various angles between the antenna and the plasma magnetic field is considered. The nature of the waves with Nz<1 excited by the grill is discussed.