DIFFRACTION EFFECTS FOR FOCUSED ELECTROMAGNETIC-WAVES IN A PLASMA

The geometric optics approximation is the commonly used description for wave propagation in media where the usual local Wentzel-Kramers-Brillouin (WKB) condition is satisfied. However, diffraction phenomena can give important corrections to a ray trajectory and scattering description when focused waves are launched. The general mathematical description for such electromagnetic beams is formulated for a general magnetized plasma and shown to be of a simple functional form. For propagation perpendicular to the magnetic field in a cold plasma the equations are shown to be particularly simple and specifically demonstrate the importance of retaining anisotropic plasma effects for focused beam experiments. In an equilibrium plasma with a particular class of RF boundary conditions the general functional form is shown to be solvable analytically and the solutions expressed in terms of solutions of the ray approximation.