The field distribution of a focused Gaussian beam reflected at 45° from a conducting plane and its effects in plasma-ignition experiments

The electrical field distribution over the focal region of a Gaussian beam, incident at 45° and focused onto a perfectly conducting infinite plane, is calculated numerically by summing the plane waves in an angular spectrum representation. The beam is plane-polarized with the transverse electric field perpendicular to the plane of incidence. The configuration and parameters were chosen to agree with our experiments on plasma initiation near a metallic conductor by 35-GHz/8.6-mm microwave radiation. The results of the calculation show that the field distribution determines the ignition threshold, spatial location, and initial development of the microwave plasma at a metal surface.