A long plasma formation tube is imbedded in a high magnetic field, with magnetic field lines passing axially through the tube, and with the tube being placed proximate or inside of a resonant cavity. Electromagnetic energy resonates in the resonant cavity representing stored microwave energy. The power density of the stored microwave energy is a function of the cross-sectional area of the resonant cavity. A portion of the stored microwave energy is concentrated to increase its power density, and coupled into the plasma formation tube, which tube has a smaller cross-sectional area than the resonant cavity. The coupled energy excites a whistler wave in the plasma formation tube that forms the plasma within the tube. In one embodiment, the stored microwave power is concentrated by funneling it through a metallic iris that forms one end of the resonant cavity, with a tip of the plasma formation tube being positioned near the metallic iris. In another embodiment, the plasma formation tube is positioned coaxially within the resonant cavity and the stored microwave energy is drawn into the plasma formation tube as plasma begins to form therein because the forming plasma represents a dominant power loss (load) for the stored energy.